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  • 151.
    Alfieri, Luisa
    et al.
    Department of Electrical Engineering and Information Technology, University of Naples Federico II, Italy.
    Bracale, Antonio
    Department of Engineering, University of Naples Parthenope, Centro Direzionale of Naples.
    Larsson, Anders
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    New Power Quality Indices for the Assessment of Waveform Distortions from 0 to 150 kHz in Power Systems with Renewable Generation and Modern Non-Linear Loads2017In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 10, article id 1633Article in journal (Refereed)
    Abstract [en]

    The widespread use of power electronics converters, e.g., to interface renewable generation systems with the grid or to supply some high-efficiency loads, has caused increased levels of waveform distortions in the modern distribution system. Voltage and current waveforms include spectral components from 0 kHz to 150 kHz, characterized by a non-uniform time-frequency behavior. This wide interval of frequencies is currently divided into "low-frequency" (from 0 kHz to 2 kHz) and "high-frequency" (from 2 kHz to 150 kHz). While the low-frequencies have been exhaustively investigated in the relevant literature and are covered by adequate standardization, studies for the high-frequencies have been addressed only in the last decade to fill current regulatory gaps. In this paper, new power quality (PQ) indices for the assessment of waveform distortions from 0 kHz to 150 kHz are proposed. Specifically, some currently available indices have been properly modified in order to extend their application also to wide-spectrum waveforms. In the particular case of waveform distortions due to renewable generation, numerical applications prove that the proposed indices are useful tools for the characterization of problems (e.g., overheating, equipment malfunctioning, losses due to skin effects, hysteresis losses or eddy current losses) in cases of both low-frequency and high-frequency distortions

  • 152.
    Alfjorden, Rikard
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Black liquor to advanced biofuel: A techno-economic assessment2019Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis looked at a biorefinery pilot plant that converted lignin in black liquor into biofuel. A heat/mass balance was made which was used to create a heat/mass balance for a theoretical large-scale plant. This then created the CAPEX for building the plant. OPEX for the largescale plant and income from sold biofuels was calculated and payback time found. This was done for three different cases with different flows and yield to optimize the plant. A sensitivity analysis was then made to find the most important parameters regarding CAPEX, OPEX and payback time.

  • 153.
    Alhalaweh, Amjad
    et al.
    Luleå University of Technology, Department of Health Sciences, Medical Science.
    Alzghoul, Ahmad
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaialy, Waseem
    Chemistry and Drug Delivery Group, Medway School of Pharmacy, University of Kent.
    Data mining of solubility parameters for computational prediction of drug–excipient miscibility2014In: Drug Development and Industrial Pharmacy, ISSN 0363-9045, E-ISSN 1520-5762, Vol. 40, no 7, p. 904-909Article in journal (Refereed)
    Abstract [en]

    Computational data mining is of interest in the pharmaceutical arena for the analysis of massive amounts of data and to assist in the management and utilization of the data. In this study, a data mining approach was used to predict the miscibility of a drug and several excipients, using Hansen solubility parameters (HSPs) as the data set. The K-means clustering algorithm was applied to predict the miscibility of indomethacin with a set of more than 30 compounds based on their partial solubility parameters [dispersion forces , polar forces and hydrogen bonding ]. The miscibility of the compounds was determined experimentally, using differential scanning calorimetry (DSC), in a separate study. The results of the K-means algorithm and DSC were compared to evaluate the K-means clustering prediction performance using the HSPs three-dimensional parameters, the two-dimensional parameters such as volume-dependent solubility and hydrogen bonding , and selected single (one-dimensional) parameters. Using HSPs, the prediction of miscibility by the K-means algorithm correlated well with the DSC results, with an overall accuracy of 94%. The prediction accuracy was the same (94%) when the two-dimensional parameters or the hydrogen-bonding (one-dimensional) parameter were used. The hydrogen-bonding parameter was thus a determining factor in predicting miscibility in such set of compounds, whereas the dispersive and polar parameters had only a weak correlation. The results show that data mining approach is a valuable tool for predicting drug–excipient miscibility because it is easy to use, is time and cost-effective, and is material sparing.

  • 154.
    Al-Hamadandy, Raied
    et al.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Goss, J.P.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Briddon, P.R.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Mojarad, Shahin A.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Al-Hadidi, Meaad
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    O'Neill, A.G.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Rayson, Mark
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Oxygen vacancy migration in compressively strained SrTiO32013In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 2Article in journal (Refereed)
    Abstract [en]

    The intrinsic properties of strontium titanate render it promising in applications such as gate dielectrics and capacitors. However, there is growing evidence that oxygen vacancies significantly impact upon its use, with the diffusion and deep donor level of the oxygen vacancy leading to electrical leakage. Where grown epitaxially on a lattice mismatched substrate, SrTiO 3 undergoes bi-axial strain, altering its crystal structure and electronic properties. In this paper, we present the results of first-principles simulations to evaluate the impact of strain in a (001) plane upon the migration of oxygen vacancies. We show that in the range of strains consistent with common substrate materials, diffusion energies in different directions are significantly affected, and for high values of strain may be altered by as much as a factor of two. The resulting diffusion anisotropy is expected to impact upon the rate at which oxygen vacancies are injected into the films under bias, a critical factor in the leakage and resistive switching seen in this material

  • 155.
    Al-Hamadany, R.
    et al.
    Newcastle University.
    Goss, J.P.
    Newcastle University.
    Briddon, P.R.
    Newcastle University.
    Mojarad, S.A.
    Newcastle University.
    O'Neill, A.G.
    Newcastle University.
    Rayson, Mark
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Impact of tensile strain on the oxygen vacancy migration in SrTiO3: Density functional theory calculations2013In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 22Article in journal (Refereed)
    Abstract [en]

    Strontium titanate is a promising dielectric material for device applications including capacitors and gate dielectrics. However, oxygen vacancies, which are inevitable donor defects mobile under bias at room temperature, lead to undesirable leakage current in SrTiO3 thin films. Epitaxially grown SrTiO3 on lattice mismatched substrates leads to strained SrTiO3, inducing structural phase transitions from a cubosymmetric non-ferroelectric geometry to tetragonal and orthorhombic structures, depending upon the sign of the strain. In this study, density functional calculations have been performed to determine the impact of isotropic biaxial tensile strain in a (001) plane upon the phase of SrTiO3 and the activation energy for the migration of oxygen vacancies in such strained SrTiO3. The phase transition of the host material yields anisotropy in oxygen vacancy diffusion for diffusion within and between planes parallel to the strain. We found a general reduction in the barrier for diffusion within and normal to the plane of tensile strain. The inter-plane diffusion barrier reduces up to 25% at high values of strain. The variation in the barrier corresponding to in-plane diffusion is smaller in comparison to inter-plane diffusion. Finally, we reflect upon how the interplay between lattice strain with native defects plays a crucial role in the conduction mechanism of thin film, strained SrTiO3

  • 156.
    Allali, Naoual
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Covalent functionalization of carbon nanomaterials for bioelectrochemical applications2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Carbon nanotubes (CTNs) are renowned for their exceptional electronic and mechanical properties. Their structure can be considered as rolling up a graphene sheet along a specific crystallographic direction, leading to a 1D confinement of the electronic wavefunction of the delocalized electrons along the perimeter of the cylindrical structure thus obtained. This confinement produces the existence of defined spikes of high intensity in the electronic density of states, called van Hove singularities. These singularities are primordial to understand both the optical and electronic properties of CNTs through electronphonon coupling processes. If the electronic density of states (DoS) is non zero at the Fermi level the nanotube is metallic, otherwise the nanotube is semiconducting. The synthesis of CNTs always produces a mixture of both metallic and semiconducting nanotubes, and this material can be useful to be incorporated at the surface of electrodes for electrochemical devices. The high specific surface area, the high mechanical and thermal stability of CNTs and the low percolation threshold for electron transport in a mat of CNTs render them very attractive for such kind of applications. There is yet a drawback of using raw CNTs: they are not compatible with solvents and modification of their surfaces by chemistry is required to make good suspensions for easy deposition at the electrode surface and to introduce specific functional groups for promoting electron transfer, called electron shuttles.

