The five basic elements of the two dimensional airfoil research program at Ames Research Center are illustrated. These elements are experimental, theoretical (including computational), validation, design optimization, and industry interaction. Each area is briefly discussed

Chapman, G. T.

English

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N79- 20033 :
OVERVIEW OF TWO-DIMENSIONAL AIRFOIL RESEARCH _,_}:
AT AMES RESEARCH CENTER _ ":
Gary T. Chapman
NASA Ames Research Center
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INTRODUCTION ";.
The five basic elements of the two-dimensional airfoil research program
at Ames Research Center are illustrated in figure i. These elements are
experimental, theoretical (including computational), validation, design
optimization, and industry interaction. The figure also shows the direction
of flow of the information, starting w_th experimental and theoretical and
moving to validation, design optimization, and industry interaction. As in
any good program, the information also flows in the reverse direction to .;
provide the needs and guides for the research. The following material treats
each area briefly and notes other Ames papers presented at this conference
that cover the topics more completely. In other topic areas, recent
publications are cited to provide more complete details.
THEORETICAL RESEARCH
The theoretical work can be divided into two major areas: computational
aerodynamics computer codes and turbulence modeling.
Computational Codes- The primary work falls into three areas: Navler-
Stokes codes, Euler codes, and multlelement airfoil codes. Deiwert and
Bailey's paper, presented at this conference, reviews the work on tlme-averaged
Navler-Stokes prediction of two-dlmenslonal airfoil aerodynamics at Ames. As
analyses tools, these codes are becoming very useful, but they are still too
time-consumlng (15-30 min on a CDC 7600) to be useful for design work. Some
very good results have been obtained with these codes when used for trarsonlc
buffet, an area that cannot be predicted by less sophisticated codes.
The paper by Olson provides a good example of work in progress on
prediction of complex multlelement airfoils at low speeds. Although the
technology is sufficiently well advanced to allow determination of optimum
slat and flap settings, the codes do not give good absolute values of the
aerodynamic characteristics, particularly the drag coefficient.
Finally, J. Steger has developed a tlme-dependent Euler code that has
been modified for easy use with an assortmei_t of airfoil problems by W. Chyu.
This code treats pitching and/or plunging airfoils with a tlme-varylng free-
stream velocity. In addition, the code can treat a spatially varying upstream
boundary condition to allow consideration of a two-dlmenslonal airfoil in a
shear flow. This can be considered as an approximation of a rotor blade
encountering a wake from a previous blade, i
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Turbulence Modeling- Turbulence modeling is a crucial area of research
for the success of any computational method that attempts to predict viscous
drag and/or strongly interacting viscous flows such as separation or shock
boundary-layer interaction. A major effort is under way at Ames, primarily
under the direction of M. Rubesln, This is a highly complex subject, and I
will not deal with it in detail. The simple algebraic turbulence models of
the eddy viscosity type do an adequate job for fully attached flows, with
more sophisticated one- and two-e%_ation models being required as the viscous
interaction becomes more important. However, computational time also increases
significantly. The simple eddy viscosity models, however, do not predict
drag better than about 5%. This m-y be sufficient for two-dimensional airfoils
since we normally wish only to select good candidate airfoils to use in three-
dimensional wing design.
A primary requirement for good turbulence modeling is good experimental
data. This is the area where much work needs to be done.
EXPERIMENTAL RESEARCH
For discussion purposes, experimental research is divided into two areas:
methodclogy and data base.
Experimental Methodology- This area deals basically with the development
of new experimental methods, particularly those that have application in
two-dimensional airfoil research, Four new experimental methods are of
interest: laser velocimetry, holography, skin friction gages, and an airfoil
oscillatory apparatus.
Laser velocimetry is covered in two papers - one from Ames and one
sponsored by Ames. These are the papers by Johnson (Ames) and Owens, respec-
tively. Johnson (Ames) shows the high degree of accuracy that has been
obtained in both potential and viscous flows with the laser velocimeter.
These included very accurate Reynolds shear stress measurements in a shock-
induced separation on an airfoil at Mach number 0.8. The Owens paper shows
the application of conditional sampling techniques to provide temporal as well
as spatial resolution of time periodic flows. The laser velocimeter has
reached the stage of a highly accurate research tool. However, at this point
it is not a production test device in the sense that a highly trained indivi-
dual is still required for operation and data interpretation.
The use of holography in two-dimensional testing is also discussed in the
paper by Johnson (Ames). Although this technique is much newer than the laser
velocimeter, it already shows excellent promise. The potential for completely
mapping the density field in a single picture makes for much more rapid data-
gathering than the laser veloclmeter. Holography has been shown to be very
accurate, both by comparison with laser veloclmetry data and with surface
static pressures. This latter comparison suggests the possibility of testing
nonpressure-instrumented models and hence cutting manufacturing costs
considerably.
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A new experimental apparatus that provides new data for unsteady
aerodynamics has been developed. This new two-dimensional airfoil oscillating
apparatus for the Ames ll-Ft Transonic Tunnel is reported in a paper by Davis
and Malcolm at this conference. The test apparatus allows for a wide range
of test conditions: reduced frequencies to 0.35; Math numbers from 0.4 to 1.2;
Reynolds numbers to 12 million; center of rotation from plus infinity to minus
infinity (pure plunge to oscillation about any point on or off the airfoil);
static angles of attack to 16°; and pitching amplitude of ±2°. The apparatus
has automated real-time data acquisition, first harmonic analysis, and display _
capability provided by a dedicated minicomputer. The apparatus is operational
and has already greatly expanded the data base in this area.
