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Optical Methods for Examining the Flow in High Speed Wind Tunnel

The paper reviews the present state of
knowledge concerning the use of schlieren,
direct-shadow, and interferometer tech-
niques for visualizing and photographing the
flow in high-speed wind tunnels. Emphasis
is placed on the most widely used techniques,
but brief details are also given of methods
which are in an early stage of development,
or which are useful only in a limited range
of investigations. Information on suitable
light sources and photographic materials is
included.

Numerous methods are available for vis-
ualizing the flow past the model under test
in a wind tunnel. These include the use of
tufts (Refs. 1 and 2), smoke (Ref. 2), an
oil film placed on the surface of the model
(Refs. 3 and 4), and the vapour-screen (Ref.
5) and tuft-grid (Ref. 6) techniques for ob-
serving the vortex pattern behind the model.
Many methods for the visualization of
boundary-layer transition are also available
(Refs. 1 and 2) and frequently depend on the
physical or chemical behaviour of a deposit
placed on the surface of the model. For
information on these techniques the reader
is referred to the references cited and to
their bibliographies.

The present paper is concerned with the
use of optical methods for observing the
changes of refractive index which accompany
the changes of density that are present in
the flow past the model at sufficiently high
Mach number. These methods are of con-
siderable value in a wide range of inves-
tigations. They enable a large field to be
surveyed rapidly without introducing ex-
ploring instruments that may disturb the flow,
and can be arranged to give almost instan-
taneous records thus avoiding difficulties
that may arise, if the flow is unsteady, from
the comparatively long response times of
other methods of observation.

We are here concerned with the use of
optical methods in routine wind tunnel experi-
ments; although much of the material is
applicable, we do not deal explicitly with
the uses of the techniques in flight or in
ballistic ranges or shock tubes. For details
of these applications the reader is referred
to Refs. 7 to 21. Also, no account is given
of the uses of the techniques for the study of
specialized problems (e.g. combustion prob-
lems and turbulence measurements) many of
which are adequately covered in Ref. 22.