76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023;
Washington, DC
Session T20: Flow Control: Structured Surfaces
4:25 PM–5:56 PM,
Monday, November 20, 2023
Room: 146C
Chair: Daniel Chung, University of Melbourne
Abstract: T20.00001 : Turbulence suppression and skin-friction drag reduction with hairy surfaces*
4:25 PM–4:38 PM
Abstract
Presenter:
Rayhaneh Akhavan
(University of Michigan)
Authors:
Rayhaneh Akhavan
(University of Michigan)
Jae Bok Lee
(University of Michigan)
A new method for turbulence suppression and skin-friction drag reduction using hairy surfaces is presented. The studies are based on direct numerical simulation, using lattice Boltzmann-immersed bounday (LB-IB) methods, performed in turbulent channel flows at a bulk Reynolds number of Reb=7200 (Reτ0≈221), with a uniform carpet of hairy surfaces implanted on both channel walls. Hairy surfaces with initial hair filament heights of 4 ≤ h0+0 ≤ 16 in base flow wall units, hair filament height to spacing ratios of 1/4 ≤ h0/s ≤ 2, hair filament diameters of d+0 ≈ 0.5, density ratios of 30 ≤ ρr ≤ 1000 and Cauchy numbers of Ca = 0,10, 20, 40, 60, 80 were investigated. It is found that the magnitude of drag reduction scales primarily with the ratio of the characteristic timescale of the hair filaments to the eddy turnover time of the large-scale turbulent eddies in the base turbulent channel flow, Tfiluτ0/H, reaching its optimal value at Tfiluτ0/H ≈ 1.5. Drag reductions of ∼ 5.5% were achieved with sparse, flexible hairy surfaces with initial filament heights of h0+0 ≈ 8, diameters of d+0 ≈ 0.5, filament height to spacing ratios of 1/2 ≤ h0/s ≤ 1, density ratio of ρr=700 and Ca=40, for which Tfiluτ0/H ≈ 1.5. The mechanism of drag reduction is found to be a disruption of the pressure-velocity-gradient correlations in the presence of hairy surfaces, which, in turn, leads to an accumulation of turbulence kinetic energy in the streamwise component of turbulent velocity fluctuations, leading to concomitant reductions in the Reynolds shear stress and turbulence production, and a drop in the streamwise turbulent vorticity fluctuations. Examination of the pre-multiplied spectra reveals that these effects are due to modifications of the largest turbulent scales in the presence of hairy surfaces, and extend to wall-normal distances far beyond the initial height of the hair filaments. These large-scale effects give optimism that the magnitude of drag reduction can be further enhanced as the Reynolds number of the flow increases. These results suggest that flexible hairy surfaces show promise for design of novel "functional surfaces" for turbulent skin-friction drag reduction, acting both as sensor and actuator without the need for any external power input.
*Supported by Martin R. Prince Foundation and NSF XSEDE Allocation TG-CTS070067N.