Three areas are discussed: oscillatory longitudinal curvature, large eddy breakup devices, and small longitudinal surface striations. The first two concepts produced sizable local turbulent skin friction reductions, but further research is required to optimize the effect, reduce parasitic device drag, and provide a net increase in vehicle performance. The surface striations, due to large wetted area increased for a given planform, have not yet yielded a planform-averaged skin friction reduction. Heat transfer data indicated such surfaces may have useful application in heat exchangers

Bushnell, D. M.

English

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https://ntrs.nasa.gov/search.jsp?R=19780012518 2020-03-22T04:22:55+00:00Z
NASA Technical Memorandum 78688
(NASA-TM-786E8)
	 RECENT TUFBUIENT CRAG,	 N78-20461
FEDUCTION FESEAFCH AT IANGIEY FESFAFCH
CENTEF (NASA)	 11 F HC AC2/MF A01	 CSCI 2CD
UncIas
G3/34	 11903
RECENT TURBULENT DRAG REDUCTION! RESEARCH
AT LANGLEY RESEARCH CENTER
D. M. BUSHNELL
PIARCH 1978
(a	
'i
(A ^^N/SA
Nauonai Ae ronautiu, ,,nd
Space Adman ^tration
Langley Research Center
Hamptun Virginia 23665
RECENT TURBULENT DRAG REDUCTION RESEARCH
AT LANGLEY RESEARCH CENTER
D. M. Bushnell
Langley Research Center
SUMMARY
This paper represents a report on a portion of the fuselage viscous drag
reduction research at NASA Langley Research Center. The paper discusses three
areas: oscillatory longitudinal curvature, large eddy breakup devices and
small longitudinal surface striations. The first two concepts produce sizable
local turbulent skin friction reductions, but further research is required to
optimize the effect, reduce parasitic device drag, and thus provide a net
increase in vehicle performance. The surface striations, due to large wetted
area increases for a given planform, have rot yet yielded a planform-averaged
skin friction reduction. However, heat transfer data indicate such surfaces
may have useful application in heat exchangers.
INTRODUCTION
One of the major goals of aerodynamic research is increased aircraft perfor-
mance. Reducing drag is an obvious method of increasing performance and at the
same time reducing fuel usage. Long haul aircraft drag is mainly drag due to
lift and turbulent skin friction. Recent advances in technology (supercritical
wings, winglets) provide sizable reductions in drag due to lift. However,
while skin friction drag reduction research is currently enjoying somewhat of a
renaissance, there is no generally applicable technique yet available which
gives a net reduction in turbulent skin friction other than driving the flow
all the way to the laminar condition (Laminar Flow Control). This forced lami-
nar option is under active study at NASA Langley Research Center for wing
application, but other metnods should be examined for the fuselage, which is
responsible for approximately one half of the aircraft skin friction drag.
Turbulent drag reduction techniques are reviewed in reference 1 and this paper
presents a progress report on research for several approaches not yet identi-
fied when reference I was prepared. The approaches include oscillatory longi-
tudinal curvature (ref. 2), large eddy breakup devices (ref. 3) and small
longitudinal surface striations (ref. 4 - paper accepted for presentation).
A chronology of the Langley viscous drag reduction research program is shown on
figure 1. This research is based primarily on concepts derived from considera-
tion of the coherent structures recently identified in turbulent boundary
layers (fig. 2). The turbulent layer is assumed to consist of the usual outer
region with large scale motion, a wall region where turbulent production occurs,
and a communication process from outer to inner layers which causes the inner
layer to erupt. Obvious possibilities for turbulent drag reduction include
breakup of the outer eddy structure, (thereby altering the outer-to-wall commu-
nication and perhaps decreasing the bursting activity) and stabilization of the
near wall layer. However, care must be taken to consider and perhaps modify
"secondary influences" of candidate drag reduction techniques to ensure that
the overall effect is the net drag reduction. There are many techniques which
reduce turbulence production, but actually increase overall drag, simple examples
being small to moderate wall suction and favorable pressure gradients. There-
fore, for each of the three candidate drag reduction techniques which will now
be discussed, the effect on overall drag will be addressed, not just the influence
on turbulent skin friction.
Oscillatory longitudinal Curvature
Early results from this study are available in reference 2. The basic concept
is to use transverse surface waviness in an attempt to produce periodic partial
relaminarization and thus a lower average skin friction. The essence of the
approach is the extreme sensitivity of turbulent flows to longitudinal curvature
effects (ref. 5). Transverse wall waves produce periodic variations in two
boundary conditions which strongly influence turbulent boundary layers, alter-
nating favorable and adverse pressure gradients and concavS and convex curvature.
The pressure variations are phase-shifted approximately 90 from the curvature
variations, but there is a region, upstream of the wave crest, where the boundary
layer is subjected simultaneously to the combined influence of convex curvature
and favorable pressure gradient, both of which are known to stabilize the turbu-
lent flow. Sufficiently large and lonq lasting convex curvature and favorable
pressure gradient effects are known to cause relaminarization of the turbulent
boundary layer.
From research performed thus far, one of the keys to obtaining an average Cff
reduction is the use of small wave 1 --ngths (order of the boundary layer thickness)
to ause large curvature effects (small radius of curvature, proportional to wave
length squared). Usual guidelines (e.g. ref. 5) indicate that d/R values greater
than 0.005 to 0.01 cause large chanqes in turbulence structure. In the trans-
verse wavy wall case relatively innocuous waves, T ` 0.03 (but small wave legth,
1	 t) produce b/R - 0(1) and therefore the boundary layer is subjected to tremen-
dous oscillatory curvature influences. The pressure gradients are also not small,
with maximum ^(
	
