**3. Aerodynamics of low-rise buildings**

Bluff body aerodynamics, and in particular fluctuating pressures on low-rise buildings immersed in turbulent flows, are associated with the complex spatial and temporal nature of winds [26]. This complexity mainly comes from the transient nature of incident turbulent winds, and the fluctuating flow pattern in the separation bubble. The flow in the separated shear layer is associated with fluctuations in the velocity field leading to the evolution of instabilities. The flow physics are dependent on upstream turbulence intensity, integral length scale, as well as Reynolds number. The later makes it difficult to scale up loads based on pressure and force coefficients as the process can be highly nonlinear, which is the case, for

example, when full-scale pressure coefficients do not meet those from small-scale aerodynamic testing (**Figure 5**). Not only free stream turbulence impacts the flow pattern around bluff bodies, but also it can impact the thickness and length of the wake, hence significantly altering aerodynamic pressures.

In order to propose mitigation alternatives to minimize damages induced by windstorms to low-rise buildings, it is vital to understand how peak loads and spatial correlation of pressures are developed. As a first step to understand this mechanism, a true simulation of flow characteristics in accordance to real full-scale winds is necessary. There are common and valuable resources for the physical investigation of wind effects on structures, including small-scale wind tunnel testing, large-scale testing an open-jet laboratory, and full-scale field measurements.

According to Ref. [27], at relatively large-scale wind tunnel models, it is very difficult to model the full turbulence spectrum, and only the high-frequency end is matched [28]. For instance, as described in Ref. [29], more than 50% discrepancies in wind tunnel aerodynamic measurements are realized from six reputable centers for roof corner pressure coefficients and peak wind-induced bending moment in structural frames of low-rise building models. Therefore, selecting an appropriate testing protocol, including model scale ratio, for physical testing to minimize discrepancies in aerodynamic loads is essential. This can be achieved by considering constraints on laboratory testing that limits producing the large-scale turbulence and the inherent issues with limited integral length scale [30].

The literature raises questions regarding the adequacy of predicting full-scale pressures on low-rise buildings tested in flows that lack the large-scale turbulence. For instance, although a good agreement was observed between a wind tunnel testing on a generic low-rise building and full-scale data, discrepancies were shown in reproducing the largest of peak pressure near roof edges [31]. **Figure 5** shows minimum pressure coefficients at a building corner and eave level for the full-scale

Texas Tech University (TTU) experimental building, along with wind tunnel measurements [14]. The local peak pressures are weaker in wind tunnel testing than those at full-scale. For instance, at 65° wind direction angle, the wind tunnel reproduced minimum pressure coefficient of 4.3, while the full-scale field measurement is 6.8, and at 250° wind direction angle, wind tunnel shows 2.2, while the full-scale data shows 5.3. Therefore, there would be a major doubt on estimating the correct wind loads for design purposes based on wind tunnel testing. To describe this mismatch, first we need to define the concept of the energy cascade

*Aerodynamics of Low-Rise Buildings: Challenges and Recent Advances in Experimental…*

*Energy cascade in a turbulent flow (adapted from Ref. [32]).*

*DOI: http://dx.doi.org/10.5772/intechopen.92794*

As depicted in **Figure 6**, the structure of a turbulent wind flow is constituted from a combination of large eddies and small eddies. In physical space, the large eddies are broken into smaller and smaller eddies with different spectral energy contents in various frequency ranges. In conventional wind tunnel testing, it can be challenging to appropriately reproduce low-frequency turbulence, which overwhelmingly contributes to the integral length scale and intensity of fluctuations. This leads to significant disparity among the wind tunnel flows and the target fullscale field flow conditions. As observed in **Figure 5**, this mismatch affected the local vorticity at edges and corners of a low-rise building model tested in a wind tunnel and resulted in local pressures weaker than those at full-scale. To alleviate these issues and to replicate the ABL flow characteristics for aerodynamics of buildings, advanced research in computational and experimental methods is essential.

