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Friday, November 20, 2015

Computational Fluid Dynamics (CFD) for Non-Mission Critical Applications: Airflow Through Aesthetic Architectural Building Elements

In the mission critical world, we are quite accustomed to using Computational Fluid Dynamics (CFD) to analyze airflow and thermal boundaries.  Health care is another field where CFD modeling is used (particularly isolation room airflow). The fact is that  CFD is a versatile tool that can be used in a multitude of applications involving airflow analysis. 

We recently completed a CFD study for a Garage Structure in which we analyzed the airflow through a new exterior metal screen mesh installed on an existing building fa├žade.  In our case, we modeled our building using Coolsim, a leading CFD software for mission critical applications. 

GOAL: We wanted to determine if the new metal mesh, with a 41% free area, will provide a level of natural ventilation airflow equal to or greater than a standard exterior garage wall constructed at the code minimum requirement of 20% free area. The CFD route was selected as it provided a robust solution to an otherwise complicated geometrical multi-dimensional problem. 

The space and wind conditions created a challenge, but the folks at Coolsim worked closely with us to adapt the model to our needs.  The software is primarily implemented for the calculation of cooling airflow within enclosed structures, but the system includes, as its analytical engine, a program that allows for a detailed examination of airflow around various physical elements.

By establishing an isolated free-body representation of a section of the parking garage, the effect of wind against a typical exterior structural bay were analyzed with various types of exterior construction elements. For our study, our research showed that a wind velocity of 5 mph was appropriate for the geographic location. 

The metal mesh screen is made of a solid sheet of steel that is “punched” to form diamond shaped openings with a uniform direction of sight. The effect is similar to an angled honeycomb shape.

A. 100% Open Garage Wall 

The first step was to model the structure with 100% open wall area. This is depicted in the first CFD image at the top of the blog and the cross section CFD for one floor and two bays is shown below. High velocity depicted in Red is expected as this is 100% free open area. The 5 MPH wind produced an average of 67,000 cubic feet per minute (cfm) of air volume.

Figure.  100% Open Garage Wall Cross Section at One Floor - Air Velocity at Two Typical 30' Bays. (Typical for the other two runs)

B.  20% Open Garage Wall

The second step was to model the structure with 20% open wall area, which is the minimum allowed by code for proper ventilation. The CFD depicts the velocity results through lighter colors of the scale as expected. The larger the obstruction, the slower the air movement.  The 5 MPH wind produced an average of 52,000 cubic feet per minute (cfm) of air volume. 

C.  41% opening with Mesh Screen

The final step was to run the model with the mesh screen. As expected, airflow velocities and airflow rates rise as the wall mesh allows more air in.  The 5 MPH wind  produces an average of 58,500 cubic feet per minute (cfm) of air volume, which exceeds the 20% requirement.  

In conclusion, the beauty of the Coolsim CFD model is that it provided proof-both visually and numerically that the airflow through the mesh exceeds minimum code requirements. As a result, (and hence the other benefit that I want to emphasize) is that that it saved the client the additional cost of installing mechanical ventilation fans throughout the garage structure. 

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