Time domain filtering

OrcaFlex sometimes requires dynamic behaviour to be filtered while the simulation is in progress. The filtering is intended to separate signals into low frequency and high frequency parts, based on some chosen dividing period. A second order Butterworth digital filter is used, as described, for example, in Bellanger, 1989.

Filter cutoff graphs

The following graphs characterise the filter performance. Time domain digital filters are not perfect: they do not offer a pure step function precisely at the dividing period. Rather, there is a fall-off of amplitude from well above the dividing period and the amplitude approaches zero some way below the dividing period. The example RAO amplitude plot here demonstrates this; the phase lag plot further shows that there is a phase shift of the filtered signal which should also be considered.