Initial measurements of the changes in local skin friction, velocity profile shape, and turbulence structure which result from the placement of tandem plates parallel to the wall in the outer region of thick turbulent boundary layers were made. Using a tunnel with a .75 m x 1.2 m x 7.3 m test section, which diverged so as to keep the pressure gradient less than 2x1000/ft, on the test wall, a skin friction reduction of approximately 30% was measured at xi/h = 62. This relaxed to a reduction of approximately 16% at xi/h = 124 for h/delta M = .6. The c sub f measurements for both the normal and modified boundary layers were obtained by measuring the slope of the velocity profile within the linear sublayer. Visual results indicated a continued presence of strong large eddy structure downstream of the devices. Local skin friction reduction of 12% at xi/h = 62 was also obtained with the manipulators above the boundary layer at y/delta m = 1.1

Falco, R. E.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19830016198 2020-03-21T04:17:44+00:00Z
(NASA-CR-170313) A STUDY CF ?EE FFFICZS OF
LEHU DEVICES ON TUREUIENT EM MI IAYEF
DRAG Final Technical Report, i Ncv. 1581 -
31 Oct. 1982 (Michigan State Univ.) 20 p
HC A02/81 A01	 CSCL OIA G3/02
P83-24469
onclaE
03594 I
Y
9
Y
A STUDY OF THE EFFECTS OF
LEBU DEVICES
ON TURBULENT BOUNDARY LAYER
DRAG
by
R. E. Falco
Principal Investigator
FINAL TECHNICAL REPORT
for the period
11/1/81 to 10/31/82
Prepared from work done under
NASA Grant NAG-1-221
Turbulcnce Structure Laboratory
Department of Mechanical Engineering
Miichigan State University
East Lansing, MI 48824
May 1983
•4
ABSTRACT
Initial measurements of the changes in local skin friction, velocity
profile shape, and turbulence structure which result from the placement of
tandem plates parallel to the wall in the outer region of thick turbulent
boundary layers have been made. Using a tunnel with a .75 m x 1.2 m z 7.3
m test section, which diverged so as to keep the pressure gradient dcp/dz
less than 2x103
 /ft, on the test wall, a skin fiction reduction of
approximately 30% was measured at {/h = 62. This relaxed to a reduction
of approximately 16% at t/h = 124 for h/6 M = .6. The of measurements for
both the normal and modified boundary layers were obtained by measuring
the slope of the velocity profile within the linear sublayer. Visual
results indicated a continued presence of strong large eddy structure
downstream of the devises. Local skin friction reduction of 129 at 4/h =
62 was also obtained with the manipulators above the boundary layer at
YAM = 1.1.
I
Y
\,
V4
PAGE 2
INTRODUCTION
The concept of achieving drag reduction via the addition of simple
flow modifying devices in the outer region of turbulent boundary layers is
extremely appealing. They are simple, light weight, inexpensive
modifications which have the potential for significant cost savings.
Using tandem flat plates, to keep the device drag low, Nagib and Corke
( see Corke 1981 ) .x;Corke 1981examined the overall picture out to 68
device heights and have measured a net drag reduction of approximately
20%. They have suggested that considerations necessary to achieve a net
reduction are:
1) The Reynolds number of the turbulent boundary layer at the
first manipulator plate must be 2000 or greater;
2) The spacing of the tandem plates must be between 8
manipulator heights ( IIT ) and 86 ( Stanford );
3) The plates must be .86 ( IIT ) to 6 ( Stanford ) long;
4) The height of the manipulators must be between .88 ( IIT )
and .56 ( Stanford ).
S) The boundary layer must be tripped in a precise way.
Although the net results appear to be sensitive to combinations of these
parameters, perhaps the most important question is the nature of the way
the modifications relax.
Corke et al have advanced a physical picture which suggests that the
manipulators interact with a hypothesized strong motion that results from
PAGE 3
the transition process. They suggest that this structure (presumably some
aspect of individual turbulent spots or combinations of spots) is thought
to control the production of new turbulence. The interaction presumably
weakens the ability of this motion to bring fluid down to the wall.
Evidence shows that 'sweeps' of high velocity fluid moving towards the
wall are responsible for the onset of the turbulence production process,
although a debate is currently underway concerning the origin and scale of
the sweeps, as well as the mechanism by which they act. However, it is
clear that interfering with the sweeps should modify the process. (See
Cantwell, Coles and Dimotakis 1978 for the spot data, and Falco 1977 for
the older large eddy boundary layer data). However, both motions appear
to have significant spanwise rotation ( for the boundary layer this
information comes from correlations, see Townsend 1975). In streamwise
planes, large scale sweeps are generated by the convected stagnation point
flow field that is established at the upstream side of a large eddy or
turbulent spot (Falco 1977 ). This is an essentially irrotational field.
