Time-resolved flow dynamics and Reynolds number effects at a wall–cylinder junction

Abstract

This study investigated the physics of separated turbulent flows near the vertical intersection of a flat wall with a cylindrical obstacle. The geometry imposes an adverse pressure gradient on the incoming boundary layer. As a result, flow separates from the wall and reorganizes to a system of characteristic flow patterns known as the horseshoe vortex. We studied the time-averaged and instantaneous behaviour of the turbulent horseshoe vortex using planar time-resolved particle image velocimetry (TRPIV). In particular, we focused on the effect of Reynolds number based on the diameter of the obstacle and the bulk approach velocity, $\mathit{Re}_{D}$. Experiments were carried out at $\mathit{Re}_{D}$: $2.9\times 10^{4}$, $4.7\times 10^{4}$ and $12.3\times 10^{4}$. Data analysis emphasized time-averaged and turbulence quantities, time-resolved flow dynamics and the statistics of coherent flow patterns. It is demonstrated that two large-scale vortical structures dominate the junction flow topology in a time-averaged sense. The number of additional vortices with intermittent presence does not vary substantially with $\mathit{Re}_{D}$. In addition, the increase of turbulence kinetic energy (TKE), momentum and vorticity content of the flow at higher $\mathit{Re}_{D}$ is documented. The distinctive behaviour of the primary horseshoe vortex for the $\mathit{Re}_{D}=12.3\times 10^{4}$ case is manifested by episodes of rapid advection of the vortex to the upstream, higher spatio-temporal variability of its trajectory, and violent eruptions of near-wall fluid. Differences between this experimental run and those at lower Reynolds numbers were also identified with respect to the spatial extents of the bimodal behaviour of the horseshoe vortex, which is a well-known characteristic of turbulent junction flows. Our findings suggest a modified mechanism for the aperiodic switching between the dominant flow modes. Without disregarding the limitations of this work, we argue that Reynolds number effects need to be considered in any effort to control the dynamics of junction flows characterized by the same (or reasonably similar) configurations.