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  • 51.
    Mattsson, Roger
    et al.
    Luleå tekniska universitet.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wåhlin, Anders
    Laser ignition of premixed gases studied by pulsed TV holography2003Inngår i: Speckle metrology 2003: proceedings ; 18 - 20 June 2003, Trondheim, Norway / [ed] Kay Gastinger, Bellingham, Wash: SPIE - International Society for Optical Engineering, 2003, s. 285-290Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Pulsed TV holography is an all-electronic version of pulsed holographic interferometry. Such a system was used to study ignition of pre-mixed flammable gases. The object was recorded in its undisturbed and disturbed state followed by an evaluation of the changes in refractive index field between those two states. The essential components of the set-up are an injection seeded, twin cavity double pulsed Nd:YAG laser and a CCD camera. A focused laser pulse from the same laser that was used for the recording of the digital hologram initiated the ignition of the pre-mixed gas. Results from the recordings show the ignition process. From the experimental phase maps the refractive index of the gas mixture could be determined. This method makes it possible to determine the proportion of components in a pre-mixed flammable gas in situ

  • 52.
    Mattsson, Roger
    et al.
    Luleå tekniska universitet.
    Kupiainen, M.
    Kungliga tekniska högskolan, KTH.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wåhlin, Anders
    Carlsson, T.E.
    Swedish Defence Research Agency, FOI.
    Fureby, C.
    Swedish Defence Research Agency, FOI.
    Pulsed TV holography and schlieren studies, and large eddy simulations of a turbulent jet diffusion flame2004Inngår i: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 139, nr 1-2, s. 1-15Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In the search for an improved understanding of jet-flame dynamics we here compare predictions from large-eddy simulations (LES) and measurements using schlieren and holographic interferometry of a round turbulent jet diffusion flame. The studies concern a turbulent propane-air (C3H 8-O2/N2) diffusion flame under ambient conditions at a Reynolds number of Re=104. The interferometric measurements were performed with an all-electronic method, pulsed TV holography, using a pulsed laser and a fast charge coupled device (CCD) camera. The LES calculations use the probability density function (PDF) flamelet approach with a beta function as the probability density function, whereas the subgrid turbulence is modeled with a one-equation eddy viscosity model. In order to validate the LES model quantitative comparisons of first-order statistical moments of the velocity were first made with available data for nonreactive jets. The LES model captures the statistics well. The next step in the validation process concerns comparing the jet-flame development between LES and the schlieren and pulsed TV holography data. To this end the results of the LES calculations were used to simulate instantaneous interference patterns using ray tracing. The LES model describes the overall behavior of the flame successfully

  • 53.
    Miroshnikova, Natalia
    et al.
    Luleå tekniska universitet.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Sarady, Istvan
    Percussion hole drilling of metals with a fourth-harmonic Nd:YAG laser studied by defocused laser speckle correlation2005Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 44, nr 17, s. 3403-8Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Defocused speckle correlation is introduced as a tool for measuring the response in metal sheets during percussion laser drilling. For this procedure the fourth-harmonic Nd:YAG wavelength (266 nm) was used in pulsed mode. The method provides a cost-efficient and robust alternative to speckle interferometry for the study of the small deformations that appear during laser processing. The accuracy was shown to be of the order of a few tens of microradians for the tilt component that is measured, which translates to a few nanometers in deflection when the component is spatially integrated. In the measurements, deflections in the form of craters as large as 50 nm were detected on the back sides of silver and copper sheets. The diameters of the craters were 300 μm in the silver and 150 μm in the copper sheet; the output diameter of the hole was ~5 μm

  • 54.
    Molin, Nils-Erik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Schedin, Staffan
    Jansson, E.V.
    Gren, Per
    Transient response of the violin body when impacted at the bridge and at the strings1997Inngår i: ISMA'97, Institute of Acoustics , 1997Konferansepaper (Fagfellevurdert)
  • 55. Molin, Nils-Erik
    et al.
    Sjödahl, Mikael
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andersson, A.
    Luleå tekniska universitet.
    Digital speckle photography combined with speckle interferometry2001Inngår i: Second International Conference on Experimental Mechanics, 29 November - 1 December 2000, Singapore, Bellingham, Wash: SPIE - International Society for Optical Engineering, 2001Konferansepaper (Fagfellevurdert)
  • 56. Molin, Nils-Erik
    et al.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Andersson, A.
    Luleå tekniska universitet.
    Digital speckle photography combined with speckle interferometry2000Inngår i: Proceedings of the IOP Applied Optics and Opto-Electronics Conference: Loughborough, 2000, IOP Publishing Ltd , 2000Konferansepaper (Fagfellevurdert)
  • 57. Molin, Nils-Erik
    et al.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Svanbro, Angelica
    Speckle photography combined with speckle interferometry2004Inngår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 41, nr 4, s. 673-686Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    By combining speckle interferometry (SI) measurements with speckle photography, the fringe visibility can be kept high despite the presence of a large bulk or rotating motion of the object. This combined technique improves the usability and measuring range of both pulsed and phase-stepped Sl-methods. This paper reviews the theory of fringe formation in Sl and shows some recent applications of this combined technique

  • 58. Molin, Nils-Erik
    et al.
    Svanbro, Angelica
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Combined digital speckle photography (DSP) and interferometry2001Inngår i: Second international conference on experimental mechanics, Bellingham, Wash: SPIE - International Society for Optical Engineering, 2001Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A general problem in optical metrology is to measure a deformation field when this field is added to a translation or a rotational motion. Methods like Speckle Photography (SP) do handle large rigid body motions but the results might be of too poor accuracy to resolve the deformation field. Interferometric methods on the other hand might measure the deformation field but the bulk motion makes the fringes disappear. By combining Digital Speckle Photography, Dsp, (also called digital correlation) with Speckle Interferometry, SI, (also called ESPI, DSPI, TV holography, pulsed TV holography) or with shearography (TV shearography) such measuring situations can be mastered.

