CFD-based shape optimization of hovering rotors using global and local parameters

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 used as the surface control and deformation method, and is linked to a radial basis function global interpolation, 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 so both the geometry control and volume mesh deformation problems are solved simultaneously. This method is independent of mesh type (structured or unstructured) or size, and optimization dependence 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 work has applied the methods to hovering rotors using only twist parameters, using minimum torque as the objective, with strict constraints on thrust, internal volume and pitching moments applied. The effects of global and local twist parameters were investigated, and showed that significant torque reductions could be achieved using only three global and 15 local twist parameters. This paper extends the parameterization to allow 63 local and global parameters, and results are presented for two transonic tip Mach numbers. Large geometric changes are demonstrated, resulting in significant torque reductions.