Isosurface of the Discriminant of the velocity gradient tensor for an instantaneous realization of a Retau=1900 turbulent channel (J.C. del Alamo, J. Jimenez, P. Zandonade and R.D. Moser 2006, Self-similar vortex clusters in the turbulent logarithmic region. J. Fluid Mech. 561, 329-358)