On the influence of wall boundary layers in closed transonic test sections

Abstract

The boundary layers at the test section walls of a transonic wind tunnel are known to reduce the wall interference. In the present paper this effect is studied by means of small perturbation theory, assuming viscosity to be negligible when perturbing a turbulent boundary layer. An approximation for thin boundary layers leads to a modified boundary condition at the wall of the test section, expressing the normal streamline slope induced by changes in mass flow density and crossflow within the boundary layer. This boundary condition is applied to the linearized equations of subsonic flow and to the non-linear transonic equations at choking, the cases of plane and circular test sections only being treated in detail. The results of linear theory show that all corrections except the three-dimensional angle-of-attack correction are considerably reduced by the presence of the boundary layers at Mach numbers greater than 0.9, the essential part of their influence being due to the change of mass flow density with pressure. In the case of choking the analysis indicates that the presence of boundary layers will increase the maximum model size for which the flow can be interpreted as corresponding to Mach number one in free flight. Finally, the technique of using artificial thickening of the wall boundary layers for reduction of wall interference is considered, though without reaching a definite conclusion as to its value compared to other techniques.