    The final aim of this thesis is therefore the covalent functionalization of CNTs by electron shuttles and their incorporation at the surface of glassy carbon electrodes for electrochemical devices application. A strategy of chemical grafting in three steps has been chosen: i) a controlled oxidation step in acidic media assisted by microwave irradiation in order to keep the structural integrity of CNTs, so as to save their useful electronic properties; ii) a chloration step to produce acid chloride groups and iii) reaction of these groups with electron shuttles modified by specific linkers. The study was first conducted on very clean HiPCO single-walled CNTs (SWCNTs). This enabled to avoid any disturbing effects of carbonaceous impurities or residual catalytic particles, since their possible effects are extremely controversial in the literature. Once validated, this approach was then conducted with cheaper material including few-walls carbon nanotubes (FWCNTs). The use of FWCNTs compared to SWCNTs was not only beneficial for the production of costeffective electrochemical devices but also for a better durability ofthe final device, the inner nanotubes being not functionalized.

    The challenge was to obtain a functionalization process with enough grafted electron shuttles to obtain a good electrocatalytic activity but maintaining CNTs integrity. The first step is predominant to reach this goal, and requires a very accurate understanding of the nature and the number of defects created in the CNTs structure versus the physico-chemical conditions used. The introduction of defects in the crystallographic structure of CNTs has strong consequences both for the electronic DoS and for the phononic properties of the material. Spectroscopic methods are essential in probing these consequences. UV-visible-near IR absorption spectroscopy is the method of choice to directly probe the existence of van Hove singularities and the oscillator strength associated with the authorized electronic transitions between theses ingularities. Covalent grafting of chemical groups at the surface of CNTs changes both the energy and the intensity of these transitions. However, this spectroscopic method requires solubilizing CNTs in non-absorbing solvents using adequate surfactants. Interactions between surfactant molecules and CNT sidewalls may also alter the position and intensity of electronic transitions between van Hove singularities unrelated to the chemical groups covalently grafted.

    Raman spectroscopy of CNTs involves the electronphonon coupling processes through the resonant electronic enhancement of Raman modes. Double resonance processes are also observed in Raman spectrum of CNTs, for instance with the D-band mode that is actually related to the existence of defects in the graphene structure of CNTs. Therefore, Raman spectroscopy is a widespread analytical method to characterize the structural defects created by covalent functionalization processes. Indeed, the intensity ratio of the D and G bands in the Raman spectrum is correlated to the number of defects. However, CNTs are used as bundles when chemical functionalization is performed, which produces a heterogeneous distribution of chemical species grafted on CNTs. Therefore, we have developed a new protocol to obtain statistically significant data for most of the samples made in this thesis. Nevertheless, this statistical approach is still limited for samples slightly functionalized, whence the idea to use spectroscopic ellipsometry as an alternative method to characterize these samples.

    More specifically, ellipsometric data were collected from UV to the IR part of the electromagnetic spectrum for CNTs functionalized in different conditions. The complex dielectric function was retrieved from the experimental data. A Drude model was used to model the infrared part of the data for raw and acid oxidized CNTs. The optical conductivity of the samples was obtained. These results, combined with other information collected using a set of complementary analytical techniques (Raman scattering, UV-visible-NIR absorption, X-ray photoelectron spectroscopy, thermogravimetric analysis coupled to mass spectrometry, transmission electron microscopy and rare gas volumetric adsorption), show that the microwave-assisted oxidation process actually consists in removing amorphous carbon deposits away from the surface of CNTs and transforming the already existing defects in the CNT structure to oxygen-containing groups such as carboxylic acids.

    Rare gas volumetric adsorption was also used to compare the distribution of chemical groups at the surface of CNT bundles when two different acids are used (HNO3 and H2SO4). The chloration step was also studied by these methods, as well as the final grafting of electron shuttles. Finally, these functionalized CNTs were deposited at the surface of glassy carbon electrodes and used as electron mediators for diaphorase-catalysed oxidation of nicotinamide adenine dinucleotide (NADH). This was a good example of mediated electron transfer for development of electrochemical devices based on NADH recycling and it validated the good electrocatalytic properties of functionalized CNTs for making electrochemical sensors and actuators, opening new perspectives with potential market applications.

  • 157.
    Allali, Naoual
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. LCPME, UMR CNRS-Université de Lorraine, Villers-lès-Nancy, France. SRSMC, UMR CNRS-Université de Lorraine, Faculté des Sciences et Technologies, Vandoeuvre-lès-Nancy, France.
    Urbanova, Veronika
    LCPME, UMR CNRS-Université de Lorraine, Villers-lès-Nancy, France.
    Etienne, Mathieu
    LCPME, UMR CNRS-Université de Lorraine, Villers-lès-Nancy, France.
    Devaux, Xavier
    IJL, UMR CNRS-Université de Lorraine, Nancy Cedex, France.
    Mallet, Martine
    LCPME, UMR CNRS-Université de Lorraine, Villers-lès-Nancy, France.
    Vigolo, Brigitte
    IJL, UMR CNRS-Université de Lorraine, Faculté des Sciences et Technologies, Vandoeuvre-lès-Nancy Cedex, France.
    Adjizian, Jean-Joseph
    IMN, UMR CNRS-Université de Nantes, Nantes, France.
    Ewels, Chris
    IMN, UMR CNRS-Université de Nantes, Nantes, France.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Soldatov, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    McRae, Edward
    IJL, UMR CNRS-Université de Lorraine, Faculté des Sciences et Technologies,Vandoeuvre-lès-Nancy Cedex, France.
    Fort, Yves
    SRSMC, UMR CNRS-Université de Lorraine, Faculté des Sciences et Technologies, Vandoeuvre-lès-Nancy, France.
    Dossot, Manuel
    LCPME, UMR CNRS-Université de Lorraine, Villers-lès-Nancy, France.
    Mamane, Victor
    Institut de Chimie de Strasbourg, UMR CNRS-Université de Strasbourg, Strasbourg, France.
    Accurate control of the covalent functionalization of single-walled carbon nanotubes for the electro-enzymatically controlled oxidation of biomolecules2018In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 9, p. 2750-2762Article in journal (Refereed)
    Abstract [en]

    Single-walled carbon nanotubes (SWCNTs) were functionalized by ferrocene through ethyleneglycol chains of different lengths (FcETGn) and the functionalized SWCNTs (f-SWCNTs) were characterized by different complementary analytical techniques. In particular, high-resolution scanning electron transmission microscopy (HRSTEM) and electron energy loss spectroscopy (EELS) analyses support that the outer tubes of the carbon-nanotube bundles were covalently grafted with FcETGn groups. This result confirms that the electrocatalytic effect observed during the oxidation of the reduced form of nicotinamide adenine dinucleotide (NADH) co-factor by the f-SWCNTs is due to the presence of grafted ferrocene derivatives playing the role of a mediator. This work clearly proves that residual impurities present in our SWCNT sample (below 5 wt. %) play no role in the electrocatalytic oxidation of NADH. Moreover, molecular dynamic simulations confirm the essential role of the PEG linker in the efficiency of the bioelectrochemical device in water, due to the favorable interaction between the ETG units and water molecules that prevents π-stacking of the ferrocene unit on the surface of the CNTs. This system can be applied to biosensing, as exemplified for glucose detection. The well-controlled and well-characterized functionalization of essentially clean SWCNTs enabled us to establish the maximum level of impurity content, below which the f-SWCNT intrinsic electrochemical activity is not jeopardized.

