Analysis of the Effects of Nozzle Geometry on the Cavitation Water Jet Flow Field Using Orthogonal Decomposition

This study focuses on analysing the influence of nozzle geometry on the structure of a cavitation cloud. The differences among the cavitation clouds of cylindrical, organ pipe, and angular nozzles were studied through high-speed visualization. The proper orthogonal decomposition (POD) method was used to investigate the different morphologies and distributions of the cavitation cloud. The line-average greyscale and the spectral analysis of the weighting coefficients of modes 1 and 3 are used to evaluate the shedding frequency of the cavitation cloud. The results show that the nozzle geometry affects cavitation performance. The existence of large-scale structures is related to the concentration of the cavitation clouds. In contrast, the small-scale structure is the main factor affecting the instability of the cavitation cloud shedding frequency. At P = 5 MPa, the mode of the angular nozzle shows many small-scale structures, so its cavitation cloud is shorter and thinner. The modes of the cylindrical and organ pipe nozzles show many large-scale structures; therefore, the cavitation cloud is wider and longer. The spectral analysis of the weighted average coefficient of mode 1 is the best way to identify the shedding frequency of the cavitation cloud. The cavitation cloud shedding frequencies of the cylindrical, organ pipe, and angular nozzles are 1357, 1755, and 1899 Hz, respectively, which is consistent with the shedding period obtained through image analysis. With the pressure increasing, the length of the cavitation cloud increased, and the shedding period was slightly extended. Compared to the other two types of nozzles, the cavitation performance of the angular nozzle is high at pressure of 10 MPa.