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Effect of drag reducing plasma actuators using LES
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-3194-5141
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work performed in this thesis explores new ways of reducing the drag of ground vehicles. Specifically, the effect of plasma actuators are investigated numerically with the intention to delay separation around a half-cylinder, a geometry chosen to represent a simplified A-pillar of a truck.

The plasma actuators have to be included in turbulent flow simulations. Therefore, emphasis is first put on finding a numerical model that can reproduce the effect of the plasma without increasing the computational cost. This effect is modeled through a body force term added to the Navier-Stokes equations. To determine the strength and spatial extent of this body force, optimization was performed to minimize the difference between experimental and simulated profiles of plasma induced velocity. 

The plasma actuator model is thereafter used in Large Eddy Simulations (LES) of the flow around a half-cylinder at Reynolds number Re=65*10^3 and Re=32*10^3. Two types of actuation cases are performed. In the first case, a single actuator is used. In the second case, a pair of consecutive actuators are used, and their position on the half-cylinder is changed. It is found that a drag reduction of up to 10% is achievable. Moreover, the ideal location for actuation is determined to be near the separation point of the non-actuated flow. 

Finally, dynamic mode decomposition (DMD) is investigated as a tool to extract coherent dynamic structures from a turbulent flow field. The DMD is first used to analyze a channel flow where pulsations are imposed at a known frequency. It is found that DMD gives similar results to phase averaging done at the oscillation frequency. However, the presence of turbulence noise hinders the ability to identify modes at higher harmonics. The DMD is also used to post-process the half-cylinder flow case. There, it is found that the spectrum of the wake is broadband. Nevertheless, modes within distinct frequency ranges are found to be located in distinct spatial regions.

Abstract [sv]

Arbetet som utförts i denna avhandling undersöker nya sätt att minska luftmotstånd hos markfordon. Speciellt undersöks numeriskt effekten av plasmaaktuatorer med avsikten att uppnå fördröjd separation av strömningen kring en halvcylinder, en geometri vald för att representera en förenklad A-stolpe på en lastbil. 

För att kunna utföra studien behöver plasmaaktuatorer kunna ingå i beräkningar av turbulenta strömningsfält. Därför undersöks först sätt för att hitta en numerisk modell som kan reproducera effekten av plasma utan att öka beräkningskostnad. Plasmaaktuatorn  modelleras i detta arbete genom att ett källterm adderas till Navier-Stokes ekvationer. För att bestämma styrkan och den rumsliga utbredningen hos källtermen, utförs en optimering för att minimera skillnaden mellan experimentella och simulerade profiler av plasma inducerad strömningshastighet. 

Plasmaaktuatormodellen används därefter i Large Eddy Simulations (LES) för att beräkna strömningen kring en halvcylinder med Reynolds tal Re=65*10^3 och Re=32*10^3. Två typer av fall studeras. I det första fallet används en enda aktuator. I det andra fallet, är ett par på varandra följande aktuatorer placerade, där aktuatorernas position på halvcylinder ändras. Resultaten visar att en luftmotståndsminskning på upp till 10% kan erhållas. Den idealiska platsen för aktuatorn bedöms vara nära den punkt där strömningen utan aktuator separerar.

Slutligen undersöks Dynamic Mode Decomposition (DMD) som ett verktyg för att extrahera koherenta dynamiska strukturer i en turbulent strömning. DMD används först för att analysera pulserande kanalströmning där pulsationen har en känd frekvens. Resultaten visar att DMD ger liknande resultat som då fas-medelvärdesbildning görs vid oscillationsfrekvensen. Förekomsten av turbulens buller hindrar dock möjligheten att identifiera moder vid högre övertoner. DMD används också för att analysera strömningen kring halv-cylindern. I avhandlingen visas att spektrat i vaken är bredbandigt men att även moder inom distinkta frekvensintervall fanns vara belägna i avgränsade områden i vaken.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 62
Series
TRITA-AVE, ISSN 1651-7660 ; 2017:02
Keyword [en]
flow control, drag reduction, plasma actuator, DMD, LES, optimization, pulsating flow
Keyword [sv]
strömningskontroll, motståndsminskning, plasmaaktuator, DMD, LES, optimering, pulserande strömning
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-199873ISBN: 978-91-7729-261-6 (print)OAI: oai:DiVA.org:kth-199873DiVA, id: diva2:1065860
Public defence
2017-02-03, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 34186-1
Note

QC 20170117

Available from: 2017-01-17 Created: 2017-01-16 Last updated: 2017-01-17Bibliographically approved
List of papers
1. Effect of a SDBD on the drag of a half-submerged cylinder in crossflow
Open this publication in new window or tab >>Effect of a SDBD on the drag of a half-submerged cylinder in crossflow
2014 (English)In: ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting, FEDSM 2014, Collocated with the ASME 2014 12th International Conference on Nanochannels, Microchannels, and Minichannels, 3 August 2014 through 7 August 2014, ASME Press, 2014, Vol. 1CConference paper, Published paper (Refereed)
Abstract [en]

In this paper the effect of a SDBD-type plasma actuator on the flow over a half-submerged cylinder is investigated numerically. The actuator is modeled via a body force, which is steady in time and where an exponential decay in space is assumed. First, the parameters in the numerical actuator model are determined for the case of no flow by optimization relative to experimental data. Thereafter, numerical solutions for the case with flow are studied numerically with and without actuation. A grid study is performed to check that the flow structures are resolved in both space and time. The effect of the actuator is examined. Although no significant change is observed when using the optimized parameters, using a stronger body force yields a reduction in drag of the order of 5%.

