High Fidelity Noise Prediction for Open Rotors

The objective of this proposal is the implementation of a high-fidelity Computational Fluid Dynamics-Computational Aeroacoustics (CFD-CAA) simulation capability for accurate noise prediction of installed contra-rotating rotor (CROR) configurations. The key technology that allows for the success of the project is the use of an acoustic analogy based on surface and volume integrals coupled to a dual-mesh, dual-solver paradigm, where an unstructured mesh near-body solver is coupled to a Cartesian, adaptive Discontinuous Galerkin (DG) off-body solver using an overset domain connectivity algorithm. The use of unstructured overset meshes affords the necessary flexibility for handling complex moving geometries while at the same time enabling efficient capturing of blade, hub, and other high-Reynolds number wall boundary layers. The use of a high-order accurate method combined with adaptive mesh refinement in off-body regions enables the accurate resolution and convection of vortices and wakes over long distances. Far-field acoustic signatures are obtained using an acoustic-analogy approach based on the Ffowcs Williams-Hawkings (FW-H) equation. Together with the commonly used permeable-surface integration of the FW-H equation, the feasibility of the direct volume integration of the quadrupole term will be demonstrated for moving overset meshes. The quadrupole integration enables accurate characterization of the dominant sources of broadband noise in CROR propulsion systems, such as rotor-wake/rotor interaction and rotor trailing-edge noise. Furthermore, the direct integration of the quadrupole term in the FW-H equation obviates the need to identify a proper permeable integration surface common of other approaches. Hence, our approach results in a noise-propagation methodology that is naturally suited for the prediction of sound generated by the complex moving geometries that are the ultimate interest of this project.

The proposed techniques will provide a novel cost-effective high-fidelity tool for open rotor noise prediction capable of handling complex geometries. This is an important application area for the NASA Aeronautics Mission Directorate, both for fixed wing applications with CROR propulsion and for extensions to rotary wing aircraft. Our surface and volume integration FW-H code will be written in a modular fashion which will be delivered to NASA for inspection and coupling with internal NASA codes.

The immediate customers for CROR noise prediction will be the aerospace propulsion OEMs. However, we anticipate significant opportunities in the rotorcraft industry as well as for propeller and fan driven aircraft. Particularly, in the emerging eVtol industry, noise is a principal driver towards community acceptance, offering a significant additional commercial market opportunity to be exploited.

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