Numerical Simulations of Unsteady Flow Fields around Falling Graupel, Hailstones and Snowflakes

When ice hydrometers are falling in air, they produce flow fields around them. These flow fields influence many important cloud microphysical processes such as the diffusion and collision growth of ice hydrometeors. The flow fields for small ice particles are relatively simple whereas that for large ice particles can be quite complicated. Most of the previous studies of the flow fields around falling ice particles are limited to steady state flow around highly symmetrically-shaped particles. The flow fields around large falling ice particles, such as graupel, hailstones, and snowflakes are unsteady and tend to be turbulent. Except a few simple cases, such flow fields have not been reported mainly due to their complexity.

In this paper, we report the results of our recent study of the unsteady flow fields past large ice particles, including conical graupel, hailstones and snowflakes. We performed computer simulations of these flow fields using the CFD software ANSYS to numerically solve the unsteady Navier-Stokes equations for the Reynolds number range (from a few hundreds for snowflakes to ~ 200,000 for large hailstones) relevant to these ice hydrometeors. For the case of graupel and hailstones where the horizontal translational motion is not pronounced, the particle is assumed to be fixed in position and orientation although the flow is unsteady. For the case of snowflakes where horizontal translation and oscillation may be important, we used the dynamic mesh technique to simulate their zigzag motions. We will show videos of the simulated flow fields during the talk and explain their characteristics.

To illustrate the impact of these flow fields on the cloud microphysical processes, we will show some preliminary results of the ventilation coefficients and collision efficiencies based on these computations as opposed to pervious results based on steady state calculations.