Aerodynamic shape optimization of rectangular and elliptical double-skin façades to mitigate wind-induced effects on tall buildings

https://doi.org/10.1016/j.jweia.2021.104586Get rights and content

Highlights

  • Wind-induced effects of rectangular and elliptical double facades on tall buildings.

  • Applying the CFD-based RANS modeling to study aerodynamics of a CAARC building.

  • Aerodynamic shape optimization of double façade design with the Genetic algorithm.

  • Implementation of response surface model (RSM) and deign of experiment (DOE).

Abstract

Double-skin façades (DSFs) are high-efficiency systems traditionally used to improve the natural ventilation of buildings for energy-saving purposes and their aesthetic improvement. There are opportunities to enhance their functionality by applying them to improve building aerodynamics. This study aims at developing the preliminar data set that would assist with design of smart morphing facades (also known as Smorphacades). For this purpose the a DSF for rectangular and elliptical tall buildings is studied with the final goal of minimizing the aerodynamic loads on such structures. For this purpose, an integrated framework including numerical modeling and statistical analysis was developed. The design of experiment (DOE) method was applied to produce a sufficient dataset based on computational fluid dynamics (CFD) simulation of two-dimensional (2D) scaled models that were validated with available wind tunnel data. According to the data points recommended by the DOE method, a considerable number of CFD simulations were performed, and the drag coefficients of the building and double-skin façade were separately calculated. A prediction model based on the response surface methodology (RSM) was developed to estimate the drag coefficient for other cases inside the design space. Based on the fitted RSM, the Genetic algorithm was applied to search for the optimized Smorphacade shapes. The results indicated that the integrated smorphacade system could significantly mitigate wind-induced drag forces on the building at all attack angles by modifying wind-induced pressure around the building and weakening vortex shedding by interrupting flow separation and ejecting airflow into a lower-pressure area. This research proves that there are opportunitie to integarte the architectural and energy applications of smart double skin facades with wind-reducing effects as a promising solution for overcoming existing challenges for controlling wind-induced load and response of tall buildings.

Introduction

Recent advances in construction techniques along with increasing urbanization have resulted in an increasing trend toward worldwide construction of tall buildings. Such structures are becoming more and more slender and flexible due to an increase in their height and lighter construction material, making them vulnerable to wind-induced vibrations (Hou and Jafari, 2020; Jafari and Alipour, 2020, Jafari et al., 2019, Micheli et al., 2017, Micheli et al., 2019, Micheli et al., 2020a, Micheli et al., 2020b, Micheli et al., 2020c, Micheli et al., 2021). Double-skin façades (DSFs), also known as double façades, ventilated façades, and adaptive skins, have gained growing attention within the architectural engineering community (see Fig. 1). DSFs are commonly implemented to improve indoor climate, save energy, and block sunlight (Pomaranzi et al., 2020). Farrokhzad and Nayebi (2014) showed that the double-skin glass façades could effectively balance energy transfer between indoor and outdoor spaces in high-rise buildings. Stec et al. (2005) placed plants inside double-skin façades, with results indicating that such a combined system could considerably improve the indoor climate and save energy. Moreover, the application of double façades for harvesting energy by the installation of vertical-axis wind turbines between façades (Hassanli et al, 2017, 2018a, 2018b) and building-integrated photovoltaics (BIPVs) has gained much attention (Agathokleous and Kalogirou, 2016; Domjan et al., 2020; Yang et al., 2019). For more information about other applications of such systems, readers are referred to review papers published by Barbosa and Ip (2014) about natural ventilation, by Jiru et al. (2011) on heat transfer aspects, and by Pomponi et al. (2016) related to climate improvement. Both advantages and existing challenges in using these structures can be reviewed in a study by GhaffarianHoseini et al. (2016).

In addition to architectural and energy-saving applications of DSFs, conceptual designs have shown that they can efficiently alleviate wind-induced loads acting on a high-rise building (Abdelaziz et al., 2021; Hu et al., 2019; Moon, 2009). There have been a few studies focusing on such effects of double façades on wind response of tall buildings, and most have used wind-tunnel testing on very limited configurations specific to the project at hand. Moon (2009) studied the impact of double-skin façades in reducing wind-induced motion of tall buildings by solving the equations of motion for the primary structure and the façade system and calculate the dynamic response due to wind load. While it was found that a proposed low-stiffness connector for a DSF system can significantly mitigate wind-induced vibration of the building, this system does not necessarily use the DSF to change the structure’s aerodynamics and has severe design limitations due to significant motion of the DSF’s outer skins. In another study, Moon (2011) installed an extra small mass inside the DSF to provide a damping mechanism similar to a tuned mass damper to overcome vibration issues associated with a previously-designed system. Hu et al. (2019) performed a series of wind-tunnel experiments to assess the effectiveness of attached DSFs with vertical openings in their external skin to alleviate wind-induced pressure on the building’s cladding. It was shown that a DSF without such an opening resulted in an increase in mean suction pressure and fluctuating pressure on the leeward face and both sides of the building and led to unsatisfactory performance for the DSF system under extreme wind conditions. Conversely, the pressure was reduced on the leeward face and sides for a DSF with opening(s), so it was concluded that DSFs with openings could effectively improve the wind resistance of claddings in tall buildings if they were designed with vertical openings.

