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Effect of hydrodynamic pressures near solid surfaces in the incubation stage of cavitation erosion

Ultrasonic vibration cavitation erosion experiments were performed to study the erosion on a steel specimen at different pitch angles. Experimental results show that the number and size of the erosion pits decrease as the pitch angle increases. This result is thought to be related to the hydrodynamic pressures generated near the solid surface. Numerical analyses from the Reynolds equation prove that the hydrodynamic pressures are generated in the fluid film when a bubble is moving towards the surface of the specimen. Simulation of the bubble collapse process shows that the hydrodynamic pressures shorten the collapse time of a bubble and strengthen the microjet at the moment of bubble collapse. As the pitch angle of the specimen surface increases, the squeeze pressure due to normal velocity decreases, and the increased dynamic pressure due to tangential velocity cannot compensate for the loss of the squeeze pressure. Thus, the total hydrodynamic pressure decreases, and a decrement in the number and the size of the erosion pits occurs.