Scale effects in wind tunnel modelling

Scale effects in wind tunnel modelling are studied by comparing the results from models with various scales with the atmospheric boundary layer. It is found that the scale ratio of roughness height should be the same as the scale ratio of boundary layer thickness. The individual roughness element exhibits local effect only in the lower zone of two times the roughness height. Lego bricks are used as the roughness elements. The best similitude of the velocity profile in the wind tunnel can be obtained if the scale ratio of the boundary layer is in the range 200-400. The scale effects of pressure on the surface of a bluff body are studied by using three models of different scale ratios. As long as the roughness height is correctly modelled, correct results of the pressure coefficient on the building surface can be obtained even if the scale ratio of the body size is not correctly selected. The scale ratio of the diffusion coefficient is the same as the scale ratio of the boundary layer thickness. The diffusion coefficient is proportional to the thickness of the boundary layer, and distributions of the dimensionless diffusion coefficient in wind tunnels with different scale ratios and in the atmospheric boundary layer are the same.