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Drag reduction using 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
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is motivated by the application of active flow control on the cabin of trucks, thereby providing a new means of drag reduction. Particularly, the work presented strives to identify how plasma actuators can be used to reduce the drag caused by the detachment of the flow around the A-pillars. This is achieved by conducting numerical simulations, and is part of a larger project that also includes experimental.

The effect of plasma actuators is modeled through a body force, which adds very little computational cost and is suitable for implementation in most CFD solvers. The spatial distribution of this force is described by coefficients which have been optimized against experimental data, and the model was shown to be able to accurately reproduce the wall jet created by a single plasma actuator in a no-flow condition.

A half cylinder geometry - a simplified geometry for the A-pillar of a truck - was used in a preliminary Large Eddy Simulation (LES) study that showed that the actuator alone, operated continuously, was not sufficient to achieve a significant reduction of the drag. Nevertheless, a significant drag reduction was obtained by simply increasing the strength of the body force to a higher value, showing that this type of actuation remains relevant for the reduction of drag.

In the course of finding ways to improve the efficiency of the actuator, dynamic mode decomposition was investigated as a post-processing tool to extract structures in the flow. Such structures are identified by their spatial location and frequency, and might help to understand how the actuator should be used to maximize drag reduction. Thus a parallel code for dynamic mode decomposition was developed in order to facilitate the treatment of the large amounts of data obtained by LES. This code and LES itself were thereafter evaluated in the case of a pulsating channel flow. By using the dynamic mode decomposition it was possible to accurately extract oscillating profiles at the forcing frequency, although harmonics with lower amplitude compared to the turbulence intensity could not be obtained.

Abstract [sv]

Denna avhandling behandlar tillämpningen av aktiv strömningskontroll för lastbilshytter, vilket är en ny metod för minskning av luftmotståndet. Mer i detalj är det övergripande målet att visa på hur plasmaaktuatorer kan användas för att minska luftmotståndet orsakat av avlösningen runt A-stolparna. In denna avhandling studeras detta genom numeriska simuleringar. Arbetet är en del av ett projekt där även experimentella försök görs.

Effekten av plasmaaktuatorer modelleras genom en masskraft, vilket inte ger nämnvärd ökning av beräkningstiden och är lämplig för implementering i de flesta CFD-lösare. Den rumsliga fördelningen av kraften bestäms av koefficienter vilka i detta arbete beräknades utifrån experimentella data. Modellen har visat sig kunna återskapa en stråle nära väggen med god noggrannhet av en enskild plasmaaktuator för en halvcylinder utan strömning.

Samma geometri - en halvcylinder som här används som förenklad geometri av A-stolpen på en lastbil - användes i en preliminär LES studie som visade att enbart aktuatorn vid kontinuerlig drift inte var tillräckligt för att uppnå en signifikant minskning av luftmotståndet. En signifikant minskning av luftmotståndet erhölls genom att helt enkelt öka styrkan på kraften, vilket visats att denna typ av strömningskontroll är relevant för minskning av luftmotståndet.

I syfte att förbättra effektiviteten hos aktuatorn, studerades dynamic mode decomposition, som ett verktyg för efterbehandling för att få fram flödesstrukturer. Dessa strukturer identifieras genom deras rumsupplösning och frekvens och kan hjälpa till att förstå hur aktuatorerna bör användas för att minska luftmotståndet. En parallelliserad kod för dynamic mode decomposition utvecklades för att underlätta efterbehandlingen av de stora datamängder som fås från LES-beräkningarna. Slutligen, utvärderades denna kod och LES-beräkningar på ett strömningsfall med pulserande kanalflöde. Metoden, dynamic mode decomposition, visade sig kunna extrahera de oscillerande flödesprofilerna med hög noggrannhet för den påtvingade frekvensen. Övertoner med lägre amplitud jämfört med turbulensintensiteten kunde dock inte erhållas.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 10, 52 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2015:10
Keyword [en]
flow control, drag reduction, plasma actuator, DMD, LES, optimization, pulsating flow
Keyword [sv]
strömningskontroll, motståndsminskning, plasma ställdon, DMD, LES, optimering, pulserande flöde
National Category
Vehicle Engineering
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-161409ISBN: 978-91-7595-479-0 (print)OAI: oai:DiVA.org:kth-161409DiVA: diva2:794548
Presentation
2015-03-27, Vehicle Engineering Lab, Teknikringen 8 KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 34186-1
Note

QC 20150312

Available from: 2015-03-12 Created: 2015-03-11 Last updated: 2015-03-12Bibliographically 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. 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. 36 p.
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
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

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