A variety of wall turbulence control devices which were experimentally investigated are discussed; these include devices for burst control, alteration of outer flow structures, large eddy substitution, increased heat transfer efficiency, and reduction of wall pressure fluctuations. Control of pre-burst flow was demonstrated with a single, traveling surface depression which is phase-locked to elements of the burst production process. Another approach to wall turbulence control is to interfere with the outer layer coherent structures. A device in the outer part of a boundary layer was shown to suppress turbulence and reduce drag by opposing both the mean and unsteady vorticity in the boundary layer. Large eddy substitution is a method in which streamline curvature is introduced into the boundary layer in the form of streamwise vortices. Riblets, which were already shown to reduce turbulent drag, were also shown to exhibit superior heat transfer characteristics. Heat transfer efficiency as measured by the Reynolds Analogy Factor was shown to be as much as 36 percent greater than a smooth flat plate in a turbulent boundary layer. Large Eddy Break-Up (LEBU) which are also known to reduce turbulent drag were shown to reduce turbulent wall pressure fluctuation

Balasubramanian, R.

Beeler, George B.

Goodman, Wesley L.

Lindemann, A. Margrethe

Mcginley, Catherine B.

Wilkinson, Stephen P.

English

NASA Technical Reports Server

WALL TURBULENCE CONTROL
/,!' 72 _/
Stephen P. Wilkinson, A. Hargrethe Lindemann, George B. Beeler,
Catherine B. RcGinley, Wesley L. Goodman, and R. Ba]asubramanian
NASA Lang]ey Research Center
, Hampton, Virginia
PRECEDING PAGE BLA_( P_OT FiLET_'D
ELM__IN.[ENTiONALLY BLANK
347
https://ntrs.nasa.gov/search.jsp?R=19880005561 2020-03-20T07:44:22+00:00Z
Abstract
A variety of wall turbulence control devices which have been
experimentally investigated are discussed; these include devices for burst
control, alteration of outer flow structures, large eddy substitution,
increased heat transfer efficiency and reduction of wall pressure fluctuation
intensity.
Control of pre-burst flow has been demonstrated with a single, traveling
surface depression which is phase-locked to elements of the burst production
process. It was shown that the near-wall streamwise flow could be accelerated
and thereby reduce the tendency of a retarded streamwise velocity profile to
inflectionally break down (burst).
Another approach to wall turbulence control is to interfere with outer
layer "coherent structures." Studies have shown that a cylinder adjacent to a
flat plate produces a modified Karman vortex street. If the cylinder is
sufficiently close to the plate, one component of shed vorticity will be
suppressed altogether. Such a device in the outer part of a boundary layer
was shown to suppress turbulence and reduce drag by opposing both the mean and
unsteady vorticity in the boundary layer.
Large eddy substitution is a method in which streamline curvature (known
to suppress turbulence) is introduced into the boundary layer in the form of
streamwise vortices. Several systems of streamwise vortices were generated
in a turbulent boundary layer. It was shown that boundary layer entrainment
rates were reduced below normal flat plate values and indicated the successful
suppression of turbulence.
Riblets, which have already been shown to reduce turbulent drag, have
also been shown to exhibit superior heat transfer characteristics. Heat
transfer efficiency as measured by the Reynolds Analogy Factor was shown to be
as much as 36 percent greater than a smooth flat plate in a turbulent boundary
layer.
Large Eddy Break-Up devices (LEBU) which are also known to reduce
turbulent drag have been shown to reduce turbulent wall pressure fluctuation.
348
Wall Turbulence Control
Research conducted by the Viscous Flow Branch/High-Speed Aerodynamics
Division has shown that it is now possible to reduce or enhance a number of
turbulent boundary layer flow properties. This presentation will review our
progress in the wall turbulence control area starting with new uses for
existing turbulence control devices and following with a variety of new
concepts aimed primarily at turbulent, viscous drag reduction. New uses for
existing devices include riblets used as high-efficiency heat transfer
surfaces and LEBU's (large eddy break-up device) used to control wall pressure
fluctuations. New concepts for drag reduction include Large Eddy Substitution
which shows that the favorable influence of wall curvature on turbulence may
also be obtained with streamline curvature on a flat plate; Opposing Unsteady
Vorticity which shows the feasibility of altering large-scale structures in
the boundary layer by introducing opposite sense vortices; and Active Phase-
