Visualizing How Buildings Breathe Presented By: Ahmed Mohamed Gendia By: Adrian Tuluca, R.A Web Address: html html Topic Number: 169 Date: 03 January 2001

Introduction The constant movement of air through the spaces of a building affects its thermal environment and, thus, its energy performance, yet this invisible "breathing" has been difficult to understand or predict using traditional tools. Expensive wind tunnel models were used before for measurement as models needed to be reconstructed every time a modification occurs. Thus Computational fluid dynamics (CFD) emerged to describe the complex movement of air molecules and temperatures within spaces. Wind tunnel model

CFD modeling was applied to special, large-scale spaces, such as airport terminals or multistory atria. These CFD models were expensive and could take weeks to build in the computer. Now CFD is being used for all kinds of buildings. CFD is no longer the budget buster. Small buildings can be modeled for $5,000 to $10,000; a 2D CFD simulation can be done for $2,000 to $4,000. CFD uses colored arrows to simultaneously display air speed, direction, and temperature.

There are two kinds of analysis: A- Two dimensional analysis: A- Two dimensional analysis: B- 3D dimensional analysis: B- 3D dimensional analysis: The following scenarios show a variety of conditions and the different approaches of analysis: Oberlin's Environmental Studies Center Oberlin's Environmental Studies Center(1255-square-meter) Well environmentally designed. Highly insulated walls and roofs; energy- efficient lighting; photovoltaic panels on the roof to generate electric power… Fresh as no air from one space is mechanically induced into another. Fresh as no air from one space is mechanically induced into another. For an atrium space at Oberlin College, CFD analysis modeled the path of air streams that moved from the front door to windows above the mezzanine.

CDF targets: 1-Minimum area of operable windows with best performance. 1-Minimum area of operable windows with best performance. 2- Detection of most efficient location of windows. 2- Detection of most efficient location of windows. 3- Elimination of still air zones. 3- Elimination of still air zones. Solutions: 1-CFD studies showed that a roof-top monitor in the atrium would provide a "chimney effect".(too costly) 1-CFD studies showed that a roof-top monitor in the atrium would provide a "chimney effect".(too costly) 2-CFD recommended that the designers place more operable windows low on the south wall to vent heat gain. 2-CFD recommended that the designers place more operable windows low on the south wall to vent heat gain. Oberlin's Environmental Studies Center Analysis of the air velocity at four feet (122 centimeters) above the floor uses color to show still air (dark blue) next to fast-moving air (red) through the door. Windows were added to the building's east wall to mitigate the stagnant pocket.

Train station. Rensselaer Train Station in Albany, New York. double-height waiting room, crowned with a sloped roof. No natural ventilation was possible in the space because the outside air is polluted. analysis of the waiting room. Targets: Determine how comfortable temperatures Determine how comfortable temperatures could be maintained without excessive air flow. could be maintained without excessive air flow. Model was simplified to keep the cost of the analysis down. Rensselaer Train Station An exterior computer rendering of the Rensselaer Train Station.

It was not necessary to accurately predict temperatures at roof level. The heat gain was accurately calculated. CFD revealed temperature differences ranging from 105 degrees Fahrenheit (40 degrees Celsius) near the ceiling to 70degrees F. (21 degrees C.) at floor level. Thus the temperature is comfortable. Rensselaer Train Station The sloped ceiling of the train station was simply modeled as rectilinear to save time and cost. The rendering of the waiting room volume shows temperature variation.

HVAC designers will have two options: (1) Specify smaller fans (1) Specify smaller fans OR OR (2) Specify the same size of fans, but with variable-speed drives (2) Specify the same size of fans, but with variable-speed drives Variable-speed drives fans is was found the best solution as it to meet the variable needs of the space. Rensselaer Train Station A streamline diagram indicates the air moving from the supply ducts to the return grilles. Notice that each streamline changes in color as its temperature changes. Two slices through the train station space show temperature variations. The abrupt change in color indicates the location of the mezzanine.

Although CFD was born from the need to model large-scale, complex spaces, there is no reason this simulation can't be used to simulate small spaces, such as one room in a house. The goal was to study how changes in a builder's routine construction techniques could lead to a more energy-efficient house. (150-square-meter) prototype house in Rochester, New York. Modifications to more energy efficient housing (low-e glazing…) First experiment involved removing the heating supply registers from under the windows. A Single Room For the prototype house built by Ryan Homes, CFD revealed temperature variations within a bedroom. Note the temperature spike at the floor register.

Registers beneath windows mitigate air cooled by the surface of the windows, which otherwise flows to the floor and creates drafts. Efficient air distribution could be achieved from registers placed in the ceiling or high on the wall. Radiant improved to more comfortable level. CFD improved the ventilation efficiency in the room and those modifications were taken as a prototype. A Single Room A new location for the bedroom supply duct, near the ceiling, allows heated air to be thrown into the room at 30 degrees (0.5 radians).

CFD computer modeling has come a long way from the time-consuming, costly simulations that were the norm only a few years ago. CFD is an accurate measurement tool. Faster than conventional tools. More illustrative. Applied on different scales. Could be used as a checking tool. Time conserver. Money conserver. Conclusion

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