Direct Numerical Simulation of Turbulent Vertical Channel Flow Using Large-Scale Control with Buoyancy Forces

This study investigates skin-friction drag reduction in fully developed turbulent vertical channel flows using direct numerical simulations. Spanwise-periodic wall heating and cooling are imposed to generate buoyancy-driven large-scale vortical motions near the walls. The drag-reduction performance is evaluated for different Richardson numbers, spanwise wavelengths, and phase differences between the two walls. The out-of-phase condition yields greater drag reduction than the in-phase condition, and the drag reduction further increases with increasing wavelength. Flow visualisations show that out-of-phase forcing produces a quasi-stationary large-scale coherent structure, whereas in-phase forcing leads to an unsteady response without a persistently phase-locked pattern. A three-component decomposition and the FIK identity indicate that drag reduction results from a balance between a decrease in the turbulent contribution and an increase in the coherent contribution to the friction coefficient.