Structure and flow patterns in turbulent wakes

A pattern recognition technique is used to detect and identify patterns and structures embedded in the velocity field of a turbulent wake behind a cylinder. Two component velocities u and ν or u and w are measured with eight X‐wire anemometers aligned with the vertical and spanwise directions in the wake at x/D=420 and Re=1600. The velocity patterns educed as averaged patterns or instantaneous structures are consistent with the presence of double‐roller eddies, whose legs are continuously stretched by the rate of strain. Experimental u and ν data suggest that the rollers are connected at the top, with vorticity parallel to the cylinder axis, forming a horseshoe vortex. The fine‐scale activity, estimated by the envelope of the second derivatives, with respect to time of the velocity signals, confirms that this kind of structure can account for both the uν correlation and the continuous entrainment of potential flow into the wake.