Transport of Inertial Droplets in a Turbulent Boundary Layer*

The horizontal transport of inertial droplets in the presence of a turbulent boundary layer is studied experimentally in a wind tunnel. Water droplets with a wide range of initial conditions (diameter, speed, and injection angle) are injected into the boundary layer at free-stream velocities ranging from U_∞ = 2.4 to 6.6 m/s with a boundary layer thickness ranging from δ = 11.4 to 12.5 cm. The number, size, and speed of droplets is measured using a cinematic in-line holographic system operating at 1250 Hz and positioned 162 cm downstream of the nozzle at several wall-normal locations, approximately coverning the thickness of the boundary layer. Using the experimental data, the dominant transport mechanisms for droplets are evaluated. The dominant transport mechanism for high Stokes number droplets (St_η >> 1) is inertia, where the initial conditions of these droplets plays a key role in determining their likelihood of transport. In contrast, the dominant transport mechanism for low Stokes number droplets  (St_η << 1) is found to be droplet interactions with turbulence. A ballistic transport model which includes the effects of droplet inertia and a turbulence is presented to predict the probability of droplet transport.