SCALES OF A TURBULENT COUETTE-POISEUILLE FLOW WITH VANISHING MEAN WALL SHEAR

摘要

This study is focused on a turbulent Couette-Poiseuille flow which is driven by a streamwise pressure gradient and relative wall motion. The parameters were carefully prescribed to yield a specific flow with vanishing mean shear at one wall. The distinct shear conditions at the opposite walls result in asymmetry of wall-normal distributions of statistical flow properties such as mean and rms velocity/vorticity and shear stresses with respect to the channel center. The mean streamwise velocity profile demonstrates a longer log-scale region than a conventional Poiseuille flow. In addition, the commonly observed coherent near-wall turbulent structures in the form of streamwise streaks only occur near the fixed wall. Using a spanwise two-point correlation coefficient of the streamwise velocity fluctuation, a growth of the spanwise length associated with the large-scale structures is observed throughout the whole channel. The small-scale streaky structures are only confined near the fixed wall (local) but are strong, while the large-scale structures occupy the whole channel height (global) but are weak. The dominating role switches from the small-scales to the large-scales in the buffer layer next to the fixed wall.