Rotorcraft simulations: a challenge for CFD

This paper gives an overview of CFD techniques developed and used at ONERA for rotorcraft applications. First, the complex multidisciplinary environment around helicopters, in which aerodynamics, flight dynamics, aeroelasticity and aeroacoustics strongly interact, is highlighted. Rotorcraft simulations are thus performed by comprehensive codes capable of dealing with the whole system efficiently, using integrated simplified models for each discipline, e.g. the aerodynamics. However, fast aerodynamic models cannot accurately represent the full complexity of rotorcraft aerodynamics, in particular as far as nonlinear phenomena are concerned, contrary to CFD. Nevertheless, helicopter problems are particularly demanding for numerical methods, requiring efficient simulation of unsteady flows with shock waves, massive flow separation, concentrated vortex structures and deforming bodies with large amplitude relative motion, while allowing fine description and analysis of local flow phenomena impacting the vehicle behaviour. Helicopter trim in the CFD solution is obtained by iterative coupling with comprehensive analysis, so that the global multidisciplinary simulation can be achieved with an advanced aerodynamic model. The approaches taken by ONERA for the comprehensive code and the CFD solvers are outlined in the paper. Examples of applications typical of rotorcraft problems are given to illustrate current possibilities and difficulties. They include an isolated rotor in hover, the dynamic stall of an oscillating wing, an isolated rotor in descent flight with Blade-Vortex Interactions, the dynamic-aerodynamic coupling of a rotor in high-speed forward flight and the simulation of a complete helicopter in forward flight. Finally, expected and needed developments are reviewed in order to make CFD a more efficient tool in the design office of helicopter manufacturers.