To better understand flow characteristics around a tall building covered by DSFs, Hu et al. (2017) performed a series of boundary-layer wind tunnel experiments. They presented useful information about wind-induced response and pressure distribution around a tall building by monitoring an aeroelastic/flexible building model. Experiments both with and without vertical openings for double façades showed a negligible impact on the building response in the along-wind direction, while in the across-wind direction, it was found that existing openings can notably reduce the wind response compared to a building with a flat side with no façade. In contrast, solid façades with no openings escalated the wind-induced response, and vertical openings in the center produced the most significant effects in terms of reducing fluctuating pressure. Dependency of wind response on façade configuration was also discussed through a cross-correlation analysis along the building height. In another wind-tunnel testing, Da Silva and Gomes (2008) examined DSFs for various multi-story layouts and wind directions while testing for small to large gap depths. Similar to results obtained by Potangaroa and Aynsley (2003), they found that the pressure coefficient within a DSF’s gap is always negative for all wind angles. In their façade system, solid with no opening, they found that angles of attack ranging from 0° to 45° significantly affected the pressure distribution pattern. Basaran and Inan (2016) experimentally evaluated pressure loss due to double façades using perforated plates. To this end, they tested different perforated plates and discussed the Reynolds number effect. Gerhardt and Janser (1994) systematically varied building dimensions, façade porosity, and gap width to study their influence on wind loading on a tall building with a double façade. They compared the pressure coefficients for the different cases and validated their wind-tunnel data with field measurement. In another study, the same authors investigated the impact of wind-permeable façades on forces acting on the building (Gerhardt and Kramer, 1983), mainly focusing on the probability distribution of the pressure coefficient and the sensitivity of peak pressure with respect to incoming flow conditions.

Lou et al. (2012) conducted a series of studies on the effects of DSFs on wind-pressure characteristics of tall buildings through numerical modeling and experimental testing (Lou et al., 2012). They captured the pressure distribution for different layouts, incident wind angles, and air corridor width, then applied a zonal approach to model inner-gap pressures over the DSFs. They found that the zonal modeling, while computationally very efficient, produced acceptable results for replacing the CFD modeling and wind-tunnel testing. In other studies (Lou et al, 2008, 2009), they performed wind-tunnel testing to compare the mean and fluctuation pressure distributions of circular and rectangular tall buildings for single- and double-skin façades with arc-shaped and L-shaped configurations and observed no significant difference between single and double façades in terms of wind load acting on the whole structure.

Pomaranzi et al. (2020) tested the aerodynamic performance of a porous double-skin façade through wind-tunnel experiments. Their research was aimed at evaluating the effectiveness of the DSFs on wind-induced pressure on the cladding surface, and it was found that a façade system can reduce both negative and positive peak pressures of the inner glazed façade by up to 40%. They also observed that filtration of the pressure signal positively affected absolute mean values and standard deviations as a result of flow passing through the porous media. Samali et al. (2014) proposed a smart double-façade system for controlling the building’s wind-induced load and response that could significantly dissipate wind-induced energy and accordingly damp out the building vibration. They concluded that a smart façade system could reduce the wind-induced response and acceleration of buildings by up to 50% if an efficient system capable of adjusting stiffness was designed. Kwok et al. (2014) tested aerodynamic performance of an innovative façade system with a specific shape for mitigating the wind response, and their wind-tunnel experiments showed that along- and across-wind responses along with the torsional excitation were significantly reduced by DSFs with vertical openings. A study by Montazeri et al. (2013) is one of few studies using CFD techniques to investigate the effectiveness of a staggered semi-open double-skin façade placed in front of the balcony on the enhancement of wind comfort on high-rise buildings. They implemented a three-dimensional (3D) steady simulation using the Reynolds-Averaged Navier Stokes (RANS) model both with and without the façade. Comparing the obtained results with the occupant comfort thresholds by Dutch Wind Nuisance Standard (2006) indicated that local wind speed significantly reduces due to pressure gradient drop across the façade width.

