3-DIMENSIONAL PATTERNS OF STREAMWISE VORTICITY IN THE TURBULENT NEAR-WAKE OF A CYLINDER

A cinematographic technique of high-image-density particle image velocimetry combined with a method of space-time image construction is introduced as a technique for extracting coherent vorticity concentrations in the turbulent (Re = 10(4)) near-wake of a cylinder. This technique leads to the first instantaneous representations of streamwise vorticity and their evolution with time. These streamwise (omega(x)) concentrations have a lifetime the order of half the period of the Karman vor tices and an order of magnitude larger than the period of the Kelvin-Helmholtz vortices initially formed in the separating shear layer. Moreover, the omega(x) concentrations have a spanwise wavelength that is approximately equal to the cylinder diameter, and average and maximum values of circulation about one-tenth and one-third the circulation of the spanwise Karman vortices.