-
yrt
ntaintyra
and-rotare-rotde
rticyerhee vor cthery lise-bowi
1
da
Ogaerodynamic characteristics are affected 1,2. In order to
ascer-tain the underlying performance characteristics of race car
frontw
fa
flim
s
v
lw
c
e
ta
gdt
ber of different fields, such as vortex generators VGs, VG
jets,spanwise cylinder, and wall heat transfer techniques 1518.
JsK
J
Downloaings, a number of experimental investigations have been
per-ormed for an isolated wing in ground effect; a
single-element35, a double-element wing 68, a gurney flap fitted
wing9,10, a wing in a wake flow 11,12, interactions between a
wingnd wheel 13, and a dynamically motioned wing 14.
The experiments of Zerihan and Zhang 4,5 revealed importantow
characteristics of a single-element wing in ground effect us-
ng a number of experimental methodologies, including
forceeasurement, surface flow visualization, surface pressure
mea-
urement, laser Doppler anemometry LDA, and particle
imageelocimetry PIV. A moving ground was used to properly simu-ate
the conditions on a race track, where a moving belt rig in theind
tunnel runs at the same speed as the freestream in order to
ontrol the boundary layer on the ground. Zerihan and Zhang
4xamined the ride height sensitivity of the downforce
characteris-ics; the downforce behavior with respect to the ride
height shows
downforce enhancement as a wing is moved closer to theround, and
then a downforce reduction is shown by further re-uction in the
ride height. Surface flow visualization revealed thathe downforce
reduction phenomenon is induced by a breakdown
Lin 19,20 suggested that effective devices for separation
con-trol are those that generate streamwise vortices, such as
thoseproduced by VGs. Large-scale vortex generators LVGs,
whosedevice height is of the same order as the boundary layer
thickness,have been used to control flow separation by transferring
the mo-mentum from the outer flow to boundary layer flow.
Sub-boundary layer vortex generators SVGs, however, appear to
bemore advantageous in terms of their effectiveness with a
lowerdevice height, which is just fraction of the boundary layer
thick-ness 1921. Lin 19 exhibited that optimally placed vane
typeSVGs with a device height of 20% of the boundary layer
thick-ness performs as well as vane type LVGs with device height
of80% of the boundary layer, but with less device drag.
Pauley and Eaton 22 studied the mean streamwise develop-ment of
pairs of longitudinal vortices, generated by delta vanetype VGs
with a device height of 150% of the boundary layerthickness, in a
turbulent boundary layer. Several vortex configu-rations were
investigated, i.e., corotating vortices and counter-rotating
vortices, inducing common-downwash and common-upwash. The
streamwise decay of the peak vorticity showed thatthe corotating
vortices decay quicker than the common-downwashvortices. The
arrayed counter-rotating vortices show a slightlyslower decay rate
in the vortex circulation than the other configu-rations. Godard
and Stanislas 23 attempted an optimization fordelta vane type SVGs.
The optimized counter-rotating sub-boundary layer vortex generators
CtSVGs shows higher effec-tiveness than the corotating sub-boundary
layer vortex generatorsCoSVGs.
1Present address: Williams F1.2Present address: TotalSim
LLC.Contributed by the Fluids Engineering Division of ASME for
publication in the
OURNAL OF FLUIDS ENGINEERING. Manuscript received March 20,
2009; final manu-cript received October 1, 2009; published online
November 19, 2009. Editor: Josephatz.
ournal of Fluids Engineering DECEMBER 2009, Vol. 131 /
121103-1Copyright 2009 by ASMEYuichi Kuyae-mail:
[email protected]
Kenji TakedaSenior Lecturer
e-mail: [email protected]
Xin ZhangProfessor
e-mail: [email protected]
Scott Beeton1
Ted Pandaleon2
School of Engineering Sciences,University of Southampton,
Southampton, Hampshire SO17 1BJ, UK
Flow PhWith VoEffectThis paper experimeon an inverted wingflow
visualization. Atunnel over a widesub-boundary layercomprising
counterheight and spacingerators and counterat the center of
eachinduce horseshoe voing sub-boundary lacontrol. Increasing
tsignificant horseshoscale vortex generatwing equipped withrotating
sub-boundaance in the spanwcounter-rotating subcontrol for race
car
IntroductionRace car performance can be significantly enhanced
by the ad-
ition of inverted wings to create downforce, improving
tractionnd cornering ability. For open-wheel race series such as
Formulane and Indy Racing, inverted wings and diffusers operate
inround effect, that is, in close proximity to the road, and
theirded 11 Mar 2010 to 152.78.214.194. Redistribution subject to
ASMsics of a Race Car Wingex Generators in Ground
lly investigates the use of vortex generators for separation
controlground effect using off-surface flow measurements and
surface
pical racing car wing geometry is tested in a rolling road
windnge of incidences and ride heights. Rectangular vane type
oflarge-scale vortex generators are attached to the suction
surface,
ating and corotating configurations. The effects of both
deviceexamined. The counter-rotating sub-boundary layer vortex
gen-
ating large-scale vortex generators suppress the flow
separationvice pair, while the counter-rotating large-scale vortex
generatorses between each device where the flow is separated. The
corotat-vortex generators tested here show little evidence of
separation
spacing of the counter-rotating sublayer vortex generator
inducesortices, comparable to those seen in the counter-rotating
large-ase. Wake surveys show significant spanwise variance behind
thecounter-rotating large-scale vortex generators, while the
counter-ayer vortex generator configuration shows a relatively
small vari-direction. The flow characteristics revealed here
suggest thatundary layer vortex generators can provide effective
separationngs in ground effect. DOI: 10.1115/1.4000423
of edge vortices at the end plates and flow separation on the
suc-tion surface of the wing, which is induced by the large
adversepressure gradient.
