Experimental investigations on flow over a circular cylinder placed in a circular pipe

Abstract

Flow over a circular cylinder placed inside a circular pipe is studied experimentally to understand the influence of Reynolds numbers (ReD = DUav/υ, where D is the diameter of the pipe, Uav is the average velocity in the pipe, and υ is the kinematic viscosity of the fluid) and blockage effects. In the present configuration, the influence of confinement, aspect ratio, upstream turbulence, shear, and end conditions coexists together. The wake dynamics of such a configuration are seldom reported in the literature. The Reynolds number range covered in the present study is ReD = 200-2.0 × 105. Four different flow regimes are defined based on the Reynolds number: steady, unsteady laminar, transition, and turbulent. In the unsteady laminar regime, the Strouhal number (St = fd/Uav, where f is the frequency of vortex shedding and d is the diameter of the bluff body) increases gradually. In the transition regime, a gradual fall in St is observed for all blockage ratios (d/D). In the turbulent regime, the upstream flow becomes fully turbulent, and the Strouhal number remains constant. The spanwise pressure distribution is influenced due to the blockage effects near the wall. The separation point moves 20° toward the rear stagnation point close to the wall compared to the center in the turbulent regime. A universal relation for the drag coefficient is proposed based on the pressure at the stagnation and separation points. The universal Strouhal number defined based on the wake width and velocity at the separation (Us) is shown to be independent of the blockage ratio. The results presented in the manuscript are relatively new in the domain of confined bluff body flows and will serve to enhance the general understanding of confined bluff body vortex dynamics.