  • 59. Niemi, Jan
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Löfqvist, Torbjörn
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    On a new sensing strategy using a combination of ultrasonic and photoacoustic techniques2006Inngår i: Proceedings: 2006 IEEE Ultrasonics Symposium : Vancouver, Canada, 3 - 6 October 2006, Piscataway, NJ: IEEE Communications Society, 2006, s. 1797-1800Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The process industry is today aiming for more advanced process control strategies. These strategies rely on quick and accurate sensing of process variables. Considering processes where particles are suspended in a fluid, e.g. paper and pulp industry, there is a need for development of a new or improved in-line sensor technique determining different properties of the particles. In this study we present a sensing strategy where a combination of acoustical and optical measurement techniques is used. For this purpose have we designed and built a measurement cell which utilises the ultrasonic signal generated from an ultrasonic transducer as well as the ultrasonic, or photoacoustic, signal generated using a pulsed laser. The photoacoustic method is using the same transducer as in the transducer based method above. In the transducer based method, the well known pulse-echo technique is used. The pulse is generated by an ultrasonic transducer, travels through the suspension and reflects at a steel reflector. In the photoacoustic method, the ultrasonic transducer receives the ultrasound that is generated when a pulse of laser light is scattered and absorbed in the suspension. The laser beam enters the cell through fused silica windows. The laser beam is crossing the cell orthogonally to the ultrasonic transducer. The photoacoustic signal contains two echoes. The first echo is the acoustic signal which is generated when photons are absorbed by the medium. The second echo is the signal that has been travelling through the suspension and reflected at the steel reflector. The two measurement techniques were tested experimentally in the cell using aqueous suspensions of Nylon 66 fibres. The samples were mixed from fibres of three different diameters 17, 51 and 55 µ m with the length of 1.2mm for the 17µm and 1.5 mm for the 51 and 55µm. The fibres were suspended in distilled and degassed water to mass fractions of 0.12% and 0.25%. Distilled and degassed water was also used as a reference medium when determining the calibration constants for the cell. The photoacoustic signal is generated using a double-pulsed laser having pulse energy of 10 mJ and the laser beam is focused at the centre of the cell. The suspension was pumped around in a closed loop using a peristaltic pump and the temperature of the suspension was monitored during the measurement cycle. The result shows that the signal generated using the transducer based method scales linearly with mass fraction for each type of fibre. The results also show that the photoacoustic signal is sensitive to the number density of scatters in the suspension, i.e. the amplitude of the signal decreases with decreasing fibre diameters. These preliminary results indicate that a combination of these two techniques might be used in resolving the proportion of different particles with different diameter in a suspension.

  • 60.
    Norman, Peter
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Engström, Hans
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Kaplan, Alexander
    Correlation between photodiode monitoring and high speed imaging of the dynamics causing laser welding defects2008Inngår i: Congress proceedings: ICALEO, 27th International Congress on Applications of Lasers and Electro-Optics : October 20 - 23, 2008 Pechanga Resort & Casino, Temecula, CA, USA ; [includes] Laser Materials Processing Conference, Laser Microprocessing Conference, Nanomanufacturing Conference / [ed] Yongfeng Lu, Orlando, Fla: Laser institute of America , 2008, s. 829-837Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Today merely a few monitoring systems for in-process detection of laser welding defects are commercially available. Despite a trend towards cameras, industrially more robust is still a photodiode, measuring in an optically filtered spectral window either the thermal emissions from the melt and vapour or the laser beam reflections. The monitoring rule for each application is identified empirically through correlations between the signal dynamics and welding defects, as the linking mechanism is non-trivial and therefore hardly understood. Thus the method does not provide a systematic guideline to detect a certain welding defect.The here presented approach studies the context between the photodiode signal, the welding defects and the vapour, melt pool, keyhole and temperature dynamics. Simultaneous laser-illuminated high speed imaging is compared to photodiode monitoring at three spectral windows in order to identify through simultaneous timing any linking dynamics in a qualitative manner. Supportive methods are emission modelling and thermal imaging. Several cases of joints, materials and defects were studied to develop an illustrated theoretical description of the defect-signal correlations.

  • 61.
    Olsson, Erik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Inferometrisk mätning av optoakustiska tryckvågor genererade i blod2009Konferansepaper (Annet vitenskapelig)
  • 62.
    Olsson, Erik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Fotoakustisk holografisk avbildning av kolbaserade absorbenter i en vävnadsfantom2011Konferansepaper (Fagfellevurdert)
  • 63.
    Olsson, Erik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Photoacoustic holographic imaging of absorbers embedded in silicone2011Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 50, nr 17, s. 2551-2558Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Light absorbing objects embedded in silicone have been imaged using photoacoustic digital holography. The photoacoustic waves were generated using a pulsed Nd:YAG laser, λ=1064 nm, and pulse length=12 ns. When the waves reached the silicone surface, they were measured optically along a line using a scanning laser vibrometer. The acoustic waves were then digitally reconstructed using a holographic algorithm. The laser vibrometer is proven to be sensitive enough to measure the surface velocity due to photoacoustic waves generated from laser pulses with a fluence allowed for human tissue. It is also shown that combining digital holographic reconstructions for different acoustic wavelengths provides images with suppressed noise and improved depth resolution. The objects are imaged at a depth of 16.5 mm with a depth resolution of 0.5 mm.

