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National Advisory Committee for Aeronautics, Technical Notes - Aerodynamic Characteristics of Damping Screens

The experimental investigation of damping screens described herein
was undertaken primarily to test theories of the effects of damping
screens and to obtain information on the performance of screens in
oblique flow. Data on normal- and tangential-force coefficients are
given for a variety of square-mesh screens. Damping theories are
reviewed and performance is compared.with theory. The characteristics
investigated include the damping of longitudinal and lateral components
of turbulence, the effect of screens on scale, the conditions for the
production of turbulence and eddies by screens, and the damping of
spatial variations of mean Speed.

Within the last decade screens have found everJincreasing use in
wind tunnels to reduce turbulence. As the result, the turbulence in
many modern wind tunnels has been reduced to levels unknown 10 years
ago. Along with the attainment of low turbulence came the recognition
that low turbulence was a necessity for many wind-tunnel investigations.
Much importance is therefore attached to the damping screen, and as
much as possible should be known about its characteristics.

In earlier times, in fact dating from some of the earliest wind
tunnels, screens were used to reduce differences in mean speed in
order to obtain a more unifonm stream. Prandtl (reference 1) appears
to have been the first to obtain an expression for the reduction of
these differences in terms of a coefficient, now commonly known as the
pressure-drop coefficient. Collar (reference 2) obtained a different
expression for the reduction of differences in mean speed involving
this coefficient and showing fair agreement with his experimental
results.

A few years ago Taylor pointed out that there is not only a change
in pressure but also a change in direction if a stream passes through
a screen obliquely. Based on these two effects, a general theory of
damping has recently been developed by Taylor and Batchelor
(reference 3) which may be applied to the damping of turbulence
as well as to the damping of mean spatial differences in velocity.

In l9h0 a systematic investigation of the damping of turbulence
was conducted at the National Bureau of Standards (reference L).
Screens consisting of various wire diameters and various numbers of
wires per inch were placed in different—numbers in the settling
chamber of the tunnel shown in figure 1, and turbulence measurements
were made in the test section. A simple theory, involving only the
pressure-drop coefficient of a screen, was proposed on the assumption
that a screen reduced the energy of turbulence irrespective of the
distribution of the energy between longitudinal and lateral components.
This theory was in good agreement with the experimental results.