Date: August 19, 2008
Time: 10:00am
Location: NIA, Room 137
Additional Info:

The Connection Between Acoustics and
Unsteady Aerodynamics
F. Farassat

Senior Research Scientist, Aeroacoustics Branch
NASA Langley Research Center, Hampton, Virginia
It has been known for a long time that acoustics and linearized unsteady aerodynamics are
closely related to each other. Acoustics and aerodynamics are usually viewed as two different
disciplines and each field has developed its own specific analytical and computational techniques.
However, in some problems of aeroacoustics, such as in the case of the airframe noise
prediction, the acoustics and aerodynamics are so intertwined that the distinction between the
two disciplines seems to disappear. Although computational aerodynamics is the main tool of
aircraft designers, linearized unsteady aerodynamics is still of much value to acousticians and
aeroelasticians because their problems are four dimensional and, thus, inherently computer
intensive. Historically, Theodorsen, Sears and von Karman, and Küssner were first to propose
unsteady aerodynamic theories that stimulated much theoretical and experimental effort around
the world. More recently, Morino, Amiet, Goldstein, Atassi, Adamczyk, Verdon and many
others worked on the refinements and extension of existing theories. We want to take a fresh
look at the connection between acoustics and unsteady aerodynamics starting from the direction
of acoustics and utilizing the advances made in the last three decades in the analysis of the
aeroacoustics of moving surfaces. We feel that we can learn a lot from this study. Our approach
will be mainly analytic and limited to linear unsteady aerodynamics. Our aim is to understand
and to unify many available unsteady aerodynamic approaches and results that are primarily
utilized in current aeroacoustic and aeroelastic problems. For uniform motion of a body, either
the frame fixed to the medium or the frame fixed to the body can be used for the formulation of
the aerodynamic problem. It will be shown that the most natural frame to study unsteady aerodynamics
is the one fixed to the stationary fluid medium. Regardless of how the problem is formulated,
studying the unsteady aerodynamic problem from the point of view of acoustics can help
in solving and understanding fully the physics of the problem. As is well known, unsteady aerodynamics
can be developed in the time or the frequency domain. We will show that from the
point of view of acoustics, time domain analysis is the more natural approach to unsteady aerodynamics
and it can even shed light on the frequency domain approach. We will end our talk by
declaring that linearized unsteady aerodynamics is all acoustics! Our talk will include some
interesting historical facts and stories about personalities involved in the development of
unsteady aerodynamics.