Frequency Properties of the Rainflow Filter—Some Examples in Oceanography

The rainflow filter is a nonlinear filter which can be used to eliminate small-amplitude oscillations in a signal. An advantage of the rainflow filter, well known in applications to fatigue, is that a considerable reduction of data is made, while the calculated total damage is not affected. An approximation of the filter in the form of a transfer function can be obtained from spectral simulations. To validate the so derived filter, the capability of filtering out small cycles is examined. Further, we pay attention to calculation of distributions for characteristic wave parameters. These methods need as input a spectral density, where high frequencies often are cut off. Numerical examples indicate that the rainflow filter is advantageous to use when filtering records of sea-elevation.