WALL-ATTACHED STRUCTURES IN A TURBULENT CHANNEL FLOW WITH NAVIER SLIP

Abstract

We explore wall-attached structures in a drag-reduced turbulent channel flow with the Navier slip boundary condition. The three-dimensional coherent structures of the streamwise velocity fluctuations (u) are examined in an effort to assess the effects of wall-attached u structures on drag reduction. We extract the u clusters from the direct numerical simulation (DNS) data; for comparison, the DNS data for the no-slip condition are included. The wall-attached structures, which are physically adhere to the wall, in the logarithmic region are self-similar with their height and contribute to the presence of logarithmic behavior. The formation and hierarchy of wall-attached self-similar structures (WASS) are not affected by a streamwise slip, of which influences on them are limited up to the lower bound of logarithmic region, supporting Townsend's outer-layer similarity hypothesis. Weakened mean shear by the streamwise slip results in the decrease in the population density of wall-attached structures within the buffer layer. This leads to lower population of WASS, whereas the space occupied by them in the fluid domain increases. The conditional statistics are obtained to analyze the WASS in the vicinity of the wall. The streamwise slip induces long tails in the near-wall part of WASS, reminiscent of the footprints of large-scale motions. The present results reveal that the characteristics of WASS is irrespective of the streamwise slip and their near-wall part is main energy-containing motions.