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In this paper, simultaneous impact of two parallel drops on a thin liquid film is investigated using the lattice Boltzmann method. The purpose of this study is to investigate the effects of surface tension (characterized by Weber number), distance between two drops, and gas kinematic viscosity on the impact. The developed numerical model in this paper which is based on the Shan and Chen pseudo-potential two-phase model makes it possible to access large density ratios, low viscosities, and tunable values of surface tension independent of the density ratio. The model is validated by comparing the coexistence densities with those of Maxwell analytical solution, evaluating the Laplace law for a droplet, and simulating single droplet impact on a thin liquid film. Simulation results of two drops simultaneous impact show that after impact, two jets raised between the drops join each other and form a central jet. Height of this jet increases with time leading to separation of secondary droplets from its tip. When the surface tension value is decreased, the central jet height is increased, but size of the separated droplets is reduced. The crown shape observed in single drop impact is also seen in simultaneous impact of two drops. Increasing distance between two drops leads to a smaller central jet height and an increase in the crown radius. The crown height, however, was found to be independent of the distance. Finally, increasing gas kinematic viscosity reduces the central jet rising speed and delays separation of secondary droplets from the jet.

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The growing and diverse applications of low Reynolds number operating vehicles impose the need for their accurate study. Optimization is an important part of computational science that can improve the performance and increase the efficiency of the initial geometry. most of the research studies on aerodynamic optimization were focused on high Reynolds number airfoils. But for aerodynamic devices that have small dimensions, like MAVs, usually the flow speed is low and thus the unsteady effects caused by boundary layer separation cannot be neglected. In this article, oscillating airfoil with pitching motion in turbulent and low Reynolds flow has been optimized with the continuous adjoint method. Drag to lift ratio was chosen to be the objective function and free form deformation parameters is adopted for the surface geometry perturbations. Since aerodynamic optimization generally consists of two parts, first solving the flow equation and then computing the gradient of the objective function, in this article in order to evaluate the accuracy of the optimization process both has been validated. The results show that the adjoint equation converges well and with specifying the suitable constraints, the designed shape approaches to the most optimized level without the loss of performance.