  • 158.
    Allali, Naoual
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Urbanova, Veronika
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Etienne, Mathieu
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Mallet, Martine
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Devaux, Xavier
    Département P2M, Institut Jean Lamour, UMR 7198 CNRS-Université de Lorraine.
    Vigolo, Brigitte
    Département CP2S, Institut Jean Lamour UMR 7198 CNRS-Université de Lorraine.
    Fort, Yves
    Laboratoire de Structure et Réactivité des Systèmes Moléculaires Complexes, UMR 7565 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Walcarius, Alain
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Noël, Maxime
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    McRae, Edward
    Département CP2S, Institut Jean Lamour UMR 7198 CNRS-Université de Lorraine.
    Soldatov, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Dossot, Manuel
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Mamane, Victor
    Laboratoire de Structure et Réactivité des Systèmes Moléculaires Complexes, UMR 7565 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Electrocatalytic effect towards NADH induced by HiPco single-walled carbon nanotubes covalently functionalized by ferrocene derivatives2013In: 2012 MRS Fall Meeting: Symposium YY – Low-Voltage Electron Microscopy and Spectroscopy for Materials Characterization, Cambridge University Press, 2013Conference paper (Refereed)
    Abstract [en]

    The present work reports the covalent functionalization of single-walled carbon nanotubes (SWCNTs) by ferrocene derivatives with polyethyleneglycol linkers. A very clean initial sample was chosen to avoid any residual catalyst and carbon impurities. Functionalized SWCNTs (f-CNTs) are deposited on the surface of a glassy carbon electrode (GCE) and this modified electrode is used for oxidizing the cofactor NADH (dihydronicotinamide adenine dinucleotide) in the presence of diaphorase. A clear electrocatalytic effect is evidenced, which can only be attributed to the f-CNTs.

  • 159.
    Allali, Naoual
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Urbanova, Veronika
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Mamane, Victor
    Laboratoire de Structure et Réactivité des Systèmes Moléculaires Complexes, UMR 7565 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Etienne, Mathieu
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Mallet, Martine
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Devaux, Xavier
    Institut Jean Lamour, Department P2M, UMR 7198 CNRS–Université de Lorraine, Ecole des Mines, 54042 Nancy.
    Vigolo, Brigitte
    Institut Jean Lamour, Department CP2S, UMR 7198 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Fort, Yves
    Laboratoire de Structure et Réactivité des Systèmes Moléculaires Complexes, UMR 7565 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Walcarius, Alain
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Noël, Maxime
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Soldatov, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    McRae, Edward
    Institut Jean Lamour, Department CP2S, UMR 7198 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Dossot, Manuel
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Covalent functionalization of few-wall carbon nanotubes by ferrocene derivatives for bioelectrochemical devices2012In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 249, no 12, p. 2349-2352Article in journal (Refereed)
    Abstract [en]

    The present work reports the covalent functionalization of few-wall CNTs (FWCNTs) by ferrocene derivatives to (i) improve their dispersion efficiency in water and (ii) graft electroactive chemical groups on their side-walls in order to promote electron transfer to biomolecules. The functionalized CNTs (f-CNTs) are used to modify a glassy carbon electrode and this modified electrode is used for oxidizing the cofactor NADH (dihydronicotinamide adenine dinucleotide).

  • 160.
    Allali, Naoual
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Urbanova, Veronika
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Mamane, Victor
    Laboratoire de Structure et Réactivité des Systèmes Moléculaires Complexes, UMR 7565 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Waldbock, Jeremy
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Etienne, Mathieu
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Mallet, Martine
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Devaux, Xavier
    Département P2M, Institut Jean Lamour, UMR 7198 CNRS-Université de Lorraine.
    Vigolo, Brigitte
    Département CP2S, Institut Jean Lamour UMR 7198 CNRS-Université de Lorraine.
    Fort, Yves
    Laboratoire de Structure et Réactivité des Systèmes Moléculaires Complexes, UMR 7565 CNRS–Université de Lorraine, 54506 Vandoeuvre-les-Nancy.
    Walcarius, Alain
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Noël, Maxime
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Soldatov, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    McRae, Edward
    Département CP2S, Institut Jean Lamour UMR 7198 CNRS-Université de Lorraine.
    Dossot, Manuel
    Département CP2S, Institut Jean Lamour UMR 7198 CNRS-Université de Lorraine.
    Few-wall carbon nanotubes covalently functionalized by ferrocene groups for bioelectrochemical devices2012In: MRS Online Proceedings Library, Cambridge University Press, 2012Conference paper (Refereed)
    Abstract [en]

    The present work reports the covalent functionalization of few-wall CNTs (FWCNTs) by ferrocene derivatives to i) improve their dispersion efficiency in water and ii) to graft electroactive chemical groups on their side-walls in order to promote electron transfer to biomolecules. The functionalized CNTs (f-CNTs) are used to modify a glassy carbon electrode and this modified electrode is used for oxidizing the cofactor NADH (dihydronicotinamide adenine dinucleotide).

  • 161.
    Al-Maqdasi, Zainab
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Luleå University of Technology.
    Development of Constituents for Multi-functional Composites Reinforced with Cellulosic Fibers2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Bio-basedcomposites are being increasingly used in applications where weight saving,and environmental friendliness is as important as structural performance. Obviously, bio-based materials have their limitations regarding durability and stability of the properties,but their potential in use for advanced applications can be expanded if they were functionalized and considered beyond their structural performance.

    Multifunctionalityincomposites can be achieved by modifyingeither of the composite constituents at different levelsso that they can perform energy-associated roles besides their structural reinforcement in the system. For the fibers, this can be done at the microscale by altering theirmicrostructure during spinning process or by applying functional coatings. As for the matrix, it is usually done by incorporating additives that can impart the required characteristics to the matrix. The nano-sized additives that mightbe considered for this objective are graphene and carbon nano-tubes. A big challenge with such materials is the difficulty to reachthe dispersionstate necessary for formation ofstable network to overcome the percolation threshold for conductivity. However, once the network is formed, the composite can have improved mechanical performance together with one or more of the added functionalities such as barrier capabilities,thermal and/or electrical conductivities or electromagnetic interference ability.

    Enormous work has been done to achieve the functionality incomposites produced with special care in laboratories. However, when it comes to mass production, it is both cost and energy inefficient to use tedious,complex methods for the manufacturing. Hence there is a need to investigate the potential of using scalable and industrial-relevant techniques and materials with acceptable compromise between cost and properties.

    The work presented in this thesis is performedwithin two projects aiming to achieve functional composites based on natural and man-made cellulosic fibers suitable for industrial upscaling. Conductive Regenerated Cellulose Fibers (RCFs) were produced by coating them with copper by electroless coating process using commercial materials. On the other hand, commercial masterbatches based on Graphene Nano-Platelets (GNPs) were used to produce wood polymer composites (WPC) with added multifunctionality by melt extrusion process. The process is one of the conventional methods used inpolymerproductionand needsno modifications for processingfunctional composites. Both materials together can be used to produce hybrid functional composites.

    The incorporation of the GNP into HDPE has resulted in improvement in the mechanical propertiesof polymer as well as composite reinforced with wood fibers. Stiffness has increased to a large extent while effect on the strength was less pronounced(>100% and 18% for stiffness and strength at 15%GNP loading). The enhancement of thermal conductivityat higher graphene loadingswas also observed. Moreover, time-dependent response of the polymer has also been affected and the addition of GNP has resulted in reduced viscoplastic strains and improved creep behavior.

    The copper-coated cellulose fibers showed a significant increasein electrical conductivity(<1Ω/50mm of coated samples) and a potential in use as sensor materials. However, these results come with the cost of reduction in mechanical properties of fibers (10% and 70% for tensile stiffness and strength, respectively) due to theeffect ofchemicals involved in the process.

