Hydrodynamic Phenomena During High-Speed Collision Between Liquid Droplet and Rigid Plane

Abstract

The dynamics of high-speed impact between a compressible water droplet and a rigid solid surface is investigated analytically. The purpose of the study is to examine the mechanism leading to the erosion of a material due to liquid impingement. A Compressible-Cell-and-Marker (ComCAM) numerical method is developed to solve the differential equations governing the unsteady, two-dimensional liquid-solid impact phenomena. The method is designed to solve this unsteady portion up until the flow reasonably approaches the steady-state solution. The validity of the method is confirmed by comparing its numerical results with the idealized exact solution for the classical one-dimensional liquid impact problem. The accuracy of the numerical reresults is found to be very good in that only slight numerical oscillations occur. Viscosity and surface tension are neglected as seems resaonable with the relatively large drops and high velocities considered. Pressure and velocity distributions are solved as a function of time. The deformation of a drop is also recorded for three different shapes: cylindrical, spherical, and a combination of the two, which may more closely model the actual droplet shapes to be encountered in such impacts. Typical liquid impact Mach numbers of 0.2 and 0.5 (sonic velocity referred to water) were studied. Thus impact velocities of about 980 and 2450 fps are considered. Compression predominates during the early stages of the impact, while rarefaction governs later, during which time the radial lateral flow velocity exceeds the initial impact velocity. The reflection of compression waves and the lateral flow leads to the possibility of cavitation within the drop, due to the consequent generation of negative pressures, exists. The maximum pressure calculated in this two-dimensional liquid impact problem is found to be less than the one-dimensional maximum pressure for all three different droplets in various degrees. It is found that droplet shape impact angle and liquid impact Mach number are the only important parameters of the problem for a flat fully-rigid target surface. As more time elapses, i.e., up to 2–3 μsec for a 2.0 mm-dia drop, the maximum pressure shifts from the center of the contact area radially outward, while the pressure at the center attenuates rapidly toward conventional stagnation pressure.