The fine scale three-dimensional structures usually associated with streamwise vortices in the near wake of a circular cylinder have been studied at Reynolds numbers ranging from 170 to 2200. Spatially continuous velocity measurements along lines parallel to the cylinder axis were obtained with a scanning laser anemometer. To detect the streamwise vortices in the amplitude modulated velocity field, it was necessary to develop a spatial decomposition technique to split the total flow into a primary flow component and a secondary flow component. The primary flow is comprised of the mean flow and Strouhal vortices, while the secondary flow is the result of the three-dimensional streamwise vortices that are the essence of transition to turbulence. The three-dimensional flow amplitude increases in the primary vortex formation region, then saturates shortly after the maximum amplitude in the primary flow is reached. In the near-wake region the wavelength decreases approximately like Re−0.5, but increases with downstream distance. A discontinuous increase in wavelength occurs below Re = 300 suggesting a fundamental change in the character of the three-dimensional flow. At downstream distances (x/D = 10-20), the spanwise wavelength decreases from 1.42D to 1.03D as the Reynolds number increases from 300 to 1200.

Detailed velocity measurements have been made to investigate the structure of the low-frequency vortex dislocations that appear in the indigenous wake of a circular cylinder. It is shown that one mechanism for the production of vortex dislocations is the superposition of two waves with slightly different frequencies, where the higher frequency wave is parallel to the cylinder axis and the lower frequency is an oblique wave. In contrast to the discrete vortex concept that implies discontinuous shedding frequencies, the measured amplitude and phase distribution of the interacting waves are shown to be continuous along the span of the cylinder. Both waves (parent modes) exist simultaneously and overlap in the wake. The difference mode associated with the vortex dislocations is produced by the nonlinear interaction of the parent modes, and its maximum amplitude is at the location where the parent modes have comparable magnitude. The spanwise phase and amplitude distributions of the difference mode are ‘‘predicted’’ by taking the product of the complex wave amplitudes of the parent modes.

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