Abstract
In order to assess the role of latent heat flux in urban climates, it is necessary to understand the evaporation rate in complex air flow around buildings. For this purpose, we made wind tunnel experiments in turbulent flow with the use of evaporation measurement by salinity change in the salt water vessels. In this paper, we report the distribution of mass transfer coefficient' (k) on flat plane and around two dimensional models. As for flat plane, evaporation rate by forced convection is in proportion to -1/5 power of Reynolds number. In spite of the difference of vertical profiles in boundary layer, k is determined by mean wind velocity at the top of boundary layer. On the contrary, k is proportional to R. M. S. of turbulent velocity near the surface and the relations to mean wind velocity vary with roughness condition. Regarding the surroundings of buildings, k is in accord with air flow around it. Namely, downdraft along windward wall increases k in front of building (1.3-1.5), and about leeward k is small in wake (0.5-0.7) and has a peak in reattachment area (1.5-1.7). Between buildings, k is tend to be large in downstream because of circular flow, and average value of k is maximum on the condition of L/H≒2. As a whole, the deterioration of mean wind velocity around buildings reduces k by a factor of 0.3-0.6, while for same mean wind velocity k is one and half times as large as that of flat plane owing to the increment of turbulence.
