# Numerical Simulation of Random Cavitation Suppression Based on Variable NACA Airfoils
## 1 Introduction  
Cavitation flow is a complex multiphase turbulence, including phase transition, flow separation and multi-scale vortex structure. Cavitation often occurs at the low-pressure surface of a flow passage component (such as the suction surface of a pump or the lifting surface of marine propeller [1, 2, 3]). It is the main cause of performance deterioration, vibration, noise, and even surface erosion of underwater equipment [4, 5, 6].  
In recent years, with the rapid development of computers, a numerical simulation method has provided important assistance for complex experiments, which has become the key technology for researchers to use to analyze flow fields. CFD technology has become very common to study the inherent laws and mechanism of fluid movement. [PERSON] [7] analyzed the time-averaged velocity field and fluctuations of annular jet flow, which proved that the re-circulation region was accompanied by low-frequency oscillations. [PERSON] [8] carried out numerical simulation of hydrofoil flow; he used a finite mass transfer model of cavitation to capture the condensation shock wave, and the three-dimensional process of the shock wave could be clearly observed in the simulation. [PERSON] and [PERSON] [9] used compressible large eddy simulation (LES) to simulate cavitation on the same geometry of [PERSON] et al. [10]. They found that the results were in good agreement with the experimental results, which confirmed the existence of bubble shock propagation and the effect of compressibility. [PERSON] [11] developed a numerical method to research the tip vortex cavitation and flow noise.  
In terms of cavitation inhibition, many scholars inhibit cavitation by adding inducers [12], modifying blade shape, size [13, 14], and other special methods, such as blade gap jet [15, 16, 17], protruding modification [18, 19], microtexture surface modification [20], and so on. In addition, scholars have obtained quantitative data about cavitation and turbulence through experiments and high-speed camera observations [21, 22, 23, 24, 25]. For example, [PERSON] [26] researched the effects of the outlet pressure, the operating temperature, and the installation angle of the two cages on the cavitation and found out the best method to suppress cavitation. [PERSON] [27] researched the physical characteristics of cavitation initiation, the rotational motion of the tip vortex cavitation flow, and the characteristics of induced noise during vortex development. [PERSON] [28] observed the whole process of cavitation flow, vortex cavitation, tip leakage vortex cavitation, and void formation in a turbine pump inducer. On the basis of the study of blade shape control, and referring to the blade control technology studied by predecessors, a novel NACA airfoil with a covering is proposed to improve the cavitation performance. This new cavitation method can effectively block the re-entry jet to improve the flow field around the cover, and it restrain the development of the low-pressure area. Additionally, it is a good reference for other hydraulic machinery to suppress cavitation.