**4. Computational fluid dynamics in aerodynamics of buildings**

In recent years, computational fluid dynamics (CFD) simulations have witnessed a spread use and applications as a potential tool in aerodynamic investigations of buildings. However, by considering the constraints of experimental testing in wind tunnels that limit producing the low-frequency large-scale turbulence and the inherent issues with limited integral length scale, implementing appropriate turbulence closure in CFD and developing a proper inlet transient

in a flow.

**37**

**Figure 6.**

**Figure 5.** *Minimum pressures at building corner (adapted from Ref. [14]).*

*Aerodynamics of Low-Rise Buildings: Challenges and Recent Advances in Experimental… DOI: http://dx.doi.org/10.5772/intechopen.92794*

#### **Figure 6.** *Energy cascade in a turbulent flow (adapted from Ref. [32]).*

example, when full-scale pressure coefficients do not meet those from small-scale aerodynamic testing (**Figure 5**). Not only free stream turbulence impacts the flow pattern around bluff bodies, but also it can impact the thickness and length of the

In order to propose mitigation alternatives to minimize damages induced by windstorms to low-rise buildings, it is vital to understand how peak loads and spatial correlation of pressures are developed. As a first step to understand this mechanism, a true simulation of flow characteristics in accordance to real full-scale winds is necessary. There are common and valuable resources for the physical investigation of wind effects on structures, including small-scale wind tunnel testing, large-scale testing an open-jet laboratory, and full-scale field measurements. According to Ref. [27], at relatively large-scale wind tunnel models, it is very difficult to model the full turbulence spectrum, and only the high-frequency end is matched [28]. For instance, as described in Ref. [29], more than 50% discrepancies in wind tunnel aerodynamic measurements are realized from six reputable centers for roof corner pressure coefficients and peak wind-induced bending moment in structural frames of low-rise building models. Therefore, selecting an appropriate testing protocol, including model scale ratio, for physical testing to minimize discrepancies in aerodynamic loads is essential. This can be achieved by considering constraints on laboratory testing that limits producing the large-scale turbulence

The literature raises questions regarding the adequacy of predicting full-scale pressures on low-rise buildings tested in flows that lack the large-scale turbulence. For instance, although a good agreement was observed between a wind tunnel testing on a generic low-rise building and full-scale data, discrepancies were shown in reproducing the largest of peak pressure near roof edges [31]. **Figure 5** shows minimum pressure coefficients at a building corner and eave level for the full-scale

wake, hence significantly altering aerodynamic pressures.

*Aerodynamics*

and the inherent issues with limited integral length scale [30].

**Figure 5.**

**36**

*Minimum pressures at building corner (adapted from Ref. [14]).*

Texas Tech University (TTU) experimental building, along with wind tunnel measurements [14]. The local peak pressures are weaker in wind tunnel testing than those at full-scale. For instance, at 65° wind direction angle, the wind tunnel reproduced minimum pressure coefficient of 4.3, while the full-scale field measurement is 6.8, and at 250° wind direction angle, wind tunnel shows 2.2, while the full-scale data shows 5.3. Therefore, there would be a major doubt on estimating the correct wind loads for design purposes based on wind tunnel testing. To describe this mismatch, first we need to define the concept of the energy cascade in a flow.

As depicted in **Figure 6**, the structure of a turbulent wind flow is constituted from a combination of large eddies and small eddies. In physical space, the large eddies are broken into smaller and smaller eddies with different spectral energy contents in various frequency ranges. In conventional wind tunnel testing, it can be challenging to appropriately reproduce low-frequency turbulence, which overwhelmingly contributes to the integral length scale and intensity of fluctuations. This leads to significant disparity among the wind tunnel flows and the target fullscale field flow conditions. As observed in **Figure 5**, this mismatch affected the local vorticity at edges and corners of a low-rise building model tested in a wind tunnel and resulted in local pressures weaker than those at full-scale. To alleviate these issues and to replicate the ABL flow characteristics for aerodynamics of buildings, advanced research in computational and experimental methods is essential.