If a flow is in a large adverse pressure gradient region,
thestreamwise momentum of the flow is reduced, and the flow
mayseparate from the wall 15,16. A number of separation
controlmethods exist and have been successfully implemented in a
num-E license or copyright; see
http://www.asme.org/terms/Terms_Use.cfm
-
ibpa2wswRtrtm
2
fvnsaccbra0evic
suT
profile, type LS1-0413, and is manufactured by carbon
fibercomposite. The model has a span of 1100 mm, constant chord c
of0.4
Freestream U
inVG
1
DownloaAs described in Ref. 24, the use of VGs for an inverted
wingn ground effect for separation control can be effective in
terms ofoth downforce and efficiency. This paper investigates the
flowhysics that induce such force improvements, examining
time-veraged and unsteady on- and off-surface flow features.
Sectiondescribes the experimental testing procedure in a wind
tunnel,here a moving ground condition is properly simulated. The
re-
ults of surface flow visualization and PIV measurement of theake
are described in Sec. 3, followed by discussion in Sec.
4.ectangular vane type of SVGs and LVGs are tested on the suc-
ion surface of the wing, comprising counter-rotating and
co-otating configurations. Different device spacings are also
studiedo see how this affects the flow characteristics. Concluding
re-
arks are included in Sec. 5.
Experimental Setup2.1 Test Facility. The experiments described
here are per-
ormed in the 2.1m1.5m closed section wind tunnel at the
Uni-ersity of Southampton. This wind tunnel has been used for
aumber of ground effect aerodynamics studies by different
re-earchers, including Zerihan and Zhang 4,5,7,8,10 and Zhangnd
coworkers 2527. The tunnel is of conventional return cir-uit
design, and is equipped with a moving belt rig and a three-omponent
overhead balance system. The 3.2m1.5m movingelt is controlled by
slots and suction system for boundary layeremoval, which gives
99.8% of the freestream velocity at 2 mmbove the belt. The
turbulence intensity of the freestream is about.3%. Further
descriptions of the wind tunnel are given by Burgint al. 28. For
the experiments presented here, the freestreamelocity U and moving
belt speed are set at 30 m/s, correspond-ng to the Reynolds number
Re of 450,000 based on the winghord c.
2.2 Experimental Models. Figure 1 shows a schematic of
theingle-element wing geometry and installation used. The modelsed
is an 80% scale model of the main element of the 1998yrrell 026 F1
car front wing, which is based on a NASA GAW
0.2x/c
0.40 0.6 0.8 1.0 1.2
0.2
0
-0.2
y/c
h VG U
Moving ground
End plate
2.6
Fig. 1 Schematic of single-element wing, end plate, and VG
za(a) zb
4hVG dVG
4hVG
15
x/c=0.537
Fig. 2 Configurations of VGs on wmeasurement: a
counter-rotatingfrom bottom to top.
21103-2 / Vol. 131, DECEMBER 2009ded 11 Mar 2010 to
152.78.214.194. Redistribution subject to ASM223.4 mm, and finite
trailing edge of 1.65 mm, and is the samemodel used by Zerihan and
Zhang 4,5,10. The origin of thecoordinate system is set at the
leading edge of the wing. For thesurface flow visualization,
generic end plates of 250mm100mm4mm are attached on both ends of
the wing, mean-while a size of 400mm170mm4mm glass end plates are
usedfor the PIV measurement. The span of the wing is long
enoughsuch that edge vortices induced around the end plates do not
affecta flow around the center portion of the wing span where the
PIVmeasurement is performed. Therefore, the size difference of
theend plates between the PIV measurement and flow visualization
isdeemed negligible for wake flow characteristics at the center
por-tion. The incidence is measured relative to a line from
thetrailing edge to the most swelled point on the pressure
surfacewhich corresponds to 2.6 deg relative to the chord line. The
trueincidence is therefore equal to the measured incidence plus
2.6deg. When the incidence is 1 deg, corresponding to the true
inci-dence of 3.6 deg, the upper and lower edges of the end plates
areparallel to the ground. The incidence is varied by rotating
aboutthe quarter chord position. The ride height h is defined by
thedistance from the lowest point on the suction surface of the
wingto the moving ground as the incidence is fixed at 1 deg. The
rideheight and incidence are fixed at h /c=0.090 and 1 deg in all
themeasurements conducted here, respectively.
