Systematic acoustic loading of an aeroacoustic whistle

Abstract

Strong self-sustained acoustic oscillations may occur in a gas pipe network under certain gas flow velocities within the network. The pipe network under consideration consists of a main pipe, with a variable mean airflow, with two closed coaxial side branches of variable but equal length joined to the main pipe at right angles. Coupling between resonant acoustic standing waves and instabilities of the shear layers separating the flow in the main pipe from the stagnant gas in the closed side branches leads to strong acoustic oscillations (easily 10% of the mean pressure) at a frequency corresponding to the half-wavelength acoustic mode defined by the total side-branch length. The acoustic power available in the resonant mode has been measured to compare with theoretical predictions and previous two-microphone measurement techniques as well as to inspire confidence when designing an aeroacoustically driven thermoacoustic heat pump. The acoustic load consists of a variable acoustic resistance and coupled acoustic compliant volume, which is similar to devices used to measure power in thermoacoustic engines. Data will be presented which characterize the performance of the aeroacoustic sound source under variable acoustic loading. [Work supported by Shell International Exploration and Production B.V.]