Reynolds number effects on a velocity-vorticity correlation-based skin-friction drag decomposition in incompressible turbulent channel flows

A form of skin-friction drag decomposition is given based on the velocity-vorticity correlations, < v omega(z) > and <-w omega(y) >, which represent the advective vorticity transport and vortex stretching, respectively. This identity provides a perspective to understand the mechanism of skin-friction drag generation from vortical motions and it has better physical interpretability compared with some previous studies. The skin-friction coefficients in incompressible turbulent channel flows at friction Reynolds numbers from 186 to 2003 are divided with this velocity-vorticity correlation-based identity. We mainly focus on the Reynolds number effects on the contributing terms, their scale-dependence and quadrant characteristics. Results show that the contributing terms and their proportions exhibit similarities and the same peak locations across the wall layer. For the first time, we find that the positive and negative regions in the spanwise pre-multiplied spectra of the turbulent inertia (< v'omega' z > + <-w'omega' (y) >) can be separated with a universal linear relationship of lambda(+)(z) = 3.75y(+). The linear relationship is adopted as the criterion to investigate the scale dependence of the velocity-vorticity coupling structures. It reveals that the negative and positive structures dominate the generation of friction drag associated with the advective vorticity transport and vortex stretching, respectively. Moreover, quadrant analyses of the velocity-vorticity correlations are performed to further examine the friction drag generation related to different quadrant motions.