Short cylindrical nozzles in a jet-driven Helmholtz oscillator

Abstract

The present paper elaborates on the experimental study of jet-excited pressure fluctuations in a Helmholtz oscillator model with two openings in a cylindrical cavity. The length of the cylindrical nozzle in the front cover ℓN normalized by the nozzle diameter dN was ℓN/dN = 0.125, 0.33, 0.47, and 0.67. The diameter of the outlet opening in the back cover dOUT was in the range dOUT/dN = 1–2.5. The length of the cylindrical cavity LCH determined the jet length LJET in the spacing between the covers, LCH/dN = 0.5–3.5. The amplitude–frequency spectra were studied when the oscillator configuration was changed in the indicated intervals. From the generation amplitude, the best ratio of the sizes of the nozzle, chamber, and outlet was determined. The appearance of the jet tone of the hole and the alternation of acoustic modes were observed with a smooth increase in the Reynolds number to ∼8 · 104. The measurements showed very high amplitude of pressure fluctuations in an oscillator with a short nozzle and a short chamber at a significant jet velocity. A slight increase in the length of the chamber led to a rapid decrease in the generation amplitude. It is determined that the tone frequency is usually much lower than the resonance frequency in the chamber. Moreover, the tone frequency gradually increases with increasing jet velocity, while the resonance frequency remains unchanged, close to the natural frequency of the cavity chamber.