Mechanism of Pipe-Tone Excitation by Flow through an Orifice in a Duct

Pipe-tone is excited by flow through a duct containing an orifice with square edges of thickness within a certain range. The mechanism of this process is investigated by time-resolved flow visualization with the orifice at the end of the duct. Vortices are primarily shed at the orifice when the acoustic pressure at that place reaches a maximum in time. Successive vortices shed during a cycle of the dominant pipe-tone mode and merge further downstream of the orifice. Under some conditions, the excitation of pipe-tone forces the jet issuing out of the orifice to fork into two trains, even for the ratio of the densities of the jet fluid to the ambient exceeding unity, unlike previously reported results. Within the range of the orifice thickness required for intense excitation of pipe-tone, the shear layer separating from the upstream edge of the orifice appears to re-attach at the downstream edge. Several mode shifts occur within the range of flow velocities tested when the orifice is placed within as well as at the end of the duct, with the amplitude maximized in the velocity range between consecutive shifts. Two modes are simultaneously present during the transition at relatively low amplitudes. Lesser number of shifts is observed when the orifice is progressively located upstream in the duct, but the simultaneous multiple-mode excitation spans a wider velocity range. The acoustic pressure amplitude of pipe-tone is found to increase linearly with the mean pressure in the duct.