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National Advisory Committe for Aeronautcs, Research Memorandum - Investigation of Shock Diffusers at Mach Numbers 1.85 - I - Projecting Single Shock Cones

In an investigation conducted in the Cleveland 18- by 18-inch
supersonic tunnel to determine design conditions for optimum perform-
ance of shock diffusers results were obtained at a Mach number of 1.85
with a series of projecting single-shock cones having angles of 20°,
30°, 40°, 50°, 60 , and 70 . Each cone was tested with a curved and
with a straight diffuser-inlet section. The variation of total
pressure recovery with tip projection and outlet area was investigated
for each cone to determine optimum contraction ratios and shock
locations. The effect of angle of attack was also investigated for
several configurations.

The maximum total-pressure recovery was obtained with the_50°
cone using a straight inlet. At an angle of attack of 0°, an outlet
total pressure of 92.2 percent of the free-stream value was attained.
At an angle of attack of 5°, this value was reduced to 90.8 percent
of the free-stream value. These total-pressure recoveries correspond
to efficiencies of kinetic-energy conversion of 96.6 and 95.6 percent,
respectively. Several other configurations gave total—pressure
recoveries greater than 90 percent at an angle of attack of 0°.

In many tests, particularly with the larger cone angles, the
total-pressure recovery in the vicinity of the maximum recovery was
insensitive to changes in outlet area. The highest total-pressure
recoveries were obtained with subsonic entrance flow.

For efficient conversion of the kinetic energy of a supersonic
air stream into ram pressure, the flow must be decelerated to low
supersonic Mach numbers before the normal shock occurs. Deceleration
may be accomplished with small total-pressure less by contracting
the flow in a converging channel or by locating one or more oblique
shocks ahead of the diffuser inlet. With the first method, the ammunt
of deceleration allowable before the occurrence of the normal shock
is limited because the normal shock will not enter the diffuser when
the contraction ratio of the convergent channel is great enough to
accelerate the subsonic flow behind the normal shock to sonic velocity.
(See reference 1.) With the second method (that is, with a shock
diffuser) no such theoretical limitation exists. The supersonic
stream may be theoreticalhy reduced to sonic velocity with negligible
total-predsure loss if a sufficient number of oblique shocks of small
intensity can be located ahead of the diffuser inlet.

Experiments with shock diffusers have been conducted by Oswatitsch
(references 2 and 5), who determined the performance of shock diffusers
having several types of projecting cone and several diffuser-inlet
designs. One of these configurations yielded efficiencies greater
than the theoretical maximum attainable with convergent— divergent dif-
fusers at the same Mach numbers.

An investigation is being conducted in the Cleveland lB— by 18—inch
supersonic tunnel to'deterndne the effect_on the performance of shock
diffusers of varying the.form of.the projecting cones, the contraction
ratios, and the inlet design; The results obtained with a series of
single-shock cones in combination with a straight and with a curved
inlet section are presented in this report. The effect of angle of
attack was also investigated for several configurations.