  • 64.
    Olsson, Erik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Sjödahl, Mikael
    Photoacoustic waves generated in blood studied using pulsed digital holography2010Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 49, nr 16, s. 3053-3058Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We studied photoacoustic waves using pulsed digital holography. The acoustic waves were generated in a reindeer blood target by absorption of an IR laser pulse, λ=1064 nm and pulse length=12 ns. The acoustic pressure waves were then imaged in water using a second collimated laser pulse at λ=532 nm2 μs after the first IR pulse. Quantitative information on acoustic wave properties such as three-dimensional shape and pressure distribution was calculated by applying the inverse Radon transform on the recorded projection. The pressure pulse had a flat and sharp front parallel with the blood surface, which indicates that the pressure was generated at the blood surface. The generated pressure was proportional to the laser fluence with the proportionality constant equal to 1.8±0.3 cm-1. According to existing data, the proportionality constant should be 1.4 cm-1 for oxygenated human blood, which made our calculations probable

  • 65.
    Olsson, Robin
    et al.
    Aeronautical Research Institute of Sweden.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Pilot study of the dynamic impact response of a composite plate1990Rapport (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    An experimental pilot study of the dynamic response of composite plates in low mass impacts is described. The purpose was to make an initial investigation of the validity of an analytical impact solution recently presented. In the experiments a small spherical pendulum was used to strike a carbon fiber/epoxy laminate at various impact velocities. The time histories of the impactor acceleration (contact force), plate displacement and flexural strains were recorded. The plate displacement history was studied using holographic laser interferometry. Only a limited number of displacement results were obtained, since the holographic method leads to a very large number of densely spaced interference fringes even at moderate impact velocities. The theoretical predictions of contact force and plate displacement were found to be very good, although the flexural strains were only predicted qualitatively. An almost linear relation was found between impact velocity and the maximum values of contact force, plate displacement and flexural strains, which is in excellent agreement with the analytical predictions.

  • 66.
    Pappadà, Silvio
    et al.
    Departments of Materials and Structures Engineering, Technologies and Processes Area, Consorzio CETMA.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Tatar, Kourosh
    Gustafson, Tord
    APC Composite, Karlsvik, Luleå.
    Rametta, Rocco
    Departments of Materials and Structures Engineering, Technologies and Processes Area, Consorzio CETMA.
    Rossini, Ettore
    Extreme Materials S.r.l., Costa Masnaga.
    Maffezzoli, Alfonso
    Department of Engineering for Innovation, University of Salento.
    Mechanical and vibration characteristics of laminated composite plates embedding shape memory alloy superelastic wires2009Inngår i: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 18, nr 5-6, s. 531-537Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Currently, there is a great interest in the study of shape memory alloy (SMA) composites, since SMA wires with a small diameter have become commercially available. Many potential uses have been found for SMA composites in shape control, vibration control, and for the realization of structures with improved damage tolerance. In this work, two types of SMA-hybridized composites are presented for investigating the mechanical and vibration characteristics. The first one contains unidirectional superelastic SMA wires, while the other has been realized with embedded knitted SMA layers. The samples from these laminates have been tested according to "Charpy method" (ASTM D256) and static flexural test method (ASTM D790) to evaluate the influence of the integration of thin superelastic SMA wires on the impact behavior and the mechanical properties of the hybrid composites. Moreover, since the SMA wires are expected to give damping capacity, by measuring the vibration mode of a clamped cantilever using laser vibrometry, the influence of both SMA arrangements on the vibration characteristics has been investigated. Finally, further tests have been carried out on composite panels realized by embedding unidirectional steel wires to distinguish the influence of the martensitic transformation from the pure introduction of a metallic wire into the polymeric matrix.

  • 67.
    Rosendahl, Sara
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dual-wavelength holographic shape measurement with iterative phase unwrapping2011Inngår i: Optical Measurement Systems for Industrial Inspection VII / [ed] Peter H. Lehmann; Wolfgang Osten; Kay Gastinger, Bellingham, Wash.: SPIE - International Society for Optical Engineering, 2011, s. 80820B-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In order to measure the shape of a large number of identical components in a manufacturing industry we propose a method where digital holography is used to capture an image of the object and then the shape of the object is achieved by using information from the CAD-model. The holographic recording of the object is done using dual wavelengths giving a synthetic wavelength of about 400 μm. This gives a phase map where the phase intervals represent a depth distance on the object of about 0.2 mm. To find the shape of the object the phase map has to be unwrapped. Since the surface contains discontinuities we use information from the CAD-model of the measured object and unwrap the phase iteratively. The result becomes a digital point representation of the measured surface that can either be used just as a description of the object shape or as a way to describe how well the object has been manufactured compared to the CAD-model. The measurement process that is proposed is adapted for on-line purposes; hence it is fast and reliable.

  • 68.
    Rosendahl, Sara
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hällstig, Emil
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Phase errors due to speckles in laser fringe projection2010Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 49, nr 11, s. 2047-2053Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When measuring a three-dimensional shape with triangulation and projected interference fringes it is of interest to reduce speckle contrast without destroying the coherence of the projected light. A moving aperture is used to suppress the speckles and thereby reduce the phase error in the fringe image. It is shown that the phase error depends linearly on the ratio between the speckle contrast and the modulation of the fringes. In this investigation the spatial carrier method was used to extract the phase, where the phase error also depends on filtering the Fourier spectrum. An analytical expression for the phase error is derived. Both the speckle reduction and the theoretical expressions for the phase error are verified by simulations and experiments. It was concluded that a movement of the aperture by three aperture diameters during exposure of the image reduces the speckle contrast and hence the phase error by 60%. In the experiments, a phase error of 0.2 rad was obtained.