dvalues of order 10 - ► 15. If such influences were mona-
tonicall y aoolied tic the himindarly layer, the effects would be ultimately catas-
trophic, either complete relaminarization or separation, depending upon the sign.
The mit-ya.ing influence which keeps the turbulent flow relatively well behaved
is the oscillatory nature of these influences. The turbulence is kept in a
highly nonequilibrium state; as soon as the flow begins to adjust to the local
gradients the spatially periodic surface condition alters them. In fact, this
2
opF PWR
OF	
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nonequilibrium effect may not allow the usual Gortler vortices to form in the
concave region, and delayed formation of these vortices (made probable by small
X) may be necessary for wavy walls to reduce average skin friction. If the
vortices formed they would probably continue on into the area where partial
relaminarization could occur and perhaps seriously disrupt the relaminarization
process.
Reference 2 documents a calculation technique deve l oped for wave-shaped walls of
small wave length with first order nonequilibrium effects included via rate
equations for the wall damping, wall layer and curvature modeling "constants".
The method was developed and calibrated to reproduce wall shear (fig.3) and
velocity profiles measured in small a wavy wall experiments. Note that the
average skin friction for figure 2 is approximately 20 percent lower than for a
flat plate. This apparent reduction in average surface skin friction is not yet
a net drag reduction, as the oscillatory (nonequilibrium) d* variation over the
wall causes a phase shift in the pressure field induced by the wavy surface and
therefore a form or pressure drag occurs which is larger than the C reduction.
The calculation procedure just discussed can predict this oscillatory  6* varia-
tion and the current Langley Research Center research is aimed at testing models
of "skewed" wavy walls where the surface wave is "corrected" for this oscillatory
b*. On paper, such optimized surfaces can produce a reasonable (10
	
