Kinematics of Large Scale Uniform Momentum Zones*

Internal shear layers divide wall-bounded turbulence into distinct uniform momentum zones (UMZs) that provide important insight into the coherent structure and dynamics of the near-wall flow. But traditional techniques for detecting these shear layers and UMZs rely on fixed measurement windows that are too small to capture large-scale motions (LSMs). In this study, we extended a recently proposed moving window approach to UMZ detection in order to identify large-scale UMZs in moderate Reynolds number, wall-bounded flows. We first developed a robust algorithm for the identification of large-scale UMZ boundaries and applied it to streamwise velocity fields from a DNS channel flow. The resulting UMZ boundaries were then classified into linear boundaries (consistent with individual large-scale hairpin packets) and piecewise-linear boundaries (consistent with the superposition of multiple packets). We extracted the size, location, inclination angle, and convective velocities of these UMZ boundaries, and identified correlations between the geometry and kinematical properties of the UMZs. Consistent with our previous work in the spectral domain, we found that the inclination angle of LSMs with respect to the wall is closely connected to their convection velocity.