  • 69.
    Rosendahl, Sara
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hällstig, Emil
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Sjödahl, Mikael
    Phase errors in speckle reduced laser fringe projection2010Inngår i: Speckle 2010: optical metrology : 13 - 15 September 2010, Florianópolis, Brazil / [ed] Armando Albertazzi Gonçalves, Bellingham, Wash: SPIE - International Society for Optical Engineering, 2010, s. 73871E-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    When measuring 3D-shape with triangulation and projected interference fringes it is of interest to reduce the phase error in the fringe pattern. A study has been carried out concerning parameters that will affect the phase error and an analytical expression has been derived. It is concluded that the phase error depends on the speckle contrast, C, and the modulation, M, of the fringes and since the phase in this investigation is determined using the spatial carrier method the phase error also depends on the filtering of the Fourier spectrum. To reduce the phase error this work has been focusing on suppressing the speckle contrast. For this the method with a moving aperture is used; a disk with several apertures is rotated in the aperture plane of the camera lens. To verify the derived expression for the phase error and the method to suppress speckles both numerical simulations and experiments have been performed. In the measurements made it was concluded that after an aperture movement of three aperture diameters the speckle contrast and hence the phase error was reduced by 60 %. A phase error of 0.15 radians was obtained in the experiments, thus approximately 1/40 of a fringe period

  • 70.
    Rosendahl, Sara
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hällstig, Emil
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Sjödahl, Mikael
    Shape measurement with one fringe pattern recording including a digital master2010Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 49, nr 14, s. 2622-2629Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a method in which the 3D shape of an object can be measured and compared to the shape of the digital master of the object, e.g., the computer-aided design model. The measurement is done using a stereo camera system and a single projected fringe pattern. Because the digital master is available, i.e., the expected shape is known, only one projection and image recording is necessary; thus, the method becomes fast. The idea in this work is to find homologous points in the cameras, i.e., points corresponding to the same object point, using the object information. An algorithm to find the homologous points is presented and a method to calculate shape is described. Given the ambiguity due to the fact that the phase in the images is wrapped, there is a maximum deviation from the master that can be correctly detected. An analytical expression for this deviation is derived. Results from the shape measurement of an object both with and without deviations from the digital master are also presented. In these measurements, where the measurement volume is approximately 1 dm3 and the fringe period on the object plane is about 1 mm, the accuracy is ≈± 40 μm, and a deviation of max ≈± 1.6mm can be correctly detected.

  • 71.
    Saarela, Juha
    et al.
    Department of Electrical and Information Engineering and Infotech Oulu, University of Oulu.
    Löfqvist, Torbjörn
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Ramser, Kerstin
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Signaler och system.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Detection of laser induced dielectric breakdown in water using a laser doppler vibrometer2010Inngår i: Central European Journal of Physics, ISSN 1895-1082, E-ISSN 1644-3608, Vol. 8, nr 2, s. 235-241Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study is focused on exploring the feasibility of an all-optic surface scanning method in determining the size and position of a submerged, laser generated, optoacoustic source. The optoacoustic effect was here generated when the absorption of a short electromagnetic pulse in matter caused a dielectric breakdown, a plasma emission flash and a subsequent acoustic wave. In the experiment, a laser pulse with l = 1064 nm and 12 ns pulse length was aimed at a volume of deionized water. When the laser beam was focused by a f = 16 mm lens, a single dielectric breakdown spot occurred. When a f = 40 mm was used several breakdowns in a row were induced. The breakdowns were photographed using a double shutter camera. The acoustic wave generated by the dielectric breakdowns were detected at a point on the water surface using a laser Doppler vibrometer (LDV). First, the LDV signal was used to calculate the speed of sound with an accuracy of 10 m/s. Secondly, the location and length of the dielectric breakdown was calculated with an accuracy of 1 mm. The calculated position matched the breakdown location recorded by a camera. The results show that it is possible to use LDV surface measurements from a single spot to determine both the position and length of the OA source as well as the speed of sound in the medium. Furthermore, the LDV measurements also show a secondary peak that originates from the OA source. To unravel the origin and properties of this interesting feature, further investigations are necessary.

  • 72. Sarady, Istvan
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Investigation of interaction mechanisms in laser processing using digital speckle photography2003Inngår i: Congress proceedings: Laser Materials Processing Conference [and] Laser Microfabrication Conference [at] ICALEO 2003, 22nd International Congress on Applications of Lasers & Electro-Optics, October 13 - 16, 2003, Adam's Mark Hotel, Jacksonville, Florida, USA / [ed] Xiangli Chen, Orlando, Fla: Laser institute of America , 2003, s. 314-321Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Digital, defocused speckle photography was used for the in-situ, real-time determination of the strain field round the focus spot of a pulsed Nd:YAG laser. These surface strains can cause cracking during laser cutting and drilling of advanced engineering ceramic materials. A CW HeNe laser was used for illumination, images being captured by CCD camera with a 633 nm band pass filter, synchronised to operate between the individual laser pulses. Targets included alumina and stainless steel.

