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Analysis of the wake of a half-cylinder by dynamic mode decomposition
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
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-9061-4174
(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: urn:nbn:se:kth:diva-199872OAI: oai:DiVA.org:kth-199872DiVA, id: diva2:1065830
Funder
Swedish Energy Agency, 34186-1
Note

QC 20170117

Available from: 2017-01-16 Created: 2017-01-16 Last updated: 2017-01-25Bibliographically approved
In thesis
1. Effect of drag reducing plasma actuators using LES
Open this publication in new window or tab >>Effect of drag reducing plasma actuators using LES
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
Keywords
flow control, drag reduction, plasma actuator, DMD, LES, optimization, pulsating flow, strömningskontroll, motståndsminskning, plasmaaktuator, DMD, LES, optimering, pulserande strömning
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-199873 (URN)978-91-7729-261-6 (ISBN)
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

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