| The near-wake of a cylinder with a surface nonuniformity is addressed. A helical wire pattern about the surface of a cylinder, whereby the wire has a diameter an order of magnitude smaller than the cylinder diameter, induces substantial alterations of both the quasi-two-dimensional (sectional) and the three-dimensional wake structure. A technique of high-image-density particle image velocimetry is employed in order to characterize the instantaneous and averaged flow patterns in three orthogonal planes in the near-wake.; In order to provide a basis for interpretation of the wake induced by the helical wire pattern, reference states of the wake from the plain cylinder are defined over a range of Reynolds number, based on cylinder diameter, up to 10,000.; For the case of the cylinder with the helical winding, extreme cases of quasi-laminar and turbulent near-wakes from the corresponding plain (smooth) cylinder are considered. Quasi-two-dimensional (sectional) patterns of instantaneous or phase-referenced vorticity show large variations with the effective angle of incidence of the helical wire at a given spanwise location. Extreme types of patterns involve: (i) a relatively narrow wake with lack of large-scale vorticity concentrations; and (ii) a wide wake with well-defined, large-scale concentrations that are formed well downstream of the cylinder. The effective width of the near-wake can vary by as much as 50%, despite the very small diameter of the helical wire. A distinctive, additional feature at Re = 104 is the occurrence of coherent patterns of small-scale, shear layer vortices, which are evident at all values of incidence angle.; Corresponding patterns of the flow structure in planes oriented along the span of the cylinder show deterministic features of three-dimensionality, which are linked to the aforementioned sectional patterns. For the quasi-laminar wake at low Reynolds number, the three-dimensional structure of the near-wake undergoes a well-defined transformation with increasing distance from the base. Space-time volumes of vorticity and velocity evolve from a predominantly in-line form to a staggered form. For the turbulent wake at higher Reynolds number, the three-dimensional patterns are less deterministic, but they show a spanwise spatial periodicity that is correlated to the helical wire pattern. |
