A theoretical decomposition of mean skin friction generation into physical phenomena across the boundary layer

Abstract

A theoretical decomposition of mean skin friction generation into physical phenomena across the whole profile of the incompressible zero-pressure-gradient smooth-flat-plate boundary layer is derived from a mean streamwise kinetic-energy budget in an absolute reference frame (in which the undisturbed fluid is not moving). The Reynolds-number dependences in the laminar and turbulent cases are investigated from direct numerical simulation datasets and Reynolds-averaged Navier-Stokes simulations, and the asymptotic trends are consistently predicted by theory. The generation of the difference between the mean friction in the turbulent and laminar cases is identified with the total production of turbulent kinetic energy (TKE) in the boundary layer, represented by the second term of the proposed decomposition of the mean skin friction coefficient. In contrast, the analysis introduced by Fukagata et al. on a streamwise momentum budget in the wall reference frame, relates the turbulence-induced excess friction to the Reynolds shear stress weighted by a linear function of the wall distance. The wall-normal distribution of the linearly-weighted Reynolds shear stress differs from the distribution of TKE production involved in the present discussion, which consequently draws different conclusions on the contribution of each layer to the mean skin friction coefficient. At low Reynolds numbers, the importance of the buffer-layer dynamics is confirmed. At high Reynolds numbers, the present decomposition quantitatively shows for the first time that the generation of the turbulence-induced excess friction is dominated by the logarithmic layer. This is caused by the well-known decay of the relative contributions of the buffer layer and wake region to TKE production with increasing Reynolds numbers. This result on mean skin friction, with a physical interpretation relying on an energy budget, is consistent with the well-established general importance of the logarithmic layer at high Reynolds numbers, contrary to the friction breakdown obtained from the approach of Fukagata et al. based on a momentum budget. The new decomposition suggests that it may be worth investigating new drag reduction strategies focusing on TKE production and on the nature of the logarithmic layer dynamics. The decomposition is finally extended to the pressure-gradient case and to channel and pipe flows.

Turbulence in intermittent transitional boundary layers and in turbulence spots

“…transition region, and decays towards the expected value downstream as the turbulence production rate approaches that of a fully turbulent flow. This observation can be explained using the analysis by Renard & Deck (2016) who related the wall shear stress, for a plate propelled at constant speed in quiescent fluid, to the sum of three normalized quantities: the power input into accelerating the mean flow, dissipation rate associated with the mean-flow profile, and production rate of turbulence kinetic energy. Using that interpretation, the herein reported increase in the rate of production within the transition zone beyond the levels for fully turbulent boundary layers contributes to the higher wall shear stress in that region.…”

Assessment of the IDDES method acting as wall-modeled LES in the simulation of spatially developing supersonic flat plate boundary layers

“…The effect of shear stress can be analyzed in an alternative way using the theory of Renard and Deck (2016), which suggests that the turbulent kinetic energy (TKE) production −ρ u v total ∂u/∂y = (−ρ u v resolved + μ t ∂u/∂y)∂u/∂y in the buffer layer dominates the mean skin friction of low Reynolds number flow. Figure 11 illustrates the TKE production of Cases 2.1-2.3.…”

Towards a Physical Scale Decomposition of Mean Skin Friction Generation in the Turbulent Boundary Layer

“…where τ /ρ = ν ∂ u /∂y − u ′ v ′ . The discussion of this new decomposition suggests that at very high Reynolds number, the turbulence-induced excess friction may be essentially related to the production of turbulent kinetic energy (represented by C f,b ) in the logarithmic layer ( [7]). The present study investigates the resolved contribution of turbulence to mean skin friction in a spatially developing flat-plate boundary layer up to Re θ = 13 000 described by a wall-resolved large-eddysimulation database [2] (fig.…”

“…to the overall production of turbulent kinetic energy, as discussed in ref. [7]. The final paper will consequently present streamwise spectra of turbulent kinetic energy production.…”