  We study numerically the propagation of a hot thermal transient through a
U-bend via an ensemble of wall-resolved large eddy simulations. Conjugate heat
transfer between fluid and solid domains is accounted for. The flow is in a
fully turbulent mixed convection regime, with a bulk Reynolds number of
$10,000$, a Richardson number of $2.23$, and water as the working fluid
(Prandtl number = $6$). These conditions lead to strong thermal stratification,
with buoyancy-induced secondary flows, and the generation of a large and
persistent recirculation region.
  The evolution of Dean vortices as the thermal transient passes is studied. It
is found that baroclinic vorticity generation dominates over a large period of
the transient, due to the thermal inertia of the wall. Gravitational buoyancy
leads to a reversal of the counter-rotating vortex pair. The impact of this
reversal on the swirl-switching and secondary-current losses is assessed. It is
found that low frequency modes are suppressed in the reversed-vortex state.
