CFD Prediction of Rotor Loads Using Time-Spectral Method and Exact Fluid-Structure Interface

Abstract

The primary objective of this paper is to study time-spectral method for simulating helicopter rotor ows in steady ight. The intent is to compare the accuracy of predicted vibratory loads (both airloads and structural loads) with time-accurate computations and quantify the aliasing error and convergence behavior in a precise manner. The CFD/Comprehensive Analysis coupling method in this paper is dierent from the state-of-the-art. We implement an exact uid-structure interface for rotors and formulate a modied delta coupling procedure, that is generic for advanced geometry blades, underlying structural models, and unstructured surface grids. The Counter 8534 ight from the U. S. Army/NASA Airloads Program, the highest vibration ight of this helicopter, is used for validation. The fastest and minimal implementation of the method, with 11 time instances for a 4-bladed rotor, predicts the vibratory normal forces and pitching moments within 5{10% accuracy with respect to time-accurate simulations in about onefth the computational time. The largest errors occur in vibratory chord forces (10 to 20%). This level of error generates un-satisfactory levels of error in structural loads. However, the primary source of error is aliasing, which for this ight decreases asymptotically with an increasing number of time instances. We demonstrate an accuracy level of 1% and 0.1% in airloads with 17 and 25 time instances respectively. These correspond to one-third and one-half the computational time of a time-accurate solution. It is concluded that time spectral method in CFD can be used eectively for the prediction of rotor vibratory loads. However, without any anti-aliasing lter, reliable prediction of structural loads requires a number of time instances at least four times the blade number { still at one-third the time, approximately, compared to a time-accurate solution.