  • 73.
    Schedin, Staffan
    et al.
    Luleå tekniska universitet.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Phase evaluation and speckle averaging in pulsed television holography1997Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 36, nr 17, s. 3941-3947Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Double-pulsed (image-plane) TV holograms of transient bending waves in plates are recorded on separate frames in a CCD camera. A small angular offset between the reference and object beams, giving a spatial-frequency bias to the recorded pattern, permits quantitative evaluation of the phase of the interference. The Fourier spectrum of the image exhibits distinct parts that can be filtered out and inverse transformed to yield the phase information. Three different apertures of the imaging system are tested: a single slit, a double slit, and a three-hole aperture. Spatial speckle averaging is possible in the cases of the double-slit and three-hole apertures

  • 74.
    Schedin, Staffan
    et al.
    Luleå tekniska universitet.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Pulsed TV holography: system description and some applications1997Inngår i: Sensors, sensor systems and sensor data processing: [Conference on Sensors, Sensors Systems, and Sensor Data Processing], June 16-17, 1997, Munich, FRG. Sponsored by EOS, the European Optical Society... / [ed] Ottmar Loffeld, Bellingham, Wash: SPIE - International Society for Optical Engineering, 1997, s. 14-21Konferansepaper (Fagfellevurdert)
    Abstract [en]

    An all electronic double-pulsed TV holography system is described. Two separate image-plane holograms are recorded using a CCD camera. A small angular offset between the reference and object beams introduces carrier fringes in the image plane. Thus, the Fourier spectrum of the image contains distinct parts that can be filtered out and inverse transformed to yield the phase information. Experiments showing propagating transient bending waves in an aluminum plate generated by a focused laser pulse are performed. Phase maps of good quality are unwrapped and presented as 3D plots of the out-of-plane displacement. The system has also been used to evaluate the 3D distribution of transient acoustic fields in air generated by an electrical discharge. Several projections of the acoustic field is recorded and the 3D pressure field is reconstructed by computerized tomography.

  • 75. Schedin, Staffan
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Finnström, Marie
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Measurement of the density field around an airgun muzzle by pulsed TV holography1999Inngår i: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, nr 3823, s. 13-19Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The density distribution in the air surrounding an airgun muzzle is visualized using pulsed TV holography. A ruby laser emitting short coherent light pulses (30 ns) is used as light source. Pulsed holograms are captured by a CCD-camera and their optical phase difference is evaluated by means of the Fourier transform method. A number of experiments are performed where the event is recorded at different instants of time as the lead bullet is about to leave the muzzle. Phase maps showing the integrated density distribution are presented. A jet with decreasing density ahead of the bullet can be observed. At the moment the bullet leaves the muzzle, a spherical sound pressure wave starts to propagate out into the air. The density in the jet in front of the traveling bullet can be calculated by means of full shock-wave theory. The theory confirms the decrease in density observed in the measurements

  • 76. Schedin, Staffan
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Rossing, T.D.
    Transient wave propagation in a cymbal1997Inngår i: ISMA '97 Conference: ISMA '97, [International Symposium on Musical Acoustics, University of Edinburgh, 19th - 22nd August 1997, Institute of Acoustics , 1997, s. 201-206Konferansepaper (Fagfellevurdert)
  • 77.
    Schedin, Staffan
    et al.
    Luleå tekniska universitet.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Rossing, Thomas D.
    Northern Illinois University.
    Transient wave response of a cymbal using double-pulsed TV holography1998Inngår i: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 103, nr 2, s. 1217-1220Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using an electronic system for pulsed TV holography with a double-pulsed ruby laser, transient wave propagation during intervals from 30 to 480 µs after impact is recorded. The first observable bending waves, having wavelengths of about 5 mm, propagate at about 1700 m/s, and reach the edge of the cymbal in about 60 µs. These are quickly followed by waves of longer wavelength which scatter at the outer edge of the cymbal and also the central dome and result in standing waves. A phase unwrapping procedure is used to obtain a three-dimensional map of the wave field. Holographic film recordings similarly show scattering of transient bending waves at the central dome

  • 78.
    Schedin, Staffan
    et al.
    Luleå tekniska universitet.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wåhlin, Anders
    Shock waves in an elliptical cavity with varying height1997Inngår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 7, nr 6, s. 343-350Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Transient shock waves in a confined elliptical chamber are experimentally investigated. Quantitative results of the pressure distribution are obtained for an air-filled cavity. Lower bounding surfaces of different geometrical shapes can be inserted making it possible to get chambers with varying height. An electrical discharge across a pair of electrodes inside the cavity gives rise to the shock waves. Double pulsed holographic interferometry is used to study the propagation and focusing process of the waves. The results are quantitatively evaluated by using the method of two-reference-beam holography. The angular pressure distribution behind the converging wave front is presented for different geometries of an air-filled cavity. The pressure distribution is non-homogeneous but symmetric along the wave front. The pressure level is higher for the geometry where the height of the chamber decreases with the radial distance from the outgoing focus and lower for increasing height of the chamber. In addition, shock waves in a water-filled cavity are studied. In this case qualitative results are obtained.

  • 79. Schedin, Staffan
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wåhlin, Anders
    Visualization of transient acoustic fields generated by schock loaded plates1995Inngår i: Proceedings of the 15th International Congress on Acoustics: Trondheim, Norway, 26-30 June 1995, Trondhjeim: Tapir Akademisk Forlag, 1995Konferansepaper (Fagfellevurdert)
  • 80.
    Schedin, Staffan
    et al.
    Luleå tekniska universitet.
    Wåhlin, Anders
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Transient acoustic near field in air generated by impacted plates1996Inngår i: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 99, nr 2, s. 700-705Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The transient acoustic near field in air from impacted cantilever plates of two different geometrical shapes and materials is studied using two-reference-beam double-pulsed holographic interferometry. The acoustic waves, observed in two directions simultaneously, are generated by traveling flexural waves in the plates and by the impact process itself. A subsequent phase-stepping procedure is used for quantitative evaluation of the registered two-dimensional projections of the acoustic fields. An interesting observation is the acoustic radiation at the free end of the plate, where sound waves propagate into the air in oblique directions from the plate. At the region right above the free end no acoustic waves are observed. Supersonic flexural waves in the plates generate trace-matched acoustic waves in the surrounding air. Calculations of the corresponding two-dimensional transient acoustic fields are performed. Those are based on an analytical solution of the Kirchhoff plate equation. Calculated results reproduce the character of the observed transient acoustic waves quite well