Place, publisher, year, edition, pages
ASME Press, 2014
Keyword
flow control, plasma actuator, sdbd, LES, drag reduction, half cylinder
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-161269 (URN)10.1115/FEDSM2014-21958 (DOI)2-s2.0-84919935823 (Scopus ID)978-0-7918-4623-0 (ISBN)
Conference
ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting, FEDSM 2014, Collocated with the ASME 2014 12th International Conference on Nanochannels, Microchannels, and Minichannels, Chicago, United States, 3 August 2014 through 7 August 2014
Funder
Swedish Energy Agency, 34186-1
Note

QC 20150312

Available from: 2015-03-11 Created: 2015-03-11 Last updated: 2017-01-17Bibliographically approved
2. Reduction of the wake of a half-cylinder using a pair of plasma actuators
Open this publication in new window or tab >>Reduction of the wake of a half-cylinder using a pair of plasma actuators
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper, the effect of plasma actuators on separated flows is studied via Large Eddy Simulations (LES) of the incompressible flow over a half-cylinder at a Reynolds number of 32*10^3. One plasma actuator is modeled by a steady body force distribution which is able to replicate the effect of the actuator in a quiescent environment without adding any significant complexity to the numerical simulations. This model is applied at two locations in order to simulate a pair of plasma actuators placed on the surface of the halfcylinder, separated by 20 degrees. Several simulations have been performed with the pair of actuators placed at different angles on the half-cylinder, and the drag reduction is reported for each configuration. It is determined that the actuation is able to achieve up to 10% of drag reduction when one actuator from the pair is placed a few degrees downstream of the separation point of the non-actuated flow. Mean flow quantities obtained in the wake and on the surface of the half-cylinder reveal that the reduction in drag is coupled to a reduction in the size of the recirculating zone as well as a delay of the separation point of up to 10 degrees.

Keyword
LES, plasma actuator, separation delay, half-cylinder, drag reduction, flow control
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-199870 (URN)
Funder
Swedish Energy Agency, 34186-1
Note

QC 20170117

Available from: 2017-01-16 Created: 2017-01-16 Last updated: 2017-01-25Bibliographically approved
3. Numerical study of the Stokes layer in oscillating channel flow
Open this publication in new window or tab >>Numerical study of the Stokes layer in oscillating channel flow
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Oscillating turbulent channel flows present particular physics that proves to be particularly difficult to understand. In this paper, a case where the amplitude of the oscillations at the center of the channel is approximately 15% of the mean velocity and the dimensionless angular forcing frequency is 0.01 was studied using several numerical methods. DNS was performed to serve as reference to which the results from an LES were compared. The LES data was post-processed using both phase averaging and the more recent dynamic mode decomposition (DMD), which extracts coherent structures based on their frequency. It was found that the DMD is not able to extract faint harmonic components of the oscillations, which have been observed with phase averaging and Fourier transforms. It is, however, able to extract accurate profiles of the mean and forcing frequency quantities. Compared to the DNS, the accuracy of the LES results was similar to analytical models, although no single model gives accurate result for every quantity investigated.  

Keyword
pulsating channel flow, LES, DNS, DMD
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-161407 (URN)
Note

QC 20170117

Available from: 2015-03-11 Created: 2015-03-11 Last updated: 2017-01-17Bibliographically approved
4. Analysis of the wake of a half-cylinder by dynamic mode decomposition
Open this publication in new window or tab >>Analysis of the wake of a half-cylinder by dynamic mode decomposition
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This paper analyzes the dynamic structures in the wake of a half-cylinder protruding from the ground. This relatively simple and smooth geometry allows to create a signicant wake, yet the the location of the detachment point is not predictable from the geometry. The flow over the half-cylinder has a Reynolds number of 32*10^3. It is considered to be incompressible and is simulated by Large Eddy Simulations (LES). The flow field is first described in terms of the time-averages of velocity, pressure, and turbulent kinetic energy. This is the most traditional way to study turbulent flows, and it enables to identify the recirculation regions upstream and downstream of the half-cylinder. The locations of separation and reattachment are also obtained. Then, dynamic structures are extracted by means of dynamic mode decomposition (DMD). The DMD modes have the particularity to oscillate in time at a single given frequency, which renders the dynamics of the flow field more intelligible. It is found that despite a broadband spectrum, all the DMD modes reveal the same type of phenomenon that varies only in scale. By observing the modes at different frequencies, vortices can be followed from their creation in the upstream recirculation region. As they are convected downstream, they merge with bigger and bigger vortices, until they are big enough to influence the whole wake.

National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-199872 (URN)
Funder
Swedish Energy Agency, 34186-1
Note

QC 20170117

Available from: 2017-01-16 Created: 2017-01-16 Last updated: 2017-01-25Bibliographically approved
5. Dymode: A parallel dynamic mode decomposition software
Open this publication in new window or tab >>Dymode: A parallel dynamic mode decomposition software
2015 (English)Report (Other academic)
Abstract [en]

Dymode is a parallel program that computes dynamic mode decompositions. The code is written in C++ and relies on a number of libraries. Several parameters can be specified in order to control the computational aspects of the program as well as the input and output of the decomposition, particularly how the modes are sorted. Finally, dymode is almost entirely parallel and is therefore particularly suitable for computing the dynamic mode decomposition of large datasets.

The dymode package also includes dymodem, a Matlab implementation of the code which accepts the same arguments as dymode, when they are relevant, and produces the same output. It can be useful to use dymodem when dealing with smaller datasets, or to validate the output from dymode.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. p. 36
Series
TRITA-AVE, ISSN 1651-7660 ; 2014:78
Keyword
dynamic mode decomposition, dmd, parallel
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-159643 (URN)978-91-7595-386-1 (ISBN)
Note

QC 20150206

Available from: 2015-02-06 Created: 2015-02-06 Last updated: 2017-01-16Bibliographically approved

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