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National Advisory Committee for Aeronautics, Technical Notes - Influence of the Orifice on Measured Pressures

The influence of different orifices on the result of meas—
uring the same pressure distributions is the subject of this
note. A circular cylinder is exposed to an air stream perpen—
dicular to its axis and its pressure distribution is repeatedly
determined._ The pressure on the greater part of the upstream
half of the cylinder apparently increases when the orifice size
increases. The pressures measured on the downstream half of
the cylinder do not change for the orifice sizes used in the
tests. Rounding the edge of an orifice has the same effect as
increasing its size.

The maximum value of the ratio of orifice diameter to radi—
us of curvature of the surface in the plane of motion, for which
no measurable error was found, is given. Values of this ratio
for orifices as used in aircraft and model airfoils were found
to be much less than the maximum ratio.
Introduction

The air pressure acting on an aircraft in flight is usually
measured by providing an orifice at the investigated point and
connecting this orifice to a manometer by means of a tube. When
measuring pressure distributions over the surfaces of model air—
foils in the wind tunnel the same method is used. In the latter
case the ratio of orifice diameter to the radius of-curvature of
the surface at the point under investigation.may be quite dif~
ferent than in the full—size airplane. In either case no stand—
ard size of orifice is used nor is any information available
which will tell the investigator what errors he,may expect when
he varies the size of the orifice or in some other way changes
its shape.

The present investigation deals with the effect of orifice
size and shape on the pressure recorded at a designated point
on a body placed in an air stream. This effect was measured by
making pressure distributions over a circular cylinder placed
in an air stream so that its axis is perpendicular to the direc—
tion of the stream.