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Investigation of the Boundary Layer About a Symmetrical Airfoil in a Wind Tunnel of Low Turbulence

SUMMARY

. An extensive series of boundaryélayer surveys was
made overlthe surface of an N.A.C.A; 0012 airfoil at zero
5 - lift. The surveys were made at-Reynoldstumbers, based,

3 on the chord, of 2,675,000. 3,780,000. 5,350,000.~and

{. _ 7,560,000.. The drag of the airfoil was measured by the

f .wake—survey method throughout a range of Reynolds Numbers
“ from 225.000 to-7.560,000. The distribution;of skin fric-
tion ofer~the surface of the airfoil was found from the
boundary—layer-surveys and the results are'compared with
those calculated‘according to the method of Squire and
Young developed in England in 1937.

A comparison of the conditions associated with tran—
sition in this investigation and those prevailing in pre-
vious unpublished tests in the.N.A.C.A. 8-foot high-speed
tunnel and the N.A.C.A. 19-foot pressure tunnel indicates
that the turbulence of the air stream used in the present
tests is less than in these tunnels. It appears that the
critical Reynolds Number of a sphere cannot be‘used as a
measure of the effects of small amounts of turbulence on
the flow about an airfoil.

'The distribution of turbulent skin friction calculated
.according to the method of Squire and Young is in fair
agreement with the experimental results.. Both theory and
experiment show-that the skin‘friction along the surface
of the N.A.C.A. 0012 airfoil is approximately 80 percent
of the profile drag. The calculated profile drag is in
good agreement with that found from the wake surveys”

INTRODUCTION

Because the position and the nature of the transition
region on a given body are controlled to a large extent
by the turbulence of the air stream in which the tests are-
made, boundary—layer determinations about the same body

'in several tunnels should giVe an indication of the tur-

bulence of their air streams. ,Furthermore, if the body
chosen for testing is an airfoil, the effect of turbulence
will be shown on a body that is applicable to practical
aerodynamics. -

The flow about the N,£.C.A. 0012 airfoil has been the
subject of several investigations. Tests made in the
N.A;C.A. full-scale tunnel included the determination of
the position of the transition region and the general
characteristics of the boundary layer about this airfoil
(reference 1); Similar studies, as yet unpublished, of
the same airfoil were also carried out in the N.A.O.A. 8?
foot high-speed and 19-foot pressure tunnels,

These tests permit a comparison'of the effects of
turbulence in the air stream in which the present inves-
tigation was conducted with those of the air streams in
which the previous tests were made. The extensive region
of laminar boundary layer in the presence of an adverse
pressure gradient that exists on the N.A.C,A. 0012 airfoil
affords an excellent opportunity to check experimentally
the laminar-boundary-layer theory of reference 2.

The purposes of this investigation are to deduce the
relative turbulence characteristics of the loveturbulence
tunnel from determinations of boundary-layer transition!
to compare experimental with theoretical'velocity profiles

'in the laminar boundary layer, to find experimentally the

skin—friction drag and thus to obtain an estimate of the
proportion of pressure drag to total drag, and to compare
the-skin friction calculated according to the method of
Squire and Young (reference 33 with that found from bound-
ary-layer determinations.

» The tests were carried out on the N.A.C.A. 0012 air—

foil set_at_zaro_liflta Boundary-layer surveys were made

at 12 positions along the surface for each of four Reynolds
Numbers ranging from 2,675,000 to 7,560,000. The varia—
tion of the position of the transition region with Reynolds
Number was determined. The profile drag of the airfoil was
determined from wake surveys over a range of Reynolds Num-
bers extending from 225,000 to 7,560,000.