Pedestrian Level Wind Environment
Downloads
Understanding Comfort Criteria for Pedestrian Level Wind Environments (PDF 210KB)
Pedestrian level wind environment studies are carried out to ensure the safety and comfort of people using a development and its surrounding areas; often as a requirement for planning consent to be given. The wind environment also affects the commercial success of a project which depends on the perception of the public - for example, the popularity of a retail site may suffer if it is thought to be windy. Open-air cafes and seating areas are especially vulnerable. For prestigious buildings, especially corporate headquarters, the approach to the main entrance sets the tone for the whole image of the building and the company.
There are three components to an assessment of wind environment:
- Meteorology: the statistics of the incident wind climate
- Wind Engineering: the impact of the building
- Comfort Criteria: the reaction of the public
Only the wind-engineering component is under the control of the designer, but the other two are essential parts of the assessment, using wind records and established acceptability criteria.
Meteorology
The statistics of the wind climate at the site are assessed from meteorological records. The UK is fortunate in having a closely-spaced network of anemometers which have recorded the wind continuously since 1970. Worldwide we also have access to a great many other meteorological stations. The relative frequency of winds is needed in each level of the Beaufort scale, by direction, for the whole year at a typical site. In northern Europe, for example, the strongest winds come most frequently from the west, but there is a secondary peak from the north east. These statistics are also broken down to determine seasonal trends.
Wind Engineering
Desk Studies
Desk studies are often perceived to be quicker and cheaper than wind tunnel testing, but this depends on the complexity of the site and the nature of the wind tunnel testing. When dealing with the interaction between a few buildings, desk-top assessment methods give reliable results. An example of the Maruta method for a simple building is shown in the picture to the right. With experience, the effect of more complex shapes can be deduced; however, when many buildings surround a site, the interactions between buildings become too complex to determine accurately.
CFD - Computational Fluid Dynamics
RWDI Anemos have access to CFD programs, many of them developed in-house, but we are still very cautious about their use for the wrong type of project. Even with the more sophisticated solution methods (such as Large Eddy Simulation) they do not predict the wake region behind bluff bodies well. Clearly this is very important in urban landscapes where the interactions between multiple buildings compound the errors. We are strong believers in CFD for environmental winds as an illustrative tool but urge extreme caution when using it to provide any quantitative data. Not only will it probably be wrong, it may even be more expensive than a wind tunnel test to do properly.
Wind Tunnel Testing
Wind tunnel tests are mistakenly thought to be expensive and time consuming, but this again depends on the complexity of the site and the type of testing. Wind tunnel models need not be expensive when the test is confined to environmental winds only, because the buildings can simply modelled. It is essential to get the plan shapes and elevations correct, but roof and architectural details are often less important. One advantage of a physical model is that it can be retained indefinitely, and updated as neighbouring buildings are redeveloped. Altering a physical model can often be cheaper and quicker than re-meshing a numerical grid.
The "sand-erosion method", is a simple and quick method that gives quick, visual results on any horizontal surface and allows the whole model to be tested at one time. A granular material clears progressively from the windier areas as the wind speed in the tunnel is increased.
More detailed measurements are taken with special pressure probes that allow multiple measurements to be taken very quickly. The measured speeds are then combined with historic wind speed and direction data to compare against established Comfort Criteria. This establishes the suitability of the site for its intended purpose.
When mitigation is required, we recommend cost-effective methods based on our combined experience of more than 50 years. Options may include simple measures such as screening or planting, and only rarely involve reshaping certain key parts of the development. These can be accommodated in physical models to confirm their effectiveness.
