The Influence of Variable Flow Velocity on Unsteady Airfoil Behavior

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Date
1991Author
van der Wall, Berend G.
Advisor
Leishman, J. Gordon
DRUM DOI
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Show full item recordAbstract
The importance of unsteady aerodynamics for prediction of rotor dynamics
is unquestioned today. The purpose of unsteady aerodynamic models is
to represent the effect of unsteady airfoil motion on the lift, moment and
drag characteristics of a blade section. This includes unsteady motion (arbitrary
motion) of the airfoil in angle of attack (pitch) and vertical movement
(plunge), as well as the effects of an airfoil traveling through a vertical gust
field. However, the additional degrees of freedom, namely the fore-aft motion
and the unsteady freestream variations commonly are acknowledged,
but neglected in virtually all analyses.
Since the effect of unsteady freestream results in a stretching and compressing
of the shed wake vorticity distribution behind an airfoil, it will have
an effect on the airfoil characteristics. The subject of this thesis is to provide
a review of the analytic and experimental work done in the area of unsteady
freestream and unsteady fore-aft motion, to clarify the limits of the various
theories, and to show the differences between them. This will be limited to
the attached flow regime since all theories are based on the small disturbance assumption in incompressible flow. As far as possible the theories are compared
with experimental data, however most of the available experimental
data are confined to stalled flow conditions and are not useful here.
In addition to the theories, a semiempirical mathematical model will be
used based on the aerodynamics of indicial functions. The purpose is to show
the differences of using the theories of unsteady airfoil motion in a constant
flow, and those accounting for unsteady freestream flow. This will help to
justify whether it is necessary to include the unsteady freestream effect in
comprehensive rotor codes.
Finally, a generalisation of Isaacs unsteady aerodynamic theory for an
airfoil undergoing a frequency spectra in pitch and plunge in a freestream
oscillating with the fundamental frequency is presented here for the first
time. Therein the axis of rotation of the airfoil is a free parameter.