What is wind flow?
Wind flow is a dynamic and random phenomenon that is complex to model. It is composed of eddies of varying size and rotational characteristics that are carried along in a stream of air that moves relative to the earth’s surface. These eddies give wind its gusty or turbulent character, with speeds and forces that vary considerably in both time and space. In order to model wind flow the wind vector at a point may be regarded as the sum of the mean wind vector (static component) and the dynamic, or turbulent, component due to wind speed variations from the mean.
In the lower levels of the atmosphere the turbulence of strong winds largely arises from contact with surface features shown in Figure 1. Therefore wind flow is particularly gusty in urban areas. This is because as wind impacts a building, further chaos is added to the already unstable nature of wind by the separation of flow, the distortion of the mean flow, the formation of vortices, and the development of a wake. These effects generate large, fluctuating wind pressures on the surface of the building that depend on the interactions of the flow characteristics (such as wind speed, wind height, ground surface features, air properties) with the building configuration (i.e. its shape, location, and dynamic and physical structural properties). As a result of these pressures, large aerodynamic loads get imposed on the building that may cause significant structural excitation and vibration provided that the natural frequency is low enough (less than 1 Hz. Above this value the structure is considered to be dynamically ‘rigid’). Therefore understanding the action of wind around dynamically sensitive buildings is of prime concern.
Wind forces on structures
Under the action of wind flow structures can experience two different responses, namely a static response and an aerodynamic (or dynamic) response. Aerodynamic forces on the structure include the drag (along-wind) force, which acts in the direction of the mean wind, and the lift (cross-wind or transverse) force, which acts perpendicular to the direction, as illustrated in Figure 2. These forces arise from different wind effects, described in more detail below. It has been shown that the along-wind response of slender structures is mostly due to the action of turbulence buffeting and that these along-wind accelerations are larger for structures with lower fundamental frequencies. The cross-wind response is more likely to arise from vortex shedding or galloping. This response is likely to exceed along-wind accelerations if the building is slender about both axes.
In general, the force due to wind-induced vibrations can be considered to consist of three components, as shown in Figure 3: a static part due to the 10-minute averaged extreme wind velocity; a static part due to fluctuating wind (turbulence); and a dynamic, or resonance, response caused by the inertial forces of a structure as it vibrates.
The overall response of a structure to wind can therefore be expressed in terms of a mean component, a broadband response, and a narrow-band response. The broad-band term is the component at the background frequencies of the wind and is largely quasi-static, while the narrow-band relates to the vibration at the natural frequency of the structure.