Building Ventilation and Dispersion
Ventilation
Whether a proposed building is ventilated mechanically or naturally an important design consideration is the effect of the wind on ventilation via the surface pressure distribution over the building.
In the case of mechanical ventilation the operation of the system may be affected by adverse pressure gradients. Uprated fans may be necessary with an initial cost penalty and subsequent long term economic implications of a system that is working at high capacity to overcome the effects of the wind.
In the case of naturally ventilated buildings, with their associated environmental advantages, it is even more important that the surface pressure of the building is understood and used to optimise the performance of the building.
Finally, for smoke ventilation in the case of a fire careful consideration has to be given to the sense of flow through proposed extract openings; it is necessary to demonstrate that for all wind directions there is an outflow through high level vents in order to avoid cool air entering the building resulting in a loss of buoyancy of smoke.
1 - External Pressure Measurements
Each of the above scenarios can be conveniently tested in the wind tunnel by pressure tapping a scale model of the building. To make the best use of the test results it is important to identify both proposed and alternative sites for the pressure taps (inlet/exhaust). This allows a data base of pressures to be built that can be manipulated to optimise the ventilation system to meet the design specification. Mass flow rates and the direction of flow through openings can be calculated for each wind direction, but the full analysis combines this with meteorological information for the site in order to provide the full probabilistic treatment.
In general, the mean pressure distribution is of primary interest in terms of building ventilation, but the arrival of simultaneous pressure scanning allows more sophisticated analyses can be carried out using "snapshot" pressure distributions around the building. The database of external pressures can also be used to supply realistic boundary conditions for computer modelling of the internal flows.
2 - Smoke Tracer Studies
An alternative/complementary and relatively quick assessment of internal building ventilation can also be made using smoke. The calculations based upon external pressure distributions are useful to determine overall ventilation rates, but for large, spaces this process takes no account of regions where the local ventilation rate is significantly and unacceptably lower than required.
Transparent wind tunnel models can be used and flooded with smoke. The geometric scale of the model and the velocity scale of the test typically set a time scale of 1:30 for the test, i.e. things occur 30 times faster in the wind tunnel than in real-life. The supply smoke is switched off and the rate and pattern of the smoke clearance is observed and recorded. Localised pockets due to poor ventilation are readily identified. The main advantage of this type of test is that the whole flow field is assessed and at the detail design stage measures can be incorporated to treat problem areas.
Dispersion
Dispersion is used here to describe the way in which effluent from chimney stacks or other building exhaust is transported and diluted by the wind as it passes across the proposed development and immediate neighbours. Anemos is generally interested in near-field dispersion, which is most relevant to new building developments and we commonly employ two techniques to quantify these issues: smoke and the measurement of tracer gas concentrations. The sensitivity of the project, compare a nuisance kitchen odour with effluent from a University fume cupboard, will determine which technique is most applicable.
The smoke test is convenient because it gives a whole flow field view of the processes and one application is in determining the minimum stack exit velocity necessary to avoid impact of effluent on an adjacent building or re-ingestion into the building ventilation system. However, there is a degree of subjectivity introduced into these assessments.
The alternative technique is to seed the exhaust with a very low concentration hydrocarbon tracer gas and measure the dispersed concentration of hydrocarbon using flame ionisation detectors at selected locations around the development.
