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National Advisory Committee for Aeronautics, Research Memorandum - An Experimental Investigation of the Zero Lift Pressure Distribution Over a Wedge Airfoil in Closed, Slotted, and Open Throat Tunnels at Transonic Mach Numbers

Pressure distributions and schlieren photographs of the flow about
a lO-percent-thick diamond airfoil at zero lift in two-dimensional closed,
slotted, and open—throat tunnels are presented and discussed. Uncorrected
airfoil pressures obtained in %~ and é-open slotted throat tunnels are
compared at subsonic Mach numbers with corrected results from open and
closed test sections of the same dimensions. The effect of varying the
slot width has been investigated at Mach numbers up to 0.92. At Mach
numbers up to 1.18, data obtained in test sections whose upper and lower
boundaries were slotted to provide openings the combined width of which
was equal to one—eighth of the tunnel width, are shown to be consistent
with theory and available experiments.

The removal of the choking limitation of conventional closed throat
tunnels by the introduction of longitudinal slots in the tunnel boundaries
makes possible the testing of relatively large models throughout the
transonic range. Previous investigations have shown that slotting of
the test section walls also effects substantial reductions in the jet-
boundary interference effects (references 1 to 3). The current investi-
gation, conducted in the Langley Internal Aerodynamics Section, is part
of the continuing research on the problems of boundary interference in
transonic tunnels. In conducting this research an airfoil whose chord
was eight—ninths of the tunnel height has been used to amplify the bound—
ary interference effects although the airfoil is much larger than would
normally be considered appropriate for wind-tunnel testing.

Reported herein are the results of two-dimensional tests of a non-
lifting symmetrical wedge airfoil in a tunnel whose upper and lower
boundaries were closed, open, or slotted. In the slotted configuration,
three different slot arrangements were used. Chordwise pressure distri-
butions obtained from tests in each of the slotted wall configurations
are presented without correction and compared with both uncorrected and
corrected subsonic results of tests in open and closed Jets of the same
size. Comparisons of the pressure-drag'curves from each facility and
of schlieren photographs of the flow in the vicinity of the airfoil are
also presented. Tests at supersonic Mach numbers, M = 1.02 to 1.18,
were made in only one of the slotted configurations; surface pressure
distributions obtained in these tests are compared with the calculated
results of references h and 5 and with available experiment.