Wall interference effects on helicopter rotor in closed and open wind tunnels

Abstract

The wall interference effects substantially contribute to systematic errors in wind-tunnel experiments, which greatly impedes the accurate measurements of aerodynamic performance, hindering the development and optimization of helicopter rotors. This paper numerically investigates the wall interference effects on helicopter rotors in both closed and open wind tunnels based on a virtual blade model. Comprehensive analyses and comparisons are performed regarding the flowfield characteristics (including downwash flow and wake flow) and helicopter aerodynamic performance (including rotor torque coefficient and lift coefficient) under freestream, closed wind-tunnel, and open wind-tunnel conditions. Results indicate that the open and closed test sections have opposite impacts on the aerodynamic performance of the helicopter rotor. Specifically, the open test section gives rise to a more inclined rotor downwash flow, resulting in an overestimated measured rotor torque coefficient, which is however underestimated for the closed test section. At the low advance ratio, the open test section remarkably intensifies the negative lift on the fuselage, increasing the load while reducing the flat tail load. Conversely, the closed test section weakens both the fuselage and flat tail loads. Furthermore, at low advance ratios, flow breakdown is observed for both open and closed wind tunnels. Additionally, the open test section experiences wake escapement, which deflects the flow away from the return circuit, affecting its operation.