Although space time correlation have suggested that the large scale
motions have strong internal rotation, results of conditionally sampled
measurements show that there is little wallward motions in the large scale
motions.
It has become clear from recent experiments at NASA Langley that
attempts to set up "identical" experimental conditions will not guarantee
similar results. Apparently, a number of factors which have previously
been considered as independently satisfiable, must be considered in
relation to each other. Furthermore, others must be very precisely
9
PAGE 4	 Y
regulated. These include:
Rem -- the momentum thickness Reynolds number at the manipulator;
h/SM -- plate height / b at the first plate;
s/SM -- spacing between the plates / 5 at the first plate;
n/dM -- distance of first plate from trip / S at the first plate;
htrip/SL -- trip height/laminar boundary layer thickness at trip;
L/SM
 -- the plate length / S at the first plate;
trip characteristics;
spanwise uniformity with trip in place--no manipulator;
spanwise uniformity with trip and manipulator in place.
We feel that a sy stematic exploratii
necessary. Clearly, examination by
a broad sense to see if the overall
but can not be used to optimize the
this investigation we have only had
parameter map.
)n of the above parameters is
flow visualization can only be used in
situation has changed significantly,
situation. During the time span of
time to begin to explore this
1i
1
PAGE 5
RESULTS OF OUR PRELIMINARY INVESTIGATION
During the past year we have made a preliminary investigation of the
effects of tandem plate manipulators on the drag of a turbulent boundary
layer. Using a .75 m x 1.2 m x 7.3 m wind tunnel with .35% free stream
turbulence le-el, measurements were made at 2.44 m and 4.41 m downstream
of the manipulators, for a nominal free stream velocity of 3m/s. The
plates spanned the 1.2 m width of the tunnel and were put under
reproducible tension ( measured by strain guages ) by supports outside of
the tunnel. They were made of .5 mm stainless steel sheet metal. Figure
1 shows the experimental arrangement and position of the measurements.
The two dimensionality of the tunnel was qualitatively judged to be good
because of the ability to create a laminar boundary layer of the 24 foot
length of the tunnel, which when marked with smoke showed no spanwise
variations.
A large part of our initial effort was spent on wind tunnel design
changes, the writing of software, and in the taking of exploratory
velocity profiles and movies to get a feel for the new situation. It
became apparent that each of the possible permutations of the parameter
set discussed in the introduction could lead to different details. We
thus li-.ted our scope to reproduce and varify the results of Corke's
investigation. We chose to arrive at those answers vio a slightly
different avenue. As we shall see, differences were fo ,ind in practically
all of the measures we investigated.
PAGE 6
The technique we used was to set the flow speed and do a complete
velocity profile first with, then without the manipulator in place. The
data was acquired via an LSI 11/23 through a 12 bit A/D converter and
stored on disc. The wires were calibrated using a three parameter fitting
routine ( to a Collis and Williams form of equation -- E_ = A + BQn ),
Eleven thousand data points per position were averaged.
Because the wall slope method can be used to measure the local skin
friction coefficient for both normal and manipulated boundary layers, and
because it would allow a comparison on a station by station basis, we
decided to use it as our primary standard, with the intention of doing a
momentum balance to back up the results. The wall slope method, depends
upon the existence of the linear sublayer, but does not depend upon the
three—dimensionality of the shear layer or any non—uniformities, and
furthermore does not depend upon differencing or gap/end plate problems.
Unlike surface hot—films, it does not depend on a calibration which is
extremely insensitive to small variations (the 1/3 power calibration). It
does however, have its own set of demands.
The drawbacks of the technique are all related to the criteria that a
thick sublayer is needed. These are: 1) heat transfer from the wire to
the wall, 2) the number of points obtainable in the linear region, and the
statistical reliability of each point, 3) the acc-iracy with which we can
measure the distance from the wall.
In the thick boundary layer of our tunnel (d = 5 in ) we were able to
PAGE 7
safely get into the sublayer while still keeping our wire more than 100
wire diameters above the wall, beyond the point where heat conduction from
the wire to the wall has been determined to be important. We have further
lessened the heat transfer problem by using a plywood test wall insert,
and by using lower resistance ratios. We have had good results using 1.2,
with ambient temperature variations of WC. Measurements taken in our
tunnel using this technique did indicate significant c f
 differences from
those obtained using Cole's law of the wall and Clauser's fitting
technique in the non--manipulated case. Figure 2 shows the statistical
fluctuations in individual profiles which are directly attributable to the
length of the data records ( 11,000 data points ). However, because of
the procedure we used, the differences in the slopes of the lines should
only be a function of the changes in drag. It is also important to note
that the recent work by Bhotia, Durst and Jovanovic strongly supports our
results on wall heat transfer about the small effect of the wall on the
hot-wire's response to y+ = 2 if the precautions we have taken are
followed. Velocity profiles taken in the Gottingen oil channel
( Eckelmann 1974, and subsequent work ) have also shown that very accurate
results can be obtained with the probe as close to the wall as y + _ .80
when care is taken to alleviate the heat transfer problem is minimized.
