Experimental and numerical study of noise generation caused by acoustic resonance in a cavitating orifice

In order to study cavitation-induced noise generation in a hydraulic system, cavitation is generated at a planar orifice. The sudden condensation (implosion) of vapor results in shock waves which excite the connected hydraulic pipes with pressure fluctuations. The synchronization between the continuous condensation and evaporation process and shock wave reflection can result in a standing wave with super elevated pressure amplitudes. The fluid-borne sound is transferred through the mechanical structure into the air, where it can be perceived as a distracting whistling noise. In addition, CFD simulations of the test rig are used to investigate the cause-effect relationship of the whistling noise. With regards to representative operating points, the numerical results confirm the measured shedding vapor frequency, particularly the pressure pulsation frequency in the pipe and its amplitude. The conjunction of the experimental and numerical investigations provides the following findings: By reducing the discharge pressure the void volume increases which leads to a reduction of the resonance frequency of the pipe downstream of the orifice. For large void volumes, the pressure wave reflection at the void volume can be identified in the amplitude spectrum. The whistling noise depends on the history of the flow field. So, the increase and the reduction of the discharge pressure level leads to different whistling ranges. The whistling range decreases with the increasing length of the pipe resonator. Besides that, the order of the dominant resonance frequency increases. The experimental and numerical results indicate that the whistling noise only occurs when the jet downstream of the orifice and hence the vapor is perpendicular to the sound propagation in the pipe resonator.