Book - School of Science and Technology

48 The building in winterTable 2.10 Values for coefficient k s and exponent aTerrain k s aOpen flat country 0.68 0.17Country with scattered windbreaks 0.52 0.20Urban 0.35 0.25City 0.21 0.33whereV ˆ mean wind speed at height z (m/s)V m ˆ mean wind speed at 10 m in open country (m/s)z ˆ height above ground (m)k s ˆ a coefficient representing the terrain (Table 2.10)a ˆ an exponent representing height (Table 2.10)It is sensible, in establishing infiltration values, to adopt a datum wind speed at the higherend of the scale and, for the greater part of the British Isles, an hourly mean speed of 8 m/sis exceeded for only 10% of the time.The pattern of wind flow over an exposed building takes a form such as that shownin Figure 2.10, but this, of course, is a generalisation since the effect of surroundingbuildings and obstructions may well disrupt the pattern in an unpredictable manner, asmay the aerodynamics of the building shape. However, it would appear that the sum ofthe positive pressure on the windward side and the negative pressure on the leeward sideapproximates to unity and that the pressure difference (P) is thus numerically equal tothe velocity pressure of the mean wind speed calculated as above, i.e. p v ˆ 0:6 V 2 Pa.The method may be summarised as shown in Table 2.11 where the individual values,for a limited range of building heights, are represented as the heat requirements per metrerun of window opening joint, for an air temperature difference inside to outside of onekelvin. The table lists basic figures which may require adjustment to take account of:. The reductions which may be appropriate in a building which is liberally compartmentedby partitions, as mentioned above. It must be emphasized that these reductionsapply to a whole building and not to individual rooms.. The need in a corner room to make an addition of 50% to the tabulated figure to takeaccount of cross-flow.Figure 2.10 Wind currents about a tall building