# Research on Cavitation Characteristics of Two-Throat Nozzle Submerged Jet
###### Abstract  
Ship fouling not only increases ship resistance and fuel consumption but is equally a type of biological invasion, which causes severe ecological damage. Submerged cavitation jet cleaning is an environmentally friendly, high-efficiency, and energy-saving cleaning method. The nozzle structure has an essential influence on the cleaning effect. Thus, a two-throat nozzle was designed for application in submerged cavitation jet cleaning. To investigate the cavitation characteristics of the two-throat nozzle, a high-speed photographic visualization experiment and an erosion experiment concerning the submerged cavitation jet were carried out in this study. The frame-difference method (FDM) was used to analyze the dynamic changes in the cavitation cloud in a single period. The dynamic changes in the cavitation cloud and the characteristics of the submerged cavitation jet were investigated under different inlet pressures. The sample mass loss and the macroscopic and microscopic changes in surface morphology were used to evaluate the cavitation intensity of the two-throat nozzle submerged jet. The experimental results demonstrate that the two-throat nozzle has a good cavitation effect, and the cavitation cloud of the submerged jet has obvious periodicity. With the increase in inlet pressure, the length, width, and area of the cavitation cloud continue to increase, and the shedding frequency of the cavitation cloud continues to decrease. The intensity of cavitation erosion is related to target distance and impact time. There is an appropriate target distance by which to achieve the optimal cavitation effect. The collapse of cavitation bubbles near the sample surface is related to the erosion distribution on the sample surface. Moreover, the magnitude of the absolute values of the root-mean-square surface roughness and surface skewness increase with cavitation intensity. The results in this paper are helpful for a better understanding of the cavitation characteristics of the two-throat nozzle submerged jet. two-throat nozzle; cavitation jet; high-speed photographic visualization; frame-difference method; cavitation cloud +
Footnote †: journal: Medicalcan reach 70-180 m/s, the hitting pressure can reach 140-170 MPa, and the resonance frequency is about 100-1000 times/(s-cm\({}^{2}\)) [4]. Moreover, cavitation jet cleaning has the characteristics of environmental protection, high efficiency, energy-saving properties, and high safety. It is widely used in underwater cleaning [5], cutting [6], material testing [7], underground drilling [8], and deep-sea mining [9], etc. According to previous studies, the performance of a submerged cavitation jet is closely related to the cavitation intensity. In order to improve the erosion ability of the cavitation jet, it is necessary to enhance the cavitation intensity. The cavitation erosion performance of cavitation jets is influenced by several factors, such as working conditions (upstream and downstream pressure, cavitation number), the temperature of the working fluid, the geometric parameters of the test material (offset and angle of attack), and the structural parameters of the nozzle [10; 11; 12; 13]. To enhance the cavitation performance, many researchers have investigated the growth, development, and collapse mechanisms of cavitation bubbles and the characteristics of the cavitation clouds. [PERSON] et al. [14] conducted a visualization study of cavitation jets of different types of nozzles (conical, cylindrical, and trumpet) using high-speed photography technology. It was found that the shedding frequency of the cavitation cloud decreases with the increase in inlet pressure and the maximum length of the cavitation cloud. [PERSON] et al. [15] found that the cavitation cloud motion morphology of the cavitating jet varied with the shape and size of the nozzle. [PERSON] et al. [16] investigated the structure of the cavitation clouds and the erosion effect on aluminum foils during the collapse of vacuolated clouds by visualization experiments.