1063-7842/00/4512- $20.00 © 2000 MAIK “Nauka/Interperiodica” 1529 Technical Physics, Vol. 45, No. 12, 2000, pp. 1529–1537. Translated from Zhurnal Tekhnicheskoœ Fiziki, Vol. 70, No. 12, 2000, pp. 18–27. Original Russian Text Copyright © 2000 by Chizhov, Schmidt. INTRODUCTION An impact of a liquid drop on an obstacle at a veloc- ity comparable to the sound speed in the liquid gener- ates shock and expansion waves interacting with each other and with free surfaces, the evolution of a cumula- tive jet, and the formation and collapse of cavitation bubbles. Therefore, the phenomenon of high-velocity drop impact is closely related to many fundamental problems in mechanics of continua and physics of strength. This phenomenon is also of interest in applied problems, such as the erosion action of liquid–gas streams, coating application, mining art, working of hard materials, cleaning of surfaces, production of new materials, astrophysics problems, etc. [1–6]. Comprehensive reviews on impacts of drops on obstacles can be found in [1, 5, 7–9]. However, phe- nomenon of high velocity has not been adequately stud- ied even qualitatively. The objective of this paper is to describe the detailed pattern of a drop impact on an absolutely rigid surface and on a thin liquid layer under conditions at which liquid compressibility shows up most vividly, that is, at impact velocities comparable to the sound speed in the liquid. Emphasis will be to the effect of liquid viscosity and surface tension, interac- tion between impact products and ambient gas, mecha- nism of initiation and disruption of the cumulative jet, jet velocity evaluation, formation mechanism of cavita- tion cavities, loads experienced by the obstacle, differ- ences between plane and axisymmetric impacts, change in the flow structure upon striking the liquid layer, and reasons for experimental spread and discrep- ancy between analytical and experimental estimates. DROP IMPACT PATTERN Generally, drop–obstacle interaction produces a liq- uid flow with the well-developed wave structure and heavily deformable free surface (Fig. 1). One interesting feature of a convex-drop impact is that, at the initial impact stage, the free surface, which does not contact the rigid one, does not deform. The compression region of the drop is bounded by a shock wave adjacent to the contact domain boundary (see Fig. 1a). This is because the velocity of motion of the contact boundary V E = V 0 (t) (where V 0 is the ini- tial velocity of the drop and β(t) is the angle between the free surface and the rigid wall), being infinitely large at the contact instant t = 0, decreases, remaining greater than the shock wave velocity up to a certain instant t c . Therefore, disturbances propagating from the wall have no time to interact with the free surface. The compression of the liquid is maximal at the contact periphery and continues to grow with time. At the critical instant t c , the shock wave leaves the contact boundary and interacts with the free surface to form a shock wave in the ambient gas and an expansion wave propagating inside the drop. The free surface starts to deform and a near-wall high-velocity cumula- tive jet is formed (Fig. 1b). The time of jet formation depends on viscous and surface effects in the liquid near the wall, and the jet velocity far exceeds the impact one. When the shock wave in the drop approaches its top, the expansion wave, following the shock wave, causes the formation of a toroidal cavitation area whose cross section is shown in Fig. 1c. At the final stage of interac- tion, the expansion wave collapses at the symmetry axis, and a vast cavitation area with the greatest rarefac- tion near the axis is formed (Fig. 1d). As the expansion wave propagates toward the rigid surface, the cavitation area occupies nearly the whole drop excepting a thin layer near the free surface and the near-wall jet region. The instabilities develop, the liquid coating breaks down, and the drop takes a shape of a crown and disin- tegrates into small fragments [10]. β cot GASES AND LIQUIDS Impact of a High-Velocity Drop on an Obstacle V. Chizhov and A. A. Schmidt Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia E-mail: Anton.Chizhov@pop.ioffe.rssi.ru Received March 1, 2000 Abstract—Processes arising when a high-velocity liquid drop strikes a rigid obstacle or a liquid layer were investigated using numerical simulation. The flow pattern being formed features a complicated interaction of compression shock and expansion waves between each other and with free surfaces, the initiation of a cumula- tive jet flow, and the formation of cavitation areas. Factors governing the interaction process are analyzed. Obtained results are compared with experimental data. © 2000 MAIK “Nauka/Interperiodica”.