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Over 50 years ago it was proposed that dry thermals entrain because of buoyancy (via a constraint which requires an increase in the radius  a ). However, this runs counter to the scaling arguments commonly used to derive the entrainment rate, which rely on either the self‐similarity or a turbulent entrainment hypothesis. The assumption of turbulence‐driven entrainment has been investigated and it has been found that the entrainment efficiency  e  varies by less than 20 %  between laminar ( Re =630) and turbulent ( Re =6300) thermals. This motivated us to utilize the argument of buoyancy‐controlled entrainment in addition to the thermal's vertical momentum equation to build a model for thermal dynamics which does not invoke turbulence or self‐similarity. We derive simple expressions for the thermals' kinematic properties and their fractional entrainment rate  ϵ  and find close quantitative agreement with the values in direct numerical simulations. In particular, our expression for entrainment rate is consistent with the parametrization  ϵ ∼ B / w  2 , for Archimedean buoyancy  B  and vertical velocity  w . We also directly validate the role of buoyancy‐driven entrainment by running simulations where gravity is turned off midway through a thermal's rise. The entrainment efficiency  e  is observed to drop to less than one third of its original value in both the laminar and turbulent cases when  g =0, affirming the central role of buoyancy in entrainment for dry thermals.

Buoyancy‐driven entrainment in dry thermals