OVERSET GRID METHODS FOR PREDICTING
ROTORCRAFT AERODYNAMICS AND ACOUSTICS
Earl Duque
Roger Strawn
US Army Aeroflightdynamics Directorate
Jasim Ahmad
MCAT Inst.
NASA Ames Research Center
Moffett Field, CA 94035-1000
Overset grids and Kirchhoff surface for a two-bladed rotor system
Research Objective:
Future rotorcraft must have low noise if they are to operate near heavily
populated areas. This is particularly true for tiltrotors as they descend
for landings. This project aims to predict the aerodynamics and acoustics
of rotorcraft by solving the Navier-Stokes equations on a series of overset
computational grids that surround the rotor blades.
Approach:
The overall scheme uses a finite-difference Navier-Stokes flow solver to
compute the aerodynamic flowfield near the rotor blades. The equations are
solved on a system of overset grids that allow for prescribed cyclic and
flapping blade motions and capture the interactions between the rotor blades
and wake. The far-field noise is computed with a Kirchhoff integration
over a surface that completely encloses the rotor blades. Interpolation of
flowfield data onto this Kirchhoff surface uses the same overset-grid
techniques that are used in the flow solver.
Accomplishment Description:
As a demonstration of the overall scheme, aerodynamic and acoustic
solutions were generated for a two-bladed rotor in forward flight.
Far-field acoustic results were compared to experimental microphone data for
two different flight conditions. Computed high-speed impulsive noise showed
good agreement with experimental data, but it is clear that the Navier-Stokes
flow solver requires improved grid resolution in order to accurately model the
details of blade-vortex interaction noise. Four overset grids were used in
the flow solver with a total of 1.25 million grid points. Each periodic
flowfield solution required approximately three blade revolutions and 24
CRAY C-90 CPU hours.
Significance:
The overset-grid CFD scheme provides a powerful new framework for the
prediction of rotorcraft noise. Two important features of the new formulation
distinguish it from other approaches. First, the rotor wake is captured as
part of the overall solution. Second, the formulation includes realistic
rotor blade motions. These features provide a highly flexible computational
tool for the prediction of rotor airloads and noise.
Future Plans:
Future efforts will aim to reduce numerical dissipation in the rotor
wake. High-order spatial and temporal differencing schemes will be
used with solution-adaptive grids to improve the rotor wake resolution.
High-fidelity rotor wake simulations will dramatically improve
our blade-vortex interaction noise predictions for helicopters and tiltrotors.
Related Publications:
Ahmad, J., Duque, E. P. N. and Strawn, R. C., "Computations
of Rotorcraft Aeroacoustics Using a Combined Navier-Stokes/Kirchhoff Method,"
presented at the 22nd European Rotorcraft Forum, Brighton, UK, Sept. 16-19,
1996.
Duque, E. P. N., Strawn, R. C., Ahmad, J. and Biswas, R., "An
Overset Grid Navier-Stokes/Kirchhoff-Surface Method for Rotorcraft
Aeroacoustic Predictions," AIAA-96-0152, presented at the 34th Aerospace
Sciences Meeting, Reno, NV, Jan. 15-18, 1996.
Point of Contact:
Roger Strawn
NASA Ames Research Center
Moffett Field CA 94035-1000
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