In the development of taller and ever-more complex buildings, a blend of CFD (Computational Fluid Dynamics) and Wind Tunnel testing is used to ensure that buildings are structurally safe, unlikely to cause discomfort or be susceptible to fatigue and failure. Often theoretical calculations will not be representative of reality, especially in a highly complex structure. In addition to the integrity of the structure itself, consideration must be given to the effects on buildings in the surrounding vicinity. An initial desk study may determine the necessity for use of a powerful simulation tool, the Wind Tunnel.
Where wind tunnel testing is required, a highly-detailed scale model (usually 1/200th or 1/400th, depending on available wind tunnel constraints) is constructed representing the structure under test and the buildings and landscape nearby. This is constructed in a model shop from detailed architectural plans, either by assembly of sectional fabrications or by 3D printing of models, which are often mounted on a rotating platform. This platform can usually be rotated through 360o to assess directional wind actions that would otherwise be difficult to visualise. The model is installed in the wind tunnel and subjected to extended testing over a wide range of wind directions and velocities, often up to 60 m/s. Data is gathered from numerous locations on the model to identify any areas that require further analysis or evaluation.
The Measurement Challenge:
Models vary considerably in design and diversity and may represent a building, bridge, chimney, sports stadium, offshore platform or wind turbine tower, where use criteria are being considered. These models are fitted with numerous sub-1mm pressure tappings that allow the localised pressure to be accurately measured at, often, 300-500 measurement points. The model and platform are usually supported on a multi-directional force balance. The data is used to determine whether the structure will perform as expected over a spectrum of wind conditions. Testing can be expensive and time-consuming, so ensuring efficient gathering of high quality, valid data – every time – is vital.
Accurate and consistent data from a proven and reliable measurement device ensures that measurement effects are minimised and the data truly represents the effects of wind and turbulence on a full-size structure. Based upon the analysis of the multitude of data gathered, decisions can be taken as to whether the structure is acceptable or needs design modifications, such as the addition of aerofoil sections, vortex disruptors, dampers or other such devices.
Challenges in the measurement of so many local pressures can be addressed with the use of pressure scanners which gather vast amounts of synchronous data very quickly and accurately, transferring this to a data acquisition system. Self-zeroing scanners with purge facilities are used to ensure that the data is consistently and accurately measured with minimal downtime or intervention. Often the pressure sensors will be regularly checked, calibrated and zeroed to ensure accuracy and repeatability of the scanned pressure data in-situ between test points and data frames. Usually multiple pressure scanners are multiplexed to gather data from many models and often space constraints may prove to be an issue in the wind tunnel environment.
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