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How to Evaluate and Analyze Air Movement

How to Evaluate and Analyze Air Movement

by Allen Mei

August 29, 2016

Chicago is widely known as the “Windy City”.  While the origin of that nickname is debatable, lots of people might wonder why it is so windy. Also, most of us spend the majority of our time staying inside of buildings – are you sometimes curious about the air around us?

Why do we care about air movement? There are many reasons. In most cases, we want to know the air distribution such as directions, velocities, pressure, and so on, to design more comfortable spaces both indoors and outdoors. We also want to know how pollutants move with air to design healthier spaces, which is very important for some seriously polluted regions. Some structural engineers might be interested in the pressure that wind puts on buildings, especially high-rise ones. The scope can be as big as the whole earth and as small as ducts.

However, air cannot be seen. How can we evaluate it then? If we look back to our college years, some of us might have taken fluid mechanics, which most likely does not bring back happy memories.  There were some complicated equations that could be used to evaluate air, but It is impossible to calculate these complicated fluid dynamics partial differential equations manually (well, you can for small scaled problems, as long as you are highly skilled in advanced mathematics and have enough time). But thanks to the development of modern computers, we now have a method called computational fluid dynamics (CFD) to achieve what we want. Also, we have many highly integrated commercial software programs such as ANSYS Fluent, Airpak, Pheonics, and many others that are capable of doing these kinds of calculations. Some of them are very easy to use and handy.

In this blog, I will use a previous project to show you how CFD can be used to inform design teams.

The project was a stadium renovation. The stadium is located in West Lafayette, IN, facing southeast with a capacity of 57,236 people. It is a huge open stadium surrounded by a lot of buildings, as shown in Fig. 1 (left), and a renovation plan was developed in 2015 by Purdue University. The design team was planning to add a multifunctional extension on the southeast part of the stadium and a large screen on the northwest side, as indicated in the red circles shown in Fig. 1 (right). Also, one of the surrounding buildings was planned to expand, which could also affect the wind flow pattern around and inside the stadium. The renovation plan is shown in Fig. 2.

From the figures, the stadium was changed from an open circle to a closed shape. The design team was worried about wind being accelerated by the shape and affecting the wind flow pattern above the field. So wind flow CFD simulation was necessary for the design team to evaluate their design concept and visualize the wind flows in their decision making process.

Figure 1Fig. 1. Plans for (left) current stadium and (right) renovation.
Figure 2aFig. 2(a). View from southeast.
Figure 2bFig. 2(b). Bird’s view of the stadium of the renovation.

For a CFD simulation, a 3D model must be well developed to maximize its accuracy. AutoCAD was selected in this study to develop the 3D model because AutoCAD is highly compatible with the software I used, which was ANSYS Fluent. Almost all details of the site, including edges, overlapping, and others, can be preserved when exporting from AutoCAD to the ANSYS Fluent. The 3D models built in AutoCAD for the current stadium and the renovation are shown in Fig 3. The imported 3D models in ANSYS are shown in Fig 4.

Figure 3aFig. 3(a). 3D models in AutoCAD for Current Stadium
Figure 3bFig. 3(b). 3D models in AutoCAD for Design Concept model.
Figure 4aFig. 4(a). Current stadium mesh setting in ANSYS.
Figure 4bFig. 4(b). Design Concept mesh setting in ANSYS.

The images in Fig 5 are the visualization of the software results. Red and yellow are higher velocities while green are lower. This color coding makes it easier to quickly compare results. Through these images, you can see the wind velocity distributions and directions in the different scenarios and how the renovation plan or the design concept may change wind flow.

Figure 5Fig. 5. Wind blows from southwest at 25mph (11m/s), (left) current Stadium; (center) design concept; (right) solution.

The simulation results indicated that larger areas of high velocities and vortexes were found in the future design concept model compared to the current stadium, which is not acceptable for a stadium. The closed shape of the design concept was regarded as the major reason for these high velocities and vortexes. To verify this assumption, a modified model was developed. With the same settings, simulation results indicated that, when the extension at the southeast end is moved up by 25 feet (7.5 meters), the areas of high velocities and vortexes were much smaller compared to those in the design concept and more similar to those in the current stadium model. Therefore, this modified design was proposed to the design team.

The entire process, from site assessment, developing 3D model, to the simulation in ANSYS Fluent, can be used as a general method for engineering project in wind flow analysis.

 

Reference:
Weijun Mei; Ming Qu.  Evaluation and Analysis of Wind Flow for a Football Stadium. International Conference on Sustainable Engineering 2016, 774-781.