continued

The Wind Blows through It

To realize the potential for natural ventilation in Dominican architecture, one must first understand the wind. In Las Terrenas, the easterly trade winds are predictable. There is a constant breeze from the northeast in the daytime and from the southeast at night.

Without the wind, the climate of this village might be unbearable. Predictably, natural ventilation is the most frequently used environmental control system in vernacular Dominican architecture.

To study the behavior of this wind relative to architectural forms, I used the CFD software, ANSYS, a mathematical simulation of the flow of a "fluid" over a given geometric shape. My goal was to optimize the passive ventilation of the cabana, day and night, all year long, by testing how different forms and building orientations would interact with the wind.

Conceptual Design

The cabana concept began with three metaphors: the windsock that responds to the wind's shift in direction, the palm tree that flexes in high winds but continues to stand, and the camera aperture that opens and closes.

In my early windsock investigations, I built models that were inverted airfoils literally pivoting to respond to changes in wind direction. I spent hours investigating drag and aerodynamics, trying to derive an optimum architecture.

To mimic the resilience of palm trees, I modeled flexible walls that would move on pivoting joints in reaction to the wind and change the shape of the interior space. Such a solution would also respond well during hurricanes, creating less resistance and therefore suffering less damage.

For the camera aperture approach, I controlled the flow of wind through an opening by adjusting the opening's size. These adjustments created a Venturi effect, which accounts for the acceleration of air movement through a constricted opening.

I tested each approach with CFD software. As the design developed, the virtues of airfoil, flexibility, and the Venturi effect prevailed in different ways.

As an airfoil, the cabana created minimal surface drag because of its contoured shapes and surfaces. In lieu of literal palm-tree-like flexibility, I introduced redundancy, locating the main living spaces of the house on two mirrored axes to capture the wind as it changed direction throughout a 24-hour cycle.

I optimized the Venturi effect by varying the size and shape of the cabana's main openings. The fenestration system allows the cabana to work like a camera aperture by allowing users to adjust the size of the opening.

Design Development

I based the cabana's utilitarian architectural program elements on my personal experience of coastal living in the Dominican Republic. I pared the program down to only the necessities. Modesty of scope became a key virtue of the cabana.

The program initially took shape as an abstract rectangle, but this form did not positively affect the airflow. The velocity of the wind entering the space was the same as its velocity exiting. When I implemented the Venturi effect by increasing the windward opening size and decreasing the leeward opening, I effectively doubled the wind velocity.

However, this increased velocity had no effect on thermal comfort within the living space because the increase only occurred at the exit. From this I concluded that the building's smallest cross-sectional area would have to be located just upwind of the living area and not at the exit opening. This suggested a roof form with its lowest point near the middle of the building, which could also serve for rainwater collection.

Testing confirmed that shifting the point of minimum area to the middle of the section would improve the ventilation in the main spaces. When testing for degree of pitch, I learned that too much constriction would cause turbulence. In turbulent areas, shown in blue in the diagrams, the airflow actually reverses direction, negating the benefits of the ventilation.

This called for a more streamlined roof line; the reduced roof pitch eliminated the unwanted eddies and made the air flow more efficiently. In early tests, the air velocity increased only near the ceiling. But by flattening the roof/ceiling at the point of entry I was able to distribute the air more evenly.

Because the prevailing night winds are 90 degrees from the daytime winds, I decided to apply this inverted roof form in both directions. I organized the program of the cabana into nighttime and daytime activities. The wing for daytime use connects visually to the ocean and its cooling breeze, while the bedroom wing takes advantage of the night airflow. The inverted roof form successfully doubles interior airflow from both directions.

The results of the CFD studies are clear in the final form of the cabana, with passive ventilation optimized in plan, section, and elevation. The building's flared shape to the north and east are visible in plan and section. The elements used in elevation are jalousie windows, commonly found in tropical climates. The jalousie's louvers can be adjusted very quickly to control the interior environment and respond to changing weather conditions

Even the furniture is arranged around the predominant air patterns. The circulation space is located where the velocity is highest to allow the air to flow freely through the space. The furniture is placed off to one side, but where it can still benefit from the circulating air.

These calculations demonstrate that no further changes to the shape of the plans can improve airflow. The result is plenty of fresh air and cross ventilation: an achievement of science in service to modest design.

Gregory Marx De Peña, originally from the Dominican Republic, is an architectural designer in Tucson, Arizona, where he works for Line and Space, LLC Architects. An animated presentation of this project is available on his Web site which was created by Michael Sablone .

Discuss this article in the E-Design Forum...

Southeast-facing section through the Dominican cabana. Image: Gregory Marx De Peņa Floor plan of the cabana, with north to the bottom left. The east wing, with a bedroom, is oriented to the prevailing nighttime breezes. The north wing, with the living room, is in line with the daytime wind. Image: Gregory Marx De Peņa The CFD analysis in plan shows wind flowing from left to right. The contours of red and tan show areas of highest velocities. Image: Gregory Marx De Peņa In section, the double-pitched roof creates an even air flow from floor to ceiling, but it requires a drop in the floor at the point of air entry, and the second pitch at the rear of the building is too high and causes turbulence. Image: Gregory Marx De Peņa With the double-pitch roof reduced, the air velocity increases only at the ceiling in a very small concentrated area and thus is ineffective in ventilating the living space. Image: Gregory Marx De Peņa In the final section study, a modified tipped roof creates an area where the high-speed air is distributed evenly through the living space. Image: Gregory Marx De Peņa Northwest elevation of the cabana model. Photo: Gregory Marx De Peņa Northwest elevation of the cabana. Image: Gregory Marx De Peņa   Click on thumbnail images to view full-size pictures.