Wind Pressure & Flow Around Building
Wind has pressure due to its velocity. This pressure is calculated from the equation
Vp = 0.6V², where Vp is the velocity pressure in Pascals (Pa) and V = velocity, in m/s. 0.6 is a constant, derived from the density of the air, at 20°C and average RH. A fresh gale blowing at Beaufort scale force 8 (80 km/h or 48mp/h) will exert a pressure of 296 Pascals on a building.
As the air passes over and round a building, the pressures are positive on the windward side and negative on the lee or sheltered side. It is often the negative pressures that suck roofs off buildings and windows out of walls. When wind is stopped, its velocity pressure is converted into static pressure.
The effect of wind on simple domestic extractor fans can be considerable. Wind speeds above 25 km/h can stall a fan on the windward side of a building and cause “free wheeling†on the lee side. If a fan is in the wall or window of a dwelling that is recessed into the building, forming a balcony or well, the trapped positive pressure will find the only route available to balance the pressure, this could be through the fan. Even the most efficient backdraught shutter will not prevent air pressure equalisation.
A mushroom shaped cowl will afford better protection than a louvred grille, but even this will allow air transfer in a positive pressure area.
Wind has pressure due to its velocity. This pressure is calculated from the equation:
V = Velocity in m/s
| Wind Speed | Description | Beaufort Scale | Velocity Pressure | ||
|---|---|---|---|---|---|
| m/h | km/h | m/s | Pa | ||
| 1 | 1.5 | 0.42 | Calm | 0 | 0.10584 |
| 6 | 10 | 2.8 | Light breeze | 2 | 4.704 |
| 12 | 20 | 5.6 | Gentle breeze | 3 | 18.816 |
| 24 | 40 | 11.1 | Fresh breeze | 5 | 73.926 |
| 48 | 80 | 22.2 | Fresh gale | 8 | 295.7 |
Duration of Average Wind Speeds
Wind speeds are connected to a standard height of 10m (33ft) above ground level. Figures can vary widely from different parts of the British Isles.
| Under 8 km/h (5 m/h) | Under 16 km/h (10 m/h) | Situation |
|---|---|---|
| 20% | 45% | Coast |
| 35% | 55% | Rural |
| 45% | 75% | City Suburbs |
| 55% | 95% | City Centre |
Wind Flow Around Buildings
If there are no obstructions, wind normally flows in a very turbulent state, but in generally straight direction in spite of veering and backing. When an obstruction like a building is encountered, the flow pattern is markedly changed.
+ Positive Pressure - Negative Pressure
Large towns rarely have wind gusts exceeding 110 km/h (Vp=560Pa)(70 mph).
Most coastal areas can have gusts of 145 km/h (Vp = 970 Pa) (90 mph)
Some Scottish areas can have gusts exceeding 160 km/h (Vp = 1185 Pa) (100 mph)
The maximum wind force exerted on a building can be considerable.
| Example: For a building with 15m (50′) wide × 6m (20′) high wall exposed to 160km/h gusts in an exposed Scottish coast area. | ||
| Wind Force = | 1185 Pa 9.80665 |
= 120.8 kg/m² |
| Actual force transmitted to building will be between 50% and 80% of theoretical maximum, depending on surface roughness of the building. | ||
| ∴ Max total force on building = | 15 x 6 x 120.8 x 80% 1000 |
= 8.7 tonnes (8.2 tons) |
Variation of Wind Velocity Due To Height Above Ground Level
In cities, the large number of buildings, some of them quite tall, have the effect of slowing down the wind. This causes a big difference between low and high level wind velocities. In open country there is far less difference between low and high level wind velocities.
From:Vent Axia Ventilation Hand Book