A new statistical coherent structure (CS), the velocity-vorticity correlation structure (VVCS), using the two-point cross-correlation coefficient Rij of velocity and vorticity components, ui 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 |Rij (yr; x, y, z)| ≥R0 to define a topologically invariant high-correlation region, called VVCSij. 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 yr) 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.
- Boundary layer structure
- Turbulent boundary layers