  • 162.
    Al-Maqdasi, Zainab
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gong, Guan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. RISE SICOMP.
    Nyström, Birgitha
    RISE SICOMP.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Characterization of wood and Graphene Nanoplatelets (GNPs) Reinforced Polymer CompositesManuscript (preprint) (Other academic)
  • 163.
    Al-Maqdasi, Zainab
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gong, Guan
    Swerea SICOMP AB, Piteå.
    Nyström, Birgitha
    Swerea SICOMP AB, Piteå.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wood Fiber Composites With Added Multi-Functionality2018Conference paper (Refereed)
    Abstract [en]

    Graphene nanoplatelets (GNPs) are used to enhance the mechanical properties and functionality of wood plastic composite (WPC) targeting applications such as de-icing or anti-icing and fast thermal diffusivity. The GNPs are integrated into neat polymer using a masterbatch containing functionalized graphene by melt compounding through a twin-screw extruder without the use of any coupling agent or compatibilizer. The same manufacturing process (melt compounding) but with the use of compatibilizer is employed to produce WPC with nano-doped matrix. The effect of different GNP loadings (up to 15 wt.%) on morphology, crystallinity, mechanical and thermal conductivity of the nanocomposites and the WPCs was investigated. It was found that both strength and modulus of nanocomposites, in tension and bending, were increased with the addition of GNPs. With the aid of MAPE compatibilizer WPCs show higher flexural strength and modulus than neat polymer. GNP has marginal effect on the flexural stress but further increases flexural modulus of WPC. The preliminary results related to the thermal conductivity of studied materials indicate that the incorporation of GNP may be beneficial for faster and more uniform heat distribution in WPC.

  • 164.
    Al-Maqdasi, Zainab
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hajlane, Abdelghani
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Materials Science and Nano-engineering, Mohammed VI Polytechnic University, Benguerir, Morocco.
    Renbi, Abdelghani
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Ouarga, Ayoub
    Materials Science and Nano-engineering, Mohammed VI Polytechnic University, Benguerir, Morocco.
    Chouhan, Shailesh Singh
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Conductive Regenerated Cellulose Fibers by Electroless Plating2019In: Fibers, ISSN 2079-6439, Vol. 7, no 5, article id 38Article in journal (Refereed)
    Abstract [en]

    Continuous metallized regenerated cellulose fibers for advanced applications (e.g. multi-functional composites) are produced by electroless copper plating. Copper is successfully deposited on the surface of cellulose fibers using commercial cyanide-free electroless copper plating package commonly available for manufacturing of printed wiring boards. The deposited copper is found to enhance the thermal stability, electrical conductivity and resistance to moisture uptake of the fibers. On the other hand, involved chemistry results in altering the molecular structure of the fibers as is indicated by the degradation of their mechanical performance (tensile strength and modulus).

  • 165.
    Al-Mashikhi, S.O.
    et al.
    University of Notttingham.
    Powell, John
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Voisey, K.T.
    University of Notttingham.
    An explanation of ‘striation free' cutting of mild steel by fibre laser2009In: Lasers in manufacturing 2009: proceedings of the Fifth International WLT-Conference Lasers in Manufacturing, LIM 2009 : Munich, Germany, June 15th - 18th, 2009 / [ed] Andreas Ostendorf, Stuttgart: AT-Fachverlag , 2009Conference paper (Refereed)
    Abstract [en]

    This paper presents the results of an experimental and theoretical investigation into the phenomenon of ‘striation free cutting', which is a feature of fibre laser cutting of thin section mild steel. The paper concludes that the creation of very low roughness edges is related to an optimisation of the cut front geometry when the cut front is inclined at angles close to the Brewster angle for the laser - material combination. For purely geometric reasons this particular type of cut front optimisation is not possible for CO2 laser cutting of mild steel.

  • 166.
    Al-Mashikhi, S.O
    et al.
    Salalah College of Technology, Engineering Department, Salalah, Oman and Faculty of Engineering, University of Nottingham.
    Powell, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Voisey, K.T.
    Faculty of Engineering, University of Nottingham.
    Heat affected zones and oxidation marks in fiber laser–oxygen cutting of mild steel2011In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 23, no 4Article in journal (Refereed)
    Abstract [en]

    The effect of cutting speed and sheet thickness on surface oxidation and heat affected zones (HAZs) has been investigated for laser–oxygen cutting of mild steel sheet with a fiber laser. Optical and scanning electron micrographs were used to determine the extent of surface oxidation and HAZ from plan and cross-sectional views, respectively. The HAZ is consistently wider at the bottom of the cut compared to the HAZ at the top of the cut. With increasing speed, the width of the HAZ at the top of the cut decreases whereas the HAZ width at the bottom of the cut generally increases. No simple, direct relationship between HAZ width and surface oxidation was seen. However, it is possible to state that in each case considered here, the HAZ would be completely removed if they are machined back by a depth equal to the extent of the surface oxidation.

  • 167.
    Almeida, Nuno A.F.
    et al.
    Mechanical Engineering Department & TEMA, University of Aveiro, Campus Universitário de Santiago.
    Rodrigues, Joana
    Physics Department & I3N, University of Aveiro, Campus Universitário de Santiago.
    Silva, Patricia
    Mechanical Engineering Department & TEMA, University of Aveiro, Campus Universitário de Santiago.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Soares, Manuel J.
    Physics Department & I3N, University of Aveiro, Campus Universitário de Santiago.
    Monteiro, Teresa
    Physics Department & I3N, University of Aveiro, Campus Universitário de Santiago.
    Lopes-da-Silva, José A.
    Chemistry Department & QOPNA, University of Aveiro, Campus Universitário de Santiago.
    Marques, Paula A.A.P.
    Mechanical Engineering Department & TEMA, University of Aveiro, Campus Universitário de Santiago.
    Pressure dependent luminescence in titanium dioxide particles modified with europium ions2016In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 234, p. 137-144Article in journal (Refereed)
    Abstract [en]

    Particles of titanium dioxide were prepared in the presence of europium ions (TiO2:Eu) by a solvothermal method and thermal annealed in air at 500 °C. The spectroscopic properties of TiO2:Eu particles were analyzed indicating that the Eu3+ ions are likely distributed at the surface or near the surface of the titanium dioxide particles. The photoluminescence analysis showed that the intraionic emission was strongly sensitive to reduced pressure conditions, as seen by its absence under vacuum conditions. The ion emission was re-established as soon as the atmosphere was restored. Additionally, the ion integrated emission intensity follows a linearly dependence with pressure in the range of 150 to 800 mbar revealing a high sensitivity to small variations in pressure, which is an unprecedented result. This innovation will allow the study of new technologies in the area of low vacuum sensors where TiO2:Eu may act as the active element of an optical sensor for a pressure device.

  • 168.
    Almer, J.
    et al.
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Lienert, U.
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Peng, R.L.
    Linköping universitet.
    Schlauer, C.
    Linköping universitet.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Strain and texture analysis of coatings using high-energy x-rays2003In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 94, no 1, p. 697-702Article in journal (Refereed)
    Abstract [en]

    We investigate the internal strain and crystallographic orientation (texture) in physical-vapor deposited metal nitride coatings of TiN and CrN. A high-energy diffraction technique is presented that uses synchrotron x rays and an area detector, and which allows the strain and intensity distributions of multiple crystallographic planes to be measured by a single x-ray exposure. Unique texture states and nonlinear sin2 strain distributions are observed for all coatings investigated. Quantitative analysis indicates that existing micromechanical models can reasonably predict strain and corresponding stress for mixed-hkl reflections but are inadequate for fully describing measured data. Alternative mechanisms involving deposition-induced defects are proposed

  • 169.
    Almer, J
    et al.
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Håkansson, G
    Bodycote Värmebehandling AB.
    Microstructure and thermal stability of arc-evaporated Cr-C-N coatings2004In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 84, no 7, p. 611-630Article in journal (Refereed)
    Abstract [en]

    The role of C incorporation in the microstructure and thermal stability of arc-evaporated Cr-C-N coatings is explored via reactive growth in a mixed C2H4-N2 environment. C is found to react more readily than N at both the Cr cathode and the coating surfaces, so that a C2H4-to-N2 flow ratio of only 1% yields a C-to-N ratio of approximately 10% within the coatings. The as-deposited microstructures consist primarily of the δ-Cr(C, N) phase and possess high compressive residual stresses, which decrease with increasing C content. Post-deposition annealing up to 700°C results in depletion of lattice defects, and concomitant reductions in stress and coating hardness, together with phase transformations which suggest metastable phase formation during growth. Apparent activation energies for this lattice defect are found to be in the range expected for bulk diffusion of N and C (2.4-2.8 eV). The results suggest that inclusion of small amounts of C in this system offers the ability to reduce internal stresses while maintaining defect-related hardness increases, permitting growth of thicker and thus more wear-resistant coatings.