Rectangular vane type VGs are employed here, comprisingthree
configurations, which are the CtSVGs, counter-rotatinglarge-scale
vortex generators CtLVGs, and CoSVGs. The SVGand LVG have a device
height of 2 mm hVG /c=0.009 and 6mm hVG /c=0.027, respectively. The
VGs are made of alumi-num plate with 0.6 mm thickness and built in
pairs separated by4hVG at the trailing edge of the VGs. The vanes
are oriented at15 deg relative to the streamwise direction,
comprising thecounter-rotating or co-rotating VG configuration.
Pairs of VGs areput side by side along the span of the wing, as
shown in Fig. 2.The VGs are attached on the suction surface of the
wing such thatthe trailing edge of the VGs is fixed at x /c=0.537.
The height tolength ratio of the vanes is fixed at 1:4. For the
CtLVG andCoSVG configurations, the device spacing dVG between each
de-vice pair of the VGs is fixed at 4hVG, while close- and
wide-spacings of 2hVG and 8hVG are also examined with the
CtSVGconfiguration in addition to the reference-spacing of 4hVG.
Unlessthere is a particular notation, CtSVG represents a CtSVG
configu-ration with the reference-spacing of 4hVG, which is the
same de-vice spacing as the other VG configurations.
2.3 Experimental Methods. The flow visualization is under-taken
using a mixture of liquid paraffin and fine powder InvisibleBlue
T70 painted on the suction surface of the wing.
PIV measurement is performed in the xy-plane along the
centerline of the wing using a Dantec Dynamics FlowMap
PIV2100Processor Denmark 29. The laser is a New Wave ResearchGemini
dual Nd-YAG laser Fremont, CA, USA mounted down-stream of the wing
to illuminate the wake section. The laser sheet
(b)
4hVG
zc
4hVG
15
x/c=0.537
dVG
g and laser sheet positions in PIVs and b co-rotating VGs. Flow
is
Transactions of the ASMEE license or copyright; see
http://www.asme.org/terms/Terms_Use.cfm
-
ptdatisHalo1icsmfiT
B
J
Downloaosition is altered for different VG geometries, as shown
in Fig. 2,o compare the difference of the wake structure in the
spanwiseirection. For the counter-rotating VGs, two laser sheet
positionsre employed, which are at the center of the device pair za
and athe center between the device pair zb. A single laser sheet
positions used at the center of the device pair zc for the
co-rotating VGs,ince the vanes are mounted periodically. A Dantec
Dynamicsi-Sense CCD camera with a resolution of 12801024 pixels
nd a 105 mm lens is used. The time interval between the twoaser
pulses is 15 s, and 500 pairs of snapshots are continuouslybtained.
The flow is seeded by smoke particles with a size of1.5 m.
FlowManeger 4.10 software is used for postprocess-
ng to calculate the velocity field with the adaptive
cross-orrelation algorithm, which uses iteration steps for
offsetting theecond window for cross-correlation analysis, using
four refine-ent steps with a local median validation function to
obtain anal interrogation area size of 3232 pixels without
filtering.he overlap between interrogation areas is set at 50%.
(a)
(c)
C
D
Fig. 3 Surface flow visualization on suction=1 deg and h
/c=0.090: a clean, b CtSVtom to top.
ournal of Fluids Engineeringded 11 Mar 2010 to 152.78.214.194.
Redistribution subject to ASM2.4 Uncertainty. The uncertainty of
the PIV measurement isestimated by assessing the normalized maximum
velocity deficitof the clean wing at x /c=1.5. The 500 PIV
snapshots for eachmeasurement are divided into five subsets
containing 100 snap-shots, and the uncertainty is assessed between
the five subsetsusing procedures described in Refs. 30,31.
According to the pro-cedures, the standard error of the measurement
is given as 0.008,and hence, the uncertainty with the 95%
confidence is given as0.017, where the coverage factor of 2 is
used.
3 Results3.1 Surface Flow Visualization. Surface flow
visualization
results are presented here to ascertain key flow physics that
drivethe force and pressure characteristics presented by Kuya et
al.24. Figure 3 illustrates results of the surface flow
visualizationon the suction surface around the centre portion of
the wing span
(b)
(d)
(b)
A
rface around centre portion of wing span atc CtLVG, and d CoSVG.
