Dynamics of cavitation bubbles in compressible two-phase fluid flow

  • Dynamik von Kavitationsblasen im kompressiblen Zweiphasenströmung

Bachmann, Mathieu; Müller, Siegfried (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2012, 2013)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2012

Abstract

In liquid flow, the liquid vaporizes when the pressure drops below vapor pressure and cavities form. This phenomenon is called cavitation. Cavitation bubbles occur in several applications, e.g., at ship propellers, where the rotation of the blades accelerates the flow field and leads to a pressure drop behind them. These can collapse near rigid structures, emanating a strong shock wave and generating a high energy that can exceed the resistance of the nearby structure and, thus, cause material damage. In 1917, Lord Rayleigh conjectured that collapsing bubbles emanating pressure waves can cause material damage at ship propellers. This was confirmed later on by experiments with laser-induced cavitation bubbles. In particular, the formation of a liquid jet by collapsing bubbles near a rigid structure has been associated to its erosion. Although numerous experimental investigations have revealed several efects, their influence on material damage is still open due to limited measurement techniques. Here, numerical simulations can improve the understanding of the complex dynamics of cavitation bubbles because they provide insights in the multidimensional flow field in both the liquid and the gas. However, there are several dificulties to overcome arising in physical modeling, numerical discretization and experimental validation. Another problem is the lack of accurate initial data that cannot directly be determined from the experiments. Therefore a quantitative comparison of experimental and numerical results is hardly to achieve. The aim of the thesis is to provide an accurate description and prediction of the wave dynamics occurring in the flow field of collapsing cavitation bubbles. A better understanding of the process might give new insights in the causal connections with material damage that cannot be easily assessed by experiments. For the purpose of this thesis, an adaptive finite volume solver has been extended to liquid-gas applications in order to overcome the instabilities at the phase boundary. This solver is validated using exact solutions and experimental results. In particular, the quasi-one-dimensional symmetric collapse of a spherical cavitation bubble in a free field and the one-dimensional interaction of a shock wave with a bubble are considered. For comparison with experiments appropriate initial data are needed. For this purpose a strategy is developed in order to determine this state. The validated code is applied to the investigation of lithotripter shock waves interacting with a collapsing bubble and the asymmetric collapse of a gas bubble near a solid wall. The results are compared with experimental results that have been performed for the case of laser-induced cavitation bubbles at the university of Göttingen: (i) the shape of the bubble and the propagation of waves, (ii) the direction of the flow and its velocity in the surrounding liquid flow field and (iii) the pressure away from the bubble provided by high-speed photography, particle-image velocimetry (PIV) and pressure measurements, respectively.

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