The Fluid Mechanics Of High-Speed Liquid/Solid Impact

Abstract

The research described in this dissertation includes the design and development of a gas gun to produce controlled solid against liquid impacts. Such impacts are relevant to important practical problems such as rain erosion damage of aircraft materials, steam turbine blade erosion and cavitation erosion. A technique has been developed whereby a rectangular-bore gas gun projects a slider at high velocity (up to 300 m s- 1) against a two-dimensional liquid target held between transparent plates. The liquid targets used are liquid/gel layers ( 95i water, 5% gelatine) which allow any chosen shape to be cut from the sheet and placed between the transparent plates. The impact is viewed at high magnification using high-speed photography at microsecond framing rates and the shock waves are visualised using schlieren photography. The great advantage of the essentially two-dimensional configuration is that processes occurring inside the drop can be observed (for example shock waves, cavitation effects etc ... ) without the problems of refraction inherent for a spherical liquid drop. It is also possible to study the growth of the liquid/solid contact periphery and study phenomena such as the onset of jetting in the gap between liquid and solid. However, a disadvantage of impact with a drop both from the viewpoints of analysis and experimental interpretation is that there is a constantly changing angle between the liquid and solid surfaces. For this reason, wedge geometries are used for which the contact angle is constant. This allows detailed study of the conditions for the commencement of jetting. Lesser ( 1981) has made theoretical predictions of these conditions and a major part of the work described is aimed at testing this theory. Lesser has also analysed the effect of target compliance on the start of jetting and the experimental evidence presented confirms these predictions. The experimental results are also compared with theoretical predictions of Finnstrom and Lesser (1983) for the velocity of jets produced in the gap between an impacting solid and wedges of various geometries. Another application of the technique has been to jet formation in cavity collapse. This occurs when the cavity is either collapsed by a shock wave or collapses under hydrostatic pressure near a solid surface. A single cavity or array of cavities, is formed in the gelatine layer and an advantage of the technique is that the cavity size and spacing can be accurately controlled. This provides a convenient arrangement for studying the details of collapse and interaction effects that occur in an array of cavities. For example, it is possible to observe how the collapse of one cavity can trigger off the collapse of a neighbouring one which has been shielded from the initial shock wave.