Circular cylinders fitted with small-scale straight and helical wires: A comparative study on the wire-induced critical effects

• Critical effects of small-scale straight and helical protrusions on cylinder flows are studied.
• While passing from a critical location, the helical wires induce a bistable separation behavior.
• The straight and helical wires affect both the shear layer and near wake when at the critical location.
• They trigger early onset of shear-layer instability while passing the critical location.
• They induce significant extension in the near-wake bubble while passing the critical location.

An experimental investigation is carried out to compare critical flow effects induced by a single straight wire and a system of three-start helical wires, protruding from the surface of a circular cylinder in uniform cross flow. The cinema technique of Particle Image Velocimetry (PIV) is employed. The Reynolds number is 10,000 and the diameter of the wires is 1.2% of the diameter of the cylinder. This size wire is a small-scale wire on the basis of a comparison with the boundary-layer thickness forming around a smooth cylinder. The cylinder fitted with the single straight wire was tested for the specific case when the wire is at θ = 60° on the cylinder surface. This is a critical location, where the spanwise wire yields: (i) the greatest extension in the time-averaged near-wake bubble, (ii) a bistable instability in the shear layer separating at the wire, and (iii) early onset of shear-layer instability on the wire-side shear layer. Spectral analysis shows that spanwise application of this wire on the cylinder at the critical location exerts no significant influence on the strength and coherence of the Karman instability. As for the cylinder-helical wire model, the three wires were arranged to pass, in the plane of visualization, from the critical angle and the base of the cylinder (i.e., θ = +60°, −60° and 180°). Distinct similarities are identified in the flow structure induced by the single spanwise wire and the helical wires in the plane where the wire(s) is/are at the critical location (θ = ±60°). Similar to the same-size spanwise wire, the helical wires under consideration do not alter the coherence of the Karman instability; however, while passing from the critical angle, they induce several intriguing effects to the shear layer and the overall near-wake structure. In the plane where the helical wires pass from the critical location: (i) a bistable phenomenon develops in the shear layer, (ii) significant extension in the near-wake is accompanied by a reduction in the peak levels of Reynolds stress and rms velocity fluctuations, and (iii) the onset of shear-layer instability advances to upstream locations.