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National Advisory Committee for Aeronautics, Report - A Theory for Stability and Buzz Pulsation Amplitude in Ram Jets and an Experimental Investigation Including Scale Effects

A theory based on quasi-one-dimensional flow has been
developed for determining factors aflecting stable flow through
ram jets with supersonic difiusers. Stable flow for such ram
jets is shown to depend upon the instantaneous values of mass
flow and total pressure recovery of the supersonic diffuser
and immediate neighboring subsonic difi’user. Conditions for
stable and unstable flow are considered. The theory is found to
be in agreement with the experimental data of NAOA TN
3506‘ and NAOA RM L50K30. This theory indicates that
the model assumed in the resonator analysis of NAOA TN
3506 may be considered a rough approximation to the actual
phenomena when applied for purposes of obtaining the general
trends and orders of magnitude of frequency and amplitude of
oscillation provided that the wave length of the highest frequency
component of the oscillation be much larger than the length
of the ram jet. The resonator analysis is not applicable to a
determination of the initial instability of the inlet without
combustion and should not be so applied to obtain such
stability criteria.

A simple theory for predicting the approximate amplitude of
small pressure pulsation in terms of mass-flow decrement from
minimum-stable mass flow was developed and found to agree
with experimental data.

Cold-flow tests at a Mach number of 1.94 of ram-jet models
having scale factors of 3.15:1 and Reynolds number ratios of
4.75 :1 with several supersonic difiuser configurations showed
only small variations in performance between geometrically
similar models. The predominant variation in steadyzflow
performance resulted from the larger boundary layer in the
combustion chamber of the low Reynolds number models.
The conditions at which buzz originated were nearly the same
for the same supersonic difl'user (cowling—position angle)
configurations in both large- and small-diameter models.
There was no appreciable variation in stability limits of any
of the models when the combustion—chamber length was in-
creased by a factor of three. The unsteady-flow performance
and wazw patterns were also similar when considered on a
reduced-frequency basis determined from the relative lengths of
the model. The negligible eject of Reynolds number on side
bility of the ofi—design conflguratiom was not anticipated in
view of the importance of boundary layer to stability, and this
result should not be construed to be generally applicable.