The potential accuracy of this method is significantly greater than
obtainable with surface films because of the 1/3 power dependence of thu
surface film calibration.
Our study of the modifications to the velocity profiles showed that
the presence of the manipulators did effect the logarithmic region of the
9
PAGE 8	 Y
velocity profile. Figure 3 a and b shows two examples of the changes
found. In all cases u:
 was determined by the wall slope method. At both
{/h - 62 and 124 the manipulators displaced the points upward in the
logarithmic region ( where C is the distance of the measurement station
from the downstream manipulator plate ). It appears from many similar
profiles that a straight line region continued to be present. By C/h =
124 the upward displacement was considerably less. Corks also found the
persistence of a linear region in semi-logarithmic coordinates, which was
displaced upward. However, when non-dimensionalized by wall coordinates,
he found no change in displacement with distance downstream of the
manipulator up to 67h, and suggested a new set of log region constants.
It should be noted that the constants calculated for our line at CA - 62,
do not agree with his. Furthermore, we have found that the logarithmic
constants change with C/h in the range investigated.
In the tunnel that was available this past year we were able with the
help of a trip ( 1.5 mm threaded rod ) to obtain boundary layers at % of
2-3,000 with sublayers approximately 1.3 mm thick. This allowed from four
to six points to be obtained in the linear sublayer. Figure 4 shows our
average results for the % c f
 reduction at 62 and 124 device heights. The
conditions of the boundary layer just upstream of the first manipulator
plate were: % = 1200, H = 1.461, o f
 - .00435. The manipulator was at
h/EN = .6. L/Sx = 1.3, and s/h - 15.15. It should be noted that at 4/h =
62 our results are consistent with Corks's and also Nagib's ( see Appendix
B of Corks's thesis ) overall C f
 values ( Corks measured downstream to CA
68, Nagib to t/h = 90 ). However, we used different trips, our boundary
PAGE 9
layer at the manipulator was 63low the Re limit that Nagib suggests would
work, and we used a different method to obtain the drag. Furthermore, the
conditions that were described above had relatively longer plates and
longer distances between plates. Thus we have shown that a manipulator
can reduce local skin friction to this level with a weaker set of
restrictions ( Re and trip ) and a wider range of plate configurations.
It is of interest to report anotaer experiment that was performed
under the same conditions but with the plates at y /SN - 1.1. At 4/h - 62,
we found 1211 local skin friction reduction. The fact that we found an
effect is not surprising, because instantaneously many of the bulges
extend further out than this, but the magnitude of the effect is larger
than would be expected if the mechanism was indeed a reduction in the
rotation of the large eddies.
Some preliminary flow visualization experiments were performed to see
if overall structural picture of reduced intermittency found by Corke
could be reproduced. Using a laser sheet parallel to the flow direction
and perpendicular to the wall, we could see the instantaneous boundary
layer over the whole length of the tunnel. Both the "live" visual
impression and observations viewed in slow motion on film showed that the
outer edge was highly intermittent, and it appeared to be similar to the
normal turbulent boundary layer. Figure S shows two photos of the
manipulated boundary layer centered around C/h - 42. It is clear that the
intermittency is very similar to that found in a normal layer. This is in
sharp contrast to the visualizations if Corke. In part the different
URIGINAL i;	 ­ 3	 !
OF P^'JI	
ivy	 r
PAGE 10
impression is a result of our different techniques. Corks introduced
smoke directly into the wake of the manipulator plates by p:acin` a smoke
wire across the flow, as well as into the boundary layer, whereas we only
introduced smoke into the boundary layer. Cotke, of course, sees far leas
intermittency than we do downstream of the manipulator. Although we
believe the indications of this visualization better reflect the state of
the large eddies, it is clear that i:rtaltaneous flow visualization and
hot—wire anemometry with Reynolds stress and cross— stream vorticity probes
will be needed to determine tLe charges in the state of the large scale
motions due to the presence of manipulators.
