CFD/KIRCHHOFF METHODS FOR COMPUTING ROTORCRAFT NOISE
US Army Aeroflightdynamics Directorate
NASA Ames Research Center
Moffett Field, CA 94035-1000
Civilian helicopters and tiltrotors that operate in urban areas must have low noise if they are to meet with public approval. In order to achieve these low noise designs, rotorcraft designers need high-accuarcy methods for predicting far-field noise. Euler and Navier-Stokes CFD methods provide excellent models for the near-field nonlinear acoustic propagation, however, near-field CFD grids cannot be extended to the far-field without excessive computational cost. Linear propagation methods such as Kirchhoff integrals are much more economical in the far-field, but fail to accurately model the near-field nonlinearities. An excellent compromise is to use the nonlinear CFD methods in the near-field and Kirchhoff methods in the far-field. This research aims to combine these two approaches into an accurate and efficient acoustics prediction scheme.
The Navier-Stokes CFD equations are solved in the near-field on an overset grid system for the rotor blades and wake system. The nonlinear CFD solution is then interpolated onto a nonrotating Kirchhoff surface that completely surrounds the rotating blades. Computational efficiency is ensured by using the same interpolation techniques for both the Kirchhoff-surface interpolations and for the interpolations between the overset grids. The Kirchhoff surface data for one unsteady cycle is written to disk for later postprocessing. This data is then used in the Kirchhoff integrations to compute the far-field noise.
The overall approach has been demonstrated for three test cases. The first case consists of in-plane high-speed impulsive noise and the other two cases show idealized parallel and oblique blade-vortex interaction noise. The computed results show excellent agreement with available experimental data and also convey much more information than the experiments about the far-field propagation of rotor noise. The Kirchhoff integrations have been run on an IBM SP-2 parallel computer with excellent parallel efficiency.
The combination of CFD solutions in the near field and Kirchhoff integrals in the far-field offers high-accuracy with reasonable computational resource requirements. Because the overall acoustic prediction method computes the rotor wake system directly in the CFD solution, it offers a clear path for computing blade-vortex interaction noise without the need for external wake models.
These new analysis tools will be applied to rotor blades with self-generated blade-vortex interactions (BVIs). Also, the Kirchhoff integration will be improved to account for non-uniform flow through the Kirchhoff surface.
Strawn, R. C., Biswas, R., and Lyrintzis, A. S., "Helicopter Noise Predictions Using Kirchhoff Methods," presented at the 51st Annual Forum of the American Helicopter Society, May 9-11, 1995 (see also Journal of Computational Acoustics, Vol. 4, No. 3, Sept. 1996, pp. 321-339).
Point of Contact:
NASA Ames Research Center
Moffett Field CA 94035-1000