  • 81.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sarady, Istvan
    Miroshnicova, Natalia
    Luleå tekniska universitet.
    Laser hole drilling process studied using laser speckle correlation2006Inngår i: Speckle06: speckles, from grains to flowers ; 13 - 15 September 2006, Nimes, France ; [proceedings] / [ed] Pierre Slangen; Christine Cerruti, Bellingham, Wash.: SPIE - International Society for Optical Engineering, 2006, s. 63412M-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This paper revisits defocused speckle correlation as a tool for measuring the response in metal sheets during percussion laser drilling processing. For the processing the 4th harmonic Nd:YAG wavelength (266 nm) was used in pulsed mode. It is shown that the method provides a cost efficient and robust alternative to speckle interferometry for the study of the small deformations that appear during laser processing. The accuracy was shown to be in the order of a few tens of μrads for the tilt component being measured which translates to a few nm in deflection when being spatially integrated. In the measurements deflections up to 50 nm was detected on the backside of silver and a copper sheet, respectively.

  • 82.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Tatar, Kourosh
    Olsson, Erik
    Laser metrology in experimental mechanics2005Inngår i: 10th NOLAMP Conference: the 10th Nordic Laser Materials Processing Conference, 17-19 August 2005, Luleå Sweden / [ed] Alexander Kaplan, Luleå: Luleå tekniska universitet, 2005, s. 41-52Konferansepaper (Fagfellevurdert)
  • 83.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Amer, Eynas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Depth resolution of phase gradients using pulsed digital holography2010Inngår i: Biomedical optics (BIOMED): digital holography and three-dimensional imaging (DH) ; April 11 - 14, 2010, Miami, Florida, USA ; technical digest, Washington, DC: Optical Society of America, 2010Konferansepaper (Fagfellevurdert)
    Abstract [en]

    technique to gain depth information from a single pair image-plane Digital Holographic recording of a transient phase object positioned between a diffuser and an imaging system has been demonstrated

  • 84.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Amer, Eynas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Depth-resolved measurement of phase gradients in a transient phase object field using pulsed digital holography2009Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 48, nr 34, s. H31-H39Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A technique to gain depth information from an image-plane digital holographic recording of a transient phase object positioned between a diffuser and an imaging system is demonstrated. The technique produces telecentric reconstructions of the complex amplitude throughout the phase volume using numerical lenses and the complex spectrum formulation of the diffraction integral. The in-plane speckle movements as well as the phase difference between the disturbed field and a reference field are calculated in a finite number of planes using a cross-correlation formulation. It is shown that depth information about in-plane phase gradients can be determined in two planes using reconstructed speckle fields from four different depths. In addition, the plane of optimum reconstruction for calculating the phase difference with maximum contrast is detected from the technique. The method is demonstrated on a measurement of a laser ablation process.

  • 85.
    Tatar, Kourosh
    et al.
    Division of Mechanical Engineering, University of Gävle.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Estimation of the in-plane vibrations of a rotating spindle, using out-of-plane laser vibrometry measurements2016Inngår i: Mechanical systems and signal processing, ISSN 0888-3270, E-ISSN 1096-1216, Vol. 72-73, s. 660-666Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A method for estimating the in-plane vibrations of a rotating spindle using out-of-plane laser vibrometry measurements is described. This method enables the possibility to obtain the two orthogonal radial vibration components of a rotating spindle. The method uses the fact that the laser vibrometer signal is a total surface velocity of the measurement point in the laser direction.Measurements are conducted on a rotating milling machine spindle. The spindle is excited in a controlled manner by an active magnetic bearing and the response is measured by laser vibrometer in one of the two orthogonal directions and inductive displacement sensors in two orthogonal directions simultaneously. The work shows how the laser vibrometry crosstalk can be used for resolving the in-plane vibration component, that is the vibrations in the laser vibrometer cross direction. The result is compared to independent measurement signals from the displacement sensors.The measurement method can be used for vibration measurements on rotating parts, for example, where there is lack of space for orthogonal measurements.

  • 86. Tatar, Kourosh
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Measurement of milling tool vibrations during cutting using laser vibrometry2008Inngår i: International journal of machine tools & manufacture, ISSN 0890-6955, E-ISSN 1879-2170, Vol. 48, nr 3-4, s. 380-387Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Spindle and tool vibration measurements are of great importance in both the development and monitoring of high-speed milling. Measurements of cutting forces and vibrations on the stationary spindle head is the most used technique today. But since the milling results depends on the relative movement between the workpiece and the tool, it is desirable to measure on the rotating tool as close to the cutters as possible. In this paper the use of laser vibrometry (LDV) for milling tool vibration measurements during cutting is demonstrated. However, laser vibrometry measurements on rotating surfaces are not in general straight forward. Crosstalk between vibration velocity components and harmonic speckle noise generated from the repeating revolution of the surface topography are problems that must be considered. In order to overcome the mentioned issues a cylindrical casing with a highly optically smooth surface was manufactured and mounted on the tool to be measured. The spindle vibrations, radial tool misalignment, and out-of-roundness of the measured surface were filtered out from the signal; hence the vibrations of the cutting tool were resolved. Simultaneous measurements of cutting forces and spindle head vibrations were performed and comparisons between the signals were conducted. The results showed that vibration velocities or displacements of the tool can be obtained with high temporal resolution during cutting load and therefore the approach is proven to be feasible for analyzing high-frequency milling tool vibrations.

