Computational Fluid Dynamics

When applied to buildings CFD is essentially the study of heat, air velocity and pressure variations through a space. Software is used to divide the space into many small cells, and the values for temperature, air velocity and pressure can be calculated for each one using the laws of conservation of mass, energy and momentum. These values are calculated repeatedly for each of these cells as the cells next to each other affect their values. The process is repeated until the values get to a point where they are no longer changing significantly with each sweep - this process is called convergence.

The actual mathematics involved becomes very complicated and requires the use of various mathematical models and approximations. Depending on the size of system being modelled, or, more accurately on the number of cells being used, the computing time required to run a model to convergence varies from minutes to hours. The output is displayed using coloured contours or arrows to represent the varying values of the chosen parameter.

CFD is useful for looking at:

• Air velocity where fast moving or stagnant air patches are to be highlighted.
• Temperature variations through a space where potential hot/cold spots need to be identified.
• Ventilation strategies to confirm that they will perform as expected.
• Smoke/contaminant movement/density in fire situations.

The changing images above show CFD analysis for three different shaped wind cowls. The models were used to analyse the amount of air that would be drawn from such a cowl depending on how it was shaped, and the size of the opening.

The following image shows work done to simulate Earth tubes. It shows the summer case where very warm air enters the tube at the right hand end and is cooled by the buried concrete walls of the tube. The earth surrounding the tube will maintain a temperature close to the annual average air temperature all year round.

In winter cold air enters the tube and is warmed as it passes through since the temperature of the ground is warmer than the winter ambient air temperature.