NASA's Advanced Subsonic Technology (AST) program is developing
new technologies to increase the fuel efficiency of commercial
aircraft engines, improve the safety of engine operation, and
reduce engine emissions and noise. With the development of new
designs for ducted fans, compressors, and turbines to achieve
these goals, a basic aeroelastic requirement is that there should
be no flutter or high resonant blade stresses in the operating
regime. To verify the aeroelastic soundness of these designs,
we need an accurate prediction and analysis code. Such a two-dimensional
viscous propulsion aeroelastic code, named TURBO-AE, is being
developed at the NASA Lewis Research Center.
The TURBO-AE aeroelastic code is based on a three-dimensional
unsteady aerodynamic Euler/Navier-Stokes turbomachinery code TURBO,
developed under a grant from NASA Lewis. TURBO-AE can model viscous
flow effects that play an important role in certain aeroelastic
problems, such as flutter with flow separation (or stall flutter)
and flutter in the presence of shock and boundary-layer interaction.
The structural dynamics representation of the blade in the TURBO-AE
code is based on a normal mode representation. A finite-element
analysis code, such as NASTRAN, is used to calculate in-vacuum
vibration modes and the associated natural frequency.
A work-per-cycle approach is used to determine aeroelastic (flutter)
stability. With this approach, the motion of the blade is prescribed
to be a harmonic vibration in a specified in-vacuum normal mode.
The aerodynamic forces acting on the vibrating blade and the work
done by these forces on the vibrating blade during a cycle of
vibration are calculated. If positive work is being done on the
blade by the aerodynamic forces, the blade is dynamically unstable,
since it will extract energy from the flow, leading to an increase
in the amplitude of the blade's oscillation.
Initial calculations have been done for a configuration representative
of the Energy Efficient Engine fan rotor. The accompanying figure
shows the work-per-cycle after each cycle of vibration. It can
be seen that the work-per-cycle does not vary much after the fourth
cycle. The negative sign of the converged work-per-cycle shows
that the fan blade is dynamically stable and will not flutter.
TURBO-AE will provide a useful aeroelastic prediction/analysis
capability for engine manufacturers. It will reduce design cycle
times by allowing new blade designs to be verified for aeroelastic
soundness before blades are built and tested. With this prediction
capability, it will be possible to build thinner, lighter, and
faster rotating blades without encountering aeroelastic problems
like stall flutter and high-cycle fatigue due to forced vibrations.
Bakhle, M.A., et al.: Development of an Aeroelastic Code Based on an Euler/Navier-Stokes Aerodynamic Solver. ASME Paper 96-GT-311, 1996.
Previous articleLast updated April 30, 1997
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