Investigation of drop and spray impingement on a thin liquid layer accounting for the wall film topology

The single drop impingement processes upon steady and wavy liquid surface films of finite thickness and the spray impingement process onto rigid walls was studied both experimentally, as well as computationally and analytically. The global aim of the present study was to develop a mathematical model of the single drop and spray impingement process onto liquid wall films. To achieve this aim, the focus was put in particular on the description of the hydrodynamics of the wall surface film produced by spray impingement onto the wall, on a broader understanding of the physics involved in modeling of spray impingement processes and on the formulation of mathematical models based on experimental data and numerical simulations of the single drop impingement. Two separate approaches were applied to obtain the necessary experimental and numerical results. The first approach focused on the single drop impingement process upon steady and wavy liquid films of finite thickness, to understand in detail the physics behind the splash mechanism, the corona formation, the evolution of the cavity below the liquid surface and the typical time and length scales of the impingement process. It was observed that for the single drop impingement onto steady wall films the penetration velocity of the cavity was constant and independent of all the investigated parameters, resulting in a direct relation between the time at which the cavity reached the bottom of the liquid film and the film thickness. A thicker film and/or a lower Weber number led to a lower radial expansion of the cavity, due to the larger surface tension forces acting on the cavity, resulting in a lower maximum diameter of the cavity. A faster receding and retraction of the cavity was observed for lower Weber numbers, but the time instant of retraction was independent of the film thickness. The study of the impingement of single drops onto solitary surface waves led to several important outcomes for spray impingement. It was found that the inclination and propagation of the surface film had a significant effect on the impingement outcomes, in particular on the time evolution of the shape of the cavity. The solitary surface wave introduced a relative velocity component upon impingement, which, together with the velocity distribution inside the liquid film, resulted in a clear increasing inclination of the cavity with time. Depending on the phase of the wave at impingement, a weak or strong interaction of one side of the expanding rim with the wave surface was observed, leading to an asymmetrical expansion of the cavity. The formation, strength and motion of the capillary waves changed magnificently for different phases of the wave, where the presence or absence of the capillary waves at the left and/or right side of the cavity, in combination with the unequal distribution of the surface tension forces over the cavity surface, induced an asymmetrical receding and retraction of the cavity, as well as an inclined Worthington jet. With these results mathematical models of the impingement processes could be developed and implemented into the numerical codes for single drop and spray impingement. The second approach was the direct measurement of the velocity distributions inside the wall film, produced by the impingement of an ultrasonic spray onto this rigid wall. A volumetric micro-Particle-Image-Velocimetry technique was developed to obtain the distributions of the mean and fluctuating three-dimensional film velocities under bad light conditions. By measuring the wall film velocity distributions under realistic conditions, not only the unsteady wall film flow, generated by the surrounding drop impingements, was taken into account, but also the interactions of drops with other impinging drops, both in the spray and during the interaction with the wall film. It was found that the swirling motion of the spray was transferred directly to the liquid film in the form of large vortices inside the liquid film. Furthermore, it was shown that for larger volume flows the areas with relatively high mean radial film velocities, as well as the values of the maximum absolute film velocities, increased