Physical Scale (Wind Tunnel) Modeling

RWDI has two boundary layer wind tunnels located at our main office in Guelph, Ontario Canada. The larger of the two wind tunnels is used for exhaust dispersion studies. This tunnel is 16 feet wide and has an upwind section (between the fans and the study model area) that is about 50 feet in length. Typically, the models constructed for exhaust dispersion studies are constructed on an 8 foot disk. However, in cases where the surrounding area beyond the 8 foot disk is considered to be important for simulation of the wind flow behavior (e.g., tall buildings, complex terrain, etc.), the model can be constructed on a 16 foot disk (the full width of the wind tunnel).

To perform the modeling, a scale model of the proposed site and all surrounding buildings, structures and topography within a given radius (determined by the model scale) is constructed on a circular disk. Surroundings beyond the limits of the proximity model will be appropriately simulated by spires and roughness blocks situated on the wind tunnel floor upwind of the study model. This will ensure an accurate representation of the wind speed and turbulence profiles of wind approaching the study model. To view examples of models in our wind tunnel (for complex terrain and urban settings) go to the following link: photographs.

Study Building Test Model

An acrylic model of the proposed study building(s) are constructed to include all surface details measuring greater than 1 ft at full scale. The scale models are then instrumented with exhaust sources and sampling points (receptors) to be tested. Receptors will be installed on the model at selected air intakes, pedestrian areas, neighboring residences, and other sensitive locations where an impact may occur.

The model will then be tested by simulating the appropriate exhaust emission behavior on the model while subjecting it to approaching winds in the boundary-layer wind tunnel. The locations of the receptors are discussed with the design team prior to testing to ensure that areas of concern are addressed.

Tracer Gas Testing

To perform the tests, the scale model (on the circular disk) is mounted on a turntable in the boundary-layer wind tunnel. During the tests, a tracer gas (carbon monoxide) will be mixed with air to a specified concentration and pumped out to simulate the emissions from various exhausts. Concentrations of the source emissions will then be measured at the tested receptor locations by an on-line gas analyzer and recorded by a computer data acquisition system.

The tests for each source will be conducted for a wide range of wind angles and wind speeds. Different wind angles are simulated by rotating the model of the turntable. Since the highest impacts tend to occur relatively close to the sources, worst-case impacts are likely to be missed if only three or four wind directions and only one or two wind speeds are tested. Tests per source involve measurements for up to 24 wind directions at five wind speeds.

The wind tunnel results will be compared to recommended design criteria to determine the acceptability of each exhaust source. If modifications are required to the mechanical design to ensure that the design criteria are met, we may include additional tests to improve the exhaust impacts such as fine tuning stack height, increasing exhaust velocity, or evaluation of alternate exhaust and intake locations.

Flow Visualization Tests

Flow visualization tests are often conducted using smoke to illustrate exhaust plume behavior. This exercise can help to demonstrate the results observed and conclusions made from an analysis of the wind tunnel data collected. An unedited working video of the smoke visualization can be provided and would contain footage highlighting worst case impacts and/or flow patterns around the site. Examples of flow visualization footage can be found at the following link: smoke tests.