  We determine the smallest instantaneous increase in the strength of an
opposing wind that is necessary to permanently reverse the forward displacement
flow that is driven by a two-layer thermal stratification. With an
interpretation in terms of the flow's energetics, the results clarify why the
ventilation of a confined space with a stably-stratified buoyancy field is less
susceptible to being permanently reversed by the wind than the ventilation of a
space with a uniform buoyancy field. For large opposing wind strengths we
derive analytical upper and lower bounds for the system's marginal stability,
which exhibit a good agreement with the exact solution, even for modest
opposing wind strengths. The work extends a previous formulation of the problem
(Lishman & Woods, 2009, Building and Env. 44, pp. 666-673) by accounting for
the transient dynamics and energetics associated with the homogenisation of the
interior, which prove to play a significant role in buffering temporal
variations in the wind.

Fluid Dynamics