  • 170.
    Almgren, Karin M
    et al.
    STFI-Packforsk AB, Box 5604, SE-114 86 Stockholm.
    Gamstedt, Kristofer
    Department of Fiber and Polymer Technology, Royal Institute of Technology - KTH.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Contribution of wood fiber hygroexpansion to moisture induced thickness swelling of composite plates2010In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 31, no 5, p. 762-771Article in journal (Refereed)
    Abstract [en]

    One of the main drawbacks of wood fiber-based composite materials is their propensity to swell due to moisture uptake. Because the wood fibers are usually the main contributor to hygroexpansion, it is of interest to quantify the hygroexpansion coefficient of wood fibers, to compare and rank different types of fibers. This investigation outlines an inverse method to estimate the transverse hygroexpansion coefficient of wood fibers based on measurements of moisture induced thickness swelling of composite plates. The model is based on composite micromechanics and laminate theory. Thickness swelling has been measured on polylactide matrix composites with either bleached reference fibers or crosslinked fibers. The crosslinking modification reduced the transverse hygroexpansion of the composites and the transverse coefficient of hygroexpansion of the fibers was reduced from 0.28 strain per relative humidity for reference fibers to 0.12 for cross-linked fibers.

  • 171.
    Almkvist, Anders
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Flash and Burr Effect on Insert Tool Life2018Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    The full text will be freely available from 2027-03-01 14:44
  • 172.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Homogenization of the Reynolds equation governing hydrodynamic flow in a rotating device2011In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 133, no 2, p. 021705-1Article in journal (Refereed)
    Abstract [en]

    In this paper, a method facilitating the analysis of the effects of surface roughness on the lubrication of a rotating device is presented. The analysis utilizes homogenization—a suitable technique for averaging the effects of roughness as modeled by the Reynolds equation. The originality of this work lies in a novel way of deriving the so called local problems, also known as microbearing problems. It is clearly shown how this increases the computational efficiency by eliminating the dependence of the global coordinates on the formulation of these local problems. This does not only speed up the computation, it also means that the derived flow factors or flow tensors require less storage space. To provide for good usability, alongside the flow factors for the averaged Reynolds equation, the correction factors for the averaged friction torque (and force) and the expression for averaged load carrying capacity are presented here.

  • 173.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    MULTISCALE MODELLING OF ELASTOHYDRODYNAMIC TILTED-PAD BEARINGS: A METAMODEL APPROACH2018Conference paper (Refereed)
    Abstract [en]

    6th European Conference on Computational Mechanics (ECCM 6)7th European Conference on Computational Fluid Dynamics (ECFD 7)11 – 15 June 2018, Glasgow, UK 

    MULTISCALE MODELLING OF ELASTOHYDRODYNAMIC TILTED-PAD BEARINGS: A METAMODEL APPROACH 

    G. N. de Boer1, A. Almqvist2, L. Gao3, R. W. Hewson4 and H. M. Thompson

    1 School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK, g.n.deboer@leeds.ac.uk, https://engineering.leeds.ac.uk/staff/1036/Dr_Greg_de_Boer 

    2 Division of Machine Elements, Luleå University of Technology, E837a Luleå, Sweden, a.almqvist@ltu.se, https://www.ltu.se/staff/a/almqvist 

    3 Department of Aeronautics, Imperial College London, London, SW7 2AZ, UK, leiming.gao@imperial.ac.uk, http://www.imperial.ac.uk/people/leiming.gao 

    4 Department of Aeronautics, Imperial College London, London, SW7 2AZ, UK, r.hewson@imperial.ac.uk, https://www.imperial.ac.uk/people/r.hewson 

    5 School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK, h.m.thompson@leeds.ac.uk, https://engineering.leeds.ac.uk/staff/140/Professor_Harvey_Thompson 

    Key Words: Elastohydrodynamic Lubrication; Surface Topography; Metamodelling; Moving Least Squares; Evolving Design of Experiments. 

    Elastohydrodynamic Lubrication (EHL) refers to the contact of two surfaces in relative motion under fully flooded conditions, pressure generated in the lubricant generates deformation of the bodies and this is coupled to determine a total load carrying capacity. In such contacts the size of surface topography and film thickness are of a similar order of magnitude and this therefore has a role in describing the phenomena. However the length scales associated with surface topography and the contact region are disparate and in order to model such effects authors have developed homogenisation based methods. 

    Recently the Heterogeneous Multiscale Methods (HMM) have been employed to study the problem. This has allowed the effects of micro-EHL to be explored and coupled into the macro-scale EHL problem. Fundamental to this is the separation of scales and periodicity applied at to simulations describing surface topography. de Boer [1] outlines a method for coupling the scales of the problem using Moving Least Squares metamodels to calculate flow factors. This was further used to optimise surface topographical features to produce the minimum possible coefficient of friction in an EHL contact [2]. This research focuses on the metamodelling approach of [1, 2] to explore more complex 3D titled-pad bearing geometries than have previously been investigated. The means by which the scales of the problem are coupled is complicated by an increase in the number of design variables. Additionally the choice of Design of Experiments and how this evolves with the solution procedure is vital to the accuracy of the approach. 

    REFERENCES 

    [1] de Boer G, Gao L, Hewson R, Thompson H. Heterogeneous Multiscale Methods for modelling surface topography in EHL line contacts. Tribol Int 2017, 113:262-278. 

    [2] de Boer G, Gao L, Hewson R, Thompson H, Raske N, Toropov V. A multiscale method for optimising surface topography in elastohydrodynamic lubrication (EHL) using metamodels. Struct Mulidisc Optim 2016, 54(3):483-497. 

