Longitudinal and spanwise vortical structures in a turbulent near wake

Approximations to the three vorticity components have been simultaneously o btained using a combination of four X wires in the near wake of a circular cylinder with the view of investigating the most likely interrelationship b etween quasilongitudinal rib-like structures and spanwise vortical structur es. The conditional spanwise vorticity variance, based on detections of spa nwise structures, shows a strong peak near the detections. This peak sugges ts a longitudinal extent of 1.0-1.3 cylinder diameters for the spanwise str uctures. The conditional streamwise vorticity variance has maxima near the saddle points on both sides of the detections, while the conditional latera l vorticity variance is maximum only downstream of the detections. The obse rvations are interpreted in the context of rib-like structures wrapping aro und consecutive spanwise structures of opposite sign, a scenario which conf orms with previous observations based on flow visualizations and direct num erical simulations. The probability density function of vorticity vectors i n the plane of mean shear further suggests that the most likely inclination of the rib-like structures to the streamwise direction is about 50 degrees . About 40% of the variances of streamwise and lateral vorticity components is associated with the spanwise vortical structures, apparently resulting from the three dimensionality of these structures; the "remainder" is large ly attributed to the rib-like structures. Spanwise structures account for a bout 60%-80% of the spanwise vorticity variance. (C) 2000 American Institut e of Physics. [S1070-6631(00)51411-1].