Plasma actuators for separation control

139 Pages·2014·48.85 MB·English

Plasma actuators for separation control
– design and application
by
Julie A. Vernet
November 2014
Technical Reports from
Royal Institute of Technology
KTH Mechanics
SE-100 44 Stockholm, Sweden
Akademisk avhandling som med tillst˚and av Kungliga Tekniska H¨ogskolan i
Stockholm framl¨agges till offentlig granskning f¨or avl¨aggande av teknologie
licentiatsexamen den 19 december 2014 kl 10.30 i sal E2, Lindstedsv¨agen 3,
Kungliga Tekniska H¨ogskolan, Stockholm.
TRITA-MEK 2014:25
ISSN 0348-467X
ISRN KTH/MEK/TR-14/25-SE
ISBN 978-91-7595-354-0
(cid:13)cJulie A. Vernet 2014
Universitetsservice US–AB, Stockholm 2014
To my grandfather
who taught me his own interpretation of the Archimedes principle
“A body immersed or partially immersed in water ... gets wet!”
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Julie A. Vernet 2014, Plasma actuators for separation control
– design and application
Linn´e Flow Centre, KTH Mechanics, SE–100 44 Stockholm, Sweden
Abstract
This thesis deals with the experimental realisation of an active flow control
technique, that utilises dielectric barrier discharge (DBD) plasma actuators,
to re-energise boundary layers subjected to adverse pressure gradients. The
ultimate goal of the research is to delay flow separation occurring on the A-
pillar of trucks tractors within the Flow Research on Advanced and Novel
Control Efficiency (FRANCE) project.
The first part of the project consists of the development of manufactur-
ing techniques for in-house built DBD plasma actuators and to evaluate their
performance when placed on a curved surface. The behaviour and parameter
dependence of the electric wind in quiescent air were investigated by means
of Laser Doppler velocimetry (LDV). The electric wind was found to develop
similar to a wall jet and the results of the parametric study are in agreement
withtheliterature, viz. increaseofwindvelocitieswithincreasingdrivingvolt-
age and frequency and with decrease of dielectric sheet thickness. The mea-
sured evolution of the induced wall jets served also as a benchmark data base
for a companion numerical project. Furthermore, during the two half-periods
(strokes) of the alternating current, the electric wind was investigated through
phase-resolved LDV data, which revealed that while the velocity during both
strokes remains positive, it differs in magnitude with nearly a factor of two be-
tween the strokes. Since the phase dependence of the electric wind is observed
only in the vicinity of the actuator, the assumption of a steady induced force
in simulations seems to be justified.
Forthesecondpartoftheproject,adoubleDBDplasmaactuatorwasused
tocontrolflowseparationoccurringonacylindricalbumpapproachedbyatur-
bulent boundary layer. The uncontrolled flow that develops on the flat plate
upstream the bump, separating on the half-cylinder geometry and reattaching
on the downstream flat plate was characterised using hot-wire anemometry.
Finally, detailed pressure measurements at the wall and in the wake of the
cylinder showed that the double actuator was able to reduce the reattachment
lengthdownstreamthecylindricalbumpwhichresultedinadragreductionup
to 30% when the actuator was placed a few millimetres upstream the separa-
tion point. However, drag reduction was observed for a wide range of actuator
positions and it is hypothesised that the measured reduction could be due to
different mechanisms, either as a re-energising of the shear-layer or as addition
of momentum close to the wall thereby delaying separation and increasing the
base pressure on the backside of the half-cylinder.
Descriptors: electric wind, drag reduction, wind-tunnel experiments
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Julie A. Vernet 2014, Plasmaaktuatorer f¨or separationskontroll
– design och till¨ampning
Linn´e Flow Centre, KTH Mekanik, SE–100 44 Stockholm, Sweden
Sammanfattning
En unders¨okning av str¨omningskontroll med sk DBD plasmaaktuatorer f¨or att
p˚averka avl¨osningen av ett turbulent gr¨ansskikt i en positiv tryckgradient har
utf¨orts. Det slutgiltiga m˚alet f¨or forskningen ¨ar att f¨orsena eller i b¨asta fall
f¨orhindra avl¨osning av str¨omningen vid A-stolpen p˚a en l˚angtradares dragbil
och forskningen g¨ors som en del inom projektet FRANCE (Flow Research on
Advanced and Novel Control Efficency).
