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Edgetones and Acoustic Resonances in a Duct


Aaron, Kim Maynard (1985) Edgetones and Acoustic Resonances in a Duct. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bfy8-1995.


Undesirable sound generation in the combustion chamber of segmented solid propellant rocket motors has been attributed to vortex shedding from obstructions that are uncovered as the propellant burns back. This phenomenon has been investigated experimentally and the mechanism explained.

A pair of aluminum baffles within a lucite duct through which air is drawn models the important aspects which enable the sound generation mechanism to operate. The baffles form an edgetone system which interacts with the longitudinal acoustic modes of the chamber. Acoustic tones occur spontaneously, at frequencies determined by the acoustic resonances, when the spacing between the baffles satisfies certain criteria.

Flow visualization using smoke and a strobe light triggered by the pressure oscillations indicate that vortex shedding occurs at the first baffle in phase with the acoustic velocity oscillations there. The interaction of these vortices with the downstream baffle drives the acoustic resonance which, in turn, triggers the formation of new vortices at the upstream separation point.

The phase relations for this feedback to operate require that there be close to an integral number of wavelengths, or vortices, from the separation point to the impingement point.

A model has been developed which predicts the experimentally observed behaviour well. Pressure amplitudes are predicted within an order of magnitude. Mean flow rates and baffle spacings yielding maximum response are determined correctly by the model.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Aeronautics
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Aeronautics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Culick, Fred E. C.
Thesis Committee:
  • Kubota, Toshi (chair)
  • Marble, Frank E.
  • Roshko, Anatol
  • Acosta, Allan J.
  • Culick, Fred E. C.
Defense Date:24 May 1985
Funding AgencyGrant Number
Air Force Office of Scientific Research (AFOSR)AFOSR-84-0286
Record Number:CaltechETD:etd-01222007-092606
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:274
Deposited By: Imported from ETD-db
Deposited On:01 Feb 2007
Last Modified:16 Apr 2021 22:52

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