Transverse flow over a wavy cylinder was investigated experimentally; surface-pressure distributions and flow visualizations were obtained for a set of wavy cylinders with different axial wavelengths. Significant spanwise pressure gradients were present, resulting in three-dimensional separation lines and the formation of streamwise trailing vortex structures near the geometric nodes. Despite the symmetry of the geometries, the separated flow structures near the geometric nodes were distinctly asymmetric a large fraction of time. Integration of the pressure data revealed greater sectional drag coefficients at the geometric nodes than at the geometric saddles.

Flow-visualization methods were used to explore the structure of streamwise vortices and their interactions with the von Kármán vortices in the immediate wakes of cylinders. The experiments were conducted in a water tunnel at Reynolds numbers from 330–21 000 based on cylinder diameter. Over this entire range of Reynolds number, pairs of counter-rotating streamwise vortices were observed immediately behind the cylinders, with a mean spanwise spacing of approximately one pair per diameter. The streamwise vortices significantly distorted the von Kármán vortices, but only on their upstream-facing sides. The cylinder near-surface-flow topology was found to include a secondary separation line containing ‘‘dominant-foci structures’’ whose spanwise locations correlated with those of the streamwise vortices.

The flowfield behind the wavy-cylinder geometry has been examined using flow visualization, total pressure surveys, and two-component laser Doppler anemometry. The topology of the boundary-layer separation line is linked to the subsequent three-dimensional development of the mean and turbulent structures of the wake. The formation of trailing streamwise vortices behind the nodal points of separation results in a locally narrower wake, a more rapid wake velocity recovery, and suppression of the turbulence development within the separated boundary layer. The dynamic behavior of the trailing vortices suggests that vortex pairing is suppressed by the presence of an axial strain field. The Reynold stress field in the immediate wake exhibited large spanwise variations even though the wavy-cylinder geometry did not affect the spanwise spacing of the dominant foci structures on the leeward side of the cylinder or the spanwise spacing of the rib vortices located in the braid region between consecutive von Karman vortices.

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