Wall-pressure fluctuations in attached compressible turbulent cavitating flows

Abstract

Unsteady wall-pressure fluctuations are of great interest to gain insight into the structures and physical mechanisms of the turbulence flows that generate the pressure fields. Wall-pressure fluctuations are investigated in high Reynolds number cavitating flows in a wide range of cavitation regimes, with the aim of providing a quantitative understanding of cavitation-induced wall-pressure fluctuations and their scaling behaviors. The experiment is conducted at a high-speed water tunnel with a backward-facing wedge test model, and a simultaneous sampling technique is adopted to synchronize the transient cavity behaviors by high-speed imaging and cavitation-induced wall-pressure fluctuations using four flush-mounted unsteady pressure transducers beneath attached cavities. The results show that with decreasing σ, transient cavity length oscillations increase with cavity regimes changing from a stable inception cavity and an intermittent sheet cavity to an unsteady quasiperiodic cloud cavity, while the time-averaged cavity structures under different cavity regimes present self-similarity. The evolution of mean cavity metrics (i.e., Lc, Ltc, tc, and θ) is examined. Inspired by this geometrical self-similarity, statistics (i.e., root mean square pressure level, probability density function, pressure spectra, convective velocity) of wall-pressure fluctuations both inside and outside the attached cavitation are examined. Specifically, the root mean square value of wall-pressure fluctuations approaches its maximum near the cavity closure, and its amplitude is independent of the cavitation number. The probability density function (PDF) shape presents non-Gaussian and asymmetric behaviors. Generally, the PDF shape is positively skewed at the cavity leading edge, approaching Gaussian behaviors near cavity closure and slightly negatively skewed outside the cavity. Spectral analysis indicates that the scaling regions obtained by fast Fourier transform are usually classified as (1) low-frequency range; (2) mid-frequency range, spectra typically show the f-2 behavior; (3) transition range, with a f-7/3 relation; and (4) high-frequency range, with a f-3.2 relation. Remarkably, the Reynolds number effects significantly enhance the non-Gaussian and asymmetric behaviors of wall-pressure fluctuation PDFs. Our study can help to improve both cavitation modeling and hydraulic designs.