# Wind Forces and Flow Patterns of Three Tandem Prisms with a Small Height-Width Ratio
## 1 Introduction  
Due to wind interference effects from surrounding prisms, wind forces and flow characteristics of a group of prisms is much different from those of an isolated prism. The flow patterns and interfered wind loads of a small group of three-dimensional (3D) prisms with a large height-width ratio (\(H/W\)) and two-dimensional (2D) prisms have been widely studied both experimentally [1; 2; 3; 4; 5] and numerically [6; 7; 8]. In contrast, for a small group of prisms with a small \(H/W\), which is usually applied for factories and rural low-rise houses, few previous studies focus on the relations between its flow patterns among prisms and the wind loads acting on prisms have been conducted. When a prism is placed in line with another prism parallel to the incident flow, the arrangement is called tandem arrangement. The flow patterns among the tandem 3D prisms with a large \(H/W\) and tandem 2D square prisms were divided into four regimes depending upon the prism spacing [2; 3], namely the reattachment flow (\(S/W\leq 2\)), bistable flow (\(2<S/W\leq 3.5\)), stable synchronized flow (\(3.5<S/W\leq 50\)), and unstable synchronized flow (\(S/W>50\)), where \(S\) is the clear spacing between two prisms and \(W\) is the width of the windward wall of prisms. When the flow pattern of prisms changes from the reattachment flow to the bistable flow, the root-mean-square (RMS) fluctuating wind loads acting on 3D prisms with a large \(H/W\) and 2D square prisms rapidly increased, and two types of instantaneous wake modes were observed in the prism gap. One mode is symmetric along the wind direction, while the other mode is asymmetric along the wind direction; the time-averaged wake in the prism gap and mean pressures of the downstream prism aresymmetric along the incident flow [9], which is ascribed to the fact that the instantaneous asymmetric flow in the prism gap is alternately and periodically biased to one of the side walls of the downstream prism. [PERSON] et al. [7] showed the flow field among tandem prism with \(H\)/\(W\) = 7 in the central vertical section at 0.5 < \(S\)/\(W\)\(\leq\) 9. The results demonstrated that the upwash flow formed at the bottom behind the upstream prism had great effects on the flow between prisms. For a small group of prisms with a small \(H\)/\(W\), the interfered wind loads acting on prisms were usually studied by the pressure measurement with wind tunnel tests. [PERSON] et al. [10] discussed mean wind pressure coefficients on a small group of prisms with \(H\)/\(W\) = 0.7. The results demonstrated that the staggered arrangement increased the suction on the roof of the downstream prism at the small prism spacing. [PERSON] et al. [11] investigated the interference effects on the three prisms with a gable roof, where the \(H\)/\(W\) was about 0.13, and there was a notable increase in the magnitude of negative mean pressure coefficients on the prisms at the small prism spacing. In addition, the flow patterns among a large group of prisms with \(H\)/\(W\) < 1 were divided into three categories depending on the prism spacing [12; 13] including skimming flow, wake interference flow, and isolated roughness flow. However, few previous studies focused on the flow patterns of a small group of prisms with a small \(H\)/\(W\), and the prism number had been confirmed as a key factor determining the wind interference effects of prisms [14]. Therefore, it is important to study the mechanism of flow patterns among a small group of prisms with a small \(H\)/\(W\) and features of wind loads acting on prisms in different flow patterns. In summary, to ensure both the safety and economy of the wind-resistant design of prism-like low-rise buildings, more studies should be conducted to investigate the wind loads and the mechanism of flow patterns of a small group of prisms with a small height-width ratio, as wind loads might be larger than those of a large group of prisms.