Critical effects of a spanwise surface wire on flow past a circular cylinder and the significance of the wire size and Reynolds number

An experimental study is conducted on flow past a circular cylinder fitted with a single spanwise wire on its surface. The work investigates the dependency of the critical wire locations on the wire size and Reynolds number, and examines the near wake and vortex shedding characteristics in an effort to advance the understanding of the critical wire effects beyond the existing literature. The Reynolds number is varied from 5000 to 30 000, and the wire diameter is varied from 2.9% to 5.9% of the cylinder diameter. All wires are larger than the boundary-layer thickness forming around a comparable smooth cylinder. Constant Temperature Anemometry and hydrogen bubble visualization are used as the flow diagnostic tools. The frequency and strength of the Karman instability are shown to vary with the wire location at any given Reynolds number nearly in an inverse fashion. For all the Reynolds numbers and wire sizes considered, two types of critical locations are shown to exist on the cylinder surface for the application of a wire. These locations are associated with the attenuation and amplification of the Karman instability, and in accord with the existing literature, are denoted as θc1 and θc2, respectively. The present work reveals that θc2 consists of a wide range of locations which remains unaffected from the wire size and Reynolds number, while θc1 is a relatively distinct location on the cylinder surface and depends on both the Reynolds number and wire size. For a given Reynolds number, increasing the wire size decreases θc1. For a given wire size, increasing the Reynolds number from 5000 to 15 000 increases θc1, and past 15 000, θc1 remains unaffected from the Reynolds number. When a wire is at θc1, even though, for the majority of the time the regular formation of Karman vortices ceases, the present data also reveals intermittent, short time periods where the regular shedding resumes.