Convective rolls and heat transfer in finite-length Rayleigh-Bénard convection: A two-dimensional numerical study

Aerospace Engineerin

A direct-numerical-simulation-based second-moment closure for turbulent magnetohydrodynamic flows

A magnetic field, imposed on turbulent flow of an electrically conductive fluid, is known to cause preferential damping of the velocity and its fluctuations in the direction of Lorentz force, thus leading to an increase in stress anisotropy. Based on direct numerical simulations (DNS), we have developed a model of magnetohydrodynamic (MHD) interactions within the framework of the second-moment turbulence closure. The MHD effects are accounted for in the transport equations for the turbulent stress tensor and energy dissipation rate—both incorporating also viscous and wall-vicinity nonviscous modifications. The validation of the model in plane channel flows with different orientation of the imposed magnetic field against the available DNS (Re = 4600,Ha = 6), large eddy simulation (Re = 2.9×104,Ha = 52.5,125) and experimental data (Re = 5.05×104 and Re = 9×104, 0 ? Ha ? 400), show good agreement for all considered situations.Multi-Scale PhysicsApplied Science

Incompressibility of the Leray-? model for wall-bounded flows

This study shows that the Leray-? model does not explicitly enforce a divergence-free field for the filtered velocity. While this condition is automatically satisfied in the absence of boundaries, bounded domains require extra attention. It is shown, both analytically and through simulations of Rayleigh–Bénard convection, that incompressibility of the filtered velocity field cannot be guaranteed in the current formulation. Several suggestions are made to restore the incompressibility of the filtered velocity, and it is shown that free-slip boundary conditions for the filtered velocity do guarantee incompressibility for the domain under consideration.Multi-Scale PhysicsApplied Science

Identification of the wind in Rayleigh–Bénard convection

Using a symmetry-accounting ensemble-averaging method, we have identified the wind in unbounded Rayleigh–Bénard convection. This makes it possible to distinguish the wind from fluctuations and to identify dynamic features of each. We present some results from processing five independent three-dimensional direct numerical simulations of a ? = 4 aspect-ratio domain with periodic side boundaries at Ra = 107 and Pr = 1. It is found that the wind boundary layer scales linearly very close to the wall and has a logarithmic region further away. Despite the still noticeable molecular effects, the identification of log scaling and significant velocity and temperature fluctuations well within the thermal boundary layer clearly indicate that the boundary layer cannot be classified as laminar.Multi-Scale PhysicsApplied Science

Wind and boundary layers in Rayleigh-Bénard convection. II: Boundary layer character and scaling

Multi-Scale PhysicsApplied Science