Locked Wall Deformation which shows the possibility of controlling turbulent
wall bursting through flow-triggered, electromagnetically actuated wall
motion.
• Reduce or enhance properties of turbulent wall boundary layers
(drag, heat transfer, noise, etc.)
• New uses of existing devices
o Riblet: efficient heat transfer surface
o LEBU: reduces wall pressure (density) fluctuations
• New concepts for drag reduction
• Large eddy substitution
• Opposing unsteady vorticity
• Active phase-locked wall deformation
349
Heat Transfer Efficiency of Riblet Dra 9 Reducin_l Surfaces
In addition to the drag reducing property of a riblet surface, it also
allows for greater heat transfer efficiency than a smooth or rough surface.
This finding is based on low-speed heat transfer and drag measurements on a
flat, heated riblet model. Heat transfer efficiency is defined by the
Reynolds analogy factor. Potential application for this finding is in the
field heat exchanger design where an increase in the Reynolds analogy factor
allows for mul ti parameter optimization studies (heat transfer, pumping power,
size, weight, etc.).
• Riblets show higher heat transfer efficiency than smooth
or rough surfaces
• Efficiency determined by experimentally measuring Reynolds
Analogy factor
/2 St/Cf St = Stanton #, Cf = skin friction coefficient)
mApplication to heat exchanger optimization/efficiency
350
,Reynolds Analo_ly Factor for Riblets
This plot shows the results of measurement of the Reynolds analogy factor
for the riblet heat transfer model as well as a reference smooth flat plate.
The ordinate is the Reynolds analogy factor (the ratio of two times the
Stanton number to the skin friction coefficient). The abscissa is the
Reynolds number based on the stream velocity and momentum thickness. As can
be seen, the flat plate data remain roughly constant at approximately
1.2 which is the usually quoted value for a flat plate in air. Two types of
riblet surfaces, both exhibiting drag reduction, were tested and have Reynolds
analogy factors 30 percent higher than the smooth flat plate.
1.8 --
2 St
Cf
1.6
1.4
1.2
1.0
©
/x
I
m
I
.8 I
1400 1600
ao
A C) Riblets
Flat plate
I I I I I I
1800 2000 2200 2400 2600 2800
Re0
351
Effect of LEBU on Wall Pressure Spectra
LEBU's affect turbulent, viscous drag apparently by their effect on the
large-scale structures in the outer part of the boundary layer. Since these
same large-scale structures are responsible for a significant portion of
indicated wall pressure fluctuation intensity, reduction in the large scales
should cause a similar reduction in wall pressure intensity. To test this
hypothesis, a pinhole microphone was used to measure wall pressure spectra
downstream of a tandem LEBU at the streamwise location of maximum skin
friction reduction. It was found that in the frequency range of the large
eddies, the fluctuation intensity was reduced by 25 percent below the
reference smooth flat plate level. This finding has potential application to
boundary layer noise reduction on aircraft allowing for reduced weight of
sound insulation. Density fluctuation intensity should also be reduced to
allow for improved performance of aircraft radar domes and laser or IR
windows.
• Expect change in wall pressure due to breakup of large scale structures
• Measured wall pressure _Pvv) spectra downstream of LEBU
' reduced 0(25%)
• Pw
• Applications: Reduced self noise on sonar domes
Improved laser and IR window performance
Reduced weight of sound insulation on aircraft
708 :j
Microphone _ P_
352
Wall Pressure Spectra Downstream from LEBU
This plot shows the results of the LEBU wall pressure spectral
measurements. The ordinate is the mean-square spectral intensity; the
abscissa is frequency. For data above 2000 Hz and less than 15000 Hz, there
is a clear drop in mean-square spectral intensity. Above 15000 Hz, the LEBU
data rejoin the reference flat plate data indicating that the LEBU is
affecting the large-scale structures. Since the diaphragm-type microphone
beneath the pinhole was sensitive to structural vibrations, the indicated
pressure spectra (actually due to vibration) were determined by covering the
pinhole during a run. Results shown in the figure indicate that the data
below 2000 Hz are excessively distorted by structural vibration and are not
-1010
correct.
O (w), -II
.2 I0
psl - sec
-12I0
0
Reference plate
Vibration
reference plate
Tandem LEBU
Vibration
tandem LEBU
I
5000 I0 000 15 000
_/2m', Hz
20000 25 000
353
LarQe-Edd_ Substitution via Vortex Cancellation
The large-eddy substitution concept is an extension of turbulence
suppression by convex wall curvature. By introducing streaTline curvature (as
opposed to wall curvature)into turbulent wall flows via co-rotating streamwise
