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National Advisory Committee for Aeronautics, Technical Notes - Wind Tunnel Standardization Disk Drag

This report deals with the resistances of a series of three
similar disks placed normal to the wind as determined in the at—
mospheric wind tunnel of the National Advisory Committee for
Aeronautics. This is the first of the standardization tests to
be made in American wind tunnels using these particular disks.
The curves of drag coefficient plotted against Reynolds Number,
for this tunnel show discrepancies between overlapping values
which are to be attributed to the presence of the tunnel walls.
The disk standardization program, a part of the general
standardization program, is for the purpose of further obtain?
ing and correlating data from the several wind tunnels now in
operation in the United States. At a meeting of the Aerodynam—
ics Committee, Dr. Zahm suggested that these tunnels measure
the drag of a given normal disk to determine whether each could__
obtain the same value with the simplest model, if small enough
to obviate wall interference. The Committee then resolved to
use three similar disks to show the scale effect. The tests
herein described were carried out in the five—foot closed throat
atmospheric density wind tunnel of the Langley Memorial Aeronau—
tical Laboratory (Reference 5). The Reynolds Number varied from
33,000 to 670,000, using the diameter of the disk as the charac—
teristic length.
The disks were four, eight, and twelve inches in diameter,
and the ratio of all other dimensions was also 1 2 8 1 3. Even
the supporting wires had conforming diameters different from each
other. The disk thickness was 1/64 the diameter.. The circumferv
enoe had a 45—degree bevel to a sharp edge, the bevel being placed
downstream. The supporting spindle was-designed so that its ‘
interference with the disk would be as small as possible.
The testing was done in approximately the same longitudinal
position in the throat, and with the disks concentric with the
tunnel wall. The velocities ranged from 5 to 35 meters per sec—
ond,,taken in five—m.p.s. steps. The "service Pitot” used in
determining these velocities was of the N.A.C.Ao type with hemi—
spherical nose, and was located in the entrance cone of the tune
nel forward of the.small honeycomb. This Pitot by prior cali—
bration gave the average throat velocity for the condition of un:
obstructed flow. The effect of the 12—inch disk upon this Pitot
was investigated and found to be negligible.