Optimum placement of Helmholtz resonators for damping pressure oscillations.
Abstract
Cold flow and hot firing tests were conducted to find
the optimum design and positioning of an acoustic resonator for
damping high intensity pressure oscillations. The variables in
the cold flow tests were the position of the resonator and its
volume. In the hot firings more than one resonator was used and
the gas properties were also varying. Pressure node or antinode
locations of the resonator were emphasized in the tests and the
resonator cavity volume was varied from fully closed to three
and a half times the resonant volume calculated by the Helmholtz
resonator theory. It was found that the resonator at the pressure node
provided little or no damping while when positioned at the pressure
antinode the sound intensity in the cold flow duct was reduced
to less than 50% of the undamped amplitude. The range of
damping was found to be narrow and centered around the volume calculated
by the Helmholtz resonator theory. The results of the
phase lag between the pressures in the cavity and duct and the
velocity in the orifice, connecting the cavity and the tube closely
corresponded to those predicted by Sirignano in Reference 14. An
investigation of the effective length of the orifice in the Helmholtz
resonator indicated that a single end correction was appropriate
(rather than the double end correction for conventional
acoustic amplitudes) - this also substantiated the jet flow model.
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