PADS 11
CONCLUSIONS
These results in general appear to support those of Corks and Nagib,
although they are mucb less comprehensive, anfi do not include net drag
measurements. However, there are some suggestions that details are
different, and would lead to different conclusions. First, we found that
the decrease in local skin friction was significantly less at t/h - 12+
then at 4/h - 62. Second. we did not find a constant displacese.t of the
logarithmic region, it relaxed back towards the unmodified laser position.
Third, we were able to obtain a skin friction reduction when the plates
were 10% higher than 6 99 . Finally, our flow visualisation results were
quite different, showing significant intermittency at 62 device heights.
PAGE 12
REFERENCES
Bhatia, J. C., Durst, F., Jovanovic, J. 1982. Corrections of hot-wire
anemometer measurements near walls. J. Fluid Mech, 122:411-431
Cantwell, B. J., Coles, D. E., Dimotakis, P. E. 1978. Structure and
Entrainment in the Plane of Symmetry of a Turbulent Spot. J. Fluid Mech.
87:642-6'2
Corke, T. C. 1981. A New View on Origin, Role and M-nipulation of Large
Scales in Turbulent Boundary Layers. PhD Thesis, Illinois Institute of
Technology.
Eckelmann, H. 1974. The Structure of the viscous sublayer and the
adjacent wall region in a turbulent channel flow. J. Fluids Mech.
65:439-459
Falco, R. E. 1977. Coherent Motions in the Outer Region of Turbulent
Boundary Layers. Phys. Fluids 20:S124-132
OF
	 r
Ns
3
G
ti
N
G
G
Aj
r
L
V)
1 4	 ^,
r^
,d
•a
N
L
G
Qi
E
Q1 1r
w O
m ro
Q) a
r a
Q) c
- c
u 6
w y
O .L
u
G
G W
•^ OL
O OJ
iJ
v ^n
G
O
4-1 •p
G
v m
0^0 .00
H v
t+ ~
m Q)
u
-	 •,4
u y
G 'C
Q)
•^ N
1+ r-I
a •v
x c
W m
r-I
v
on
w
9
ORIGINAL. PAGE
	 Y
OF POCR QUALrrf
c
3
s
v^L
M ,r
Q U
.0	 >,
ro
U ^
U
•n n
7 ro
U E
^O	 1+
CN C
u
r
C ^
C O
•r u
CD ro
a
4
ro ro
3 i.
w o
V
u
W Ca
U) 3
U-
w a
O bID
:V a•  ^
^ ro
,J L
U
C
J
u C
r w u
w
v
r+ va>
e^
ro ^
k U
v
v
G
4 uO
•	 u7O	 v
Z	 00
w
IGO
L
t
N
W
v
N
cJ
U
.b
'l!
u
G
rz
N
Aj
ORIGINAL PAit L
	 9
OF POOR QUALITY
	
Y
^	 w
C
O
b
d
uC
O
O
1	
-4
T v^
U
6
a.i
OD
O
ri
0	 C1
r	 ^
V;
y
w
O
L+
G.
TL
U
O
.-1
Gl
C
!Cy
r	 ^
ro
M
v
N
ao
.r,
w
OOO
OO
T
O	 ^-+
N	 —	 V
OF POOR QUALr Y
	 9
Y
.J
a, roX
a
o G
k. ro
t;
J
u
N qJ
II	 ^
w ro
+^ c
>~	 v
o G
ro ^
o v
cl
•rl L
G
cy w
E o
^ G
O JL
k
ro
O —+
,c v
u
..	 v
v u
G
ro v
u
0z
I
00
a)
roJ
b
alL
ro
a0. aJ
G ^
ro ro7
10	 M	 C	 0N
s
v
c^
w
OF POOR QUALIIy
3
v
ro u
^ u
G J
^	 r ^
c^
s
G u
•4 yr
cr, co
ro u
v
s. u
u r.
mb
i1
L ^%
r w
V
u u
D	 " N
D	 z cc
o	 m
C
C N
'^ GL Q
L
a m
'O •rl L.
cu	 W
aj
 ro
m	 C 'tt ^
u v ti..
^ G c9
w a
►,	 .N J
.J	 .Y `ra Cl
Ci u
w
G u C
C
—+ w y
.Q
s: E v.
o w a
w o
8
U ro v,
N U:
O G O►. 3 u
O ro
G^ •fl r-1
ac	 a
ro ^ a
3r y •r-1
v > G
6 8 8
ORIGINAL PAGE 19
OF POOR QUALITY
Figure 5. Two photos of the manipulated turbulent boundary layer. They are
centered around C/h =62. Flaw is formed right to left.
I
Y


A study of the effects of Lebu devices on turbulent boundary layer drag

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server