Enhancement of aeroelastic rotor airload prediction methods .

摘要

The accurate prediction of rotor airloads is a current topic of interest in the rotorcraft community. The complex nature of this loading makes this problem especially difficult. Some of the issues that must be considered include transonic effects on the advancing blade, dynamic stall effects on the retreating blade, and wake vortex interactions with the blades, fuselage, and other components. There are numerous codes to perform these predictions, both aerodynamic and structural, but until recently each code has refined either the structural or aerodynamic aspect of the analysis without serious consideration to the other, using only simplified modules to represent the physics. More recent work has concentrated on coupling CFD and CSD computations to be able to use the most accurate codes available to combine the best of the structural and the aerodynamic codes. However, CFD codes are the most computationally expensive codes available, and although combined CFD and CSD methods are shown to give the most accurate predictions available today, the additional accuracy must be deemed worth the time required to perform the computations.;The objective of the research is to both evaluate and extend a range of prediction methods comparing accuracy and computational expense. This range covers many methods where the highest accuracy method shown is a delta loads coupling between an unstructured CFD code and a comprehensive code, and the lowest accuracy is found through a free wake and comprehensive code coupling using simplified 2D aerodynamics. From here, methods to improve the efficiency and accuracy of the CFD code are considered through implementation of grid adaptation and low Mach number preconditioning methods. Applying grid adaptation allow coarser grids to be used where high gradients in the physics are not present, reserving the denser areas for more interesting regions. For steady-state problems, clustering of the grid provides better wake resolution behind the actuator disk. This method is proven to work for the steady-state equations, but its application to rotor flows using the time-accurate equations still needs to be tested. Low Mach number preconditioning is also an efficiency and an accuracy improvement which allows the CFD code to work for a wider range of Mach numbers within a single simulation. There are many cases, especially for rotor flows, where the range of Mach numbers contained in the flow field cover both the incompressible and compressible regimes. Thus, applying the compressible equations to the entire flow field results in governing equations with high stiffness matrices. The preconditioning reduces the numerical stiffness and thus improves the quality of the results. This improved quality is demonstrated through low speed rotor-fuselage simulations.;Further efficiency improvements are obtained by modifying the codes used in the analysis to include more simplified methods. On the aerodynamic side, a coupling between a CFD code and a prescribed rigid motion module has been completed, and on the structural side a coupling between a CSD code and a combination of a 2D airfoil theory and a free wake code is shown. It is found that the rigid motion method is more appropriately applied where blade elasticity is not significant, and the CSD method is far more efficient than CFD methods, but with a penalty in accuracy. The exact formulation of the 2D aerodynamic model used in the CSD code is discussed, as are efficiency improvements to improve the speed of the free wake code. The advantages of the computationally expensive free wake code are tested against a faster dynamic inflow model, and show that there are improvements when using the more accurate wake formulation. A comparison of these methods evaluates the advantages and consequences of each combination, including the types of physics that each method is able to, or not able to, capture through examination of how closely each method matches flight test data.