# A Review on the Erosion Mechanism in Cavitating Jets and Their Industrial Applications
## 3 Erosion Mechanism in Cavitating Jets
### Effect of Standoff Distance  
In addition to geometrical parameters of the cavitating jet nozzle, the target standoff distance and the attack angle are always considered in cleaning, peening, and cutting applications. [PERSON] et al. [51] considered different standoff distances for erosion experiments with aluminum plates. Three types of nozzles were used in their study: conical, cylindrical, and venturi (horn) types, made of stainless steel SUS 304. All targets were placed normal to the jet nozzle, 30 MPa was used as the working pressure, and the duration of the experiment was 5 min. Three regions were defined for the target location, Region I: x/D = 5 (where x is the distance from nozzle exit to the target and D is nozzle diameter), Region II: x/D = 15, Region III: x/D = 100. The mass loss of the aluminum plate was recorded after the experiments. It was shown that the maximum erosion for all nozzles was achieved around x/D = 15 (Figure 4). It was thus suggested that Region II is optimal for cutting and Region III is optimal for peening. It can also be seen from Figure 5 that the erosion efficiency strongly depends on the nozzle geometry. For the same operating conditions, the cylindrical type resulted in significantly higher erosion rates as compared to the two other nozzles used in the experiments. In [52; 53], the effect of different geometrical parameters on the cavitation erosion was experimentally studied using copper samples exposed to a high-speed submerged cavitating jet for various durations. Erosion rates and eroded areas were discussed. Influences of the dimensionless standoff distance x/d (x is the distance to the target and d is the nozzle outlet diameter), the aspect ratio L/d (L is the nozzle length) and the angle of attack were investigated. Convergent nozzles were used for these experiments and the operating regimes are shown in Table 1. The jet nozzle geometry is shown along with Table 1 (Figure 5). The duration of each experiment was 30 min. Since the nozzles could not be moved, and in order to accommodate the ASTM G134 [54], increasing the specimen's thickness was used to set the standoff distance. It can be seen from Figure 6 that under the same operating conditions, the erosion rate is significantly affected by standoff distance and nozzle diameter. Therefore, the optimum position for maximum damage can be found and occurs close to the nozzle which is completely different in the case of conventional jets, where the position of the target is much less critical and can be much farther from the nozzle outlet. This can be explained by the dynamics of cavitation bubbles. [PERSON] and [PERSON] [55] developed a model of a cavitating jet and showed that the optimal distance is related to a point in the flow where the bubble has expanded to a maximum size prior to its collapse. The authors also showed  
Figure 4: Mass loss vs. Standoff distance [51]. Figure 5: Jet Nozzle geometry [52]. that the high speed of cavitation bubbles growth and the need for these bubbles to be at a maximum size prior to collapse determine the critical standoff distance for cavitating jets and therefore, the position for the maximum damage. The typical jet erosion pattern for different nozzle diameters is shown in Figure 7. It was also shown that the optimal standoff distance was a function of the cavitation number and that the cavitation efficiency increases with the upstream pressure and has a peak that changes with the downstream pressure for a cavitation number of 0.014. In order to investigate the relationship between cavitation cloud and cavitation erosion, [PERSON] et al. [56] conducted simultaneous high-speed flow visualizations and erosion tests. Proper Orthogonal Decomposition (POD) was used to analyze the spatiotemporal distributions of the cavitation cloud. In addition, both macroscopic and microscopic methods were implemented to study the morphology of the eroded specimen. The POD analysis showed that the low-order POD modes are organized in pairs, which confirms the periodicity of the cavitation phenomenon.