Velocity-vorticity correlation structure in turbulent channel flow

A new statistical coherent structure (CS), the velocity-vorticity correlation structure (VVCS), using the two-point cross-correlation coefficient R_ij of velocity and vorticity components, u_i and ω _j (i, j = 1, 2, 3), is proposed as a useful descriptor of CS. For turbulent channel flow with the wall-normal direction y, a VVCS study consists of using u-i at a fixed reference location y-r, and using |R_ij (y_r; x, y, z)| ≥R₀ to define a topologically invariant high-correlation region, called VVCS_ij. The method is applied to direct numerical simulation (DNS) data, and it is shown that the VVCS _ij qualitatively and quantitatively captures all known geometrical features of near-wall CS, including spanwise spacing, streamwise length and inclination angle of the quasi-streamwise vortices and streaks. A distinct feature of the VVCS is that its geometry continuously varies with y-r. A topological change of VVCS₁₁ from quadrupole (for smaller y _r) to dipole (for larger y_r) occurs at yr⁺=110, giving a geometrical interpretation of the multilayer nature of wall-bounded turbulent shear flows. In conclusion, the VVCS provides a new robust method to quantify CS in wall-bounded flows, and is particularly suitable for extracting statistical geometrical measures using two-point simultaneous data from hotwire, particle image velocimetry/laser Doppler anemometry measurements or DNS/large eddy simulation data.