Turbulent drag reduction using active control of buoyancy forces

• Published in: The International journal of heat and fluid flow Vol. 61; pp. 585 - 598
• Main Authors: Fuaad, P.A., Baig, M.F., Khan, B.A.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.10.2016
• Subjects: Coherent structures mitigation; Cross-flow velocity fluctuations; Periodically arrayed surface heating; Turbulent drag-reduction; Periodically arrayed surface heating; Cross-flow velocity fluctuations; Turbulent drag-reduction; Coherent structures mitigation
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2016.07.003

•Novel drag reduction method proposed using thermal forcing.•Percentage of drag reduction depends on orientation of periodic strips.•Mechanism responsible for turbulent skin-friction drag reduction is proposed.•Buoyancy forces generate large scale rolls which displace near-wall streaks upwards.•Suppression of cross-flow fluctuating velocities weaken the transient streak growth. 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.