The submerged cavitation jet is suitable for ocean engineering activities such as ship fouling cleaning, organic wastewater treatment, offshore oil drilling, and natural gas hydrate extraction due to its superior hydraulic performance and erosion capacity. As an intuitive analysis method, image processing is widely used to investigate the characteristics of submerged cavitation jets. However, due to the lack of quantitative evaluation of the cavitation cloud in image processing, it is difficult to establish the relationship between cavitation cloud image and cavitation performance. Therefore, a novel image processing method based on dimensionless grayscale intensity is proposed in this paper. This method was used under different sample spaces to obtain the maximum mass loss of the sample. The results showed that the method could accurately calculate the maximum mass loss of the sample based on the image processing results. When the sample space is 200 images and the working pressure is 20 MPa, the calculation error of the image processing method for the maximum mass loss of the sample is 1.26%. For the sample spaces of 10–5000 images, the maximum calculation error of the image processing method for the maximum mass loss of the samples is 3.29%. The image processing method proposed in this paper establishes the relationship between the cavitation cloud image and the maximum mass loss of the samples, which provides help for further understanding and application of submerged cavitation jets.

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.

In order to improve the cavitation performance of the self-excited oscillation nozzle (SEON), a novel SEON with a circular arc curve chamber was designed by changing the chamber wall profile of the SEON. The performance of the circular arc curve chamber SEON was studied numerically. Taking the vapor volume distribution and the vapor volume fraction as the evaluation indexes, the influences of the chamber wall profile on the cavitation performance of the circular arc curve chamber SEON were analyzed. In addition, it was compared with the broken-line chamber SEON. The numerical results show that the cavitation performance of the circular arc curve chamber SEON is first enhanced and then weakened by increasing the circular arc radius. The circular arc curve chamber structure can form a larger central cavitation volume in the nozzle, which improves the cavitation performance of the SEON. When the circular arc radius is 2 mm, the cavitation area and the turbulent kinetic energy of the circular arc curve chamber SEON increase by 122.5% and 16.9%.