Experimental Investigation of the Wake of a Discontinuous Cylinder

Abstract

The effects of a discontinuous cylinder geometry on the near wake structures was investigated experimentally. This ‘discontinuous’ circular cylinder has gaps so that solid segments 5D long are followed by gaps 2.5D long, in a repeating pattern, where D is the diameter of the cylinder. A thin steel plate was used to hold all of the cylinder pieces together. Thus, a three-dimensional (3D) wake was created at the origin with the intent to force the near wake flow to have similar structural characteristics as the far wake behind an ‘infinite/continuous’ cylinder, i.e., a near wake flow with horseshoes or double rollers formed by rapid kinking of Ka´rma´n-like vortices. Since the kinetic energy associated with the fluctuations of these near-wake 3D vortical structures is high, the flow system is considered suitable to clarify the role of these velocity patterns in the entrainment process of wake flows, which is still the subject of controversy. Particle Image Velocimetry (PIV) and Hot-Wire Anemometry (HWA) techniques were used to analyze the flow at two Reynolds numbers, Re = 10000 and 4000, in the wake of the discontinuous cylinder up to x/D = 190 downstream. The development of double rollers resulting from the interaction between the high momentum flow through the gaps and the Ka´rma´n-like vortices formed behind the solid cylindrical segments was confirmed. The Strouhal number of the double rollers in the wake is 0.14. These vortices have a dominant role in the initial wake growth. Thus, the overall flow dynamics are similar to the momentum transfer that takes place at the scale of the intermittent turbulent bulges that protrude from the wake in the far region and that were reported to be associated with double rollers.