  This paper presents a reduced order approach for transient modeling of
multiple moving objects in nonlinear crossflows. The Proper Orthogonal
Decomposition method and the Galerkin projection are used to construct a
reduced version of the nonlinear Navier-Stokes equations. The Galerkin
projection implemented in OpenFOAM platform allows accurate impositions of
arbitrary time-dependent boundary conditions at the moving boundaries. A
modelling technique based on moving domain and immersed boundary techniques is
proposed to overcome the challenge of handling moving boundaries due to
movements of the multiple objects. The model is demonstrated capable to capture
the complex flow fields past one and two oscillating cylinders and the forces
acting on the cylinders. Simulation time could be reduced by more than three
orders for a small case on a fine mesh as compared to an existing method and
could be more for large cases. In general, the simulation time of the reduced
model is of order of seconds as compared to hours of the full order
Computational Fluid Dynamics models.