  • 174.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    On the effects of surface roughness in lubrication2006Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Tribology is a multidisciplinary field defined as the science and technology of interacting surfaces in relative motion, and embraces the study of friction, wear and lubrication. A typical tribological application is the rolling element bearing. Tribological contacts may also be found in other types of bearings, cam-mechanisms, gearboxes and hydraulic systems. Examples of bearings inside the human body are the operation of the human hip joint and the contact between teeth during chewing. To fully understand the operation of this type of application one has to understand the couplings between the lubricant fluid dynamics, the structural dynamics of the bearing material, the thermodynamical aspects and the resulting chemical reactions. This makes modeling tribological applications an extremely delicate task. Because of the multidisciplinary nature, such theoretical models lead to mathematical descriptions generally in the form of non-linear integro-differential systems of equations. Some of these systems of equations are sufficiently well posed to allow numerical solutions to be carried out, resulting in accurate predictions on performance. In this work, the influence on performance of a surface microscopical nature, the surface roughness, in contact interfaces between different types of machine element components is the subject of study. An example is the non-conformal lubricated contact between one of the rollers and the inner ring in a rolling element bearing. The tribological contact controlling the operation of the human hip joint is also very similar to this. Another example of a non-conformal contact occurs when driving on rainy roads, where the hydrodynamic action of the water separates the tire. To enable investigations of these types of problems, different theoretical models were studied; for the selected model, a numerical solution technique was developed within this project. This model is based on the Reynolds equation coupled with the film thickness equation. The numerical solution technique involves a multilevel technique to facilitate the solution process. Results presented in this thesis, utilizing this approach, study elementary surface features such as ridges and indentations passing each other inside the lubricated conjunction. The Reynolds equation is derived under the assumptions of thin fluid film and creeping flow, and considers in its most general form shear thinning of the lubricant. This type of equation describes the hydrodynamic action of the lubricant flow and may be used when the interfaces consist of either conformal or non-conformal conjunctions. Examples of applications having conformal interfaces are thrust- and journal- bearings or the contact between the eye and a (optical) contact lens. In such types of applications the load carried by the interface is distributed over a fairly large area that under certain circumstances helps to prevent mechanical deformation of the contacting surfaces. Such applications are said to operate in the hydrodynamic lubrication (HL) regime. Lubricant compressibility and cavitation are important aspects and have received some attention. However, the main objective when modeling HL has been to investigate and develop methods that enable the influence of surface roughness to be to be studied efficiently. Homogenization is a rigorous mathematical concept that when applied to a certain problem may be regarded as an averaging technique as well as it provides information about the induced effects of local surface roughness. Homogenization inflicts no restrictions on the surface roughness representation other than the representative part of the chosen surface roughness being assumed periodically distributed and of course the assumptions of thin film flow made through the Reynolds equation. The homogenization process leads to a two sets of equations one for the local scale describing surface roughness, scale and one for the global scale describing application geometry. The unequivocally determined coefficients of the global problem, which may be regarded as flow factors, are obtained through the solution of local problems. This makes homogenization an eminent approach to be used investigating the influence of surface roughness on hydrodynamic performance. In the present work, homogenization has been used to derive computationally feasible forms of problems originating from incompressible and compressible Reynolds type equations that describe stationary and unstationary flows in both cartezian and cylindrical co-ordinates. This technique enables simulations of surface roughness induced effects when considering surface roughness descriptions originating from measurements. Moreover, the application of homogenization facilitates the interpretation of results. Numerical investigations following the homogenization process have been carried out to verify the applicability of homogenization in hydrodynamic lubrication. Homogenization has also been shown here to enable efficient analysis of rough hydrodynamically lubricated problems. Also of note, in connection to the scientific contribution within tribology, collaboration with a group in applied mathematics has lead to the development of novel techniques in that area. These ideas have also been successfully applied, with some results presented in this thesis. At start-ups, the contact in a rolling element bearing could be both starved and drained from lubricant. In this case the hydrodynamic action becomes negligible in terms of load carrying capacity. The load is carried exclusively by surface asperities, the tribo film, or both. This is hereby modeled as the unlubricated frictionless contact between rough surfaces, i.e. a contact mechanical approach. A variational principle was used in which the real area of contact and the contact pressure distribution minimize the total complementary potential energy. The material model is linear elastic-perfectly plastic and the energy dissipation due to plastic deformation is accounted for. The numerics of this contact mechanical approach involve the fast Fourier transformation (FFT) technique in order to facilitate the solution process. Investigation results of the contact mechanics of realistic surfaces are presented in this thesis. In this investigation the variation in the real area of contact, the plasticity index and some surface roughness parameters due to applied load were studied.

  • 175.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    On the effects of surface roughness in lubrication2009Doctoral thesis, comprehensive summary (Other academic)
  • 176.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Rough surface elastohydrodynamic lubrication and contact mechanics2004Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the field of tribology, there are numerous theoretical models that may be described mathematically in the form of integro-differential systems of equations. Some of these systems of equations are sufficiently well posed to allow for numerical solutions to be carried out resulting in accurate predictions. This work has focused on the contact between rough surfaces with or without a separating lubricant film. The objective was to investigate how surface topography influences contact conditions. For this purpose two different numerical methods were developed and used. For the lubricated contact between rough surfaces the Reynolds equation were used as a basis. This equation is derived under the assumptions of thin fluid film and creeping flow. In highly loaded, lubricated, non- conformal contacts of surfaces after running-in, the load concentration no longer results in plastic deformations, however large elastic deformations will be apparent. It is the interaction between the hydrodynamic action of the lubricant and the elastic deformations of the surfaces that, in certain applications, enable the lubricant film to fully separate the surfaces. This is commonly referred to as full film elastohydrodynamic (EHD) lubrication. Typical machine elements that operates in the full film EHD lubrication (FL) regime include rolling element bearings, cams and gears. Unfortunately, a cost effective way of machining engineering surfaces seldom results in a surface topography that influence contact conditions in the same way as a surface after running-in. Such topographies may prevent the lubricant from fully separating the surfaces because of deteriorated hydrodynamic action. In this case the applied load is carried in part by the lubricant and in part by surface asperities and/or surface active lubricant additives. This could also be the case in lubricant starved contacts, which is a common situation in not only grease lubricated contacts but also in many liquid lubricated contacts, such as high speed operating rolling element bearings. The load sharing between the highly compressed lubricant and the surface and/or surface active lubricant additives is the reason why this lubrication regime is most commonly referred to as mixed EHD lubrication (ML). Machine elements that while running operate in the FL regime may experience a transition into the ML regime at stops or due to altered operating conditions. It is not possible to simulate direct contact between the surfaces using a numerical method based on Reynolds equation. A parameter study, of elementary surface features passing each other inside the EHD lubricated conjunction, was performed. The results obtained, even though no direct contact could be simulated, does indicate that a transition from the FL to the ML regime would occur for certain combinations of the varied parameters. At start-ups, the contact in a rolling element bearing could be both starved and drained from lubricant. In this case the hydrodynamic action becomes negligible in terms of load carrying capacity. The load is carried exclusively by surface asperities and/or surface active lubricant additives. This regime is referred to as boundary lubrication (BL). Operation conditions could also make both FL and ML impossible to achieve, for example, in the case in a low rpm operating rolling element bearing. The BL regime is in this work modeled as the unlubricated frictionless contact between rough surfaces, i.e., a dry contact approach. A variational principle was used in which the real area of contact and contact pressure distribution are those which minimize the total complementary energy. A linear elastic-perfectly plastic deformation model in which energy dissipation due to plastic deformation is accounted for was used. The dry contact method was applied to the contact between four different profiles and a plane. The variation in the real area of contact, the plasticity index and some surface roughness parameters due to applied load were investigated. The surface roughness parameters of the profiles differed significantly.

  • 177.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Burtseva, Evgeniya
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    New insights on lubrication theory for compressible fluids2019In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 145, article id 103170Article in journal (Refereed)
    Abstract [en]

    The fact that the film is thin is in lubrication theory utilised to simplify the full Navier–Stokes system of equations. For incompressible and iso-viscous fluids, it turns out that the inertial terms are small enough to be neglected. However, for a compressible fluid, we show that the influence of inertia depends on the (constitutive) density-pressure relationship and may not always be neglected. We consider a class of iso-viscous fluids obeying a power-law type of compressibility, which in particular includes both incompressible fluids and ideal gases. We show by scaling and asymptotic analysis, that the degree of compressibility determines whether the terms governing inertia may or may not be neglected. For instance, for an ideal gas, the inertial terms remain regardless of the film height-to-length ratio. However, by means of a specific modified Reynolds number that we define we show that the magnitudes of the inertial terms rarely are large enough to be influential. In addition, we consider fluids obeying the well-known Dowson and Higginson density-pressure relationship and show that the inertial terms can be neglected, which allows for obtaining a Reynolds type of equation. Finally, some numerical examples are presented in order to illustrate our theoretical results.

  • 178.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Campañá, C
    University of Ottawa, Department of Chemistry, Ottawa, Canada K1N6N5.
    Prodanov, N
    Sumy State University, 2 Rimskii-KorsakovStr., 40007 Sumy, Ukraine.
    Persson, B N J
    IFF, Jülich.
    Interfacial separation between elastic solids with randomly rough surfaces: Comparison between theory and numerical techniques2011In: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 59, no 11, p. 2355-2369Article in journal (Refereed)
    Abstract [en]

    We study the distribution of interfacial separations at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for the distribution of interfacial separations using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) a Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches.

