Proper-Orthogonal-Decomposition-Based Reduced-Order Models for Characterizing Ship Airwake Interactions

Reduced-order models of the airwake produced by the flow over a simple frigate ship are developed using proper orthogonal decomposition (POD)-based methods. The focus is to understand the trade space between cost and accuracy, where different forms of the POD technique are concerned. Of particular importance is the upfront expense of employing "classical" or snapshot forms of the POD technique in both scalar and vector forms using either time-suppressed data (conventional POD) or kernels constructed from cross-spectral densities of the fluctuating velocity. The latter approach is referred to as harmonic POD so as not to exclude harmonic transforms in space. The flow over a simple frigate ship is an ideal testbed, given that it is unsteady, three-dimensional, and inhomogeneous in all spatial directions, as well as stationary in time. The spatial modes from all three techniques are shown to correspond to unique timescales, thereby demonstrating how the preservation of the temporal behavior associated with a particular spatial scale is not unique to the harmonic-POD approach.