Changes in turbulent dissipation in a channel flow with oscillating walls
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In this article, the authors studied the physical mechanism for skin-friction drag reduction in a turbulent plane channel flow at constant pressure gradient and found that the dominant, oscillation-related term in the turbulent enstrophy caused the turbulent dissipation to be enhanced.Abstract:
Harmonic oscillations of the walls of a turbulent plane channel flow are studied by direct numerical simulations to improve our understanding of the physical mechanism for skin-friction drag reduction. The simulations are carried out at constant pressure gradient in order to define an unambiguous inner scaling: in this case, drag reduction manifests itself as an increase of mass flow rate. Energy and enstrophy balances, carried out to emphasize the role of the oscillating spanwise shear layer, show that the viscous dissipations of the mean flow and of the turbulent fluctuations increase with the mass flow rate, and the relative importance of the latter decreases. We then focus on the turbulent enstrophy: through an analysis of the temporal evolution from the beginning of the wall motion, the dominant, oscillation-related term in the turbulent enstrophy is shown to cause the turbulent dissipation to be enhanced in absolute terms, before the slow drift towards the new quasi-equilibrium condition. This mechanism is found to be responsible for the increase in mass flow rate. We finally show that the time-average volume integral of the dominant term relates linearly to the drag reduction.read more
Citations
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Model-based design of transverse wall oscillations for turbulent drag reduction
TL;DR: In this article, the Boussinesq eddy viscosity hypothesis is used to quantify the effect of fluctuations on the mean velocity in flow subject to control, and the resulting correction to the turbulent mean velocity induced by small-amplitude wall movements is then used to identify the optimal frequency of drag-reducing oscillations.
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Effect of wall heating on turbulent boundary layers with temperature-dependent viscosity
TL;DR: In this article, the effect of viscosity stratification on the turbulence statistics and skin friction was investigated, and an empirical relation for temperaturedependent viscosities for water was adopted.
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Reynolds-number dependence of turbulent skin-friction drag reduction induced by spanwise forcing
Davide Gatti,Maurizio Quadrio +1 more
TL;DR: In this article, Quadrio et al. examined how increasing the value of the Reynolds number affects the ability of spanwise-forcing techniques to yield turbulent skin-friction drag reduction.
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Direct numerical simulation of turbulent channel flow over porous walls
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