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National Advisory Committee for Aeronautics, Technical Notes - Chocking of a Subsonic Induction Tunnel by the Flow from an Induction Nozzle

A decrease in Mach number with increase in induction—jet pressure
has been shown to occur experimentally in one type of induction tunnel
at induction-jet pressures in excess of the lowest value required to
attain the highest subsonic Mach number. This performance character-
istic was analytically demonstrated to be associated with choking of
the induced flow by the flow from the induction nozzle. The analysis
provides a means of predicting the occurrence of induction choking so
that it can be adequately considered in design. Use of an auxiliary
induction nozzle operating as a choker is suggested as a means of
controlling the speed of a tunnel.

Considerable work has been done both experimentally and analyti—
cally toward the development of induction tunnels (refs. 1 to h). In
the previous work, no unusual performance characteristic or reversal
of expected performance was reported. Recently, however, during tests
to determine the performance of the 2h-inch induction tunnel at the
Picatinny Arsenal, Ordnance Corps, Dover, N. J., a decrease in stream
Mach number was observed to occur as the pressure of the inducing air
in the induction nozzle was increased in the high—subsonic speed range.
This unusual performance was definitely established as a distinct opera-
tional characteristic of the tunnel and it has been analytically studied.

The purpose of the present paper is to present the expermmental data
demonstrating this unusual and undesirable performance and to present
an analytical method for predicting its occurrence. This information
supplements the material presented on the development and performance
of an induction tunnel in reference 3.