vortices, similar turbulence suppression may be possible. The idea is to
"wrap-up" and suppress the turbulence in vortex-induced curvature over a
streamwise processing region and then remove the vortices. Two techniques
were studied: vortex cancellation and vortex self-annihilation. Vortex
cancellation employs widely spaced rectangular strakes to generate a spanwise
array of co-rotating wall vortices. Opposite sign generators (i.e., unwinders)
are placed 20 boundary layer thicknesses downstream to remove the vortices.
Vortex self-annihilation employs closely spaced generators which produce
vortices which self-destruct downstream of the generators. To determine the
effectiveness of the devices, boundary layer growth was measured to estimate
the rate at which free-stream air was entrained by the turbulence into the
boundary layer. Lower entrainment rates indicate suppression of turbulence.
In each case, the entrainment rate was reduced below flat-plate reference
levels. Details of this work are presented in Reference I.
• Co-rotating, longitudinal vortices used to study effect of streamline
curvature (induced by vortex)on turbulent wall flow
o"Wrap-up" turbulence in curvature substituting vortex for turbulence
• Two methods employed:
• Widely spacedgenerators with unwinders (cancellation)
o CIosely spacedgenerators without unwinders (self-annihilation)
• Boundary layer entrainment decreased in both cases
U
I _ -_ Generatorj 208 ( Unwi nder )
354
Lar_e-Edd_ Substitution
This figure shows the effect of the two techniques of creating and elimi-
nating wall vortices. The left-hand figures demonstrate vortex cancellation.
The top figure shows spanwise contours of constant velocity 149 boundary layer
thicknesses downstream of the vortex generators without vortex unwinders. The
bottom figure shows the same streamwise location with vortex unwinders located
20 boundary layer thicknesses downstream of the generators. As can be seen,
the unwinders are very effective in removing the vortices.
The right-hand figures demonstrate the vortex self-annihilation concept.
The top figure shows the generated vortices 20 boundary layer thicknesses down-
stream of the generators. The bottom figure shows the absence of vortices 149
boundary layer thicknesses downstream of the generators due to the self-annihi-
lation process.
Vortex unwinding Vortex self-annihilation
x= 1496 u= x=206
80 ue _J
.99
Ymm 40 90 _-,V/_ A"J xJ _90
8O "_ "1 _ "_ "_. 800
-100 0 100 -100 0 100
'z, mm z, mm
x = 1496 u
,°I- .99
Ymm 40| _.90
-I00 0 100 -I00
z, mm
L
x = 1496
0
z, mm
U
ue
.99
•90
.80
I
I00
355
Effect of Opposin 9 Unsteady Vorticit_ on
Turbulent Structures in Wall Flows
Coherent structures in the outer part of a wall boundary layer have
fluctuating vorticity of the same sign as that of the mean boundary layer. A
possible technique for controlling these structures is to introduce vorticity
of the opposite sign to counteract the existing coherent structures. A
two-dimensional rod in a flow normally produces an alternating vortex pattern
downstream of the rod (Karman vortex street). By placing a thin control plate
at a proper distance from the rod, one side of the vortex street will be
reduced. This method may be used to introduce the vortices required to
counteract the coherent structures in the outer part of the boundary layer.
Total drag reduction on the order of 25 percent was measured with this
technique of which 20 percent was due to the momentum deficit introduced by
the device and an additional 5 percent presumably due to turbulence
modification by the device. Details of this work are presented in Reference
2.
• Counteract outer layer "coherent structures" by introducing
vorticity of opposite sign
• Use Karman vortex street from 2-D rod with one side of street
suppressedby control plate
• Viscous drag reduced 0(25%): 20% momentum deficit
5% turbulence modification
U i) _Plate
356
ORlGlNAL PAGE 1s 
.OF  QUAL^^ 
Production o f  Control Vortices for  Turbulence Modification 
This f i g u r e  shows t h e  e f f e c t  o f  t h e  vor tex  s t r e e t  c o n t r o l  p l a t e  f o r  a rod 
i n  a un i fo rm f low. The arrangement o f  t h e  dev ice i s  shown schemat ica l ly  on 
t h e  l e f t  w i t h  smoke-wire f l ow  v i s u a l i z a t i o n  o f  t h e  downstream v o r t i c e s  on t h e  
r i g h t .  Par t  A shows t h e  una l te red  Karman vor tex  s t ree t .  Par t  B shows t h e  
e f f e c t  o f  optimum placement o f  t h e  c o n t r o l  p la te .  Note t h e  reduc t ion  i n  
s t reng th  o f  t h e  upper p a r t  o f  t h e  vor tex  s t ree t .  Par t  C shows t h e  e f f e c t  o f  
p lac ing  t h e  c o n t r o l  p l a t e  t o o  c lose  t o  t h e  rod. I n  t h i s  case, t h e  rod and 
