Improved Wave Force Classification and Systems 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 from velocity time-histories, generated experimentally from various sources: regular oscillatory flow in a U-tube, cylinder oscillatory in still water and in a current, random waves in the large De Voorst wave flume and a directional sea state at the Christchurch Bay Tower. The curve fits from the Morison equation are sometimes poor while the curve fits from the extended equation are always excellent, although the corresponding "predictions" give little or no improvement on the Morison equation. The curve fits obtained by simply adding a term proportional to F/F/, where F ids force, are also significant improvements over the Morison fits enabling an improved classification of force in terms of drag, inertia and history (for each flow situation). 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 here will indicate the significance of vortex shedding through the relative magnitude of the history term.