  • 87. Tatar, Kourosh
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Digital holographic interferometry for simultaneous orthogonal radial vibration measurements along rotating shafts2008Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 47, nr 18, s. 3269-3274Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A digital holographic interferometry setup used to measure radial vibrations along a rotating shaft is presented. A continuous Nd:YAG laser and a high-speed digital camera are used for recording the holograms. The shaft was polished optically smooth to avoid speckle noise from the rotating surface. The light reflected from the shaft was directed onto a diffuser which in turn was imaged by the holographic system. Simultaneous measurements with a laser vibrometer were performed at one point and comparisons between the signals showed good agreement. It is shown that different vibration components of a rotating shaft can be simultaneously measured with this technique.

  • 88.
    Tatar, Kourosh
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Rantatalo, Matti
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Laser vibrometry measurements of an optically smooth rotating spindle2007Inngår i: Mechanical systems and signal processing, ISSN 0888-3270, E-ISSN 1096-1216, Vol. 21, nr 4, s. 1739-1745Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Laser doppler vibrometry (LDV) is a well-established non-contact method, commonly used for vibration measurements on static objects. However, the method has limitations when applied to rotating objects. The LDV signal will contain periodically repeated speckle noise and a mix of vibration velocity components. In this paper, the crosstalk between vibration velocity components in laser vibrometry measurements of a rotating dummy tool in a milling machine spindle is studied. The spindle is excited by an active magnetic bearing (AMB) and the response is measured by LDV in one direction and inductive displacement sensors in two orthogonal directions simultaneously. The work shows how the LDV crosstalk problem can be avoided if the measurement surface is optically smooth, hence the LDV technique can be used when measuring spindle dynamics.

  • 89.
    Tóth, Pál
    et al.
    RISE Bioekonomi/RISE Energy Technology Center AB, Piteå. Institute of Thermal Energy, University of Miskolc, Miskolc, Hungary.
    Ögren, Yngve
    RISE Bioekonomi/RISE Energy Technology Center AB, Piteå, Sweden.
    Sepma, Alexey
    RISE Bioekonomi/RISE Energy Technology Center AB, Piteå, Sweden.
    Vikström, Therese
    RISE Bioekonomi/RISE Energy Technology Center AB, Piteå, Sweden.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wiinikka, Henrik
    RISE Bioekonomi/RISE Energy Technology Center AB, Piteå, Sweden.
    Spray combustion of biomass fast pyrolysis oil: Experiments and modeling2019Inngår i: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 237, s. 580-591Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this work, we are the first to report a detailed comparison between the predictions of a current Computational Fluid Dynamics (CFD) model for describing Fast Pyrolysis Oil (FPO) spray combustion and results from a laboratory-scale experiment. The objectives were to assess the predictive power of the CFD model, evaluate its usefulness in a numerical optimization scenario and characterize the spray flame. The spray flame was produced by using an air-assist atomizer piloted by a CH4" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;">CH4/air flat-flame. Pyrolysis oil from a cyclone fast pyrolysis plant was combusted. The flame was characterized by using two-color pyrometry, Tunable Diode Laser Absorption Spectroscopy and high-magnification shadowgraphy. Overall, the assessed model correctly predicted flame structure and seemed appropriate for engineering applications, but lacked predictive power in estimating droplet size distributions. Numerical results were the most sensitive to variations in the initial droplet size distribution; however, seemed robust to changes in the multicomponent fuel formulation. Several conclusions were drawn regarding FPO spray combustion itself; e.g., the amount of produced soot in the flames was very low and droplets exhibited microexplosion behavior in a characteristic size-shape regime.

  • 90. Wahba, Hamdy
    et al.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Digital holographic microscopy for the study of nano-fibers2013Inngår i: Optical Measurement Systems for Industrial Inspection VIII / [ed] Peter H. Lehmann; Wolfgang Osten; Armando Albertazzi, SPIE - International Society for Optical Engineering, 2013Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The advantages of digital holographic microscopy to record not only the intensity but also the optical phase are employed. The experimental arrangement comprises a Mach-Zehnder type interferometer with a microscopic objective of magnification 100x. The used camera is a 5 Mpixels Allied Vision Guppy Pro F-503 with a pixel pitch of 2.2 μm. The lateral magnification is set to about 200x based on the standard MIL-STD-150A 1951 USAF resolution test target. The dimensions of the aggregated natural cellulose nanowhisker fibers used are in the range of some hundreds of nanometers, which are positioned in the front of the microscopic objective using a 3D translation stage in the object arm of the holographic setup. The recorded off-axis holograms are refocused using the angular spectrum method. The reconstructed complex field is used to calculate optical phase and intensity distributions of the object at different reconstructions depths. The dimensions and orientation of the fibers can be evaluated from the optical field at different depths. Then, the shape and textures along the aggregated natural cellulose nanowhisker fiber can be presented in 3D space. The nano fiber found to have the dimensions of mean width 223 nm, depth 308 nm and length of 8.1 μm. Further, the mean local refractive index of the nano fibers can be calculated (n=1.501).

