Direct Numerical Simulation of Water Droplets in Turbulent Flow

Details on the fall speeds of raindrops are essential in both applications and natural events, such as rain-rate retrieval and soil erosion. Here, we examine the influence of turbulence on the terminal velocity of two water drops of different sizes. For the first time, computations of droplets in turbulent surroundings are conducted with a direct numerical simulation code based on a volume of fluid method. Both the drop surface deformation and internal circulation are captured. The turbulence intensity at the inflow area, as well as the turbulence length scale are varied. In turbulent flow, we find a decline in the terminal velocities for both drops. Based on the study of the wake flow characteristics and drop surface deformation, the decrease in the terminal velocity is found to be directly linked to a shortening of the wake recirculation region resulting from an earlier and more drastic increase in the turbulence kinetic energy in the shear layer. The turbulent surroundings trigger substantial rises in the drop axis ratio amplitude and a slight increase in the drop oscillation frequency, but barely influence the time-averaged drop axis length.