CFD-Based Twist Optimization of Hovering Rotors

Abstract

Aerodynamic shape optimization of a helicopter rotor in hover is presented, using compressible CFD as the aerodynamic model. An efficient domain element shape parameterization method is presented which overcomes both the geometry control and volume mesh deformation problems simultaneously. Radial basis function global interpolation is used to provide direct transfer of domain element movements into deformations of the design surface and the CFD volume mesh, which is deformed in a high-quality fashion, and the parameterization method requires very few design variables to allow free-form design. This method is independent of mesh type (structured or unstructured) or size, and optimization independence from the flow solver is achieved by obtaining sensitivity information for an advanced parallel gradient-based algorithm by finite-difference. This has resulted in a flexible and versatile modular method of ’wrap-around’ optimization. Previous fixed-wing results have shown that a large proportion of the design space is accessible with the parameterization method and heavily constrained drag optimization has shown that significant improvements over existing designs can be achieved. In the present work, the method is extended to a rotor blade, and this is optimized for minimum torque in hovering flight with rigid constraints on thrust, internal volume and pitching moments. Twist optimization results are presented for three tip Mach numbers, and the effects of different parameterization levels analysed, using various combinations of two levels; global and local. Torque reductions of over 12% are shown for a fully subsonic case, and over 24% for a transonic case, using only three global and 15 local twist parameters.