## Abstract

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)| ≥R_{0} 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 VVCS_{11} 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.

Original language | English |
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Pages (from-to) | 291-307 |

Number of pages | 17 |

Journal | Journal of Fluid Mechanics |

Volume | 742 |

DOIs | |

State | Published - Mar 2014 |

## Keywords

- Boundary layer structure
- Turbulent boundary layers