European Turbulence Conference 14

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Lattice Boltzmann simulations of drag reduction by super-hydrophobic surfaces

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Drag reduction by super-hydrophobic surfaces is investigated using Lattice Boltzmann simulations in turbulent channel flow. The super-hydrophobic surface is modeled as longitudinal arrays of slip/no-slip stripes of size 4 ≤ g+0 = w+0 ≤ 128, covering both channel walls, where g+0 and w+0 denote the widths of the slip and no-slip stripes, respectively, normalized with respect to the friction- velocity of the base flow and viscosity. An additional case was also run with g+0 = 28, w+0 = 4. All simulations were performed in channels of size 5h × 2.5h × 2h at a bulk Reynolds number of Reb = Ubh/ν = 3600 (Reτ0 ≈ 230), where h denotes the channel half height. Drag reductions of 5%, 11%, 18%, 23%, 38%, 47% and 51% were observed for g+0 = w+0 = 4, 8, 16, 32, 64, 128 and g+0 = 28, w+0 = 4, respectively. Mathematical analysis shows that the magnitude of drag reduction can be expressed as DR = Us/Ub +ε, where ε is zero in laminar flow, but attains a small non-zero value in turbulent flow proportional to the magnitude of DR. Results from both the present DNS studies as well as prior experiments [1] were found to fit this scaling. The one-point turbulence statistics show characteristics of combined slip described by [3]. When normalized in wall units, the turbulence statistics and structure remains nearly unchanged outside of a layer of thickness on the order of one slip-length from the walls. Drag reduction is found to be due to a weakening of the turbulence structures accompanied by a drop in turbulence production throughout the channel, but especially over the slip surfaces.


Amirreza Rastegari    
University of Michigan, Ann Arbor
United States

Rayhaneh Akhavan    
University of Michigan, Ann Arbor
United States


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