  • 179.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Dasht, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    The homogenization process of the Reynolds equation describing compressible liquid flow2006In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 39, no 9, p. 994-1002Article in journal (Refereed)
    Abstract [en]

    This paper summarizes the homogenization process of rough, hydrodynamic lubrication problems governed by the Reynolds equation used to describe compressible liquid flow. Here, the homogenized equation describes the limiting result when the wavelength of a modeled surface roughness goes to zero. The lubricant film thickness is modeled by one part describing the geometry/shape of the bearing and a periodic part describing the surface topography/roughness. By varying the periodic part as well as its wavelength, we can try to systematically investigate the applicability of homogenization on this type of problem. The load carrying capacity is the target parameter; deterministic solutions are compared to homogenized by this measure. We show that the load carrying capacity rapidly converges to the homogenized results as the wavelength decreases, proving that the homogenized solution gives a very accurate representation of the problem when real surface topographies are considered

  • 180.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Dasht, Johan
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Sahlin, Fredrik
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization of the Reynolds equation2005Report (Other academic)
  • 181.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Essel, Emmanuel Kwame
    Department of Mathematics and Statistics, University of Cape Coast.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Multiscale homogenization of a class of nonlinear equations with applications in lubrication theory and applications2011In: Journal of Function Spaces and Applications, ISSN 0972-6802, E-ISSN 1758-4965, Vol. 9, no 1, p. 17-40Article in journal (Refereed)
    Abstract [en]

    We prove a homogenization result for monotone operators by using the method of multiscale convergence. More precisely, we study the asymptotic behavior as epsilon -> 0 of the solutions u(epsilon) of the nonlinear equation div a(epsilon)(x, del u(epsilon)) = div b(epsilon), where both a(epsilon) and b(epsilon) oscillate rapidly on several microscopic scales and a(epsilon) satisfies certain continuity, monotonicity and boundedness conditions. This kind of problem has applications in hydrodynamic thin film lubrication where the bounding surfaces have roughness on several length scales. The homogenization result is obtained by extending the multiscale convergence method to the setting of Sobolev spaces W-0(1,p)(Omega), where 1 < p < infinity. In particular we give new proofs of some fundamental theorems concerning this convergence that were first obtained by Allaire and Briane for the case p = 2.

  • 182.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Essel, Emmanuel Kwame
    Fabricius, John
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Reiterated homogenization applied in hydrodynamic lubrication2008In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 222, no 7, p. 827-841Article in journal (Refereed)
    Abstract [en]

    This work is devoted to studying the combined effect that arises due to surface texture and surface roughness in hydrodynamic lubrication. An effective approach in tackling this problem is by using the theory of reiterated homogenization with three scales. In the numerical analysis of such problems, a very fine mesh is needed, suggesting some type of averaging. To this end, a general class of problems is studied that, e.g. includes the incompressible Reynolds problem in both artesian and cylindrical coordinate forms. To demonstrate the effectiveness of the method several numerical results are presented that clearly show the convergence of the deterministic solutions towards the homogenized solution.Moreover, the convergence of the friction force and the load carrying capacity of the lubricant film is also addressed in this paper. In conclusion, reiterated homogenization is a feasible mathematical tool that facilitates the analysis of this type of problem.

  • 183.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Essel, Emmanuel Kwame
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Reiterated homogenization of a nonlinear Reynolds-type equation2008Report (Other academic)
  • 184.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Essel, Emmanuel Kwame
    Department of Mathematics and Statistics, University of Cape Coast.
    Fabricius, John
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Variational bounds applied to unstationary hydrodynamic lubrication2008In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 46, no 9, p. 891-906Article in journal (Refereed)
    Abstract [en]

    This paper is devoted to the effects of surface roughness in hydrodynamic lubrication. The numerical analysis of such problems requires a very fine mesh to resolve the surface roughness, hence it is often necessary to do some type of averaging. Previously, homogenization (a rigorous form of averaging) has been successfully applied to Reynolds type differential equations. More recently, the idea of finding upper and lower bounds on the effective behavior, obtained by homogenization, was applied for the first time in tribology. In these pioneering works, it has been assumed that only one surface is rough. In this paper we develop these results to include the unstationary case where both surfaces may be rough. More precisely, we first use multiple-scale expansion to obtain a homogenization result for a class of variational problems including the variational formulation associated with the unstationary Reynolds equation. Thereafter, we derive lower and upper bounds corresponding to the homogenized (averaged) variational problem. The bounds reduce the numerical analysis, in that one only needs to solve two smooth problems, i.e. no local scale has to be considered. Finally, we present several examples, where it is shown that the bounds can be used to estimate the effects of surface roughness with very high accuracy.

  • 185.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Essel, Emmanuel Kwame
    Persson, Lars-Erik
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization of the unstationary incompressible Reynolds equation2007In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 40, no 9, p. 1344-1350Article in journal (Refereed)
    Abstract [en]

    This paper is devoted to the effects of surface roughness during hydrodynamic lubrication. In the numerical analysis a very fine mesh is needed to resolve the surface roughness, suggesting some type of averaging. A rigorous way to do this is to use the general theory of homogenization. In most works about the influence of surface roughness, it is assumed that only the stationary surface is rough. This means that the governing Reynolds type equation does not involve time. However, recently, homogenization was successfully applied to analyze a situation where both surfaces are rough and the lubricant is assumed to have constant bulk modulus. In this paper we will consider a case where both surfaces are assumed to be rough, but the lubricant is incompressible. It is also clearly demonstrated, in this case that homogenization is an efficient approach. Moreover, several numerical results are presented and compared with those corresponding to where a constant bulk modulus is assumed to govern the lubricant compressibility. In particular, the result shows a significant difference in the asymptotic behavior between the incompressible case and that with constant bulk modulus.

  • 186.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    A new approach for studying cavitation in lubrication2014In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 136, no 1, article id 11706Article in journal (Refereed)
    Abstract [en]

    The underlying theory, in this paper, is based on clear physical arguments related to conservation of mass flow and considers both incompressible and compressible fluids. The result of the mathematical modeling is a system of equations with two unknowns, which are related to the hydrodynamic pressure and the degree of saturation of the fluid. Discretization of the system leads to a linear complementarity problem (LCP), which easily can be solved numerically with readily available standard methods and an implementation of a model problem in matlab code is made available for the reader of the paper. The model and the associated numerical solution method have significant advantages over today's most frequently used cavitation algorithms, which are based on Elrod-Adams pioneering work

  • 187.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Reynolds equation flow factor estimates by means of homogenization2010In: ASIATRIB 2010: Frontiers in tribology - knowledge & friendship . proceedings of the fourth Asia International Conference on Tribology, 5-9 December 2010, Perth, Western Australia, 2010, p. 185-Conference paper (Refereed)
  • 188.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Flow in thin domains with a microstructure: Lubrication and thin porous media2017In: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1798, article id 020172Article in journal (Refereed)
    Abstract [en]

    This paper is devoted to homogenization of different models of flow in thin domains with a microstructure. The focus is on applications connected to the effect of surface roughness in full film lubrication, but a parallel to flow in thin porous media is also discussed. Mathematical models of such flows naturally include two small parameters. One is connected to the fluid film thickness and the other to the microstructure. The corresponding asymptotic analysis is a delicate problem, since the result depends on how fast the two small parameters tend to zero relative to each other. We give a review of the current status in this area and point out some future challenges.

