CFD Based Unsteady Aerodynamic Modeling For Rotor Aeroelastic Analysis

Abstract

A Computational Fluid Dynamics (CFD) analysis is developed for 3-D rotor unsteady aerodynamic load prediction. It is then coupled to a rotor structural analysis for predicting aeroelastic blade response, airloads and vibration. The CFD analysis accounts for the elastic deformations using a dynamically deforming mesh system. All the rotor blades are assumed to be identical, therefore to reduce the computational complexity the CFD calculations are performed for a single blade. This accounts for the near wake flow field. But the far wake effects because of the trailed tip vortices from all the blades have to be included separately. This is achieved by the use of the field velocity approach, which is a method for modeling unsteady flows via apparent grid movement. In this method, the induced velocity field caused by the trailed vortex wake is included by modifying the grid time metrics.

The CFD method developed is systematically validated for a range of problems starting from simple 2-D model problems to full scale forward flight cases. The CFD analysis shows significant improvements in airloads prediction compared to a table lookup based lifting-line analysis. The CFD analysis is then used to investigate the fundamental mechanisms of rotor vibration. It is found that both the normal forces and pitching moments are dominated by three dimensional aerodynamic effects. The curvature introduced by the blade elasticity appears to play a key role in the generation of the vibratory harmonics in airloads. The pitching moments near the blade tip (85\% outboard) are significantly affected by transonic tip relief effects. The fundamental understanding of rotor vibrations gained from this study is then used to develop generic corrections for improving the accuracy of a lifting line analysis.

Finally the CFD analysis developed is coupled with an advanced comprehensive rotor aeroelastic analysis. The coupling procedure is formulated in a way such that there is an exchange of information between the structural model and CFD model every rotor revolution. The coupled CFD/structure scheme is found to considerably improve the prediction of rotor vibratory airloads compared to the baseline rotor aeroelastic analysis which uses a lifting line based aerodynamic model.