Space-Time Imaging of Streamwise Vorticity in a Turbulent Near-Wake

Space-Time Imaging of Streamwise Vorticity in a Turbulent Near-Wake. A technique of cinematographic particle image velocimetry [9] allows acquisition of a sequence of images of streamwise vorticity concentrations, similar to those shown in the preceding layout, over a period of one Karman cycle. By stacking up the instantaneous patterns of positive (red) and negative (yellow) streamwise vorticity, it is possible to obtain the space-time images shown, with the time axis aligned with the incident flow. Each of the black rings corresponds to a concentration of vorticity from the cinema sequence. These images show that the spanwise wavelength is of the order of one cylinder diameter, the identity of the streamwise vorticity is maintained over at least one-half Karman cycle, and the long wavelength of the entire pattern corresponds to the Karman wavelength [8,10]. Related studies are described in [7,9,11].