Direct Numerical Simulations of Grouping Effects in Droplet Streams Using Different Boundary Conditions

The tendency of convening and coalescing of initially distant droplets, called droplet grouping, can influence the final droplet size distribution, evaporation rates or final settling points. To better understand the factors which govern droplet grouping mechanisms, reliable direct numerical simulations are beneficial. Previous numerical studies have shown that the approaching and collision process of droplets in a stream is extremely sensitive to the applied boundary conditions. Therefore, simulations are performed regarding the influence of domain size and boundary conditions on the grouping behavior. Improved boundary conditions, so called artificial boundary conditions, are derived to allow for suitable velocity approximations in the far field. A comparison to the existing boundary conditions regarding velocity profiles and grouping behavior for different domain widths and Reynolds numbers is investigated. The proposed artificial boundary condition takes into account the decay of velocities near the side boundaries more accurately than the already implemented boundary conditions in combination with periodicity in the direction of droplet motion. Evaluating droplet spacing over time reveals that the domain width using artificial boundaries can be reduced without influencing the grouping behavior.