  • 189.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Spencer, Andrew
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Similarities and differences between the flow factor method by Patir and Cheng and homogenization2011In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 133, no 3, p. 031702-1Article in journal (Refereed)
    Abstract [en]

    Different averaging techniques have proved to be useful for analyzing the effects of surface roughness in hydrodynamic lubrication. This paper compares two of these averaging techniques, namely the flow factor method by Patir and Cheng (P&C) and homogenization. It has been rigorously proved by many authors that the homogenization method provides a correct solution for arbitrary roughness. In this work it is shown that the two methods coincide if and only if the roughness exhibits certain symmetries. Hence, homogenization is always the preferred method.

  • 190.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization of a Reynolds equation describing compressible flow2011Report (Other academic)
  • 191.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fabricius, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization of a Reynolds equation describing compressible flow2012In: Journal of Mathematical Analysis and Applications, ISSN 0022-247X, E-ISSN 1096-0813, Vol. 390, no 2, p. 456-471Article in journal (Refereed)
    Abstract [en]

    We homogenize a Reynolds equation with rapidly oscillating film thickness function hε, assuming a constant compressiblity factor in the pressure-density relation. The oscillations are due to roughness on the bounding surfaces of the fluid film. As shown by previous studies, homogenization is an effective approach for analyzing the effects of surface roughness in hydrodynamic lubrication. By two-scale convergence theory we obtain the limit problem (homogenized equation) and strong convergence in L2 for the unknown density ρε. By adding a small corrector term we also obtain strong convergence in the Sobolev norm.

  • 192.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Glavatskih, Sergei
    Larsson, Roland
    Marklund, Pär
    Sahlin, Fredrik
    Dasht, Johan
    Persson, Lars-Erik
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization of Reynolds equation2005Report (Other academic)
  • 193.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Selected papers from those presented at the 3rd International Tribology Symposium of IFToMM2014In: Lubrication Science, ISSN 0954-0075, E-ISSN 1557-6833, Vol. 26, no 5, p. 273-275Article in journal (Refereed)
  • 194. Almqvist, Andreas
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The effect of two-sided roughness in rolling/sliding EHL line contacts2004In: Transient processes in tribology: proceedings of the 30th Leeds-Lyon Symposium on Tribology / [ed] Gerard Dalmaz, Amstersam: Elsevier, 2004Conference paper (Refereed)
    Abstract [en]

    In most theoretical studies carried out to date on the effect of surface roughness in elastohydrodynamic lubrication (EHL) one surface is considered smooth and one as being rough. In real tribological contacts however, both surfaces normally have similar roughness heights. When modelling a rolling contact it is possible to simply sum the roughness of the two contact surfaces but in a sliding EHL contact, a continuously changing effective surface roughness occurs. The aim of this work was to investigate the influence of elementary surface features such as dents and ridges on the film thickness and pressure. This was done numerically using transient non-Newtonian simulations of an EHL line contact using a coupled smoother combined with a multilevel technique. Four different "overtaking" phenomena were investigated; ridge-ridge, dent-ridge, ridge-dent, and dent-dent. It was shown that the minimum film-thickness produced by a ridge is further reduced in a dent-ridge overtaking event. The squeeze effect seen in the ridge-ridge case resulted in large deformations and film-thickness heights comparable to the corresponding smooth case just before the overtaking event occurred. These local effects arising from simulating two-sided roughness were compared to simulations using a traditional "one-sided rough surface contacting a perfectly smooth surface.".

  • 195. Almqvist, Andreas
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sahlin, Fredrik
    Development of a lubrication simulation model2009In: Svenska mekanikdagarna: Södertälje 2009, Stockholm: Svenska nationalkommittén för mekanik , 2009, p. 74-Conference paper (Other academic)
  • 196.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    The homogenization process of the time dependent Reynolds equation describing compressible liquid flow2007In: Tribologia : Finnish Journal of Tribology, ISSN 0780-2285, Vol. 26, no 4, p. 30-44Article in journal (Refereed)
    Abstract [en]

    To increase the hydrodynamic performance in different machine elements during lubrication, e.g. journal bearings and thrust bearings, it is important to understand the influence of surface roughness. In this connection one encounters different approaches commonly based on some form of the Reynolds equation. They may generally be divided into deterministic- and averaging- techniques. The former regards all surface roughness information and provides a detailed understanding of the local effects that arise. The latter method is suitable when investigating how the surface roughness affects performance of the machine element as a whole. Homogenization is a rigorous mathematical concept that when applied to a certain problem may be thought of as an averaging technique also providing information about local effects. In this work the compressible time dependent Reynolds equation is homogenized. Related problems have recently been analyzed by homogenization techniques under various assumptions. In the present paper the compressibility is modeled assuming a constant lubricant bulk modulus. The formal method of multiple scale expansion is used to derive a so-called homogenized equation and a numerical solution method to solve both the deterministic problem and the homogenized problem is implemented. The numerical results clearly show that the solution of the homogenized equation is a suitable approximation to the solution of the deterministic problem. It is also demonstrated that for small values of the roughness wavelength, the homogenization technique is superior, since the solution of the deterministic problem requires an extremely fine discretization mesh. More over, the solution of the time dependent homogenized problem may in some cases be reduced to solve a stationary problem that facilitates the solution process and interpretation of results.

  • 197. Almqvist, Andreas
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    The homogenization process of the time dependent Reynolds equation describing compressible liquid flow2006Report (Other academic)
  • 198.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lukkassen, Dag
    Meidell, Annette
    Narvik University College, 8505 Narvik, Norway.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    New concepts of homogenization applied in rough surface hydrodynamic lubrication2007In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 45, no 1, p. 139-154Article in journal (Refereed)
    Abstract [en]

    This work introduces a new concept of homogenization that enables efficient analysis of the effects of surface roughness representations obtained by measurements in applications modeled by the Reynolds equation. Examples of such applications are trust- and journal-bearings. The numerical analysis of these types of applications requires an extremely dense computational mesh in order to resolve the surface roughness, suggesting some type of averaging. One such method is homogenization, which has been applied to Reynolds type equations with success recently. This approach is similar to the technique proposed by Patir and Cheng, who introduced flow factors determining the hydrodynamic action due to surface roughness. The difference is, however, that the present technique has a rigorous mathematical support. Moreover, the recipe to compute the averaged coefficients is simple without any ambiguities. Using either the technique proposed by Patir and Cheng or homogenization, the coefficients determining the averaged Reynolds equation are obtained by solving differential equations on a local scale. Unfortunately, this is detrimental when investigating the effects induced by real, measured, surface roughness, even though these local problems may be solved in parallel. The present work presents a solution by applying the technique based on bounds. This technique transforms the stationary Reynolds equation into two computationally feasible forms, one for the upper bound and one for the lower bound, where the flow factors are obtained by straightforward integration. Together with the preciseness of these bounds, the bounds approach becomes an eminent tool suitable for investigating the effect of real, measured, surface roughness on hydrodynamic performance.

  • 199.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Modelling Flows in Lubrication2020In: Modeling and Simulation of Tribological Problems in Technology / [ed] Marco Paggi & David Hills, Springer, 2020, 1, p. 229-278Chapter in book (Other academic)
  • 200.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Scientific Computing with Applications in Tribology: A course compendium2019Other (Other academic)
    Abstract [en]

    This compendium comprises models and numerical solution procedure for tribological interfaces. It describes the tribological contact and the classical lubrication regimes. A thorough derivation of the Reynolds equation, governing the fluid pressure, from the Navier-Stokes momentum equations and the continuity equation for conservation of mass, is presented along with its analytical solution for the infinitely wide linear slider bearing.

    The compilation of the compendium was conducted by the first author during his tenure as Professor at the Division of Machine Elements, Department of Engineering Sciences and Mathematics, Luleå University of Technology and by the second author during his tenure as a postdoctoral researcher at the same division.

    Although the compilation of this text is the work solely of the authors, the models and solution procedure presented herein is joint development of many good colleagues and co-authors. Our sincere gratitude is extended towards them all.

1234567 151 - 200 of 8923
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