Flow is from bot-
DECEMBER 2009, Vol. 131 / 121103-3suG,E license or copyright;
see http://www.asme.org/terms/Terms_Use.cfm
-
afl
lbTsecms
cclooctvvcsiiwaisebdsstVirieei
surd
1
Downloat =1 deg and h /c=0.090 for the four configurations.
Theow direction is from bottom to top in the figures.All of the
configurations show that the flow transitions from
aminar to turbulent at about 30% chord via a short
reattachmentubble in agreement with the results of Zerihan and
Zhang 4.he flow pattern near the end plates not shown here features
thepanwise component of flow toward the wing center due to thedge
vortices induced around the end plates. The edge vorticesreate a
downwash on the suction surface, which pumps the mo-entum into the
boundary layer flow, and thus, there is no flow
eparation around the spanwise end of the wing.For the clean wing
in Fig. 3a, characteristic horseshoe vorti-
es and flow separation can be seen between 65% and 80%hords. The
vortices induced by the counter-rotating VGs areikely to flow
straight, since the spanwise component of the sec-ndary flow
induced by the VG-generated vortices cancels eachther near the
surface. Meanwhile, the flow pattern of the CoSVGonfiguration
features the spanwise component of the flow due tohe lateral
component of the secondary flow of the VG-generatedortices. In
general, since a secondary flow induced by co-rotatingortices flows
in the same direction near a surface, the spanwiseomponent is
enhanced. Downwash regions can be seen down-tream of each pair of
the CtSVGs as represented by the region An Fig. 3b. At the center
of each device pair, the flow separations suppressed by the
vortices, where each vane induces a down-ash on the suction
surface. The vortices allow the flow to remain
ttached up to 95% chord, eventually breaking down at the
trail-ng edge; the close proximity to the trailing edge inferring
that thetrength of the VG-generated vortices is nearly optimal.
Betweenach device pair, an upwash region can be observed as
representedy the region B in Fig. 3b induced by the VG-generated
vorticesetaching from the suction surface; the upwash enhances the
floweparation. For the CtLVG configuration in Fig. 3c, the
floweparation is entirely suppressed at the center of each device
pairo the trailing edge due to the strong downwash induced by
theG-generated vortices. In this case the horseshoe vortices
are
nduced between each device pair at 70% chord, as denoted by
theegion C. The rotational direction of the counter-rotating
vorticess likely to induce the flow outwards, deviating from the
center ofach device pair, as represented by the region D. The flow
fromach device pair meets at the center of the device pairs, as
shownn the region C, where the upwash induced by the
VG-generated
(a)
F
E
Fig. 4 Surface flow visualization on suctioneffect of VG device
spacing at =1 deg anspacing. Flow is from bottom to top.
21103-4 / Vol. 131, DECEMBER 2009ded 11 Mar 2010 to
152.78.214.194. Redistribution subject to ASMvortices is enhanced
by the two vortices, resulting in the separatedflow with the
horseshoe vortices. The CoSVG configuration ap-parently shows flow
separation downstream of the VGs over thewing span at about 80%
chord, as shown in Fig. 3d. The sepa-rated flow pattern is more
two-dimensional rather than three-dimensional as featured by the
horseshoe vortices on the cleanwing. As will be discussed later,
the reduced effectiveness of theCoSVGs regarding the separation
control is likely to be due toquicker decay of co-rotating vortices
compared with counter-rotating vortices.
The close- and wide-spacings dVG=2hVG and 8hVG are exam-ined in
addition to the reference-spacing dVG=4hVG in theCtSVG
configuration. Figure 4 shows results of the surface
flowvisualization on the suction surface of the CtSVG
configurationswith close- and wide-spacings at =1 deg and h
/c=0.090. Forthe close-spacing CtSVG configuration, the flow
pattern is similarto that of the reference-spacing CtSVG
configuration; the regionsE and F in Fig. 4a correspond to the
regions A and B in thereference-spacing CtSVG configuration shown
in Fig. 3b. Forthe wide-spacing CtSVG configuration, the horseshoe
vortices canbe observed between each device pair, as represented by
the re-gion G in Fig. 4b, resulting in a flow pattern similar to
that of theCtLVG configuration. When the VGs are spaced closer
togetherrelative to the vortex size, the interaction with
neighboring vortexpairs is significantly enhanced. On the contrary,
the vortices gen-erated by the wide-spacing CtSVG configuration
appear to moveapart, inducing separated flow comprising horseshoe
vortices be-tween each device pair.
3.2 Wake Characteristics. Results of the PIV measurementof the
wake are presented here. Figure 5 shows mean streamwisevelocity
contours of the four configurations at =1 deg andh /c=0.090,
time-averaged over 500 PIV snapshots. The velocityis normalized by
the freestream velocity. The results in the vicin-ity of the
trailing edge of the wing and moving ground are ex-cluded due to
reflections from the solid surfaces. The figures forthe
counter-rotating VG configurations Figs. 5b and 5c showprofiles at
two spanwise positions z=za and zb. At z=za, thevortices from the
counter-rotating VGs induce downwash on thesuction surface, while
the upwash, which detaches from the suc-tion surface, is induced at
z=zb.
