CFD/KIRCHHOFF METHODS FOR
COMPUTING ROTORCRAFT NOISE
Roger Strawn
US Army Aeroflightdynamics Directorate
Rupak Biswas
MRJ
NASA Ames Research Center
Moffett Field, CA 94035-1000
Overset grids and Kirchhoff surface for V-22 tiltrotor noise prediction
Research Objective:
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.
Approach:
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 cyle is written to disk for later postprocessing. This data is then used in the Kirchhoff integrations to compute the far-field noise.
Accomplishment Description:
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.
Significance:
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.
Future Plans:
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.
Related Publications:
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:
Roger Strawn
NASA Ames Research Center
Moffett Field CA 94035-1000
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