Correlation between single-wire and multi-wire tripping of the flow past a circular cylinder

Abstract

An experimental study is carried out to explore if the collective influence of multiple tripwires applied spanwise on a circular cylinder can be correlated to the well-known effects of single-wire tripping from previous studies. Constant Temperature Anemometry and Particle Image Velocimetry tests were conducted on single-wire- and two-wire-fitted cylinders in subcritical flow. Noting that a single tripwire can only influence the shedding frequency when it is within a certain range of locations on the cylinder surface, two scenarios for two-wire-fitted cylinders are noteworthy: (1) One of the two wires can fall within this range of locations. This is the case where flow features for the two-wire-fitted cylinder are identical to the single-wire-fitted one. In this category, the Karman vortex shedding loses its coherence and a bistable separation behavior sets in when the influential wire is at the first critical location (+/−θc1) of the single-wire application. (2) Both tripwires can be in the range of locations where the single tripwire application influences the shedding frequency. This is the scenario where flow is controlled by both tripwires. The most dominant influence comes from the tripwire that is closer to the θc location (another critical location identified by previous studies for single-wire applications). Furthermore, in this category, the drastic attenuation of the Karman instability and the onset of the bistable shear-layer separation occur when one of the wires is at a new θc1 location that is about 5°–7° larger than the θc1 reported for single-wire tripping by previous studies. This change in θc1 comes together with a large degree of near-wake contraction to satisfy the simultaneous entrainment demand needed for the early transition of both shear layers in the two-wire-fitted case. A shortage of fluid in satisfying such an increased entrainment demand is speculated to be the cause of the observed shift in θc1.