## 1 Introduction  
Cavitation is a complex fluid dynamic phenomenon involving multi-phase variation, turbulence, and compressibility [1; 2]. At a certain temperature, when the local pressure inside a fluid falls below the saturated vapor pressure at that temperature, the microbubbles formed within the fluid are called cavitation bubbles [3]. The process from the inception to the development and ultimately the explosion of these cavitation bubbles is known as cavitation [4] in which an enormous amount of energy is generated and released, leading to corrosion and rusting of machine surfaces. The rational utilization of the energy brought about by the collapse of cavitation bubbles can be used in the field of organic matter degradation, wastewater treatment, sterilization, and toxicity, etc. [5]. Moreover, since it is a single physical movement inside the hydrodynamic cavitation generator, there will be no secondary pollution without the addition of other chemical reagents, which contributes its promising application. Common modes of cavitation generation include ultrasonic cavitation and hydrodynamic cavitation. Ultrasonic cavitation uses ultrasonic vibration to cause local pressure fluctuations. When the ultrasonic energy is high enough to make the local pressure vibration drop below the saturated vapor pressure, cavitation will occur [6]. Since the ultrasonic cavitation effect occurs only in the pressure fluctuation near the surface, the efficiency of wastewater treatment in cavitation is low. The hydrodynamic cavitation creates a low-pressure, high-flow velocity state on the pipe through which the liquid passes to form cavitation [7]. The different shapes of the fluid domain will affect the velocity of the fluid,so the structure of the hydrodynamic cavitation generator will have a great impact on the cavitation performance. There are many types of hydrodynamic cavitation generators, such as venture-type [8], orifice plate-type, vortex-type, rotary-type hydrodynamic cavitation generator, and so on. Among them, venture-type and orifice plate-type hydrodynamic cavitation generators have been widely studied due to their simple structure, and their basic principles are to reduce the fluid pressure by applying a constriction in the path of the moving fluid. The structural parameters of the Venturi tube were investigated using numerical simulation techniques [9; 10]. It was found that when the throat diameter was slowly increased from 10 mm to 16 mm with same structural parameters, the gas content ratio increased and then decreased with the increase in the throat diameter. When the throat diameter reaches 14 mm, the gas content reaches the maximum value. If the throat diameter is too small, cavitation bubbles are not easily generated. If the throat diameter is too large, the pressure will be lower, resulting in lower cavitation performance. The effects of back pressure, number of holes, hole diameter, and aspect ratio of hole plates on cavitation were investigated in a study conducted by [PERSON] et al. [11] and [PERSON] et al. [12] using a combination of experimental and numerical simulation methods. The critical cavitation number is found to increase with the increase in back pressure. The cavitation number is a dimensionless number that describes the degree of cavitation. When the cavitation number is greater than the critical cavitation, cavitation does not occur, and vice versa. Therefore, it could be concluded that cavitation does not occur easily when the pressure increases. The cavitation performance of the hole plate is influenced by parameters such as the number of holes, hole diameter, and aspect ratio. Additionally, the cavitation performance of multi-hole plates was found to exceed that of single-hole plates. Although the venturi and orifice plate hydrodynamic cavitation generators can achieve cavitation relatively easily, they are limited by parameters such as the opening rate and throat diameter ratio, resulting in the disadvantages of a small flow and difficulty in enlarging the structure size.