  With the increasing spans and complex deck shapes, aerodynamic nonlinearity
becomes a crucial concern in the design of long-span bridges. This paper
investigates the nonlinear interaction between the gust-induced and
motion-induced forces acting on bridge decks using the Vortex Particle Method
(VPM) as a Computational Fluid Dynamics (CFD) method. This nonlinear
interaction is complex and intractable by the conventional linear
semi-analytical models that employ the superposition principle. To excite such
interaction, a sinusoidally oscillating bridge deck is subjected to sinusoidal
vertical gusts. Two distinct aspects are studied: The influence of large-scale
sinusoidal vertical gusts on the shear layer of a moving body and the nonlinear
dependence of the aerodynamic forces on the effective angle of attack. For the
latter, the resultant aerodynamic forces based on a linear semi-analytical and
a CFD model are compared for the same effective angle of attack due to
sinusoidal gust and motion. The methodology is first employed to verify the
linear behavior of a flat plate and then study the nonlinear behavior of two
bridge decks. The results show that the linear superposition principle holds
for streamlined bridge decks with minimal shear layer instability. However,
this principle may not be valid for bluff bridge decks due to strong separation
vortices that dictate the shear layer, which induces nonlinearity in the
aerodynamic forces manifested through amplitude difference in the first
harmonic and emergence of higher-order harmonics. The outcome of this study
aims to provide a deeper understanding of the complex nonlinear fluid-structure
interaction occurring for bluff bodies subjected to motion and free-stream
gusts.
