Abstract
The excitation of pressure fluctuations in a model jet-driven Helmholtz oscillator is experimentally investigated. An axisymmetric channel consisted of a cylindrical chamber with a nozzle in the front cover and an outlet opening in the back one. Optimal geometric dimensions of the chamber, nozzle, and outlet were chosen to achieve the greatest amplitude of the pressure fluctuations in the chamber. The nozzle length-to-diameter ratio ln/dn varied in the 0.77 ≤ ln/dn ≤ 4.17 range. The cylindrical chamber length Lch determining the air jet length ljet in the interval between the covers and Lch/dn = 0.5–3.5. The outlet opening diameter in the back cover dout varied within the limits dout/dn = 1–2.5. The optimal nozzle length ln/dn, the corresponding chamber length Lch/dn, and the outlet diameter dout/dn are determined. The air jet development within the nozzle and the formation of an inverse flow zone between the nozzle wall and the jet periphery and a chain of vortex structures in the mixing layer are considered. The generation of a jet tone of the opening on the frequency of the feedback in the jet and a family of acoustic modes on the resonance frequency with a smooth increase in the jet velocity from 0 to 70 m/s is studied.
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