BISHOP O'DOWD WIND STUDY
In the fall 2014 Bishop O’Dowd High School (BOD) requested that Siegel & Strain Architects (S&S) investigate wind-mitigating solutions for the covered patio at the Center for Environmental Studies (CES) on the north end of campus within the school’s Living Lab. We undertook an array of analyses and conceptual studies to understand the local wind conditions and patterns, and to inform conceptual solutions aimed at improving the outdoor experience on the CES Patio.
The CES is located on the north side of the Bishop O’Dowd High School in the foothills of south Oakland in the Toler Heights Neighborhood, located on a hillside that rises up to the now decommissioned EBMUD Seneca Reservoir.The CES includes two classrooms, an office space and restrooms configured in a bar on the south side of a triangular-shaped and covered Patio. This outdoor space is approximately 1,600 square foot at 242’ above sea level, offering a great view of the San Francisco Bay to the west. As such, it has become a very popular lunch and hang-out spot for students on campus and a desirable space for campus events.
The prevailing wind blows in from the Bay right over the Living Lab. The wind speed at the site varies between 1 to 31.5 mph (see Figure 2) throughout the year, with greatest intensity during the late spring and summer season. Wind originates most consistently from due West and WSW, and occasionally from SW. It typically picks up throughout the day as the air temperature rises.
DETERMINING COMFORT VALUE
It is improbable to mitigate 100% of the wind, from 100% of the directions, 100% of the time, making it good practice in wind studies to establish a working design value for outdoor wind conditions. Ed Arens, Director of the Center for Environmental Design at UC Berkeley, advises that “the designer must judge what percentage of the time or how often in a year or season it may be exceeded”. Standard practice for wind study is to consider wind speed and its exceedance frequency (defined below) for the targeted design comfort value. Various sources agree that an appropriate design comfort value for an outdoor space should not exceed 5-6 m/s (11.1 – 13.4 mph) more than 20% of operating hours between 9am-5pm. For this 20% of the time, known as exceedance frequency, use of the patio should be scheduled around known wind patterns. (For example, records show that afternoons in May and June are windy and often extend into this exceedance frequency.)
To explore the potential solutions of the design effort, we focused on the localized conditions the building is producing given the westerly wind blowing from the SF Bay up to the Oakland Hills to understand the micro-wind effects created by the building geometry, and how best to visualize and measure them.
To avoid trivializing or generalizing aspects of wind flow on the site, we used both physical and digital means for visualizing and measuring the wind. Simultaneous testing, re-testing, and verification in an exploratory and non-linear process from both simulation domains (physical model and digital model) provided us with an understanding and well-rounded approach to our design efforts.
We used three primary Computational Fluid Dynamics (CFD) software programs – Autodesk Vasari, Autodesk Flow Design, and Autodesk Simulation CFD – to produce the graphics contained in this study. “CFD simulates how gases or fluids flow through or around objects and encompasses a large and complex body of research and algorithms used in the design of spacecraft and artificial hearts, as well as predicting airflow around and within buildings.
The necessity and reliability of physical simulations in a wind chamber has been historically validated to prevent, if not accurately predict, many wind-related issues such as undesirable conditions for pedestrian comfort, natural ventilation, aerodynamic characteristics of geometry/material, and structural damage cuased by volatile wind loading.
The theory and principles of why and how testing in the boundary layer wind tunnel works, is essentially by mathematical sleight of hand, or scaling laws. By dropping the anemometer (wind-measuring device) into different locations around the patio area we were able to record dimensionless velocity distributions or relative magnitudes of speed, enabling us to compare one point to another.
TESTING DESIGN OPTIONS
As designed and built, the CES building has horizontal steel beams supported by columns on its western side, intended for shear structural loads and future accommodation of sunscreen devices. These columns and beams provided the framework for supporting the design solutions that we experimented with in the wind tunnel and simulated in our computer models.
We concentrated our efforts on a limited combination of wind protection design approaches to address three wind directions – W, WSW and SW. We took wind speed measurements for a number of design options that combined solid panels and adjustable louvers, below and above the horizontal beams, on the west and north-facing sides of the CES Patio. Below are the results of our experimentations and analysis.
