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arXiv · 1804.09326

Entrainment in Resolved, Dry Thermals

Daniel Lecoanet·Nadir Jeevanjee
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Abstract

Entrainment in cumulus convection remains ill-understood and difficult to quantify. For instance, entrainment is widely believed to be a fundamentally turbulent process, even though Turner (1957) pointed out that dry thermals entrain primarily because of buoyancy (via a dynamical constraint requiring an increase in radius $r$), rather than turbulence. Furthermore, entrainment has been postulated to obey a $1/r$ scaling, but this scaling has not been firmly established. Here, we study the classic case of dry, turbulent thermals in a neutrally stratified environment using fully resolved direct numerical simulation. We combine this with a thermal tracking algorithm which defines a control volume for the thermal at each time, allowing us to directly measure entrainment. We test Turner's argument by varying the Reynolds number Re of our thermals between laminar (Re~600) and turbulent (Re~6000) regimes, finding only a 20% variation in entrainment rate $ε$, supporting the claim that turbulence is not necessary for entrainment. We also directly verify the postulated $ε\sim 1/r$ scaling law.

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Daniel Lecoanet, Nadir Jeevanjee. 2019-06-03. Entrainment in Resolved, Dry Thermals. https://doi.org/10.1175/jas-d-18-0320.1

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