In the recent spate of urbanization, tall buildings and skyscrapers have become increasingly complex in overall design and scale, putting them at a greater risk due to wind effects and induced forces on the building structure. To ensure safety, architects and design engineers must develop a safe, sustainable, and cost-effective design by utilizing studies concerning wind engineering technology. Typically, these studies are conducted to evaluate the dynamic effect of wind on a structure followed by design optimization in order to mitigate the consequences. Moreover, conducting studies of the like also help meeting current industry standards.
In most cases, the two primary concerns for engineers are:
1. Pressure loads on the structure and facade design
Essentially, this involves a steady analysis to identify areas that experience extreme pressures (high and low peak pressures) and require reinforcement to increase stability. Often, it is possible to derive pressure loads for simple designs via basic code methodology. However, to achieve accurate results for complex shapes, detailed wind tunnel testing or numerical analysis becomes necessary.
2. Determining and mitigating the dynamic wind load effects
The analysis of unsteady vortex shedding is crucial for tall, high aspect-ratio structures due to their potential for inducing oscillating crosswind forces with a certain frequency. If this frequency coincides with the natural frequency of the structure, resonance may occur, leading to damage or failure of the structure.
Architects and design engineers use the following design modification strategies to reduce or at least suppress vortices:
- Creating flow spoiler or disturbance
- Corner softening
- Tapering the height or varying the cross-section shape
- Adding porosity, open floors/sections or bleed slots
These modifications can be applied during the design cycle and can easily diminish wind-induced forces by 25-60%
CFD for Design Optimization
It is a common practice to investigate the above-mentioned design modifications via wind tunnel testing. For testing complex, large-scale design, models have to be scaled down. This can be a challenging, costly, and time-consuming process for studying numerous hypotheses.
Computational Fluid Dynamics (CFD) provides a numerical approach for Virtual Wind Tunnel testing that enables the user to perform a full-scale, cost-effective analysis for pressure loads and dynamic wind loading. Additionally, the effort and time involved in manufacturing a physical model to conduct testing are also saved.
The numerical analysis presents both 3D visual contouring and quantitative data for pressure, force and velocity that is easy to understand and high in detail. Areas of complex recirculating flow and localized vortices can be simulated and identified. Modeling mean wind profiles and atmospheric boundary layers is also possible. On top of that, the main benefit of virtual wind tunnel tests is that several scenarios and designs can be simulated in parallel.
The purpose of this project is to investigate and mitigate vortex shedding around a 50-story tall building at high wind speeds of 45 m/s. The building is 150 m tall and has a fixed square cross base of 20 x 20 m. Two designs are analysed, the initial design with sharp corners and the optimized with corner softening (rounded corners). To study the dynamic effects of wind loading, a transient analysis with incompressible turbulent flow is performed in SimScale.
Figure 1: Wind Optimization Project Building Model
The geometries of the building are shown in Figure 2, the initial design with sharp corners and the optimized version with corner softening using rounded corners.
Figure 2: Sharp Corners (left) and Rounded Corners (right)
A hex-dominant parametric mesh was created in SimScale. The fluid region around the building is meshed to model a Virtual Wind Tunnel.
Figure 3: Virtual Wind Tunnel in SimScale
Figure 4: Virtual Wind Tunnel (Inside View)
For this project, a transient incompressible flow analysis was carried out with KOmega-SST turbulence modeling to study the time-dependent vortex shedding phenomenon.
Results and Conclusions
The results presented show the pressure loading and velocity contours for the initial design and the comparison of the dynamic wind loading effects of vortex shedding for the modified design.
Looking at Figures 4 and 5 (below), it is clear that the building with sharp corners produces a larger vortex trail and a larger wake region compared to the building with rounded corners.
Figure 4: (Top Section) Comparison of velocity contours showing vortex shedding for the two designs.
Figure 5: (Side Section) Comparison of velocity contours showing vortex shedding for the two designs.
Another comparison to be made is regarding separation. The building with sharp corners has a wider side separation zone compared to the building with round corners. See Figure 6 below.
Figure 6: (Zoomed View) Comparison of side separation at a given time
The numerical study investigated dynamic wind loading effects on two building designs. The study showed that the original design with sharp-corners produced strong vortex-shedding phenomenon. This results in high amplitude intermittent forces in the cross-wind direction that could be damaging for the structure if the calculated frequency is comparable to the natural frequency of the structure.
In this case, the calculated frequency of the original design is about ~ 0.23 Hz, which is quite close to the typically natural frequency value of ~0.2 Hz for such a 50-story building.
Figure 7: Comparison of Forces in the Cross-Wind Direction (Y-Axis)
Based on this study, it can be shown that a rounded corner design significantly mitigates the wind-induced dynamic forces in the cross-wind direction and thus reduces the risk of damage and failure of the structure.
Figure 8: Animation of Vortex Shedding comparison for the building designs
 Peter A. Irwin, “Wind Issues in the Design of Tall Buildings,” RWDI Los Angeles Tall Building Structural Design Council, 2010.
 Peter A. Irwin, “Vortices and Tall Buildings: A Recipe for Resonance,” 2010 American Institute of Physics, S-0031-9228-1009-350-6, www.physicstoday.org