|Wind forces on Emergency Storage Structures (supplement) (NRI)|
Air flow in wind is laminar and/or turbulent. Laminar flow implies little exchange of mass between different layers, while turbulent flow has such exchange with resultant Reynolds or shearing stresses. Figure 1 below contrasts laminar with turbulent flow.
Figure 1 Laminar and turbulent flow
At the edge of the boundary layer next to the main stream, the fluid velocity is equal to the main stream speed. With laminar flow speed drops sharply, but with turbulence velocity falls only when the fluid is close to the surface. With turbulent flow there is interchange of energy between layers and therefore mean velocity is almost the main stream speed. The British Standards Institution CP3: Ch V-Part 2 (1972) defines the mean 'turbulent' wind speed for the United Kingdom as the 3-second basic gust speed to be exceeded on average once in fifty years. The Building Research Establishment (Eaton, 1981) and the Meteorological Office (1987) have provided 3-second basic gust speeds for a selection of tropical countries (Table 1). For rigid structures these basic gust speeds can be translated into wind loads based on the dynamic pressure of wind; for calculation of wind forces see Appendix 2. Gust speeds are squared for this purpose, hence the importance of accuracy at the design stage to achieve robust structures.