Amplitude of Supersonic Diffuser Flow Pulsations Page: 3 of 25
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2 NACA IRM E52124
Certain resonance criterions which reasonably predicted both the
conditions for the occurrence of diffuser flow pulsations and the fre-
quency of pulsation were evaluated assuming that a ram-jet engine acts,
in effect, as a Helmholtz resonator (reference 1). It was found that a
diffuser would resonate if the subcritical diffuser characteristic had
a positive slope of sufficient magnitude. At conditions where the
slope of this curve was less than the critical value, the ram-jet engine
would not resonate. A similar criterion for the occurrence of diffuser
pulsation was suggested from analyses presented in references 2 and 3.
CA
The occurrence of flow pulsation in external-compression diffusers
is related in reference 4 to the velocity discontinuity or vortex sheet
downstream of the intersection of two shock waves. Flow pulsations
start when this vortex sheet just enters or leaves the diffuser cowl-
ing. The entrance of the vortex sheet into the diffuser cowling marks
a change in the slope of the diffuser characteristic which arises from
the entrance of increasing percentages of low-energy mass flows. The
amount of change in diffuser characteristic required to induce flow
pulsations in any ram-jet engine may be readily determined from the
theoretical criterions of reference 1.
Although certain theoretical criterions as to the nature of dif-
fuser pulsation have been evaluated, no theoretical evaluation of the
variables affecting amplitude has been developed. This report, pre-
pared at the NACA Lewis laboratory, presents a theoretical evaluation
of the variables which govern pulsation amplitude for the cold-flow
engine. The theoretical method employed is a refinement of the
diffuser-pulsation theory presented in reference 1. The method of
reference 1 assumed small amplitudes of flow disturbances in order
that quantities such as mass flow and the slope of the diffuser char-
acteristic could be considered constant. When large finite amplitudes
are allowed, these quantities must be recomputed in a point-to-point
calculation throughout a cycle of oscillation. Several numerical com-
putations which illustrate the nonlinear solutions for pulsation ampli-
tudes and frequencies as a function of diffuser mass-flow ratio and
geometry are included herein. Experimental data are presented for
comparison with the theoretical trends where possible.
SYMBOLS
The following symbols and definitions were used in this report:
A area, sq ft
a velocity of sound, ft/secf frequency, cycles/sec
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Sterbentz, William H. & Davids, Joseph. Amplitude of Supersonic Diffuser Flow Pulsations, report, December 16, 1952; (https://digital.library.unt.edu/ark:/67531/metadc65164/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.