Enhanced drag-reduction over superhydrophobic surfaces with sinusoidal textures: A DNS study. (15th March 2019)

• Record Type: Journal Article
• Authors: Fuaad, P.A.; Arul Prakash, K.
• Is Part Of: Computers & fluids. Volume 181(2019)
• Page Start: 208
• Page End: 223
• Publication Date: 2019-03-15
• Subjects: Drag reduction -- Turbulence -- Spatial stokes layer -- Flow control; Fluid dynamics -- Data processing -- Periodicals; 532.050285
• DOI: 10.1016/j.compfluid.2019.01.022
• Languages: English
• ISSNs: 0045-7930

Abstract

Highlights: Flow over superhydrophobic sinusoidal textures induce secondary-flow leading to SSL. Vortex stretching alters secondary vortices effecting skin-frictional drag levels. Cross-stream fluctuations are damped for sinusoidal SHS compared to straight ridges. Near-wall streaks are skewed in the spanwise direction by Stokes transverse strain. Abstract: Direct Numerical Simulation ( DNS ) studies of a fully developed turbulent channel flow are performed with sinusoidal surface texture to assess its ability to enhance turbulent skin-frictional drag reductions over superhydrophobic surfaces ( SHS ). The streamwise sinusoidal microgroove structure generates asymmetric secondary flows to induce an in-plane stationary distribution of spanwise velocity that oscillates in the streamwise direction. The transverse motions over a sinusoidal texture behave analogously to an existing drag-reduction technique by spanwise wall oscillations. The friction-drag levels and slip-lengths are precisely quantified for various streamwise wavelength configurations, and an optimum wavelength for the maximum reduction in turbulent drag is determined. The response of the near-wall turbulent structures to the sinusoidal microgrooves is also analysed. The transverse Stokes strain generates spanwise tilting in the near-wall streaks for large wavelengths, inducing an apparent drop in the wall-normal ejections and sweeps, thereby reducing the turbulent contribution to the wall-shear-stress. The transverse shear strain above the sinusoidal microgroove is demonstrated to be resembling a Stokes spatial layer ( SSL ), by comparing with existing analytical solutions of strain profiles for wall-forcing by spanwise spatial oscillations.