After reviewing the materials collected from our anemometer measurements, wind tunnel smoke visualizations, and CFD simulations, congruities emerged and a short narrative began to materialize:
On a typical summer afternoon, a breeze picks up steadily from the west south westerly direction vacillating in strength until reaching its peak when the sun is high in its arc mid-afternoon. Over the course of the morning, the surface of the hills warm, creating a series of ruffled wind eddies that tumble over one another around undulations and obstructions as they move up the hillside from west to east. The CES sits at the top of one generally unobstructed mound, exposed to the western racing fetch that is both accelerating and blustery as it moves up the slope. The wind is inconsistent in its behavior – sometimes it smoothly bypasses the patio columns and sometimes it slams against the western CES façade, looking for the path of least resistance around the building. Moving downstream (or uphill), the turbulent wind muddles around the leading building corners, shedding vortices on the leeside. These vortices, speedy and stochastic, flow into the CES Patio area disturbing the air, and therefore the people and the objects occupying the outdoor space.
This narrative is a simplified caricature of what is occurring on the site, illustrating a few key conditions.
Firstly, the wind is accelerating as it is climbing up in elevation. By the time it arrives at the open patio area, it is stronger and faster in some locations than the measured laminar flow and mean velocity. It is turbulent and deviating from the mean.
Secondly, as it hits the western façade it slows down quite drastically then almost instantaneously speeds up, accelerating to the right and left of its point of impact.
Thirdly, as it accelerates around the corners the building, the wind changes its direction abruptly because of air entrapped by the general flow, i.e., the wind moving in the west to east direction through the patio. This creates a pressure differential on the west and north building faces, with lower pressure on the leeward or north side and higher pressure on the windward or west side. The pocket formed on the leeward side is also called a wind shadow. This shadow is where larger eddies and turbulence occurs.
An obvious solution for an outdoor architectural space fraught with wind discomfort during certain months is to encase it in solid panels, ideally glass to maintain view. (See Figure 15) While seemingly logical for stilling the wind, this solution will, however, reduce the feel and character of the Patio as an open and outdoor environment.
It was noted that the solid panels created the following adverse local wind conditions:
• Winds from the due west direction that were constricted through the NW entrance hit the northern wall of classroom one and created an adverse eddy on the leeward side of the western panels.
• Winds that were constricted through the SW entrance swirled around the newly designed solid panel edge. Additionally, because we’ve created an enclosed corridor space, the wind that collides with the western wall in searching for an escape route, is redirected and actually channeled through this breezeway. This was one of the more conspicuous observations made during our smoke visualization studies in the wind tunnel.
The solid panels, when tested in the WSW wind direction, showed the greatest velocity reduction, which was a 39.8 % average decrease of all measured points. (See Figure 14) With our comfort design value in mind, this puts our days per year that it exceeds 5-6 m/s at 0 days.
An adjustable louver design allows users to be active participants of the architecture. The emphasis of this cannot be stressed enough as there will certainly be occasions where flexibility is needed. The ability to leave gaps in the system allows it to be more climate responsive. For example, during an arid and almost entirely windless morning, allowing in any available breeze would be desirable to reach a comfortable state. Depending on the direction of the wind, the angles of the louvers could be adjusted to divert the cooling whisks into the patio area. (See Figure 20) Additionally, diurnal changes can drastically modify the direction and intensity of the wind throughout a day.
The louver panels allow the user to make adjustments and control amount, timing and direction of breezes. This is an advantage over a sliding and stacking panel system which could provide only an open or closed scenario. Since an adjustable louver option can conceptually and physically imitate the performance of the solid panel solution when closed, we already knew the outcome of this test as discussed previously. The next question was to determine the impact of a louver in a partially open position. The model was modified to replace all solid panels on the western and northern edges with a partially open louvered panel.
The adjustable louvers tested in this partially open position with a WSW wind direction showed a 24.4 % average decrease in velocity for all measured points. (See Figure 18) In our model, the days per year exceeding 5m/s was only 18 days and exceeding 6m/s was only 4 days a year, just meeting our target comfort value. With winds coming from the predominately WSW direction, our next question was the efficacy of the panels along the northern edge at column line A.
HIGH & LOW PANELS
It should be noted that in all wind tunnel scenarios only the combined top and bottom panels were tested, not separately tested. However, it’s reasonable to assume that more protection will be provided with both top and bottom panels mounted (See Figure 28-30).