Reduced-order models of the airwake produced by the flow over a simple frigate ship
are developed using 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 test bed given that it is
unsteady, three-dimensional, inhomogenous in all spatial directions, and stationary in time. The spatial modes from all three techniques are shown to correspond to unique time-scales, thereby demonstrating how the preservation of the temporal behavior associated with a particular spatial scale is not unique to the harmonic-POD approach.