20 percent)
net drag reduction; even when the small additional surface area associated with
the wave is included.
Large Eddy Breakup Devices
The original research on this concept is documented in reference 3. The initial
basic approach was to insert across the boundary layer (but not across the entire
flow) a screen or other device to break up the outer eddies Ffg. 4). The usual
relaxation distance for the outer flow is 60 to 100
	 and in this region the
outer eddies convalesce, heal and grow back to their usual scale. During this
healing or relaxation process the outer-to-wall communication might be disrupted,
resulting in less bursts and a lower C f
	The screen experiments of reference 3
show a large C
	 reduction (up to 50 percent) with a monatonic relaxation to the
"undisturbed 4vel" in the order of 50
	 1 103 d. The device (screen) drag in
reference 3 is probably larger than the area integrated C f reduction and there-
fore further research is required to determine whether the device drag can be
reduced below the C  reduction so a net drag reduction can be obtained.
As shown on figure 4, in unpublished tests at Langley by J. N. Hefner, we have
used honeycombs rather than screens. These honeycombs have approximately an
order of magnitude less drag than the screen of reference 3. Preliminary
direct drag (balance) measurements indicate C  reductions the order of those
observed in reference 3 (0 - 50 percent), but the device drag is still larger
(by a factor of 2 to 3) than the skin friction reduction. The current Langley
Research Center research in this area attempts to determine whether a device
such as shown on the right of figure 4 (having a smaller number of horizontal and
vertical elements) can also produce large C f reductions downstream. If so, a net
drag reduction may be obtainable. The basic question of what effect various_
device geometries have upon suppressing the outer structures in turbulent boundary
layers is essentially an entirely new line of inquiry. Most previous work
3
focused upon the opposite problem of using "vortex generators" or roughness to
excite the turbulent boundary and keep the flow attached in adverse pressure
gradient regions, or for enhanced heat transfer. Estimates based upon the form
drag of four horizontal elements with an aspect ratio of 10 indicate a net drag
reduction of 10 to 20 percent, if such a device produces large C  reductions.
The research on "large eddy breakup devices " is obviously in an early stage.
Small Longitudinal Surface Striations
The basic concept is described in reference 4. Early work by Eckert and Irvine
(ref. 6) indicate that the apex region of isosoles triangle ducts (with small
apex angle) could have laminar flow locally, although the major portion of the
duct flow wds turbulent. An obvious possible origin for this behavior is the
local dimunition in transverse scale associated with the apex region, i.e.,
locally the flow is highly viscous. With this experience in mind, and consid-
ering the wall scales associated with the bursting process, several walls were
machined with triangular (and rectangular) shaped "riblets" having dimensions
Ay	 .,z	 0(50). These longitudinal striations are considerably smaller than
any used in previous research.
Direct drag measurements indicate small drag increases (5 ; 10 percent) when
compared to a flat or smooth surface of the same planform. However, the actual
surface (wetted) area associated with the riblets was as much as 300 percent
greater than the smooth surface. Therefore, on a wetted area basis the drag was
reduced considerably (but not on a planform basis, which is the only important
consideration for aircraft drag reduction). Work is continuing to determine if
a net drag reduction can be achieved. It should be noted that because the
striations are longitudinal, there is no local pressure of form drag associated
with such surfaces.
Having evidently altered the wetted surface average C f considerably, we next
j	 checked the heat transfer behavior of these riblets. Results shown on figure 5
{	 indicate a sizable increase in Reynolds analogy factor compared to the flat plate
case. This observation may be of considerable interest for heat exchanger appli-
cations, as these small riblets evidently increase heat transfer more than they
increase pumping power for non-pressure gradient (external type) surfaces. Most
devices which increase heating do so at the expense of much greater pumping power
compared to the heat transfer increase.
CONCLUDING REMARKS
This pro qress report is on a portion of the fuselage viscous drag reduction
research at NASA Langley Research Center. Paper discusses three areas:
oscillatory longitudinal curvature, Dirge eddy breakup devices, and small longi-
tudinal	 -irface striations.
	
