THE SHAPE AND AERODYNAMICS OF LARGE RAINDROPS

Abstract

The physical factors which might be expected to control the shape of large raindrops are surface tension, hydrostatic pressures, external aerodynamic pressure, electrostatic charge, and internal circulation. Each of these is examined quantitatively, and it is concluded that under most conditions only the first three play important roles in producing the deformation characteristic of large raindrops. By analysis of high-speed photographs of water drops falling at terminal velocity, the distribution of aerodynamic pressures is deduced and is shown to imply that separation in the airflow about a raindrop has very significant effects on drop shape. The surface integral of the vertical components of the deduced aerodynamic pressures is found to be in reasonable agreement with the drop weight. The effect of boundary-layer separation on a number of physical processes occurring at the surface of falling drops is noted briefly.