Exact Coherent Structures of Sheared Double-Diffusive Convection

Abstract

The interaction between shear and double-diffusive convection (DDC) in the diffusive regime (cold fresh water on top of hot salty water) plays an important role in the heat and mass transport of polar region oceans. This study computes exact coherent structures (ECS) of diffusive-regime DDC with a uniform background shear in a vertically wall-bounded flow layer. We focus on the shear-influenced regime and present two-dimensional (2D) ECS consisting of steady-state solutions and periodic orbits. The steady-state solutions include tilted convective rolls with various horizontal wavenumbers, and they are invariant under horizontal translation. All tilted convective roll states undergo saddle-node bifurcation, leading to a stable upper branch and an unstable lower branch, suggesting that they originate from the subcritical bifurcation of conduction base states. Hopf bifurcations appear on the stable upper branch of tilted convective rolls, leading to periodic orbits. Bifurcation diagrams for dimensionless parameters, including the Rayleigh number, the Prandtl number, and the diffusivity ratio, are established, suggesting subcritical behavior. Increasing shear strength stabilizes the 2D tilted convective roll, while these tilted convective rolls continue to exist in the limit of zero density ratio corresponding to sheared Rayleigh-B\'enard convection. Extension to the three-dimensional (3D) domain leads to 3D streamwise-elongated roll solutions; one of them originates from a subcritical bifurcation of the corresponding 2D roll solution. Chaotic solutions from direct numerical simulations generally visit neighborhoods of these steady or periodic solutions, and these visits leave an imprint on the flow statistics.