## 1 Introduction  
Marine fouling is a serious problem in ship operations. The marine fouling adhering to the hull can increase navigation resistance and fuel consumption, up to 40% [1]. Submerged cavitation water jet technology is one of the cleaning methods that can be applied in the underwater cleaning of the ship hull. Submerged water jet refers to the water jet generated where the working and the surrounding media are the same. The high-speed water jet formed by the nozzle under water interacts with the low-speed water of the surrounding medium, generating strong disturbances, and forming periodic vortex rings on the periphery of the jet. Both the low-pressure region of the vortex center and the structural characteristics of the cavitation nozzle can provide favorable conditions for the occurrence and development of cavitation [2]. When the local static pressure is lower than the saturated vapor pressure, cavitation bubbles are generated. These bubbles are entrained into the jet, developing, gathering, and shedding as the jet moves. When cavitation bubbles reach the high-pressure area near the target surface, bubbles will collapse and form a \"micro-jet\" with releasing huge energy, which can cause erosion damage to the target surface. Because the action time is extremely short (in the order of microseconds) and a lot of micro-jets discontinuously and repeatedly impact the target, the cavitation erosion is severe. So the submerged water jet usually belongs to the cavitation water jet. In recent years, cavitation water jet technology has been widely used in the fields of rock breaking [3; 4; 5; 6], cleaning [7; 8], processing [9] and so on. The nozzle is a core device for generating water jets. Under different working conditions, water jets have different characteristics and erosion performance. In order to achieve a satisfactory result in underwater cleaning operations, erosion effect of the nozzle under different working conditions can be predicted and analyzed, so as to formulate an appropriate cleaning scheme according to the actual target situation, and realize real-time adjustment and control of the cleaning process. Because the process of water jet erosion is very complicated, in which continuous impact and cavitation erosion coexist, and the time of cavitation erosion is very short, it is difficult to directly measure the impact strength. The erosion effect of the nozzle is often reflected by the damage parameter of the impacted target in the water jet erosion test. The existing experimental researches [10; 11; 12; 13; 14] mostly studied the impact and cavitation effect of submerged water jets on the surfaces of pure copper, copper alloys, aluminum alloys, ti alloys and other metal materials at the jet pressure in the range of 100-200 MPa, which belongs to high-pressure conditions. However, the jet pressure in our project is lower than 10 MPa, which is under low-pressure conditions. Therefore, there is no obvious damage to the metal surface, and the erosion effect of the nozzle is difficult to measure. In order to overcome the difficulty in obtaining fouling samples, according to the simulated fouling method [7], refractory bricks are used instead of marine fouling as targets for erosion prediction research under low pressure. Refractory bricks are also brittle materials, have similar structural strength to certain marine fouling, and resist high temperatures and water. In addition, refractory bricks show good damage characteristics to water jet erosion. Under low pressure, they will neither be excessively fragmented nor fail to produce obvious damage. There are obvious differences in damage under different working conditions. Therefore, obvious damage morphology on refractory bricks can be obtained to measure damage parameters as experimental data, which has been verified by [PERSON] et al. [15] to be feasible under low-pressure conditions.