Effets des échanges de chaleur aux parois sur la dynamique des courants de gravité

Although gravity currents have been extensively studied in the scientific literature, the case of currents evolving along non-adiabatic walls has received comparatively little attention, despite the frequent occurrence of this boundary condition in both geophysical and artificial frameworks.Atmospheric gravity currents, such as sea-breeze fronts and thunderstorm outflows, as well as those generated by accidental heavy gas releases in the environment, can be affected by the convective heat flux coming from the Earth's ground. Similarly, when hot-smoke currents induced by indoor fires travel horizontally below the room ceiling, they can be cooled due to the lower temperature of the ceiling with respect to the smoke.To investigate how the heat exchange at the wall affects the dynamics of the current, a new laboratory facility has been realized in which a sustained dense current, generated by an air--CO2 mixture, propagates in a channel with a uniformly heated bottom wall. The setup enables the investigation of both the unsteady configuration, related to the advancing motion of the current front, and the steady configuration established once the front has left the channel. A dimensionless parameter, Λs, is introduced to quantify heating intensity as the ratio between the wall-normal buoyancy flux induced by the heating and the wall-tangent buoyancy flux generating the current.The results for the unsteady problem show that the front propagation velocity is progressively reduced with stronger wall heating. In strongly heated experiments, the front velocity can even be suppressed. In this case, the density difference between the current head and the downstream ambient fluid vanishes, and the head becomes a positively buoyant plume that extends indefinitely in the vertical direction.For the stationary problem, challenging experiments have been conducted performing simultaneous high-frequency measurements of two velocity components, CO$_2$ concentration, and temperature. These measurements allow the investigation of the turbulent transport of different scalar quantities and reveal, in the heated case, the formation of an unstable boundary layer along the current wall.The novel experimental dataset enables a thorough validation of large-eddy simulations (LES) of non-adiabatic gravity currents, which are then used to extend the range of experimental conditions investigated in the laboratory.Wall heating is observed to enhance the entrainment coefficient E, while having a negligible influence on the bulk Richardson number (Ri). This implies that the E(Ri) relations commonly employed to parameterize entrainment in adiabatic gravity currents are no longer exhaustive when wall heat exchange is considered.