Comment on "evolution of wall shear stress with Reynolds number in fully developed turbulent channel flow experiments"

Abstract

The recent study by Gubian et al. [Phys. Rev. Fluids 4, 074606 (2019)2469-990X10.1103/PhysRevFluids.4.074606], based on a new wall-shear-stress sensor in a low-Reynolds-number Re turbulent channel flow, came to the surprising conclusion that the magnitude of the fluctuating wall-shear stress τw,rms+ reaches an asymptotic value of 0.44 beyond the friction Reynolds number Reτ≈600. This statement is at odds with results from well-established direct numerical simulation (DNS) results that exceed the authors' highest Reynolds number by up to a factor of 5 while exhibiting a clear Reynolds-number dependence. Furthermore, they claim that "prior estimates of these quantities did not resolve the full range of wall-shear-stress fluctuations, which extended beyond 10 standard deviations above the mean."This contradicts high-quality DNS results and calls for a more in-depth explanation, which is given in the present Comment. We shows that the measurements by Gubian et al. suffer from spatial-resolution issues among others, which when accounted for invalidate the statements made of an asymptotic state at Reτ≈600 and resurrects the Reynolds-number dependence of τw,rms+ for which DNS evidence exists exceeding Reτ≈600 by an order of magnitude.