Experimental and numerical investigation of noise generation due to acoustic resonance in a cavitating valve

Abstract

Abstract 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 stationary wave with superelevated 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. A high-speed camera is used to visualize the void volume. Dynamic pressure signals are recorded in the vicinity of the orifice to analyze the pressure pulsation. The operating point is varied, the orifice geometry is changed and the reflection properties of the pipe on the up- and downstream sides are changed too. In addition, CFD (Computational Fluid Dynamics) simulations of the test bench are used to investigate the root cause 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.