Characterization of near-wake structures around wall-mounted prisms and its implications to wind force generation

Abstract

The presence of large-scale coherent structures in the wake of a generic wall-mounted prism has attracted great attention for several decades due to its close association with numerous engineering problems. In wind engineering, this flow is related to the spatio-temporal evolution process of three-dimensional and complex wind flow field around a building, which in turns governs the relationship between the fluctuating velocity field and wind loading, and the ventilation and pollutant dispersion around and behind the building. The major task of this study is focusing on how to identify and extract the energetic large-scale coherent motions with a relatively pure physical meaning. The inherent chaotic feature of the highly three-dimensional and turbulent wake flow causes huge troubles to the seeking of a proper solution due to frequency modulations, phase jitter, and oscillations. This study takes advantage of the original proper orthogonal decomposition (POD) technique to overcome the difficulties and effectively extract the energetic coherent wake structures by introducing various POD variants. The planar time-resolved particle image velocimetry (PIV) technique is adopted to obtain the time-varying turbulent wind velocity fields around wall-mounted prisms of seven different geometries. Synchronized measurement of fluctuating wind pressure on the prism faces is also conducted in several cases. As the supplement to the limitation of planar PIV velocity fields, CFD (Computational Fluid Dynamics) using large-eddy simulation (LES) is also performed to obtain the three-dimensional velocity fields around a short square prism. The current study mainly investigates the following areas: (i) the definition, identification, and significance of transient extreme wake events derived from these energetic large-scale anisotropic coherent structures; (ii) the physical understanding of antisymmetric and symmetric coherent motions and their association with typical Karman vortex shedding and arch-type vortex shedding as well as the implications to across-wind force generation mechanism; (iii) the effects of geometry parameters (aspect ratio and side ratio) on the characteristics of the three-dimensional building wake structures and coherent flow motions; (iv) the comparative study of these coherent motions obtained from experimental and CFD data and the possibility of investigation on other physical phenomena, such as the regular arch-type vortex shedding and its connection with tip vortex system, from the three-dimensional CFD data. Meanwhile, the characteristics of time-averaged mean wake patterns are also summarized and documented. In terms of findings, this study reveals the coexistence of antisymmetric and symmetric coherent structures with different occurrence probabilities, representing Karman-type and arch-type vortex shedding, respectively. These energetic low-order coherent structures are found to govern grossly the dominant features of extreme wake patterns producing peak flow events such as strongest and weakest air ventilation and pollutant transport. In wind loadings, the large-scale antisymmetric coherent motions are demonstrated to be closely associated with the generation of peak across-wind forces. Ascribe to the downwash flow, the flow unsteadiness at the free end is observed to modulate the spanwise in-phase vortex activities with a low-frequency oscillation.