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

This study examines dynamic pressure drag on rising dry buoyant thermals. 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. However, whereas the laminar thermals have C-d approximate to 0.01, the turbulent thermals have weakly negative drag (C-d approximate to -0.1). This difference is explained by the laminar thermals having near vertical symmetry but the turbulent thermals exhibiting considerable vertical asymmetry of their azimuthally averaged flows. In the laminar thermals, buoyancy rapidly becomes concentrated around the main centers of rotation located along the horizontal central axis, leading to expansion of thermals via baroclinic vorticity generation but doing little to break vertical symmetry of the flow. Vertical asymmetry of the azimuthally averaged flow of turbulent thermals is attributed mainly to small-scale resolved eddies that are concentrated in the upper part of the thermals.