c o n t r o l  p l a t e  a c t  as a s i n g l e  obs tac le  t o  t h e  f low. 
Cylinder with contr 
er 
357 
Turbulent Burst Control Throu_lh Phase-Locked Wall Deformation
Data on coherent structures close to the wall suggest the possibility of
controlled wall motion as a means of altering the bursting process. An
inflectional instability model of the bursting process was used as a basis for
design of the control mechanism. Briefly, the flow model states that low-
speed wall streaks and "typical" eddies play a synergistic role in burst
production. Wall streaks start the inflection of an initially quiescent,
preburst streamwise velocity profile near the wall and the moving, adverse
pressure gradient associated with the "typical" eddy adds to the inflection
causing turbulent breakdown (bursting). Calculations have shown that a
traveling wall depression phase-locked on the low-pressure region beneath a
convecting "typical" eddy will raise the local pressure (i.e., decreasing the
moving adverse pressure gradient) and decrease the slope of the instantaneous
velocity profile. An experimental model was constructed using a flexible,
ferromagnetic membrane with discrete, electromagnetic actuators to create the
traveling wall depression. Ideally, wall triggering should occur at the
simultaneous detection of both a wall streak and a "typical" eddy. A noisy
tunnel environment prohibited detection pressure signature of a "typical"
eddy. Therefore, wall shear stress was used as the trigger signal to evaluate
the effectiveness in stabilizing the preburst flow. No data on actual
bursting was taken. (This approach is currently restricted to low-speed flows
due to the limited frequency response of most wall motion actuators.) Details
of this work are presented in Reference 3.
• Stop turbulent bursting by stabilizing inflectional pre-burst flow
• Based on cancelling moving, instantaneous adverse pressure gradient
due to typical eddy
• Calculations of vortex convecting in laminar boundary layer show
favorable effect of phased-locked wall motion
• Experiment with electromagnetically actuated wall membrane shows
stabilizing effect of phase-locked wall motion
% J -"_']m_ U
_''_ C
358
Streawise Velocity Fluctuation Component for Phase-Locked Wall
In order to determine whether the traveling wall depression model could
stabilize preburst flow, wall motion was triggered on various levels of wall
shear-stress measured at a point just upstream of the wall device. A
depression convection speed of 0.75U was used. Hot-wire anemometry was used
to measure turbulent velocity fluctuations above the first actuator in the
wall device. These data were then ensemble averaged over three short time
intervals for 250 cycles of wall motion. The time intervals or windows were
chosen to bracket the time during which the wall was depressed immediately
beneath the hot-wire probe. The figure shows the normalized, ensemble-
averaged, streamwise velocity fluctuation component for the three time windows
both with and without wall motion. It is evident in window 2 that the effect
of the wall motion is to accelerate the streamwise flow near the wall. The
first window shows no significant effect of secondary, propagating wall waves
(caused by the impulsively started wall motion) which travel faster than the
wall depression. The third window shows that wall continued to oscillate
after the wall depression passed. The primary finding, however, is that the
traveling wall depression can stabilize preburst flow.
m
N
U
+
X
P
Wall inactive
Window 1
O< t_< 4.3
2 1
1
0
_17
 IL_Zj
-2 -I 0 I 2 3
+
= 140 Y = II
P
Window 2
4.3 < t_< 8.5
! i i
./x'_
<
I I I
-2 -I 0 I 2 3
"rw/_ w
...... z_...... Wall active
Window 3
8.5<t _< 12.8
! I !
1 I I
-2 -I 0 I 2 3
t Uoot _ __
6_
359
CONCLUSION
We have shown in these initial experiments that wall turbulence
can be reduced or amplified by a variety of techniques including
embedded bodies, non-planar wall geometries and phase-locked
control. The payoff of such control includes drag reduction,
reduced sound insulation, increased heat exchanger efficiency,
improved perform3nces of laser and IR windows and reduced self
noise on sonar domes.
References
1. McGinley, C. B.; and Beeler, G. B.: Large-Eddy Substitution via Vortex
Cancellation for Wall Turbulence Control. AIAA Paper 85-0549, March 12-14,
!985.
2. Goodman, W. L.: The Effect of Opposing-Unsteady Vorticity on the
Turbulent Structures in Wall Flows. AIAA Paper 85-0550, March 12-14, 1985.
3. Wilkinson, S. P.; and Balasubramanian, R.: Turbulent Burst Control
Through Phase-Locked Traveling Surface Depressions. AIAA Paper 85-0536,
March 12-14, 1985.
360


Wall turbulence control

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