Dynamic pressure drag on rising buoyant thermals in a neutrally stable environment

A theoretical expression for drag coefficient C-d as a function of several other nondimensional parameters governing thermal dynamics is derived based on combining the thermal momentum budget with the similarity theory of Scorer. Using values for these nondimensional parameters from previous studies, the theory suggests drag on thermals is small relative to that on solid spheres in laminar or turbulent flow.

Two sets of numerical simulations of thermals in an unstratified, neutrally stable environment using an LES configuration of the Cloud Model 1 (CM1) are analyzed. One set has a relatively low effective Reynolds number R-e and the other has an order-of-magnitude-higher R-e; these produce laminar and turbulent resolved-scale flows, respectively. Consistent with the theoretical C-d, the magnitude of drag is small in all simulations.