Investigation of vertically sheared flow in turbulent convection

Turbulent fixed-flux convection in a 2D (x-z) channel can break into strong vertically sheared flow. Using a Boussineq model, we generate convection cells by imposing antisymmetry on the horizontal vorticity and examine their stability with a linearized version as well as the fully nonlinear model. The linear model predicts oscillatory growth but cannot generate permanent shear; wave-wave interactions are necessary for the system to lock into the shear flow state. Kinematically, an imbalance in the flux from the upwelling (downwelling) plume along the top (bottom) boundary causes the plumes to wander until they come close enough to interact strongly. We also find that the critical Rayleigh number for the breakdown into the vertical shear state increases very strongly with the Prandtl number.