The effect of droplet addition on flow structure in the boundary layer of a flat plate

In this study, the turbulent droplet-laden boundary layer over a flat plate was investigated using large eddy simulation. A two-way coupled Eulerian–Lagrange point particle method was used for droplet particles. The effect of droplet addition on flow loss and the underlying mechanism of droplet modulation on the boundary layer coherent structure in the near wall turbulence were explored. It was found that the droplet-laden boundary layer exhibits an increase in displacement and momentum thickness, resulting from an increased velocity gradient on the plate surface. As a result, the flow loss rises while the shape factor drops. The underlying mechanism of droplet modulation lies in the variations in turbulent structures and energy transfer budget. It was observed that the coherent structure changes from sparsely distributed larger vortices in the unladen flow to densely packed smaller vortices in the droplet-laden flow. The presence of droplets accounts for the intensified stretching and fragmentation of large-scale vortices into smaller scale vortices, which then leads to an increase in the streamwise vorticity and enhanced velocity fluctuations in the spanwise and vertical directions. Furthermore, in the streamwise direction, the fluctuating velocity decreases due to the obstruction caused by the low-velocity droplets. The total turbulent kinetic energy is suppressed by the presence of droplets, and the suppression effect is enhanced with the droplet size. Additionally, the current simulation results exhibit ambiguous vortex periodicity in the spanwise direction. Thus, for periodic boundary conditions, a large-span configuration is recommended.