The structure of the vorticity field in the near-wall region of turbulent channel flow at high-Reynolds number

The structure of the vorticity field in turbulent channel flow is studied by using direct numerical simulations of the incompressible Navier-Stokes equations with up to $2048\times 1536\times 2048$ grid points; the maximum friction Reynolds number is $Re_\tau=2560$. Instantaneous vortex-line plots show the presence of $\Omega$-shaped hairpin vortices in the near-wall region of turbulent channel flow. The $\Omega$-shaped hairpin vortices in the near-wall region are well displayed by a bundle of vortex lines starting from points along a line near $y^+=10$ parallel to the mean stream. They suggest that the hairpin vortices are formed by instabilities and roll-ups of sheets of spanwise vorticity in the buffer layer of turbulent channel flow. The three-dimensional structure of the low-speed region of the streamwise velocity near the wall ($y^+<100$) is discussed in view of these vortex lines starting from the buffer layer.