An investigation has been conducted of the velocity coupling phenomenon reported in acoustically unstable solid propellant rocket motors. An innovative simulation facility has been built using solid carbon dioxide as the simulated propellant. The use of dry ice allows us to focus directly on the fluid mechanical aspect of the problem. Dry ice is contained in a two-meter long chamber. The acoustic disturbance in the chamber is introduced by means of a mechanically driven piston. Mass flow rate and acoustic pressure measurements indicate the existence of a coupling mechanism, other than pressure coupling, between the acoustic disturbance and dry ice sublimation process. Flow visualization using a high speed camera shows a high degree of mixing, near resonance, within the flow above the dry ice surface. The flow over the dry ice is very chaotic, probably turbulent. Hot film anemometry measurements show a broad-band spectrum at low frequencies with strong peaks at the driving frequency and its harmonics near resonance. A broad band spectrum is generally taken as an indication of turbulent flow. Acoustically induced turbulent forced convection is believed to be responsible for the increase in the sublimation rate of the dry ice (simulated burning of the propellant). We believe that turbulence is one of the principal mechanisms in the velocity coupling phenomenon.

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