**6. Conclusions**

This study aims to investigate the essential mechanism of nonlinear dynamic systems, i.e., the random pressure field and the flow field over a tall building

*Mode Interpretation of Aerodynamic Characteristics of Tall Buildings Subject to Twisted… DOI: http://dx.doi.org/10.5772/intechopen.103757*

### **Figure 16.**

*The first three POD mode patterns of the wake velocity field at horizontal plane of 1/6 H under TWP30 (a) contour of u component; (b) contour of v component; (c) streamline pattern.*

submerged in the turbulent boundary layer. The POD method is employed to extract the spatial/spectral features of the pressure pattern and flow pattern, and thus provides a physical mode interpretation of the aerodynamic characteristics of tall buildings subject to twisted winds. The primary conclusions are as follows:


**Figure 17.**

*Correlation of the first three POD mode coefficients of two horizontal planes at height of z = 1/6H and 1/2H for (a) CWP; (b) TWP30.*

and the energy proportion of the dominant modes, and this modification in the POD mode pattern causes the twisted flow to have a significantly different impact on the aerodynamic properties of buildings. Specifically,


*Mode Interpretation of Aerodynamic Characteristics of Tall Buildings Subject to Twisted… DOI: http://dx.doi.org/10.5772/intechopen.103757*

> symmetrically distributed, but experience an initially upward and then downward process from the windward edge to the leeward edge. Such finding indicates that flow separation associated with flow reattachment simultaneously occurs at the side surface of a building with a large side ratio of 3:1. Similar to the 0° wind incident angle case, the asymmetrical second mode of CWP is highly related to the vortex shedding phenomenon. Affected by TWP, the continuous varying wind directions along the building height cause the reattachment location on the rightside surface moves forward, while that on the left-side surface basically disappears. Overall, the TWP has a similar influence on the mode shape of the windward and leeward surfaces for buildings attacked by 0°and 90° approaching wind.

3.For the flow pattern, its spatial–temporal features under CWP and TWP are successfully captured by POD. The main mode pattern of CWP is found to be controlled by the ILEVS and TEVS phenomenon, which corresponds to frequencies of f11 = 0.04 and f12 = 0.102 respectively. Whereas, the predominant POD mode of TWP is highly related to AEVS with a frequency of f3 = 0.048. This modal pattern is reported to twist towards the wind direction with a more regular shape, even though it becomes less intense. Moreover, the spanwise correlation of the main POD mode at two planes of different heights is also significantly enhanced by TWP.

The study has comprehensively revealed the potential mechanisms of the twistedwind effect that underlies the pressure field and flow field of tall buildings exposed to TWP. The mode interpretation of aerodynamic characteristics is expected to shed light on the fluid-induced interaction between twisted flows and tall buildings, and provides a better understanding of the underlying mechanism of the twisted-wind effect. The obtained conclusions facilitate useful references for designing and constructing tall buildings in hilly terrain where twisted wind occurs frequently, and more importantly, for TWP-induced vibration control measures.
