Towards a 3-D Fluid-Structure Interface for Helicopter Rotors

The emergence of three-dimensional Computational Structural Dynamics for helicopter rotors warrants the development of a higher fidelity fluid-structure interface that can replace the one-dimensional sectional airload interface commonly used for coupled analysis with Computational Fluid Dynamics. Three methods of progressively higher fidelity are examined for imposing airloads onto the structure. These are defined as level-III, II, and I, based on fluid stresses, patch forces, and sectional airloads (baseline), respectively. A model problem investigating a 3-D cylindrical shell with large deformations near the boundaries is used to verify the methods. The patch force interface (level-II) approaches the stress interface (level-III) when the mesh is highly refined. Level-I (baseline) produces no solution at all (or zero solution). Level-II is then applied to a UH-60A-like rotor and compared with level-I. Only a forced response was carried out, not a full-fledged trim solution. For this practical problem, severe mismatches of the fluid and structural meshes required a special algorithm to impose the patch forces onto the structure. The key conclusion was that a higher fidelity interface is indeed feasible for rotors, and level-II appears to be the most convenient. Even though axial (bending) stresses showed approximately 5% difference from the baseline airloads interface, the errors in shear stresses were dramatic, rising to 100% and higher inside the main load bearing parts of the rotor across its mid-span.