Non-spherical symmetry development of underwater shock waves created by laser-induced breakdown

As a controllable alternative to cavitation collapse-induced shock waves,numerous cavitation studies on laser-induced breakdown have been carried out in hydromechanics.When the laser focusing region is not spherical,the shock waves caused by laser breakdown also exhibit non-spherical symmetry propagation.Recently,some researchers have proposed the linear superposition theory based on the far field measurement data to explain this asymmetry,assuming that it is essentially the linear superposition of multiple wave fronts caused by multiple points of laser-induced breakdown that leads to the asymmetric propagation of shock waves.In this study,measurements of shock wave propagation processes with different breakdown energies are carried out based on a nanosecond resolution photogrammetry system,and the propagation velocities of shock waves in different directions are directly measured using a double exposure technique on a single frame.In the experiment,the velocity of the shock wave at the beginning of the breakdown was measured up to nearly 4000 m/s.The early shock wave front was ellipsoidal,and the propagation velocity in the laser incident direction was generally slower than that in the perpendicular direction,decaying to the speed of sound in water within 1000 ns after the breakdown,and the wave front gradually approached to a circle.The variability of the shock wave front pressure ratio in the laser propagation direction and the vertical direction implies that the linear superposition theory applicable to the far field is not applicable to the near field.There may be more complex mechanism for the near-field shock wave propagation process.

Study of the influence of added water and contained water on structural vibrations and acoustic radiation using different dynamic modeling methods

Using the 3-D sono-elasticity method and the simplified nonstructural mass method,the different dynamic modeling methods of the added water for a single-hull structure are first analyzed in this study.Then,the complete internal flow field method and the simplified nonstructural mass method of the contained water between the double hulls of a double-hull structure are investigated.Finally,based on the calculation and analysis under multiple conditions,a reasonable and simplified dynamic modeling method of added water and contained water is obtained.It is indicated that the mass of added water for a single-hull structure is closely related to the mass of total underwater displacement of the structure.With the increase in the analysis frequency,the mass of added water is characterized by first decreasing rapidly and then decreasing gradually and smoothly.The contained water between the double hulls is distributed to the pressure hull and the light shell based on the ratio of the impedances of the double hulls.The results can basically reflect the acoustic radiation characteristics of the double-hull structure.

An experimental study on the impact behavior of cavitation inside tip clearance of a hydrofoil

Tip clearance cavitation is one of the most common cavitation phenomena exist on duct propellers,pumps and some hydraulic turbines,which may lead to erosion of the components.Due to the influence of the nearby wall,cavitation inside the tip clearance is more complicated than other cases without interaction.So far,the understanding about the impact mechanism of tip clearance cavitation is still limited.In this paper,to obtain the impact behavior of tip clearance cavitation,a high-speed camera was used to capture the cavitation behavior inside the tip clearance of a hydrofoil,and surface paint coating peeling method was applied to show the impact region.Results indicated that cavitation around the tip of the hydrofoil was composed of a tip separation cavity and a tip leakage vortex cavity,and the one with contribution to impact was the tip separation cavity.Through the comprehensive analysis of the paint peeling region and dynamic behavior of tip separation cavity,the impact was found to be related to the local collapse and rebound of the cloud cavitation shed from the attached part.In addition,the influence of tip clearance size on the behavior of tip clearance cavitation was also investigated.As the tip clearance size increased,the tip separation cavity tended to transfer from sheet cavitation to vortex cavitation.These findings can provide a sound basis for evaluating the erosion risk arising from the tip clearance cavitation.