At first sight, walls appear as the most relevant ingredient in turbulence confined or limited by solid surfaces, and it seems plausible to assume that they should be the origin and organizing agent of wall-bounded turbulence. Consequently, many efforts have been devoted to understand the structure of turbulence in the vicinity of walls. Particularly interesting is the region within the so-called log-layer, where most of the dissipation resides in the asymptotic limit of infinite Reynolds number.
In the present work, the role of the wall and mean momentum transfer on the outer layer of wall-bounded turbulence is investigated via direct numerical simulation (DNS) of turbulent channel flows where the no-slip wall is replaced by a Robin boundary condition for the three velocity components. The new set-up allows for non-zero streamwise, wall-normal and spanwise instantaneous velocities at the boundaries with intensities comparable to those in the bulk flow. We show that the outer-layer one-point statistics and spectra of this 'wall less' channel flow agree quantitatively with those of its wall-bounded counterpart. This suggests that the wall-parallel no-slip condition is not required to recover classic wall-bounded turbulence far from the wall and, more importantly, neither is the impermeability condition. The results are remarkable since no transpiration is assumed to be one of the most distinctive features of walls, and it is commonly understood as the mean by which the log-layer motions 'feel' the boundaries. Instead, we argue that the energy-containing eddies are controlled by the mean momentum flux rather than by the distance to the wall. The hypothesis is further supported by examination of channel flow simulations with modified mean pressure gradients and velocity profiles where the resulting outer-layer flow structures are substantially altered to accommodate the new momentum transfer. Finally, a scaling for the intensities and sizes of the momentum-carrying eddies is proposed based on the mean shear and momentum flux.