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National Advisory Committee for Aeronautics, Technical Notes - The Design, Operation, and Uses of the Water Channel as an Instrument for the Investigation of Compressible Flow Phenomena

The results of several years' experience in the operation of a
small water channel have shown that the hydraulic analogy can be used
successfully to demonstrate many two—dimensional compressible-flow
phenomena. Attempts to use the 20—inch—wide water channel at the
Langley Laboratory for research projects, however, led to difficulty in
the interpretation of the data with respect to practical flight problems
because of the low value of the Reynolds number in the water channel.

A channel permitting Reynolds numbers as large as 3,000,000 at tunnel
choking would have to be approximately 10 feet wide if the water temper—
ature were 200° F or 20 feet wide if the water temperature were 100° F.

The problem of maintaining a stable stream velocity has been solved.
by using a weir to control the total head and a variable—width Laval
nozzle to control the mass flow through the channel. This system was
capable of holding the stream velocity constant within one—half of l per-
cent for an indefinite period.

The water channel is recommended as an effective low-cost demon—
stration instrument for use in the teaching of aerodynamic compressible
flow and as an instrument for quickly checking new ideas.

The application of the hydraulic analogy tothe investigation of
compressible—flow phenomena has previously been discussed in reference 1.
Since the publication of that paper, a number of modifications have been
made on the water channel in operation at the Langley 8—foot high—speed
tunnel which have definitely increased the ease of operation of the
channel and the accuracy of the data obtained therefrom. The principles
involved in these changes are presented along with a number of suggestions
concerning the design of such a channel and its uses.

The auxiliary apparatus, such as depth—survey systems, stream—
velocity controls, nanometers, and equipment for flow observation by
optical methods, is considered in detail because the more important
advances in the water channel have been improvements in the auxiliary
apparatus. Suggestions are also included that concern the operation of
a water channel, the adjustment-of the test=section floor shape.in order
to obtain a flow in the test section free from velocity gradients, the
computation of data,'and precautions necessary for proper interpretation
of the data obtained.