Hole tone generation from highly choked jets

Abstract

The hole tone generated from the passage of underexpanded jets through a circular hole in a rigid plate and the secondary sound wave emission associated with the strong hole tone generation were investigated experimentally. Flow visualization shows that when the jet passes through a hole whose diameter is less than the nozzle diameter and interacts with the plate around the hole, two immediately succeeding strong hole tones having partially plane and spherical wave fronts are radiated in the upstream direction of the plate. A particular flow condition, where the pressure ratio R=3.90, the nozzle-to-plate distance h/d=2.40, and the hole diameter D/d=1.00, was investigated in detail. In addition to the radiation of two succeeding strong hole tones in the upstream direction, four strong secondary sound waves are radiated downstream of the plate during one cycle of the hole tone generation. Flow visualization confirmed that the six sound waves found for this flow condition are generated by two different generation mechanisms: (a) upstream of the plate, a feedback mechanism consisting of upstream-propagating sound waves in the ambient and downstream-convecting coherent vortical structures surrounding the jet; and (b) downstream of the plate, a radial expansion of the vortical structure due to the interaction with the hole and merging of two succeeding vortices. It is found that the shapes of wave fronts are changed by the refraction of the sound wave due to the vortex-induced flow near the jet boundary. The directionality of the far sound field is consistent with the result of flow visualization.