  We develop a Proper Orthogonal Decomposition (POD)-Galerkin based Reduced
Order Model (ROM) for the efficient numerical simulation of the parametric
Navier-Stokes equations in the stream function-vorticity formulation. Unlike
previous works, we choose different reduced coefficients for the vorticity and
stream function fields. In addition, for parametric studies we use a global POD
basis space obtained from a database of time dependent full order snapshots
related to sample points in the parameter space. We test the performance of our
ROM strategy with the vortex merger benchmark. Accuracy and efficiency are
assessed for both time reconstruction and physical parametrization.
