Improved wave force classification, using system identification

An extension to the Morison equation including Duffing oscillator type force terms is postulated through knowledge of the flow mechanisms. This is used to curve fit measured force time histories for velocity time histories, generated experimentally from various sources: regular oscillatory flow in a U tube, cylinder oscillation in still water and in a current, random waves in wave flume, and a directional sea state. In unidirectional flows the association of a significant history term with vortex shedding is confirmed by the occurrence of a prominent transverse or lift force. In directional seas, lift (due to vortex shedding) cannot be isolated and it is suggested that the data analysis described will indicate the significance of vortex shedding through the relative magnitude of the history term.