Direct Computation of a Hole-Tone Feedback System at Very Low Mach Numbers

Abstract

This study is concerned with an investigation into the sound produced when a jet, issued from a circular nozzle or hole in a plate, goes through a similar hole in a second plate. The sound, known as a hole-tone, is encountered in many practical engineering situation. Direct computations and experiments of a hole-tone feedback system are conducted. The mean velocities of the air-jet are 8 and 10 m/s in the computations, and 6-13 m/s in the experiments. The diameters of the nozzle and the end plate hole are both 50 mm, and the impingement length between the nozzle and the end plate is 50 mm. The computational results agree well with the experimental data in terms of the qualitative vortical structures and the relationship between the most dominant hole-tone peak frequency and the jet speed. Based on the computational results of the air-jet speed of 10 m/s, the shear-layer impingement on the hole edge, the resultant propagation of pressure waves, the associated vortical structures and a newly proposed feedback mechanism is discussed. As far as the authors know this is the first direct computation of the hole-tone feedback system that predicts its dominant frequency successfully. Non-axisymmetry observed in the present computation is also shown.