Turbulent drag reduction using active control of buoyancy forces

The present study involves Direct Numerical Simulations (DNS) of a turbulent channel flow subject to spatially modulated thermal forcing with a special interest to realize reduction of skin-friction drag. Thermal forcing has been employed using both streamwise and transverse arrays of heated strips on the bottom wall of the channel. The simulations have been carried out for a fixed friction Reynolds number Reτ=180 with friction Richardson number Riτ varying from 15 to 30. The periodic transverse strips exhibited an increase in skin-friction drag with a decrease in the width of hot strips though wider hot strips exhibit a slight decrease in skin-friction drag. Streamwise periodic strips exhibited a reduction in skin-friction drag of the order of 8% with an increasing trend in reduction of friction drag with increasing Riτ. The mechanism responsible for turbulent skin-friction drag reduction due to streamwise thermal forcing affects two processes, namely: (a) advection of streamwise kinetic energy from the buffer-layer to outer layer by the wall-normal buoyancy forces leading to formation of broader low-speed streaks over the heated regions and (b) brings about suppression of cross-flow fluctuating velocities which in turn weakens the transient growth of the turbulent streaks.