En tillverkningsteknik tagits fram f¨or att bygga DBD plasmaaktuatorer
som kan placeras p˚a en kr¨okt yta. Den elektriska vind som aktuatorerna ger
upphov till har best¨amts med laser Doppler velocimetri (LDV) i stillast˚aende
omgivande luft och hastighetsf¨altet l¨angs den kr¨okta ytan kartlagts. Resul-
taten visar att vinden uppf¨or sig likartat med vad som tidigare rapporterats i
litteraturen, n¨amligen en ¨okning av hastigheten med ¨okande drivsp¨anning och
frekvens och ocks˚a en ¨okning n¨ar tjockleken av det dielektriska materialskiktet
minskas. Fasuppl¨osta m¨atningar med LDV visade att b˚ade de positiva och
negativa halvperioderna av den drivande v¨axelsp¨anningen ger samma riktning
avvinden,dockmedenskillnadidenmaximalahastighetenmedenfaktortv˚a.
Vindens fasberoende observerades dock bara i n¨arheten av aktuatorerna och i
simuleringar antas en stationa¨r inducerad kraft vilket d¨arf¨or kan anses vara en
god approximation. Resultaten har ocks˚a anv¨ants f¨or att kalibrera en modell
f¨or aktuatorerna i ett parallellt numeriskt projekt inom FRANCE
Tv˚a plasmaaktuatorer i en dubbel (tandem) konfiguration har anv¨ants f¨or
att p˚averka avl¨osningen p˚a en halvcylinder som anstr¨ommas l¨angs en plan
yta med ett turbulent gr¨ansskikt. Basstr¨omningen, dvs str¨omningen utan
kontroll, framf¨or, ¨over och nedstr¨oms cylindern, har kartlagts med hj¨alp av
varmtr˚adsanemometri. Detaljeradetryckm¨atningarp˚aytornaochivakenned-
str¨oms cylindern visade att med str¨omningskontroll kunde l¨angden p˚a det
avl¨osta omr˚adet minskas, bastrycket p˚a nedstr¨omssidan av cylindern ¨okas och
det totala motst˚andet minskas med 30% f¨or den b¨asta placeringen av aktu-
atorn (ett par mm uppstr¨oms avl¨osningslinjen). Minskningen kunde emeller-
tid observeras f¨or ett brett intervall av positioner och uppvisade tv˚a omr˚aden
med lokalt maximal effekt. En hypotes ¨ar att detta kan bero p˚a att tv˚a
olika mekanismer ¨ar verksamma, i ena fallet genom att aktuatorn ger extra
r¨orelseenergi till sj¨alva str¨omningen utan att direkt p˚averka avl¨osningen, och
i andra fallet genom att tillf¨ora r¨orelsem¨angd n¨ara ytan och som f¨orskjuter
avl¨osningen p˚a cylindern nedstr¨oms och d¨armed minskar det avl¨osta omr˚adet.
Nyckelord: elektrisk vind, motst˚andsminskning, vindtunnel experiment
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Preface
In this licentiate thesis, the capability of Dielectric Barrier Discharge plasma
actuatorstocontrolflowseparationisexperimentallyinvestigated. Thethesisis
dividedintotwoparts. Thefirstpartgivesanintroductionandmotivationsfor
the project together with prerequisite concepts concerning plasma actuators.
The different experimental setups and measurement techniques employed are
also described together with a summary of the results. Finally, a conclusion
sectionwithdiscussionoftheresultsandplansforthenexttwoyearsisending
this part. The second part consists of three papers giving more details to the
reader about the observations made and results obtained during this project.
November 2014, Stockholm
Julie A. Vernet
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Contents
Abstract v
Sammanfattning vi
Preface vii
Part I. Overview and summary
Chapter 1. Introduction 1
1.1. Motivations 1
1.2. Flow separation and control 1
1.3. Heavy trucks aerodynamics 2
1.4. Layout of the thesis 4
Chapter 2. History, principle and review of plasma actuators 5
2.1. Plasma actuators for flow control? 6
2.2. Review of studies on the electric wind induced by DBD plasma
actuators 10
2.3. Applications of DBD plasma actuators to flow control 15
Chapter 3. Design of DBD plasma actuators and study of the
induced electric wind in quiescent air 21
3.1. Design and construction of DBD plasma actuators 21
3.2. Electric wind measurement setup 27
3.3. Results 31
Chapter 4. Flow separation control and drag reduction study 37
4.1. Flow separation control setup 37
4.2. Drag evaluation 45
4.3. Results 47
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Chapter 5. Outlook 49
Chapter 6. Papers and authors contribution 51
Acknowledgements 53
References 55
Part II. Papers 61
Paper 1. Phase-averaged measurements of the electric wind
induced by a Single Dielectric Barrier Discharge
plasma actuator 65
Paper 2. A turbulent boundary layer with pressure gradient,
curvature and separation – results from hot-wire
measurements 87
Paper 3. Flow separation delay on trucks A-pillars by means of
Dielectric Barrier Discharge plasma actuation 107
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