## Wednesday, May 25, 2011

### The aerodynamics of buildings

Civil engineering is perhaps not the most exciting application of aerodynamics, but to look at some buildings, one has to wonder if questions of airflow ever entered the minds of their architects.

The diagrams here demonstrate just how complex the aerodynamics can be around even the simplest of buildings. Anyone with a passing interest in the subject should be able to predict that a low pressure area will form at the rear of a building (relative to the prevailing wind direction), and the detachment of the boundary layers atop and to the sides of the building, will create a turbulent wake.

Less expected, perhaps, will be the vortex tubes created at the front of a building, which are then stretched down the flanks of even a simple cubular or parallelepiped construction. And look at that pair of slender vertical vortices at the rear of the building in the first diagram; you wouldn't want to put a drain or a dustbin at the bottom of one of those!

The first two diagrams here deliberately ignore the turbulent wake behind a building. Instead, they represent what's called the mean flow. The idea here is that a turbulent velocity vector field can be decomposed as the sum of a time-independent mean flow, and a fluctuating component which contains all the turbulence:

If you want to simulate a turbulent airflow, then one of the short-cuts you can use is to conduct a Large-Eddy Simulation (named after a 16-stone fella called Eddy), which calculates just the mean flow, and the largest turbulent eddies. A Large-Eddy Simulation uses a parameterized model to dispose of turbulent kinetic energy which, in reality, is only dissipated after the largest eddies have broken up into a succession of much smaller eddies.

It would be interesting to know what the airflow pattern is around a cluster of buildings; do they perhaps generate a network of vortex tubes, like wormholes, capable of escorting aerosols emitted in one place, to anywhere else in the cluster?

(Second and third diagrams here courtesy of P.A.Davidson's superb tome 'Turbulence', CUP, 2004).