  The present paper develops a reduced-order model capable of modeling unsteady
aerodynamic loads in the transonic regime using system identification
techniques. The computational fluid dynamics (CFD) calculations are based on
the Euler equations and the code uses a finite volume formulation for general
unstructured grids. A centered spatial discretization with added artificial
dissipation is used, and an explicit Runge-Kutta time marching method is
employed. For comparison reasons, unsteady calculations are performed using
mode-by-mode and simultaneous excitation approaches. System identification
techniques are employed to allow the splitting of the aerodynamic coefficient
time histories into the contribution of each individual mode to the
corresponding aerodynamic transfer function. Such methodology is applied to
model the aerodynamic terms of the aeroelastic state-space system particularly
of a NACA 0012 airfoil-based typical section. Results demonstrate the
importance of signal processing techniques to compute the aerodynamic transfer
functions and also the advantageous applicability of transonic unsteady
aerodynamic reduced-order models to perform aeroelastic analyses in the
frequency domain.