(b)
G
face around centre portion of wing span forh /c=0.090: a
close-spacing and b wide-
Transactions of the ASMEE license or copyright; see
http://www.asme.org/terms/Terms_Use.cfm
-
tflrgFwstlCtdtpebcCt
1.2
zc
Counter-rotating Co-rotating
zaU U
xz=
h
J
DownloaFor the baseline clean wing case in Fig. 5a, the wake
becomeshicker as it flows downstream, reducing the velocity
deficit. Theow between the wake and the moving ground is
accelerated as aesult of ground effect, inducing boundary layer
growth on theround see Fig. 6. For the counter-rotating VG
configurations inig. 5b, the wake profiles at z=za and zb show
different features,ith the CtLVG configuration exhibiting more
variance in the
panwise direction compared with the CtSVG configuration. Bothhe
profiles of the CtSVG configuration at z=za and zb show simi-ar
wake thicknesses to the clean wing. The velocity deficit of thetSVG
configuration tends to persist farther downstream, while
he wake of the clean wing shows relatively faster decay of
theeficit. For the wake profile at z=za, the downwash near the
suc-ion surface induced by the VG-generated vortices appears to
sup-resses the flow separation accelerating the flow near the
trailingdge, compared with the clean wing. For the accelerated
flowetween the wing and the ground, both profiles of the
CtSVGonfiguration show similar distributions to the clean wing.
ThetLVG configuration exhibits a significant effect of the VGs
on
he wake structure, the wake profiles at z=za and zb being
very
(a)
1.2 1.1
1.1
1.0
1.0
1.0
0.9
0.9
0.8
0.80.70.60.50.40.3
0.1
0
-0.1
y/c
1.11.0 1.2 1.3 1.4 1.5 1.6
1.11.2
0.1
0
-0.1
1.11.0
0.1
0
-0.1
1.11.0 1.2 1.3 1.4 1.5 1.6
1.1
1.00.9
1.2
1.00.9
1.0
0.1
0
-0.1
1.11.0
1.1
0.6
1.2
zb
(b)
y/c
y/c
y/c
x/c
(c)
1.0x/cz=za
Fig. 5 Mean streamwise velocity contours at =1 deg andz=za and
zb, and d CoSVG at z=zc
u/U0.5
-0.1
y/c
0.6 0.7 0.8 0.9 1.0 1.
0.1
0
y/c
Fig. 6 Mean streamwise velocity profile
ournal of Fluids Engineeringded 11 Mar 2010 to 152.78.214.194.
Redistribution subject to ASMdifferent, as shown in Fig. 5c. The
profile at z=za shows a nar-row wake distribution downstream of the
trailing edge as the re-sult of the suppression of the flow
separation. Meanwhile, a sig-nificantly thicker wake is shown at
z=zb. For the CoSVGconfiguration in Fig. 5d, the wake profile shows
a similar distri-bution to that of the clean wing, while the CoSVG
configurationgenerates a slightly thicker wake than the clean
wing.
Figure 6 shows the mean streamwise velocity profiles at x /c=1.5
of the four configurations at =1 deg and h /c=0.090. Forthe
counter-rotating VG configurations, profiles at the two span-wise
positions z=za and zb are given. Characteristics of the ve-locity
profiles are summarized in Table 1. The thickness of thewake is
defined as the distance between the upper and lowerpoints of the
wake where the velocity is 99% of the freestream.Adding the CtSVGs
to the wing reduces the thickness and in-creases the maximum
velocity deficit of the wake compared withthe clean wing. The
maximum velocity deficit increases fromumin /U=0.70 y /c=0.048 for
the clean wing to umin /U=0.60 y /c=0.055 at z=za and umin /U=0.61
y /c=0.053 at
/c
1.0
1.00.9
0.90.8
0.8
0.70.6
1.3 1.4 1.5 1.6
0.7
1.0
1.1
1.0
1.1
0.90.80.7
0.60.50.40.3
0.1
0
-0.1
1.11.0 1.2 1.3 1.4 1.5 1.6
0.90.8
1.0
1.0
0.1
0
-0.1
1.11.0 1.2 1.3 1.4 1.5 1.6
1.1
1.0
1.00.9
0.90.8
0.8
0.70.6
0.7
1.2
0.50.4
1.0
1.3 1.4 1.5 1.6
1.0
1.00.9
0.90.8
0.8
0.7
0.6
1.0
y/c
y/c
zax/cz=zb
(d)x/cz=zc
/czb
/c=0.090: a clean, b CtSVG at z=za and zb, c CtLVG at
Counter-rotating
za
zb
Uzc
Co-rotating
U
CleanCtSVG(z=za)CtSVG(z=zb)CtLVG(z=za)CtLVG(z=zb)CoSVG(z=zc)
t x /c=1.5 at =1 deg and h /c=0.090
DECEMBER 2009, Vol. 131 / 121103-51
s ax1.2
0.7
z=E license or copyright; see
http://www.asme.org/terms/Terms_Use.cfm
-
zfCpdCttbsdzw=
=
t
larger wake compared with the clean wing. The maximum veloc-ity
deficit increases from umin /U=0.70 y /c=0.048 for the
Table 1 Wake characteristics at x /c=1.5 at =1 deg andh
/c=0.090
u
y
x1
1xz
z
=1
1
Downloa=zb for the CtSVG configuration. The wake thickness is
reducedrom wake /c=0.19 for the clean wing to wake /c=0.17 for
thetSVG configuration. Of great interest here is that both
wakerofiles of the CtSVG configuration show a remarkably
similaristribution, thus, less variance in the spanwise direction.
