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Reduction of the wake of a half-cylinder using a pair of plasma actuators
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.ORCID iD: 0000-0002-4156-8976
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]

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.

Keywords [en]
LES, plasma actuator, separation delay, half-cylinder, drag reduction, flow control
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-199870OAI: oai:DiVA.org:kth-199870DiVA, id: diva2:1065825
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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