As shown in this review, studies on the performance of DSFs in changing aerodynamic loads on buildings have been limited in their application, considered configurations, and the research methodologies used to prove this concept. Furthermore, no prior study has addressed the effects of important parameters such as wind direction and façade shape on the aerodynamics of double façades. This calls for more focused research to evaluate the impact of DSFs as wind load altering additions to the building. As such, this paper aims to fill this gap through comprehensive application of CFD and optimization techniques. Increasing the application of CFD techniques to solve problems dealing with fluid mechanics provides an opportunity to try more combinations of DSFs. Compared to wind-tunnel testing, experimentally-validated CFD models can simulate a larger parametric space. Furthermore, CFD allows for capturing continuous data points within the domain of the desired fluid, compared to the limited number of pressure taps commonly used in wind tunnels. The advantages of CFD modeling can overcome the limitations of wind-tunnel experiments and help the research community explore the use of DSFs in decreasing wind-load effects on tall buildings. In this study, a considerable number of CFD simulations suggested by the DOE model were used to implement a robust optimization framework for finding a DSF’s optimal shape for providing the lowest drag force on building, double façade, or both.

This paper is organized as follows: Section 2 explains the proposed framework for DSF optimization. Section 3 presents details of the rectangular and elliptical façades used for optimization and numerical modeling. Section 4 describes the numerical modeling, including treatment of boundary conditions and meshing parameters with CFD results. RMS models are presented in Section 5, and a discussion in Section 6 elucidates the optimization results obtained. Concluding remarks are presented in Section 7.

Section snippets

Proposed methodology to optimize double façade

To fill the existing knowledge gap on aerodynamic application of DSFs, this study primarily focuses on two popular exterior-façade shapes, rectangular and elliptical double façades. This shape choice is based on a study conducted by Al-Share (2020) on tall buildings of nine metropolitan areas in the United States that found rectangular shapes to be one of the most popular building shapes (66% of all tall buildings). Another reason for choosing the rectangular building shape is the availability

Geometric description of double façades

This study used a series of two-dimensional (2D) numerical simulations to investigate the influence of rectangular and elliptical double façades on wind-induced drag forces exerted on the CAARC building, with the exterior façade (representing the Smorphacade) and building model with B/D ​= ​1.5 scaled down to 1:100 for numerical modeling. A schematic view of the building’s cross-section covered by rectangular and elliptical facade is displayed in Fig. 3. To design the openings (see Fig. 2), the

Numerical modeling

In CFD analysis, ANSYS Fluent 19.2 software was used for two-dimensional (2D) numerical modeling, using the boundary conditions and dimensions shown in Fig. 4. To model the moving incompressible airflow inside the computational domain, tRANS equations were solved using the kωShear Stress Transport (SST) turbulence model. The second-order upwind was used for spatial discretization, and the SIMPLE algorithm was used for coupling pressure and velocity terms. The minimum acceptable residual was

Response surface model

In statistics, the response surface method (RMS) defines the relationship between explanatory/design variables and response or target parameters. This method, employing sequences of designed experiments to determine an optimal response, was introduced in the early 1950s by Box and Wilson (1951). The RMS, a useful mathematical tool for approximating stochastic models, has other applications such as optimization, design development, and new product formulation (Myers et al., 2016). The RMS, when

Optimization to find optimal configuration of façades

The optimization procedure can be performed using the RSM model developed here to predict any data points within the defined design space of 967 simulations. The objective functions here are the drag coefficients acting on the CAARC building and the façade, as described in Equations (3), (4).CDB=FDragBuilding/0.5ρU2BDCDF=FDragFacade/0.5ρU2BDwhere Bis the building length, D is the building width, U is the upstream wind speed, andFDragBuilding and FDragFacade are the respective drag forces acting

Conclusions

Over the past few decades, double skin façades have been mostly designed to improve indoor climate, save energy, and block sunlight. While there is convincing evidence that DSFs could also be used for reducing wind-induced loads on structures, there is still a lack of coherent research studying the effect of façade shape on wind-induced load and response of tall buildings at different angles of attack. Considering the large number of buildings worldwide that utilize DSFs, there would seem to be

CRediT authorship contribution statement

Mohammad Jafari: Methodology, Validation, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization. Alice Alipour: Conceptualization, Methodology, Investigation, Writing – review & editing, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This paper is based upon work supported by the National Science Foundation under Grant No. 1826356. Their support is gratefully acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the sponsor’ ‘s views.

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