Direct numerical simulation of turbulent pipe flow by large-scale control using buoyancy force

Skin-friction drag reduction control for a fully developed turbulent pipe flow is performed using direct numerical simulations. Heating and cooling of pipe wall is homogeneous in the streamwise direction and varies in the azimuthal direction and buoyancy force induces the flow to decrease the skin-friction drag. The control parameters are the amplitude of buoyancy force and wavenumber of heating and cooling in the azimuthal direction. There are two different mechanisms of skin-friction drag reduction: stable stratification and large-scale vortical structure. In the former, the buoyancy force attenuates the turbulence directly. In the latter, the buoyancy force creates wall jets along the pipe wall, and a large-scale streamwise vortex is formed. The vortex increases the coherent contribution to the skin-friction coefficient, but it reduces the random contribution indirectly.