Drag-reduction in buoyant and neutrally-buoyant turbulent flows over super-hydrophobic surfaces in transverse orientation

• Published in: International journal of heat and mass transfer Vol. 93; pp. 1020 - 1033
• Main Authors: Fuaad, P.A., Baig, M.F., Ahmad, H.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2016
• Subjects: Cross-flow velocities; Low-speed streaks; Transverse ridges & posts; Turbulent bursting events; Turbulent skin-friction reduction; Unstable-stratification; Vorticity; Wall-shear stress; Transverse ridges & posts; Low-speed streaks; Cross-flow velocities; Vorticity; Unstable-stratification; Turbulent skin-friction reduction; Turbulent bursting events; Wall-shear stress
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2015.10.068

The present study involves Direct Numerical Simulations (DNS) of a turbulent channel flow subject to passive-control of super-hydrophobic surfaces (SHS) employed in form of ridges/posts at the bottom-wall of the channel, oriented in transverse direction to the flow. The simulations have been carried out for a fixed friction Reynolds number Reτ=180 to investigate the effect of thermal forcing in tandem with SHS at a fixed friction Richardson number Riτ=15. It is observed that on decreasing the width to depth (w/d) ratio of the ridge topology, the drag-reduction increases and this reduction is found to be maximum for the topology of SHS posts. A key effect of this control is reduction in cross-flow fluctuations in the buffer-layer (i.e. 10<z+⩽50) which lead to generation of weaker and more stable low-speed streaks which results in reduction in bursting of these near-wall streaks. This eventually causes considerable reduction in turbulent kinetic energy (TKE) which leads to turbulent skin-friction drag reduction at the controlled wall generally of the order of 10–22%. Unstable-stratification of the SHS flow results in the wall-normal convection of turbulent-vorticity which enhances cross-flow fluctuations leading to an increase in Reynolds shear-stresses near to the bottom-wall. In total the effect of heating is found to mitigate the net-reduction produced in turbulent drag by 6–7% for different cases employing ridge/post topology.