Towards the Numerical Determination of the Splashing Threshold of Two-Component Drop Film Interactions

The scenario of an impacting drop onto a film is highly relevant in many natural and technical systems. A fundamental and often required parameter of these interactions is the so called splashing threshold above which secondary droplets are generated. For interactions with differing liquid properties for the film and the impacting drop a general splashing threshold is, however, still unknown because an experimental determination is difficult to achieve. For this reason, we investigate the suitability of a numerical determination of this threshold by means of direct numerical simulation using the multiphase flow solver Free Surface 3D (FS3D). Simulations across an already existing splashing threshold are performed stemming from an empirical correlation. In order to determine the necessary grid resolution for accurately reproducing the corresponding impact regime, all interactions are simulated for several grids. A detailed grid study shows that only by using very high grid resolutions the threshold can be reproduced with a sufficient accuracy, whereas the use of coarser resolutions leads to a significant underestimation of the threshold. Additionally, simulations of highly resolved impact phenomena on thin films depend heavily on the efficient solution of the problem with most of the computational costs affiliated to solving the Pressure Poisson Equation within the FS3D framework. Therefore, the implemented multigrid solver was optimized employing advanced tree structured communication during coarsening and refinement on the levels during the solution cycle. A performance analysis of FS3D using the original and the improved multigrid solver shows that the implemented tree structured communication leads to a remarkable speed-up.