An experimental investigation of wind-tunnel wall conditions for interference-free dynamic measurements

Abstract

Results are presented of dynamic tests on two model half-wings performing pitching oscillations in the NPL 24 cm x 24 cm (9½ in x 9½ in) tunnel. The boundary condition at the ventilated roof and floor is varied by fitting perforated channels behind the longitudinal slots such that the size of the perforations can be varied from zero to 0.79 cm (0.31 in) diameter circular holes. It is found that the measured derivatives each vary systematically with changing porosity and a method is suggested for selecting a wall porosity to give interference-free damping derivatives; interference-free stiffness derivatives are not simultaneously obtained and small corrections to measured values are required. A porosity parameter for the ventilated wall is defined, values of this parameter are measured and consideration is given to the value required to minimize wall interference in the NPL tunnel and other tunnels of different shape or different slot configuration.