Due to their great potential for fuel saving, Contra-Rotating Open Rotors (CRORs) receive renewed interest by the airframers and the engine manufacturers. The inherent high efficiency of this propulsion system, however, is potentially offset by the level of noise emitted by the open blades. The acoustic impact on passengers and community may represent a major issue to their environmental acceptance. Fast and robust noise prediction tools are clearly required to support the development of quieter propellers and their integration in future civil aeronautical transport. The most common strategy for noise estimation consists in a two-step approach, based on the Lighthill analogy: unsteady near-field aerodynamic flow simulation to evaluate the noise sources, coupled to a far-field acoustic propagation code.
Focus is given here on two structured grid flow solvers employed to investigate a scale-model of a 12×10 pusher CROR. The unsteady aerodynamic three-dimensional flow is indeed computed for typical cruise conditions using both the nonlinear harmonic method (NLH) of FINE™/Turbo software and elsA’s chorochronic technique. The evaluation of the far-field noise based on the aerodynamic fields is then carried out with the KIM code, Onera’s acoustic propagation code based on the Ffowcs-Williams and Hawkings (FW-H) formulation.
The obtained results enable an analysis of the complex aerodynamic interactions between the two propellers that generate interaction tones in the acoustic signature of the propulsion system. A comparison in terms of numerical settings, computational costs and flow fields is performed between the two CFD methods, which show an excellent match of the predicted global performance of the propulsion system. Some differences in the predicted acoustic signatures are discussed in the paper.
The ONERA 7A rotor wind tunnel test data are investigated to assess the accuracy of analytical tools in the calculation of rotor airloads and structural loads. Comprehensive analysis codes, HOST and RCAS, and coupled computational fluid dynamics/comprehensive analysis codes, elsA/HOST and Helios/RCAS, are used to examine the ONERA 7A rotor blade dynamics, trim, airloads, and structural loads, and the calculated results are compared with the measured data for both high-speed and high-thrust conditions. The present elsA and Helios analyses include the test stand modeling. Comprehensive analyses show significant phase differences for the airloads and structural loads. The phase correlation is substantially improved by the coupled analyses for both high-speed and high-thrust conditions. In general, the coupled analyses also improve the half peak-to-peak correlation of structural loads compared to the comprehensive analyses. The majority of the predictions from the coupled analyses in half peak-to-peak blade structural loads are within 20% of the measured data. The present study shows unprecedented correlation of structural loads for the 7A rotor, consistently better than the previous structural loads correlation for the UH-60A rotor using Helios/RCAS analysis.
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