Characterisation of a Wavy Convective Instability in Poiseuille-Rayleigh-Be´nard Flows: Linear Stability Analysis and Non Linear Numerical Simulations Under Random Excitations

The aim of the present paper is to characterize a secondary convective instability of Poiseuille-Rayleigh-Be´nard (PRB) mixed convection flows in air that takes the shape of wavy thermoconvective rolls, for 70≤Re≤300 and 3000<Ra<15000. At first, the linear stability analysis by Clever and Busse [JFM, 1991] in the case of PRB flows between two infinite plates is extended to the case of confined channels with a 10 transversal aspect ratio. In the second part, using 3D non linear direct numerical simulations, the space and time development of the chaotic wavy rolls obtained by maintaining a permanent random excitation at channel inlet is analyzed. As the perturbation is designed to cover all the modes, it is possible to detect the modes that are naturally amplified by the flow and those that are damped. It is shown that the wavy roll characteristics obtained in this way vary a lot with Ra increasing and stabilize for Ra>3Ra*. Comparisons with the experiments by Pabiou et al. [JFM, 2005] are proposed.