To properly account for these nonlinearities the WEC modelling has to be performed in time domain. In addition, second-order slow drift forces may be also included in a time domain description of the WEC dynamics (this force must be undertaken by the station-keeping system). This requires some sort of non-linear modelling that typically consists of treating the buoyancy and the excitation loads as non-linear terms. Furthermore, under moderate to extreme waves, nonlinear effects in the wave/device hydrodynamic interaction are more relevant. These nonlinearities arise mostly from the dynamics of the mooring system, the PTO equipment and control strategy and, when present, the end-stop mechanism. Wave-to-wire models (time domain tools): Besides the interest of the frequency domain approach, in many practical cases the WEC dynamics has some parts that are strongly non-linear, and so the superposition principle is no longer applicable. Hence, this approach is generally used to optimise the geometry of WECs in order to maximize the energy capture.
![orcaflex water wave 1st order orcaflex water wave 1st order](https://www.orcina.com/wp-content/uploads/TW18A-9908.jpg)
Under these circumstances, the equations of motion become a linear system that may be solved in a straightforward manner.Īlthough frequency models have limited applicability, being restricted to linear problems where the superposition principle is valid, the frequency domain approach is extremely useful as it allows for a relatively simple and fast assessment of the WEC performance, under the aformentioned conditions. Consequently, all the physical quantities vary sinusoidally with time, according to the frequency of the incident wave. Therefore, in such a framework (which is normally fairly acceptable throughout the device’s operational regime), and with linear forces imposed by both the PTO and the anchoring system, the first step to model the WEC dynamics is traditionally carried out in the frequency domain (where the excitation is of a simple harmonic form). In this case the hydrodynamic problem is well characterised by a linear approach.
![orcaflex water wave 1st order orcaflex water wave 1st order](https://m.media-amazon.com/images/I/814pk+j-ybL._AC_UL1500_.jpg)
![orcaflex water wave 1st order orcaflex water wave 1st order](https://m.media-amazon.com/images/I/81w9LnxYCVL._AC_UL1500_.jpg)
![orcaflex water wave 1st order orcaflex water wave 1st order](https://ae01.alicdn.com/kf/HTB18yHgaiLrK1Rjy1zdq6ynnpXao/Electric-Water-Heater-Household-Intelligent-Digital-Display-Water-Heater-Fast-Heat-Storage-Water-Heater-A13.jpg)
This is the case for waves and device oscillatory motions of small-amplitude. Frequency models: The hydrodynamic interaction between WECs and ocean waves is a complex high-order non-linear process, which, under some particular conditions, might be simplified.