Abstract:
A method for the removal of the roof blockage effects for thick turbulent boundary layer flows over topographical models is presented. Predominantly, the surface pressure distributions over some selected two-dimensional models were examined and it was found that the distributions of the dimensionless surface pressure over hills of a given dimensionless shape were almost identical irrespective of hill size, slope or the wind tunnel used. Also, the alternative methods for the removal of the roof blockage effects were shown to lead to incorrect conclusions. New sections constructed for an existing small open-return win? tunnel are described. These included a 0.5 x 0.3m working section featuring an adjustable roof capable of large curvatures for hill modelling and ~lockage removal purposes. Grids of single-plane rods were tested in the small tunnel and in the department's larger 1.83 x 1.07m single-return wind tunnel. By a judicious choice of input parameters, it was found that grids designed according to the adapted Cowdrey (1967a) method were capable of generating thick logarithmic velocity profiles that conformed well to specifications. Next, the roof was transformed to an adjustable floor simply by inverting the rod grid, and then by adjusting this floor, hills of different sizes and shapes were obtained. The surface pressure distributions were compared to the linearized theory of Jackson (1975) and it was found that a correction, whose functional form was later verified in the large tunnel, was required. However, comparison with data from an independent source showed that this theory satisfactorily predicted the changes in the longitudinal velocity, no correction being needed. Tests were also conducted in the small tunnel with the grid in its original orientation and the roof adjusted according to a method based on the analysis of Jackson (1979) for the removal of the roof blockage effects. Different sizes of discrete model hills were tested with the largest being one-third of the tunnel height. Additional tests were also conducted in the large tunnel and it was found that the distributions of the dimensionless pressure were dependent only on the dimensionless hill shape consequently showing that the roof blockage effects had been effectively removed. The limitations associated with testing small models have been highlighted and explained. Also, the alternative methods for blockage removal were tested in the-small tunnel and shown to lead to incorrect conclusions. Finally, the results from the present method were compared to the predictions of a numerical simulation.