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National Advisory Committee for Aeronautics, Research Memorandum - The Use of Perforated Inlets for Efficient Supersonic Diffusion

Supersonic diffusion may be most easi y accomplished by
means of a normal shock. Associated with this discontinuous
process is a progressive decrease in the total-pressure recovery
ratio as the Mach number is increased. The losses in total pres—
sure across the shock may be minimized, however, by decelerating
the supersonic stream by means of stream contraction to a low
supersonic Mach number before the shock occurs.

The usable stream contraction and the amount of stream
deceleration is limited on some types of supersonic diffuser. If
the internal contraction ratio (the entrance area divided by the
throat area) of the diffuser is made too large, the entrance mass
flow will not pass through the throat of the diffuser, choking
will occur, and a normal shock and bow wave configuration will
form ahead of the inlet. The shock will not be swallowed again
by the diffuser until the internal contraction allows the subsonic
stream behind the normal shock to be accelerated to a Mach number
of unity at the throat. This value of the contraction ratio is
less than required for isentropic supersonic diffusion. In the
design of a supersonic diffuser, either a loss in total pressure
must therefore be accepted or some means must be provided to prevent
choking. '

One method to prevent choking that has been effectively applied
is to accomplish the supersonic diffusion ahead of the inlet (such
as on a conical shock diffuser). During the starting operation, the
subsonic mass flow behind the normal shock, which will not pass
through the throat of the-diffuser, spills over the edge (reference 1).
Investigations of shock or spike diffusers have been reported in
references 2 to 6. Although the pressure recoveries that may be
obtained with the spike diffusers are very satisfactory, the external
wave drag of this type diffuser is likely to be large unless the
position of the shocks is carefully controlled. Furthermore, the
high recoveries of total pressure may be obtained only on single
units because, if the diffuser is operated in cascade (such as on a
supersonic compressor) or if the diffuser is confined as a second
throat in a supersonic tunnel, the flow spillage that allows the
diffuser to start may be prevented.

The convergent-divergent type diffuser investigated by Kantrowitz
and Donaldson (reference 7) and by Wyatt and Hunczak (reference 8)
need not have a shock in the vicinity of the entrance. This diffuser
should therefore be less critical with respect to external wave drag
than the shock diffuser. Because no spillage is required for starting,
the diffuser may be operated at the design Mach number in cascade or
as the second throat of a supersonic wind tunnel.