  • 91.
    Wahba, H.H.
    et al.
    Department of Physics, Faculty of Science, Damietta University, New Damietta City.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    High resolution digital holographic microscopy for the study of aggregated natural cellulose nanowhisker fibers2015Inngår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 73, s. 69-74Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, digital holographic (DH) microscopy demonstrates its ability to perform a full characterization of nanofibers. The high resolution and magnification of the presented method to study the nanofibers are tested using standard MIL-STD-150A 1951 USAF resolution test target. In this investigation, aggregated natural cellulose nanowhisker fibers are positioned in the front of the microscopic objective using a 3D translation stage in the object arm of DH setup. The recorded off-axis holograms are refocused using the angular spectrum method. The reconstructed complex field is used to calculate optical phase and intensity distributions of the object at different reconstruction depths. A simple algorithm is used to define the focused image with suitable accuracy. The dimensions and orientation of the fibers can be evaluated from the optical field at different depths. Then, the shape and textures along the aggregated natural cellulose nanowhisker fiber can be presented in a 3D space.

  • 92.
    Wahl, Joel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lindbäck, Torbjörn
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Magnusson, Malin
    GKN Aerospace Sweden AB.
    Domeij, Kristina
    GKN Aerospace Sweden AB.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Comparison between process simulation and deformation measured by defocused speckle photography2018Inngår i: Speckle 2018: VII International Conference On Speckle Metrology / [ed] M. Kujawińska ; L. R. Jaroszewicz, SPIE - International Society for Optical Engineering, 2018, Vol. 10834, artikkel-id 108341KKonferansepaper (Fagfellevurdert)
    Abstract [en]

    Defocused laser speckle photography is used as a tool to measure the heat responses in a titanium component during laser heating. The evolution of the response is compared with a set of preprocessed Finite Element Simulations of the corresponding process with the aim to verify the simulation model and to find the simulation settings that best resemble the experimental results. The titanium component consists of a 300 x 100 mm2 substrate of thickness 3.2 mm on which a 200 x 30 x 11 mm3 ridge is built up using the laser metal deposition by wire process. The component is heated on the top of the ridge by a 300 W laser for 10 s and the deformation of the subtrate is followed throughout the heating-cooling cycle. The simulated deformation gradient is shown to resemble the measured response, and the magnitude of the response indicates that about 70 % of the laser power transferres into heat in the metal.

  • 93. Wåhlin, Anders
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Visualization of structure borne sound generated by transient bending waves1994Inngår i: Recent advances in experimental mechanics: proceedings of the 10th International Conference on Experimental Mechanics, Lisbon, Portugal, 18 - 22 July 1994 / [ed] J.F. Silva Gomes, Rotterdam: Balkema Publishers, A.A. / Taylor & Francis The Netherlands , 1994Konferansepaper (Fagfellevurdert)
  • 94.
    Wåhlin, Anders
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Molin, Nils-Erik
    On structure-borne sound: experiments showing the initial transient acoustic wave field generated by an impacted plate1994Inngår i: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 96, nr 5, s. 2791-2797Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The initial propagating transient acoustic field in air generated by an impacted plate is visualized. The transient traveling flexural waves in the plate created by the impact are shown, simultaneously. The experiments are performed using double pulsed holographic interferometry. It is shown that flexural waves in a plate act as a series of traveling acoustic sources. Since the flexural waves in the plate are dispersive the trace matched acoustical waves further away from the impact source propagate at a smaller angle relative to the plate than those closer to the impact. An observer situated close to the plate and away from the impact point will first receive high-frequency components of the sound. A quantitative evaluation of the acoustic field also shows that if there is a transient increase in pressure on one side of the plate there will be a similar decrease on the other side.

  • 95. Yalukova, O.
    et al.
    Miroshnikova, N.
    Gren, Per
    Sarady, Istvan
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Investigation of laser percussion hole drilling by use of speckle correlation2005Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 44, nr 30, s. 6338-44Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper speckle correlation is introduced as a tool to investigate the heat-influenced area during material processing with laser light. Two materials were investigated, a pure silver sheet and a sheet of SiC-diamond composite. The processing laser used in the experiments was a diode-pumped acousto-optical Q-switched Nd:YAG laser that allowed percussion hole drilling to be performed using green light through a second-harmonic crystal. The measurements were performed using a continuous-wave He-Ne laser and a digital camera. The experimental results show that the heat-influenced area is ~5000 times larger than the actual hole being drilled and that it reaches a steady-state condition toward the end of the processing cycle

  • 96.
    Öhman, Johan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Polarization-resolved dual-view holographic system for 3D inspection of scattering particles2019Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 58, nr 34, s. G31-G40Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A novel dual-view polarization-resolved pulsed holographic system for particle measurements is presented. Both dual-view configuration and polarization-resolved registration are well suited for particle holography. Dual-view registration improves the accuracy in the detection of 3D position and velocities, and polarization-resolved registration provides polarization information about individual particles. The necessary calibrations are presented, and aberrations are compensated for by mapping the positions in the two views to positions in a global coordinate system. The system is demonstrated on a sample consisting of 7 μm spherical polystyrene particles dissolved in water in a cuvette. The system is tested with different polarizations of the illumination. It is found that the dual view improves the accuracy significantly in particle tracking. It is also found that by having polarization-resolved holograms, it is possible to separate naturally occurring sub-micrometer particles from the larger, 7 μm seeding particles.

  • 97.
    Öhman, Johan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Polarization Resolved Dual-View Holographic System for Investigation of Microparticles2019Inngår i: OSA Technical Digest (Optical Society of America, 2019), 2019, artikkel-id Th2A.5Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A dual-view polarization resolved digital-holographic system is presented. The necessary calibration for both polarization and spatial coordinates are outlined. As an example the system is is used to track spherical microparticles in a cuvette.

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