For thetLVG configuration, the profiles obviously show different
fea-
ures from each other. The variance of the wake structure
indicateshat the wake flows are highly affected by the vortices
generatedy the CtLVGs. The velocity deficit of the CtLVG at z=za
ismaller than the others, and another small velocity deficit can
beetected around y /c=0.10. The wake thickness of the CtLVG at=za
including the small deficit is similar to that of the cleaning.
Meanwhile, a significantly thicker wake is observed at zzb. The
maximum velocity deficit changes from umin /U=0.70y /c=0.048 for
the clean wing to umin /U=0.86 y /c0.043 at z=za, and umin /U=0.63
y /c=0.037 at z=zb for
he CtLVG configuration. The CoSVG configuration shows a
Clean CtSVG CtLVG CoSVG
z=za z=zb z=za z=zb
wake /c 0.19 0.17 0.17 0.19 0.22 0.20min /U 0.70 0.60 0.61 0.86
0.63 0.62/c at umin 0.048 0.055 0.053 0.043 0.037 0.032
(c)
0.1
0
-0.1
1.0x/c
0.1
0
-0.1
1.0
(b)
y/c
y/c
(a)x/c
0.1
0
-0.1
1.1 1.2 1.31.0
y/c
0.1
0
-0.1
1.1 1.2 1.31.0
y/c
zc
Counter-rotating Co-rotating
za
zb
U U
z=za
-40z
Fig. 7 Instantaneous spanwise vorticity distributions at c CtLVG
at z=za and zb, and d CoSVG at z=zc
21103-6 / Vol. 131, DECEMBER 2009ded 11 Mar 2010 to
152.78.214.194. Redistribution subject to ASMclean wing to umin
/U=0.62 y /c=0.032 for the CoSVG con-figuration. For the
accelerated flow between the wake and ground,the counter-rotating
configurations show slightly higher velocity,whereas the CoSVG
configuration presents lower velocity, com-pared with the clean
wing. The boundary layer growth detected onthe moving ground shows
a similar rate in all four profiles.
Figure 7 shows instantaneous spanwise vorticity contours of
thefour configurations at =1 deg and h /c=0.090 to illustratesome
unsteady flow features behind the wing. While only onesnapshot is
shown, the characteristics described here are evidentin the whole
set of captured PIV data. The vorticity is normalizedby the
freestream velocity and wing chord. The figures for
thecounter-rotating VG configurations Figs. 7b and 7c show
dis-tributions at two spanwise positions z=za and zb. The
vortexstructure behind the clean wing is characterized by the
presence ofvortex shedding and a shear layer between regions of
positive andnegative vorticity, as shown in Fig. 7a. For both
distributions ofthe CtSVG configuration in Fig. 7b, the vortices
appear tospread relatively wider and the size of each shed vortex
is typi-cally smaller compared with the clean wing case. The
differencebetween the profiles at z=za and zb is not obvious. The
CtLVGconfiguration shows a significant impact of the CtLVGs on
thevorticity structure in Fig. 7c. At z=za, the vortices
distributeonly downstream of the trailing edge, and the shear layer
is ob-served clearly. The size of each vortex is relatively small.
Mean-while, a wide range of the vortex shedding is observed at
z=zb.The structure of shed vortices is much more irregular
compared
0.1
0
-0.1
x/c1.1 1.2 1.31.0
/c.1 1.2 1.3
0.1
0
-0.1
x/c1.1 1.2 1.31.0
(d)
.1 1.2 1.3/c
y/c
y/c
=za z=zb
z=zc=zb
40
deg and h /c=0.090: a clean, b CtSVG at z=za and zb,
Transactions of the ASME0E license or copyright; see
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-
wFdc
itrtcadvctoo
Bogdptttzzvmsvsvsdt
nd
J
Downloaith the others, and the shear layer cannot be observed
clearly.or the co-rotating VG configurations in Fig. 7d, the
structure ofiscrete vortices of the CoSVG configuration is similar
to thelean wing.
