Determination of the shedding frequency of cavitation cloud in a submerged cavitation jet based on high-speed photography images

Abstract

To accurately determine the shedding frequency of the cavitation cloud in a submerged cavitation jet, the spectral analysis and the proper orthogonal decomposition (POD) for high-speed photography images are performed. The spectrums of 6 different kinds of image signals (the area-averaged gray level, the line-averaged gray level, the point gray level, the cavitation length, width, and area) are calculated and compared. The line-averaged gray level is found to be optimal in determining the shedding frequency but an accurate frequency can only be obtained in the stable-frequency zone where the cavitation cloud sheds. In repeated experiments, the plateau-shape distribution of the main frequency is established with a deviation of 10.8%. A revised Reynolds number Re′ is defined and the shedding frequency can be correlated to Re′ by a power law when the cavitation number is less than 0.02. This relationship is validated by the experimental data in literature. The first mode of the POD characterizes the ensemble-average of the cavitation cloud while the second mode is the major part of the cavitation cloud transient components. The modes 2–5 are organized in pairs, which confirms the periodic feature of the cavitation cloud in the submerged cavitation jet. Near the nozzle exit, the modes 2–5 are symmetrically distributed in the jet shear layer. The shedding frequency of the cloud cavitation can also be precisely determined by performing the spectral analysis of the weighting coefficients of the mode 2. This paper shows that the two parameters, namely, the line-averaged gray level and the weighting coefficients of the mode 2, can be confidently used to calculate the shedding frequency of the cavitation cloud in a submerged cavitation jet.