Modélisation de l'aérodynamique incluant le décrochage dynamique pour les codes compréhensifs d'analyse des rotors d'hélicoptère

摘要

To fulfill the objective of design tools, comprehensive analysis codes need to be capable of providing both accurate and time-efficient predictions of rotor airloads. The coupling of high-fidelity computational fluid dynamics CFD with comprehensive analysis codes has showed improved predictions of airloads, but such approach is very expensive in terms of CPU time and cannot be used for design. The aerodynamics environment about helicopter rotors is very complex, encompassing subsonic to transonic flow with unsteady, stalled regimes and 3-D effects. Semi-empirical models of dynamic stall were created for modeling unsteady aerodynamics including stalled flow. Most of them, developed from the 1970s through the 1990s, were found unsuccessful to reproduce experimental results on the UH-60A helicopter. Most dynamic stall models also suffered problems of numerical convergence. Thus, there are two levels of difficulties for semi-empirical models: providing good physics description of aerodynamics and ensuringnumerical convergence when implemented in comprehensive analysis codes.The present communication is concerned about the revision of the ``ONERA – Hopf Bifurcation model''. The model takes into account various aerodynamic phenomena, unsteady behavior before stall onset, stall delay, vortex-shedding phenomenon and boundary-layer effects that have been overlooked.3-D effects are not still well investigated and their complexity is accounted for by sweep effects, rotation effects and transonic tip relief effects. For the extreme aerodynamic condition of high-speed flight, a correction outside of the stall model, is suggested for the induced velocity. The implementation of the model into a comprehensive analysis code ensuring the numerical convergence is presented. The experimental results obtained in the wind tunnel S1 of Modane (France) in 1991 on the rotor 7A are considered for illustration of the capabilities of the stall model. There are three test conditions considered: high-speed test point, high-thrust test point with light stall and high-thrust test point with deep stall. Since tests were made in windtunnel, corrections for test environment involving wind tunnel walls and the test stand, and for Reynolds effects are necessary. The prediction of the airloads and structural loads of all the test points considered is in reasonable agreement with the experimental results and requires very low CPU time demands.