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Numerical and experimental investigation of laser-induced cavitation bubbles and induced damage

Sagar, Hemant

Cavitation has shown its destructive effects on hydro-machinery and has been investigated since last century. Cavitation induced damages have not been very well understood in bulk cavitation. For a better understanding of the cavitation and induced damage, investigations of a single bubble collapse are useful and logical. These investigations allow describing and quantifying the physics of the cavitation bubble collapse near a solid surface and its interaction with the surface material. The generation of such single bubbles in the water using an electric spark or focused laser beam are well-known experimental methods, which were used in the past and systematic investigations of bubble dynamics near surfaces exist. However, only a few studies addressed the material damage induced by a single bubble collapse. For fluids and materials engineer, quantification of series of physical tests are necessary to have a better idea up to what extent a tiny cavitation bubble can severely induce damage on metals surface. Therefore, the objective of this dissertation is an experimental and numerical investigation of the collapse of a single collapsing cavitation bubble and induced damage on the aluminum surface. This dissertation presents experiments generating single cavitation bubbles with 3 mm diameter in the water. Cavitation bubbles were generated at various defined relative wall distances to the polished commercially pure aluminum specimen surface. The relative wall distance is a ratio of the distance between the bubble enter to a maximum bubble radius. The collapse of each laser-induced cavitation bubble was captured using a high-speed camera by the backillumination method. A detailed quantitative analysis of the surface damage was performed using 3D profilometry. A single cavitation bubble collapse results in a shallow pit with an average depth of 1 - 3 micrometer. The overall statistics of damage parameters such as pit depth, width and volume found consistent with previous investigations. Some damages obtained were correlated with the corresponding bubble collapse sequences. The image sequences also helped to identify effects of the sample surface edge on collapsing dynamics. Further, numerical investigations of the three-dimensional flow surrounding a collapsing laserinduced cavitation bubble with an initial radius of 1.45 mm were performed using two different numerical methods. The three-dimensional flow was captured by solving the Navier-Stokes equations. First method accounts for the multiphase flow (water and vapor), the Volume of Fluid (VoF) method was used. The source term of the phase transport equation of the VoF function was based on the Schnerr-Sauer cavitation model developed using the simplified Rayleigh-Plesset equation. The relative wall distance was varied ranging from 0.3 to 3.0. Computed collapse derivatives, impact velocity, impact pressure, and shape of the bubble, such as toroidal shapes of oval impacts agreed favorably to the experimental measurements. In addition, numerical investigations were performed using the compressible, two-phase Navier-Stokes equations in an Euler-Euler approach with barotropic equations of state. The bubble collapses were compared with the experiments for shapes and collapsing times. The obtained flow characteristics near the surface, such as impact velocities and pressures were also discussed. Further, a microscopic bubble collapse near the surface was also investigated to compute collapse induced wall shear rate and flow around the collapsing bubble. The results of numerical simulations were compared with the existing experimental data. The comparisons have shown a good qualitative and quantitative agreement. Overall, this work gives broad insight towards the investigation of cavitation bubble and induced damage.

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Sagar, Hemant: Numerical and experimental investigation of laser-induced cavitation bubbles and induced damage. 2018.

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