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High-resolution velocity measurement in the inner part of turbulent boundary layers over super-hydrophobic surfaces
Published online by Cambridge University Press: 26 July 2016
Abstract
Digital holographic microscopy is used for characterizing the profiles of mean velocity, viscous and Reynolds shear stresses, as well as turbulence level in the inner part of turbulent boundary layers over several super-hydrophobic surfaces (SHSs) with varying roughness/texture characteristics. The friction Reynolds numbers vary from 693 to 4496, and the normalized root mean square values of roughness $(k_{rms}^{+})$ vary from 0.43 to 3.28. The wall shear stress is estimated from the sum of the viscous and Reynolds shear stress at the top of roughness elements and the slip velocity is obtained from the mean profile at the same elevation. For flow over SHSs with $k_{rms}^{+}<1$, drag reduction and an upward shift of the mean velocity profile occur, along with a mild increase in turbulence in the inner part of the boundary layer. As the roughness increases above $k_{rms}^{+}\sim 1$, the flow over the SHSs transitions from drag reduction, where the viscous stress dominates, to drag increase where the Reynolds shear stress becomes the primary contributor. For the present maximum value of $k_{rms}^{+}=3.28$, the inner region exhibits the characteristics of a rough wall boundary layer, including elevated wall friction and turbulence as well as a downward shift in the mean velocity profile. Increasing the pressure in the test facility to a level that compresses the air layer on the SHSs and exposes the protruding roughness elements reduces the extent of drag reduction. Aligning the roughness elements in the streamwise direction increases the drag reduction. For SHSs where the roughness effect is not dominant ($k_{rms}^{+}<1$), the present measurements confirm previous theoretical predictions of the relationships between drag reduction and slip velocity, allowing for both spanwise and streamwise slip contributions.
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- © 2016 Cambridge University Press
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Ling supplementary movie
Sample original holograms recorded at 20 kHz in the JHU water tunnel showing a sample plastron for a turbulent boundary over SHSPor, a porous substrate spray-coated with F-POSS/PMMA. The rms value of roughness height is 10 μm, the freestream velocity is 2.2 m/s, the friction Reynolds number is 1408, and the pressure difference across the porous base is 12 kPa (higher above the SHS). Image size: 9.3 mm x 2.5 mm.
Ling supplementary movie
Sample original holograms recorded at 20 kHz in the JHU water tunnel showing a sample plastron for a turbulent boundary over SHSPor, a porous substrate spray-coated with F-POSS/PMMA. The rms value of roughness height is 10 μm, the freestream velocity is 4.3 m/s, the friction Reynolds number is about 2850, and the pressure difference across the porous base is 6 kPa (higher above the SHS). Image size: 9.3 mm x 2.5 mm.
Ling supplementary movie
Sample original holograms recorded at 20 kHz in the JHU water tunnel showing a sample plastron for a turbulent boundary over SHSPor, a porous substrate spray-coated with F-POSS/PMMA. The rms value of roughness height is 10 μm, the freestream velocity is 6.4 m/s, the friction Reynolds number is 4287, and the pressure difference across the porous base is -4 kPa (higher below the SHS). Image size: 9.3 mm x 2.5 mm.
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