Aeroacoustic Computations of Contra-Rotating Open Rotors Using the Nonlinear Harmonic Method and a Chorochronic Approach

Abstract

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.

Validating the Harmonic Balance Method for Turbomachinery Tonal Noise Predictions

“…If the number of blades in each blade row match, β = 0 according to Eq. (15). In this special case, it is found that Eq.…”

“…This information was used to set the phase shift according to Eq. (15). To ensure that the gusts were convected with the flow, i.e.…”

“…11 Application of frequency domain methods to the non-linear URANS equations for noise predictions have been done by several authors,. 6,[12][13][14][15] To the authors' knowledge there is however still very little work available in the literature on validating the Harmonic Balance method for turbomachinery aeroacoustics. This paper aims to expand this knowledge by presenting results obtained with the Harmonic Balance solver recently implemented into Chalmers' in-house CFD/CAA tool G3D::Flow.…”

Aeroacoustic Analysis of a Counter Rotating Open Rotor based on the Harmonic Balance Method

“…If coupling between the unsteady perturbations is to be accounted for, the nonlinear terms in the URANS equations must be retained in the analysis. A common approach in this case is to integrate the equations in time [8][9][10], possibly in combination with the Chorochronic method [11] which is used to reduce the computational domain to one blade per row. Although time accurate solutions generally are computationally expensive, this approach has the clear advantage of making few assumptions on the nature of the flow apart from the choice of turbulence modeling.…”

“…Both the Harmonic Balance method as well as similar methods have been employed for turbomachinery tonal noise predictions with good results, see e.g. [9,[13][14][15] .…”