The first two concepts produce sizable local
turbulent slain friction reductions, but farther research is required to optimize
the effect, reduce parasitic device drag and thus provide a net increase in
vehicle performance. The surface stri,rtion--, due to large wetter areas increases
4	
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for a given planform, have not yet yielded a planform-averaged skin friction
reduction. However, heat-transfer data indicate such surfaces may have useful
application in heat exchangers.
REFERENCES
1. Hefner, J. N.; and Bushnell, D. M.: An Overview of Concepts for Aircraft
Drag Reduction. AGARD R-654 "Concepts for Drag Reduction," 1977.
2. Balasubramanian, R.; Cary, A. M., Jr.; Bushnell, D. M.; and Ash, R. L.:
Influence of Transverse Surface Waves on Turbulent Boundary Layers.
Presented at 5th Biennial Symposium on Turbulence at University of
Missouri, Rolla, Oct. 1977.
3. Yajnik, K. S.; and Acharya, M.: Non-Equilibrium Effects in a Turbulent
Boundary Layer due to the Destruction of Large Eddies. Symposium on
Turbulence at Technische Universitat Berlin, Aug. 1977.
4. Walsh, M. J.; and Weinstein: Drag and He-.t Transfer on Surfaces with
Small Longitudinal Fins. AIAA 11th Fluid and Plasma Dynamics Conference,
Seattle, Washington, July 10-12, 1978.
5. Bradshaw, P.: Effects of Streamline Curvature on Turbulent Flow. AGARDo-
graph 169, 1973.
6. Eckert, E. R. G.; and Irvine, T. F., Jr.:
	 Flow in Corners of Passages with
Non-Circular Cross Sections. Transactions of the ASME, May 1956, pp. 709-
718.
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I	 Report No.	 2. Government Accession No. 	 3. Recipient's Catalog No
NASA TM-78688
Tale and Subtitle	 5	 Report Date
RECENT TURBULENT DRAG REDUCTION RESEARCH AT 	 —	 —
LANGLEY	 RESEARCH	 CENTER	 6	 Performing Organization Corte
Authorlsl
	
8	 Performing Organization Report No
D.	 M.	 Bushnell
10
	
Work Unit No
9	 Performing Organization Name and Addreu	 505-06-23-05
NASA	 Langley
	
Research	 Center	 11	 Conttact or Grant fro
Hampton, VA	 23665
_	 13
	 Type of Report and Period Covered
Technical
	
Memorandum_
12
	
Sponsoring Agency Name and Address
National	 Aeronautics	 and	 Space Administration	 I,	 Sponsoring Agency Code
Washington, DC	 20546
15	 Supplementary Notes
16	 ADSbact
This paper represents a report on a portion of the fuselage viscous drag
reduction	 research at NASA Langley Research Center. 	 The paper discusses	 three
areas:	 oscillatory longitudinal	 curvature,	 large eddy breakup devices,	 and
small	 longitudinal	 surface	 striations.	 The	 first	 two	 concepts	 produce	 sizable
local	 turbulent	 skin friction	 reductions,	 but further research	 is	 required	 to
optimize the effect,
	
reduce parasitic device drag, 	 and thus
	
provide a net
increase in vehicle performance.
	
The surface striations,
	 due to	 large wetted
area	 increases
	
for a given	 planform,	 have not yet _y ielded a	 plan.ornl-averaged
skin	 friction	 reduction.	 However,	 heat	 transfer data	 indicate such surfaces
may have useful	 application	 in heat exchangers.
bRI('7T'.^ AT, PAGF, IS
of , PUUIt QUALITY
1 7 Kav Nords l suggested by Author(fl)
	 18 D-stnbut.on Statement -- -
	 —	 -
viscous drag	 reduction
turbulent	 flow	 Unclassified-Unlimited
Subject Category 34
!	 ,
fol sale by the National 1# , lhnual Infotrnation Scr vlte Sp r nef eir] Vu 1^, 1, 1 11161I
1 9 SPC.. r ^ty el aeu It (Ol thu report
	 70 Sat uuty 1 'In,^t luf lost page) ?1 NO o) Pepe,	 22 Pace'
Unclassified	 Unc 	 i Fied 10	 53.5


Recent turbulent drag reduction research at Langley Research Center

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