In addition to the instantaneous unsteady features of the
vortic-ty, the mean spanwise vorticity contours of the four
configura-ions, time-averaged over 500 PIV snapshots, at =1 deg
andh /c=0.090, are shown in Fig. 8. The figures for the
counter-otating VG configurations Figs. 8b and 8c show
distribu-ions at two spanwise positions z=za and zb. The mean
vorticityontours clearly explain where the positive and negative
vorticesnd the shear layers are generated, and how they diffuse as
theyevelop downstream. For the clean wing in Fig. 8a, the
positiveorticity is thicker than the negative vorticity at a region
ofx /c1.3 due to the flow separation on the suction surface.
Bothontours for the CtSVG configuration show a characteristic
vor-icity distribution in Fig. 8b. Two positive vorticity regions
arebserved: one is along the suction surface of the wing, and
thether is detected underneath the suction surface at y
/c0.02.etween these positive vorticity regions, the negative
vorticity isbserved underneath the suction surface in addition to
the oneenerated around the pressure surface. The difference between
theistributions at z=za and zb is not obvious; however, at z=za,
theositive vorticity region near the trailing edge flows slightly
fur-her downstream than that at z=zb. Additionally, the positive
vor-icity underneath the suction surface at z=za shows lower
degreehan the value of the clean wing and the CtSVG configuration
at=zb. These discrepancies of the distributions between z=za andb
are due to the suppression of flow separation at z=za as re-ealed
by the surface flow visualization. The effect of the VGs isore
obvious in the CtLVG configuration in Fig. 8c, as can be
een in other wake surveys. At z=za, thin distributions of
theorticity are observed downstream of the trailing edge due to
theuppression of the separation. Meanwhile at z=zb, the
negativeorticity generated from the pressure surface and underneath
theuction surface results in a thicker distribution downstream.
Bothistributions of the CtLVG configuration show two positive
vor-icity regions, as observed in the CtSVG configurations. One
is
Fig. 8 Mean spanwise vorticity distributions at =1 deg aat z=za
and zb, and d CoSVG at z=zc
ournal of Fluids Engineeringded 11 Mar 2010 to 152.78.214.194.
Redistribution subject to ASMshown on the suction surface of the
wing, and the other under-neath the suction surface at y /c0 z=za
and at y /c0.05 z=zb. The value of the positive vorticity at y
/c0.05 z=zb is obviously higher than that at y /c0 z=za dueto the
flow separation at z=zb. The vorticity distribution of theCoSVG
configuration in Fig. 8d is remarkably similar to that ofthe clean
wing. The similarity between the clean wing andCoSVG configuration
can also be seen in the mean velocity dis-tributions.
4 DiscussionKuya et al. 24 showed that both the counter-rotating
configu-
rations can increase downforce compared with the clean wingwhen
the wing is operated in the ground effect regime, and inparticular,
the use of the CtSVGs has advantages both in thedownforce and
efficiency under some conditions, while theCtLVG configuration
indicates less efficiency. The CoSVG con-figuration deteriorates
the wing performance in all cases. The ex-perimentally investigated
characteristics, including surface flowvisualization and wake
surveys, explain the physical mechanismof the separation control
and the advantage of the CtSVGs com-pared with the others.
The vortices generated by the VGs induce downwash and up-wash to
the suction surface, which mixes the outer flow and theflow in the
boundary layer. The downwash generated at the centerof each device
pair transfers the high momentum of the outer flowinto the boundary
layer flow, leading to the suppression of flowseparation, as can be
seen in the results of the surface flow visu-alization of the
counter-rotating VG configurations. However, theinteraction between
the neighboring co-rotating vortices tend tocancel each others
downwash and upwash effects, resulting in amore rapid decay of the
vortex, and enhances the lateral compo-nent of the flow. Therefore,
the co-rotating vortices do not persistfurther downstream,
resulting in little effect of separation control.The more rapid
decay of the co-rotating vortices compared withthe counter-rotating
vortices captured here is in good agreementwith the investigation
of Pauley and Eaton 22. Thus, it is con-
h /c=0.090: a clean, b CtSVG at z=za and zb, c CtLVG
DECEMBER 2009, Vol. 131 / 121103-7E license or copyright; see
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-
ceivable that co-rotating VGs with wider device spacings,
whichhave less interaction between each vortex, could have a
morefavorable effect for separation control.
gvbcttfidtvcClwridw
ssrCpaacscC
ttcwed
ciaVc
5
ifc
tices induced by the close- or reference-spacing CtSVGs
isrestricted by the interaction of the vortices existing in
theirneighbors.