A significant amount of work is going on at Ames to develop simple and
accurate skin friction gages. Two types are under development: the "buried"
wire gage and a very s_all floating element mounted on a crystal gage. The
"buried" wire gage development is nearly complete and has been reported by
Murthy and Rose (ref. i). This gage is similar in principle to most heated
wire or film skin friction gages but differs in construction. The heated
wire, after being soldered to the leads, is pulled snuggly to the surface of
a plastic button (leads pass through holes in the button). A drop of solvent
is placed on the surface around the wire, bringing the substrate (button)
material into suspension. When the solvent evaporates, a smooth, very thin
coat is deposited over the wire. Gages constructed in this manner are
inexpensive, simple to use, repeatable, and fairly accurate.
The floating element gage is nearing the point of first test. The basic
idea is similar to all floating element gages. The major difference is that
the crystal mount (the deflecting beam) is very rigid, allowing movement of
only a few millionths of an inch. This requires a special sensing method.
The technique used here is a surface acoustic wave detection method -- the
surface acoustic wave speed being linearly proportional to the stress in the
beam surfaces caused by deflection of the beam. This device is much more
expensive that a "buried" wire gage but has the potential for much higher
accuracy and does not require in-place calibration.
Data Base- The data base work covers three primary areas: steady,
unsteady, and turbulence. Data base information is seldom isolated from some
other activity because it is normally acquired to validate a theory or develop
a better understanding on which a more complete theory can be developed. This
is particularly true of the two-dimensional airfoil research at Ames. Examples
of steady data base can be found in the papers by Lores, Burdges, Shrewsbury,
and Hicks, and by Johnson (Vought) and Hicks. Examples of unsteady data can be
found in the paper by Davis and Malcolm and for turbulence in the paper by
Johnson (Ames).
VALIDATION
Validation is the step whereby theory and experiment are brought together
for comparison. It can be uJed both to validate the theory or point out
errors in the experiment. This is a continual area of ac_ ;vity that is never
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really finished. I would like to describe briefly one example in the area of t
wind-tunnel wall interference. Much has been said about the need for a good
set of interference-free wlnd-tunnel data on an airfoil at t_ansonlc speed.
In an attempt to assess wall interference in the 2 by 2 ft transonic wind
tunnel at Ames (this i3 a slotted test section facility), some interesting
points have been discovered. This discovery results from a more complete
comparison of experiment and theory. The experiment was with an NACA 64A010
airfoil. Both pressure distribution and flow fields were measured (the latter
with the laser velocimeter). Compu=ations were made with a small disturb-_ce
transonic code under conditions for which the theory would be expected to be
valid.
First, calculations were made at the set angle of attack. At this
condition, the lift and pressure distribution were missed badly, but the flow
angle measurement (one chord height above the model) was predicted very well
upstream and over the leading edge but deviated downstream of the leading
edge. A second calculation was performed at an angle of attack that resulted
in the correct lift coefficient. At this condition, the pressure distribution,
while not exact, was realistic; however, the stream angle was not correct
anywhere although the agreement improved in the downstream portion of the
flow. This suggests that the classical angle-of-attack correction method used
for _ra,,°onic wall effect is not valid in general.
DESIGN OPTIMIZATION
The application of numerical optimization procedures-- with existing
airfoil computational methods-- to the problem of airfoil design has been
actively researched at Ame&. Work has been under way in three different areas:
transonic airfoils, low-speed improved CLmax airfoils, and rotor airfoils.
There are two papers on transonic airfoil designs that are the result of
cooperative programs with indusury. These are the papers by Lore_s (Lockheed-
Georgia) et al., and Johnson (Vought) and Hicks. These two papers illustrate
the power, as well as the limitations, of this method of design.
In the area of low-speed airfoils designed to improve CLmax, there has
been considerable success. A recent leading edge modification (30% of upper
surface) of an NACA 632215 resulted in a 20% improvement In CL at landing
conditions. Other cases have also been designed and tested. Am_sign meth-
odology has evolved and been verified, and a handbook of leading edge
modifications (theoretically defined) for the most widely used NACA airfoils
is underway.
The rotor airfoil section work is unique in that these sections require
multipoint designs. The retreating blade (shock-induced stall at M = 0.4
and 0.5) and advancing blade transonic effects must be considered. A recent
three-polnt design has been tested and found satisfactory although not the
best possible. This area of multlpoint design will continue to receive
greater attention.
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INDUSTRY INTERACTION
Industry interaction is not an area of research but rather a mode of
operation that is essential to good applied research. The two papers described
in the foregoing section regarding transonic airfoil design optimization (Lores
et al., and Johnson (Vought) and Hicks) are examples of this industry interac-
tion. The importance of the two-way nature of this interaction is crucial to
the foregoing work. NASA gets direct input of industry's needs as well as
active evaluation of the validity of the research it is conducting. Industry
gets the fastest possible transfer of new technology, often long before
complete publication or documentation is available. This is a continuing area
of activity with a shift towards interacting for complete three-dimensional
wing design problems.
CONCLUDING REMARKS
In conclusion, the two-dimensional airfoil aerodynanics research at
Ames Research Center has been briefly described. Although in many respects
it is a small program, the contributions are substantial_
REFERENCE
i. Murthy, V. S.; and Rose, W. C.: Buried Wire Gages for Wall Shear Stress
Measurements, AIAA Paper 78-798. San Diego, Calif., April 19-21,
1978.
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Overview of two-dimensional airfoil research at Ames Research Center

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