  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.
