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Highlight Lecture

Variations in cavitation bubble dynamics and their effects on the resulting surface indentations on commercially pure Aluminium

Thursday (27.09.2018)
17:30 - 17:45 S1/01 - A5
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Cavitation is the formation and collapse of gas bubbles in liquids caused by pressure changes. This phenomenon is observed under high flow velocities or in turbulent flow. Damage of technical surfaces caused by cavitation is an important engineering issue. However, difficulties are encountered to date in fundamentally modelling and predicting such material damage. When a bubble forms close to a solid surface, its collapse will not occur point-symmetrically. Instead, what takes place is a complex sequence of liquid-jet formation, movement of the bubble, its repeated collapse, re-expansion, and partitioning into several bubbles, and the formation of vortices. These processes depend on the diameter and the distance of the bubble to the solid surface. The latter is referred to as dimensionless distance gamma, which is the distance between the solid surface and the bubble centre divided by the maximum bubble radius. For fundamental investigations of cavitation, single bubbles can be created and studied, for example by focusing a short, high-power laser pulse within water. When the material is soft, the bubble’s collapse can indent a nearby solid surface in a single event. However, all metrics of the surface damage, like depth and width, at each specific gamma show pronounced scatter.

In this study, single bubbles of 3 mm diameter are created within water using a Q-switched Nd:YAG laser at gamma = 0.9, 1.0, 1.2, 1.4, and 1.8 near a sample of 99.5 wt% Al. Each cavitation event is recorded by means of a high speed camera. The resulting damage on the Al surface is quantified using white-light confocal microscopy. The dynamics of “typical” bubbles, correlating with indentations of average dimensions, as well as those corresponding to outliers of very small or very large indentations, are analysed. It is found that in particular the water jet forming during the first bubble collapse is sensitive to asymmetric initial boundary conditions. These are caused e.g. by the close-by sample edge or small, pre-existing bubbles on the sample surface. While the average of indentation dimensions varies with gamma, with a maximum at gamma = 1 as expected from the literature, indentation depths up to about 3 µm were also found for all gamma values. The results of this study show possible variants of the bubble collapse dynamics, revealing the range of non-uniformity that must be expected in fluid-flow induced cavitation in technical applications.

Speaker:
Dr. Stefanie Hanke
University of Duisburg-Essen
Additional Authors:
  • Hemant J. Sagar
    University of Duisburg-Essen
  • Chaojie Feng
    University of Duisburg-Essen
  • Martin Underberg
    University of Duisburg-Essen
  • Prof. Ould el Moctar
    University of Duisburg-Essen
  • Prof. Sebastian A. Kaiser
    University of Duisburg-Essen