Effects of streamwise aspect ratio on the spatio-temporal characteristics of flow around rectangular cylinders

Abstract

Turbulent flows over rectangular cylinders with streamwise aspect ratios (AR) of 1, 2 and 4 were experimentally studied using time-resolved particle image velocimetry at a Reynolds number based on free-stream velocity and cylinder height of 16200. These aspect ratios represent cylinders for which the mean separated shear layers from the leading edges are shed directly into the wake region (AR1), reattach over the cylinder (AR4), and an intermediate case (AR2). The effects of aspect ratio on the transport phenomena, turbulence production and its budget are investigated. The spatio-temporal characteristics and the interactions between the small-scale Kelvin-Helmholtz (KH) vortices and the energetic large-scale von-Kármán (VK) vortices are examined using frequency spectra, while the dynamics of the coherent structures are elucidated using proper orthogonal decomposition (POD). The mean flow topology reveals a massive recirculation bubble in the wake of the AR2 cylinder, compared to AR1 and AR4. While a single dominant vortex shedding frequency is observed for AR1 and AR4, a dual vortex shedding instability is manifested for the AR2 cylinder, which is intimately connected with the intermittent reattachment and detachment of flow over the AR2 cylinder. The interactions between the KH and VK vortices are shown to vary non-linearly with aspect ratio, as the flow transitions from the fully detached case (AR1) to fully reattached case (AR4). The differences in the vortex shedding process are revealed in the dynamics of the first POD mode pair. For AR1 and AR4, the fundamental shedding frequency is dominant and the first POD mode pair contain similar energy content, suggesting regular vortex shedding patterns. For AR2, dual shedding frequencies are observed with significant cycle-to-cycle variation, suggesting irregular vortex shedding patterns.