  The inverse Leidenfrost regime occurs when a heated object in relative motion
with a liquid is surrounded by a stable vapour layer, drastically reducing the
hydrodynamic drag at large Reynolds numbers due to a delayed separation of the
flow. To elucidate the physical mechanisms that control separation, here we
report a numerical study of the boundary-layer equations describing the
liquid-vapour flow around a solid sphere whose surface temperature is above the
Leidenfrost point. Our analysis reveals that the dynamics of the thin layer of
vaporised liquid controls the downstream evolution of the flow, which cannot be
properly described substituting the vapour layer by an effective slip length.
In particular, the dominant mechanism responsible for the separation of the
flow is the onset of vapour recirculation caused by the adverse pressure
gradient in the rearward half of the sphere, leading to an explosive growth of
the vapour-layer thickness due to the accumulation of vapour mass. Buoyancy
forces are shown to have an important effect on the onset of recirculation, and
thus on the separation angle. Our results compare favourably with previous
experiments.
