Evolution of highly buoyant thermals in a stratified layer

¹ NORDITA, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark
² Department of Mathematics, University of Newcastle upon Tyne, NE1 7RU, UK
³ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

Corresponding author: A. Brandenburg, brandenb@nordita.dk

Received: 7 August 2000
Accepted: 7 February 2001

Abstract

The buoyant rise of thermals (i.e. bubbles of enhanced entropy, but initially in pressure equilibrium) is investigated numerically in three dimensions for the case of an adiabatically stratified layer covering 6-9 pressure scale heights. It is found that these bubbles can travel to large heights before being braked by the excess pressure that builds up in order to drive the gas sideways in the head of the bubble. Until this happens, the momentum of the bubble grows as described by the time-integrated buoyancy force. This validates the simple theory of bubble dynamics whereby the mass entrainment of the bubble provides an effective braking force well before the bubble stops ascending. This is quantified by an entrainment parameter alpha which is calculated from the simulations and is found to be in good agreement with the experimental measurements. This work is discussed in the context of contact binaries whose secondaries could be subject to dissipative heating in the outermost layers.