**1. Introduction**

Due to nonuniform geometric features of mountainous topography (e.g., slope and curvature), hills can significantly modify wind properties not only by accelerating the wind flow but also by altering the flow direction. As a result, wind profiles in the proximity of hills exhibit both wind speed and direction variation with height, which are generally referred to as twisted wind profiles (TWP), as shown in **Figure 1** [1–4]. Tall buildings built in hilly landforms, such as in Hong Kong and Japan, have a high probability of being attacked by the topography-induced twisted wind. The varying wind directions in a twisted wind profile can produce a highly non-uniform flow field around a building. Exhibiting varying degrees of flow separation and reattachment as well as

#### **Figure 1.** *Schematics of twisted wind (TWP).*

varying vortex structures along with the building height. A non-uniform flow field can induce irregular pressure distribution on the external walls of a building and, thus, asymmetric wind loads. Asymmetric wind loads can increase the torsional loading and hence affect sway and twist responses, which are usually only minor considerations when designing buildings in conventional wind profiles. In addition, an increased correlation between wind load components will occur because the wind turbulence associated with a twisted wind profile spans from the windward face to the side face along with the height of a building as a result of the changing wind angles [5–8]. Thus, in comparison to the conventional wind profile (CWP), the aerodynamic characteristics and flow field in the presence of TWP become more complicated, forming a random and nonlinear high-dimensional dynamic system with obscure and elusive features [9, 10].

Therefore, it is of great importance to conduct a deep and systematic investigation on the pressure patterns and flow patterns of tall buildings exposed to the twisted wind instead of only focusing on the micro aerodynamic features but neglecting to reveal the mechanism. This approach is expected to provide a better understanding and physical interpretation of the twisted-wind effect; thus, it can further provide theoretical guidance to ensure wind safety and to optimize the control measures of wind-induced responses for tall buildings built in mountainous terrain.

To extract the spatial-spectral features and dynamic characteristics from the random pressure field, reduced-order models (ROMs) are recommended as an effective way [11, 12]. The goal of ROMs is to search for a relatively straightforward lowdimensional system to represent a chaotic, high-dimensional system through a process of decomposition, truncation, and error estimation [13]. As one of the most typical and effective ROMs models, Proper Orthogonal Decomposition (POD) is a multivariate statistical technique that aims to extract the dominant patterns from the intercorrelated and dependent original observations [14]. The principle of POD is to find a set of the optimal orthogonal bases called principal components in a second-order statistic sense, and then describe the important information by the superposition of the product of the POD base and modal coefficients [15]. POD has numerous advantages like extracting the most important information, compressing the data dimensionality, eliminating noise interference, and more importantly, identifying the structures and patterns contained in the seemingly disordered original observations. As a result, POD has been widely and successfully applied in the fields of both fluid

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

mechanics and wind engineering to capture coherent structures and pressure patterns contained in the flow field and the associated pressure field [16–19]. However, almost all previous studies have only addressed the topic regarding modal identification of the aerodynamic characteristics of tall buildings exposed to conventional wind profiles, Barely no study to date examines that under TWP [20, 21]. It is evident that the aerodynamic properties under TWP are significantly changed and they are not be simply equivalent to the case of CWP with a certain wind incident angle. Thus, it is necessary to identify the coherent structure and interpretate the structures hidden in the aerodynamic features of a tall building, specifically for twisted wind. Additionally, the POD technique is believed to facilitate deep understanding and physical insight into the unique flow and pressure patterns in the presence of twisted wind. This mode interpretation tool is thus helpful to elucidate the underlying aerodynamic interaction mechanism between TWP and tall buildings.

The remaining parts of this paper are organized as follows. Section 2 introduces the methodology of POD and the related notion/notations. Section 3 describes how we used wind tunnel testing and numerical simulation methods to obtain wind pressure and flow field data; Section 4 illustrates the POD analysis results on the wind pressure, and compares the pressure pattern between CWP and TWP cases; Section 5 shows the POD analysis results on the flow field, and compares the flow patterns between CWP and TWP cases. Section 6 gives concluding remarks on the mode interpretation of the aerodynamic characteristics of tall buildings subject to twisted winds as well as recommendations on future work.
