In general, it can be stated that there are 3 technologies to assess wind in the urban environment:
- Real life measurements
- Wind tunnel experiments
- Computer simulations (CFD or “Computational Fluid Dynamics”)
Real life measurements
When assessing an existing situation or problem, real life measurements of wind speeds and wind directions can be performed. When measuring in real life, no simplifications or assumptions are needed in terms of building geometries or atmospheric conditions. However, in order to have statistically reliable data, it is necessary to perform measurements during a long period in time (typically several months), so that every possible wind condition has occurred during the measurement period. Another issue with real life measurements is that you can only collect data at a limited amount of measurement locations, which means that you don’t have a complete overview of all wind phenomena that occur in a certain area. A final challenge is that it is hard to relate a momentaneous measurement of the wind, to the undisturbed wind conditions known from the national meteorological institutes, which mostly have statistical wind data for the entire country.
Real life measurements can be useful to identify possible problems or challenges but are not recommended for a complete wind assessment in cities. Definitely when assessing a future situation (e.g. a new development in the city that is not yet built), real life measurements are not an option.
Wind tunnel experiments
Instead of performing real life measurements, it is also possible to build a scale model of a part of a city and place the model in an atmospheric boundary layer wind tunnel. In such a wind tunnel, the real wind conditions can be simulated for many different wind directions, including the dynamic behavior of the wind. To understand the influence of individual buildings or vegetation, it is very easy to add or remove elements in the scale model and run the wind tunnel experiment again.
However, as with real life measurements, you can only collect wind data at the locations where the pressure probes are installed in the scale model, meaning that you don’t see or register what happens in the space between those locations. Another issue is caused by the small scale of the wind tunnel model. Local wind effects, for example on balconies of high-rise buildings or other private outdoor areas, are impossible to measure in the wind tunnel, simply because the measurement equipment is too large in relation to the building or location of interest.
CFD computer simulations
Instead of building a physical scale model, it is also an option to make a 3-dimensional computer model of a part of a city, and let the computer calculate the wind effects. Specialized software is used to simulate the wind behavior in the entire city area under varying atmospheric conditions i.e. different wind speeds, wind directions, air temperatures, etc. Also here, the influence of individual buildings or vegetation can be easily assessed by adding or removing elements from the computer model, and run a new simulation. However, the runtime of a computer simulation is typically longer then the runtime of a wind tunnel experiment.
The main advantage of CFD is that the software shows the local wind speeds and wind directions in the entire space in the city, which means that any remarkable or unexpected wind behavior can be identified. As the computer model is not scaled, all details in the flow around small balconies or other private places can be studied.
For the assessment of the mean wind velocities in cities, or for the design of wind in cities, CFD computer simulations are extensively validated and proven to be reliable, as long as the simulations are performed by an expert in the field of CFD. For the analysis of wind dynamics like gusts, CFD methods can still be improved and are under development.