USE OF PROPER ORTHOGONAL DECOMPOSITION TO INVESTIGATE THE TURBULENT WAKE OF A SURFACE-MOUNTED FINITE SQUARE PRISM

This study aims to investigate the characteristics of the instantaneous velocity field in select vertical planes located in the turbulent wake of a surface-mounted finite square prism. The instantaneous velocity field is obtained from a Large Eddy Simulation (LES); state-of-the-art post-processing methodologies namely the Proper Orthogonal Decomposition (POD) and the swirling strength criterion are used to analyse the flow structure. The study specifically considers the flow over a square prism of aspect ratio AR = 3 (AR = H/D where H is the height and D is the prism width) mounted on a ground plane and located within a thin laminar boundary layer. The Reynolds number based on the freestream velocity and cylinder width is Re = 500, and the angle of incidence is zero. POD is used to extract the dominant flow features, especially those related to vortex shedding, while the swirling strength criterion is used to visualise the smallscale turbulent structures. A principal conclusion of the study is that strong periodicity is observed in a transverse plane located 3.5D downstream of the prism, whereas the vertical mid-plane revealed a more complicated periodic structure. No compelling evidence of half-loop structures was obtained based on the analysis of the flow for a single periodic cycle. On the other hand, strong streamwise vortex structures were observed in the transverse plane in the upper region of the wake, somewhat reminiscent of but not the same as tip vortices. INTRODUCTION Flows over surface-mounted square prisms have been extensively studied in the literature. This flow is not only relevant from an industrial perspective in terms of flow over buildings and chimneys, but also from a research standpoint as the flow around the prism/cylinder and its turbulent wake exhibit multiple complex features. Many studies in the literature have focused on the time-averaged features of this flow, with relatively fewer studies investigating the instantaneous topologies of the flow field. While heuristic approaches have identified the different regions of the prism wake, some recent studies have attempted to propose a unifying flow theory based on the dynamics of the vortical structures shed from the prism. Wang and Zhou [1] suggested that the instantaneous flow tends to form arch-type vortices regardless of aspect ratio and is characterized by two spanwise vertical „legs‟ perpendicular to the ground plane and a connecting horizontal „bridge‟ at the free end. Based on probabilistic analysis and two-dimensional (2D) Particle Image Velocimetry (PIV) measurements, they developed a model of an outward-bulging arch vortex, which explains some of the mean flow characteristics in the wake downstream of the finite prism. More recently, Bourgeois et al. [2] proposed a different flow paradigm, namely the alternating half-loop vortex structure shed by the finite square prism into the wake. They indicated that the mean flow structure of the wake could be explained by averaging the quasi-periodic half-loop structure over the shedding period. A schematic of the alternating half-loop vortex structure is given in Figure 1. A single half-loop structure is made up of a principal core which is aligned approximately perpendicular to the ground plane and a streamwise connector strand. The strand connects the top of the principal core to the base of the principal core of the next half-loop located upstream. Since the principal cores occur on alternate sides of the wake, each connector strand stretches diagonally across the wake, with an orientation which also alternates. A detailed description of the half-loop and educed phase-averaged structure can be found in [2]. Figure 1. Schematic for the alternating half-loop structures in the wake of a surface-mounted square prism. Both of the studies above considered finite square prisms of larger aspect ratio for relatively higher Reynolds numbers. Saha [3] studied the flow past a finite-height surface-mounted square prism using direct numerical simulation (DNS) at a lower Reynolds number of Re = 250 for four different aspect ratios: AR = 2, 3, 4 and 5. June 30 July 3, 2015 Melbourne, Australia 9 6B-3