# Wind Forces and Flow Patterns of Three Tandem Prisms with a Small Height-Width Ratio
## 4 Conclusions  
Wind tunnel tests and LES were conducted to investigate the dependency of wind loads and the mechanism of flow patterns on the clear spacing for three tandem rectangular prisms with a small height-width ratio (\(H/W=0.4\)). The following conclusions were derived from this present study:  
The coefficients of mean and RMS drag and RMS roof uplift of downstream prisms have large local peaks and increase rapidly and then decrease rapidly at the spacing ratio \(0.4<S/W\leq 0.8\), and the mean roof uplift coefficient reaches the local peak at \(S/W=0.7\). This phenomenon is much different from that of a small group of tandem prisms with a large \(H/W\) and a large group of tandem prisms with a small \(H/W\). In addition, the above local peaks of wind forces of prisms at \(S/W=0.7\) are much larger than wind forces of prisms at the adjacent spacing ratios. If the above phenomenon is not found in the experiment, the wind loads for the design of low-rise buildings at \(S/W=0.7\) will be significantly underestimated, resulting in potential safety hazards. Furthermore, at \(S/W=0.7\), the tap pressure time histories of downstream prisms are non-stationary with abrupt changes at irregular moments. Within the tap pressure time histories without abrupt changes, mean and RMS pressure distributions on downstream prisms are asymmetric, but these are symmetric to those after the abrupt changes in pressure time histories. Compared with the non-stationary tap pressure time histories, wind forces time histories of the downstream prism are stationary at \(S/W=0.7\), and these are much different from those for a small group of tandem prisms with a large \(H/W\), where both tap pressure and wind force time histories of the downstream prism are non-stationary.  
The mechanism of the flow pattern at \(S/W=0.7\) was analyzed by LES, and the asymmetric time-averaged wake regime was observed, which reveals that the asymmetric pressure distributions on downstream prisms are attributed to the asymmetric flow between prisms. There are two types of asymmetric wake modes between prisms. The duration of each asymmetric wake mode is rules, and the irregular switching between two modes in the prism gap causes the abrupt changes in the pressure time histories of downstream prisms. Because the bias directions of two asymmetric flow are symmetric along the incident flow, the irregular switching in wake modes has little effect on the wind force pressure time histories of downstream prisms and keeps these wind forces stationary. In addition, the rapid increase and decrease in the wind forces of downstream prisms at \(0.4<S/W\leq 0.8\) are caused by the emergence and disappearance of the asymmetric wake between prisms. This special asymmetric time-averaged wake regime in this paper was not observed in previous studies on flow fields around a group of tandem prisms.  
It should be mentioned that the geometric characteristics of prisms are one key factor to affect flow patterns among prisms, and the study of this topic will be carried out in the future.  
All authors contributed significantly to this study. [PERSON] collected the data, performed the analysis, and wrote the paper; [PERSON] revised the paper. All authors have read and agreed to the published version of the manuscript. This research was funded by the Fundamental scientific research business expenses of Beijing Jiaotong University, grant numbers C17 JB00250.  
Not applicable.