# A Review on the Erosion Mechanism in Cavitating Jets and Their Industrial Applications
## 2 Cavitation Mechanism and Flow Dynamics in Cavitating Jets
### Cavitating Jet Flow Dynamics  
Recently, [PERSON] et al. [33] compared their numerical predictions of the cavitation cloud in a cavitating jet to their high-speed flow visualizations. They found almost zero vapor volume fraction in the central region of the jet near the nozzle exit and the presence of a ring-type cavitation zone in the high-speed shear layer, around the central zone. They also observed the formation of concentrated vortices similar to those found by other experimental investigations [34,35]. These vortices grow as they travel downstream before they shed off from the main cloud, then they collapse and diffuse. The authors distinguished three different zones along the jet, namely, growing, shedding, and collapsing zones (Figure 3). A good agreement was found between their numerical and experimental results. It was suggested that the shedding process greatly enhances the performance of the cavitating jet for cleaning applications and rock breaking, since the collapsing of the bubbles happens just after it sheds off the central cloud zone [36]. The cavitation bubbles with smaller diameter diffuse further downstream and the vapor volume fraction decreases despite the jet expansion. Figure 3: Vapor volume fraction in submerged cavitation jet; numerical prediction and flow visualization [33]. (**a**) growing, (**b**) shed-ding, (**c**) collapsing. [PERSON] et al. [33] also studied the effect of the diverging angle of the nozzle exit on the flow dynamics using the RANS RNG k-\(\epsilon\) turbulence model. The vapor volume fraction distribution showed (Figure 4) that the cavitation cloud is larger for a 60\({}^{\circ}\) angle than for 40\({}^{\circ}\) and 80\({}^{\circ}\). When the divergent angle is reduced (close to 0\({}^{\circ}\)), the fluid resistance to flow across the nozzle increases and thus, the jet velocity is reduced when the upstream pressure is kept constant. The shear stress that affects cavitation inception and its development is reduced with smaller velocity gradient. Using high-speed flow visualizations, [PERSON] et al. [20] distinguished three regions in the cavitating jet flow: near jet exit, pulse formation, and collapsing zone. The new pulse formation corresponds to the shedding location of [PERSON] et al. 's study [33]. They also studied the effect of the Reynolds number on the cloud propagation distance and pulsation frequency. They found a 30% increase in the cloud propagation distance when the jet diameter increased from 1.0 to 2.0 mm. The propagation distance also increases with the Reynolds number, while the frequency of the cavitating jet instability decreases when the Reynolds number increases. [PERSON] et al. [37] studied the role played by cavitation in the initial stage of a rectangular jet break-up and vaporization using LES simulations that includes a non-condensable gas component in a thermodynamic equilibrium cavitation model. The authors found that the cavitation promotes the jet break-up in the lateral direction. They suggested that, in addition to the Kelvin-Helmholtz instabilities, three main mechanisms are responsible for the jet break-up: (1) turbulent fluctuations due to the cavitation bubbles collapse near the nozzle exit, (2) entrainment of surrounding gas into the jet flow, and (3) collapsing events near the liquid-gas interface of the jet. It was suggested that the collapsing events interface are the main mechanisms in inducing momentum away from the jet axis, which results in the initiation of the jet break-up. [PERSON] et al. [38] studied the flow dynamics, pressure oscillation, and acoustic shock wave characteristics of a self-excited oscillating water jet using Particle Image Velocimetry (PIV) and unsteady pressure measurements. They found a peak pressure 2.5 times larger in the self-excited jet as compared to an unexcited water jet.