| An experimental study is carried out to investigate if the collective influence of multiple spanwise tripwires fitted on a circular cylinder in subcritical flow can be correlated to the knowledge accumulated by previous studies on the influence of only one tripwire on its own. In an effort to explore this, the vortex shedding frequency, characteristics of the near wake flow topology, shear layer separation behaviour, and the strength and coherence of the von Karman vortex shedding were determined for cylinders fitted with one, two, three, four, five, and eighteen spanwise tripwires using measurement techniques such as Particle Image Velocimetry (PIV), Constant Temperature Anemometry (CTA), and Hydrogen Bubble Flow Visualization. The single-wire tripping was selected as the baseline case because it is the simplest form of tripping, and much is known about its effects from previous studies. Based upon the effective range of angles from thetat to theta b, defined by Nebres and Batill (1992), four scenarios could be identified for a multi-wire fitted cylinder. If none of the tripwires of the multi-wire fitted cylinder were within this range, the cylinder was analogous to a smooth cylinder. If only one tripwire was within the thetat to theta b range, the flow around the multiple-wire fitted cylinder can be exactly inferred from the flow around the single-wire fitted cylinder. If two tripwires of the multi-wire fitted cylinder were within the thetat to theta b range, the Strouhal number could be inferred by considering an accumulative influence of both of the tripwires. The fundamental angles such as the first and the second critical angles could also be found for the multi-wire-fitted cases by considering the fundamental angles of the single-wire-fitted case. Here, the total amount of fluid available to be entrained in the shear layers needs to be considered to predict the near wake flow behaviour; which may cause a delay in the location of the first critical angle (thetac1 ) in certain special circumstances. For the cases considered in the present study, when more than two tripwires are within the thetat to thetab range, the Strouhal number of the multi-wire fitted cylinders is similar to that of the smooth cylinder. In the last scenario, the whole cylindrical configuration needs to be considered at once rather than using the data from a single-wire fitted-cylinder to infer the properties of the flow. |
