Sheared convection

Rayleigh-Bénard convection and wall-bounded shear flow are both canonical, extensively studied turbulence problems on their own. Sheared thermal convection combines them: a mean shear, generated by moving the top and bottom plates relative to each other (plane Couette-type forcing) or by an imposed large-scale wind, is superimposed on the buoyancy-driven convective flow. This coupling of shear and buoyancy is directly relevant to the atmospheric boundary layer, where wind shear and surface heating or cooling act together, so sheared convection serves as an idealized, numerically and experimentally tractable model for that interaction.

From: A. Blass, X. Zhu, R. Verzicco, D. Lohse and R.J.A.M. Stevens - Direct numerical simulations of sheared thermal convection, Winner SURFsara Visualization Competition 2017. For a corresponding video, see the Physics of Fluids YouTube channel.

Using direct numerical simulations, we have studied how increasing shear reorganizes the convective flow and changes the efficiency of heat and momentum transport [1]. As the shear strength grows relative to the buoyancy forcing, the large-scale convection rolls that dominate unsheared Rayleigh-Bénard convection are progressively broken down and replaced by more streak-like, shear-aligned structures, with a corresponding change in how heat is carried between the plates. We subsequently examined how this shear-buoyancy interplay depends on the Prandtl number [2], and how small-scale flow structures — not just the large-scale flow organization — contribute to the heat transport in sheared convection [3]. Bringing these results together, we derived scaling relations that connect the heat and momentum transport in sheared Rayleigh-Bénard convection across the shear-dominated and buoyancy-dominated regimes [4], providing a unifying picture that links back to the classical (unsheared) Rayleigh-Bénard scaling.