Experiments on the wavy instability in Rayleigh–Bénard–Poiseuille convection: Spatial and temporal features in the linear and nonlinear regime

An experimental study of a Rayleigh–Bénard–Poiseuille air flow in a rectangular channel is presented. The aim of the paper is to characterize a secondary instability, referred to as wavy instability and known to be a convective instability, with the objective to identify the best conditions for an optimal homogenization of heat transfers in the system. A periodic mechanical forcing is introduced at channel inlet and the spatial and temporal evolution of the temperature fluctuations are analyzed, depending on the Rayleigh and Reynolds numbers, the forcing frequency and the forcing amplitude. As the saturation state is a priori the best situation to homogenize the transfers, the objective is to expand the saturation area and to generate a maximum saturation amplitude value by conducting experiments at high Rayleigh numbers. It is shown that the change in the Rayleigh number value influences the saturation length but does not act on the saturation magnitude while the change in the Reynolds number value causes antagonist effects on the saturation parameters. The key parameter acting on the saturation amplitude is the forcing frequency. The most efficient forcing configuration is to introduce the external perturbation into the fully developed region of the longitudinal rolls and to apply a specific low forcing frequency associated with a moderate forcing magnitude.