1
DownloaFor separation control, important factors regarding the
VG-enerated vortices are their strength and size. The surface
flowisualization reveals that the vortices induced by the
CtSVGsreak down at 95% chord, indicating that the vortices are
suffi-iently strong, but not excessively so, to make the flow
overcomehe adverse pressure gradient region, leading to efficient
separa-ion control at the condition tested. Meanwhile, the CtLVG
con-guration not only suppresses the flow separation due to
theownwash on the suction surface, but also induces horseshoe
vor-ices where the flow is separated by the upwash. The wake
sur-eys performed by the PIV measurement show that the
CtSVGonfiguration possesses a small spanwise variance, while
thetLVG configuration exhibits significant spanwise variances.
The
arge variance of the CtLVG configuration indicates that
vorticesith excessive strength and size have not only favorable
effects
egarding separation control but also significant penalties.
Accord-ngly, the device height of the VGs is important not only for
theevice drag but also for the strength and size of the
vortices,hich the VGs produce.Regarding the effect of the device
spacing, Kuya et al. 24
howed that the close-spacing CtSVG configuration exhibits
aimilar effect on the downforce and efficiency with respect to
theeference-spacing CtSVG configuration, while the wide-spacingtSVG
configuration shows less downforce and efficiency, com-ared with
the other spacing CtSVG configurations. Those char-cteristics are
consistent with the results of the surface flow visu-lization
investigated here; the close-spacing CtSVGonfiguration shows a
similar flow pattern to that of the reference-pacing CtSVG
configuration, while the wide-spacing CtSVGonfiguration leads to
the horseshoe vortices, as shown in thetLVG
configuration.Accordingly, as shown in the investigation of Kuya et
al. 24,
he CtSVG configuration exhibits the best performance thanks tohe
nearly optimal strength and size of the counter-rotating vorti-es
to overcome the adverse pressure gradient region induced hereith a
small drag penalty. The CoSVG configuration, however,
xhibits a negligible separation control capability due to the
rapidecay of the co-rotating vortices.
Although the device height and spacing of the VGs arehanged, the
orientation angle is fixed at 15 deg in the currentnvestigation.
The strength of the VG-generated vortices may beltered by adjusting
the orientation angle of the vane; optimizedG parameters for the
rectangular vane type could be found via
hanging the device height, spacing, and orientation angle.
Concluding RemarksAn experimental investigation of the flow
characteristics of an
nverted wing with VGs in ground effect is performed, using
sur-ace flow visualization and wake surveys, and the following
con-lusions are drawn:
Surface flow visualization of the clean wing captures
thecharacteristic horseshoe vortices and flow separation
down-stream of 6580% chord. Both the counter-rotating
configu-rations suppress the flow separation at the center of
eachdevice pair, while the CtLVGs induce the horseshoe
vorticesbetween each device where the flow is separated. TheCoSVG
configuration shows the flow separation down-stream of the VGs over
the wing span at 80% chord.
The close- and wide-spacings dVG=2hVG and 8hVG areexamined in
addition to the reference-spacing dVG=4hVGin the CtSVG
configuration. The counter-rotating vorticesinduced by the
wide-spacing CtSVGs are likely to spreadoutward and induce the
horseshoe vortices as the CtLVGconfiguration features; meanwhile,
the spreading of the vor-
21103-8 / Vol. 131, DECEMBER 2009ded 11 Mar 2010 to
152.78.214.194. Redistribution subject to ASM Wake flow surveys
obtained by the PIV measurement revealsignificant spanwise
variances in the wake behind the wingequipped with the CtLVGs,
while the CtSVG configurationshows a relatively small variance in
the spanwise direction.The CoSVG configuration shows very similar
distributionsto those of the clean wing.
The flow physics investigated here suggests advantages of ause
of the CtSVG configuration for the separation control.
AcknowledgmentY. Kuya gratefully acknowledges the financial
support of the
Ministry of Education, Culture, Sports, Science, and
Technologyof Japan and the School of Engineering Sciences,
University ofSouthampton. The authors would like to thank Mr. Mike
Tudor-Pole for his assistance with the experiments.
NomenclatureRoman Symbols
c wing chorddVG device spacing of the vortex generatorhVG device
height of the vortex generator
h wing ride heightRe Reynolds number =Uc /U freestream
velocity
u , v Cartesian components of velocity streamwiseand lateral
directions
umin maximum velocity deficitx ,y ,z Cartesian tensor system
streamwise, lateral,
and spanwise directionsza ,zb ,zc PIV laser sheet positions
Greek Symbols wing incidence
wake wake thickness dynamic viscosity
densityz nondimensional spanwise vorticity =v /x
u /yc /U
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Downloaournal of Fluids Engineeringded 11 Mar 2010 to
152.78.214.194. Redistribution subject to ASMDECEMBER 2009, Vol.
131 / 121103-9E license or copyright; see
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