Office Building

The feasibility of a natural ventilation strategy for an office building such as this depends on a sufficient supply of fresh air to all occupied areas.

The offices are arranged around a central atrium which is designed to drive the ventilation via the 'stack' effect. In summer, on days when there is little or no wind, the atrium will draw air in from the surrounding offices and expel it upwards and out through vents on the roof. Under more windy conditions, it is hoped that the leeward offices will receive a high proportion of air from the cooler basement level rather than from the windward offices.

A consequence of the complex external shape of the building is that it is difficult to estimate the wind-pressure driven flow rate through the windows. Instead of conducting a separate external flow simulation, it was decided to go for a single complete external and internal simulation of the entire building. This bold step was possible with only minimal alterations and assumptions.

The first phase in the analysis was to conduct a dynamic thermal simulation for the building covering the summer months. This provided information regarding approximate office temperatures and air change rates. In addition, the dynamic thermal simulation provided solar gain information which was fed into the CFD analysis.

An initial CFD analysis was conducted for a typical warm summer day picked from the chosen London dataset. The mid-day solar gains and internal gains (lighting - 11 W/m2, occupants - 14 W/m2, small power - 18 W/m2) were implemented as heat flux sources. The remaining boundary conditions were the wind speed, direction and air temperature (3.0 m/s at 88 deg E of N and 24.5 deg C).

The results showed a high degree of ventilation for the windward offices, but the leeward offices received too much of their air from the windward offices. This was rectified by repositioning the atrium openings and increasing the number of openings from the cooler basement.

Once set up, it was straightforward to analyse the affects of varying the external conditions. Under still conditions, the neutral pressure level was sufficiently high to deliver some fresh air to all offices at all levels. Air change rates of approximately 16 air changes per hour were achieved in places.

In summary, the model was relatively time-consuming to set up and care had to be taken to ensure all walls and openings were resolved correctly. However, the effects of wind could be accurately modelled and once created, the effects of varying the external conditions could be analysed quickly at a low cost.

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