Free decay tests are less onerous to carry out compared to experiments that include waves. These tests can also shed light on hydrodynamic characteristics of a device, such as added mass, stiffness and damping. This knowledge can then inform numerical models set up for the device. In this paper, free decay tests carried out for a floating oscillating water column in surge, heave and pitch at a wave basin are analyzed, to extract hydrodynamic characteristics. Ordinary Differential Equations (ODE), are formulated for these motions and coefficients of these equations calibrated with experimental data. Finally, we show how these coefficients compare with those calculated from AQWA.
The Ocean Energy (OE) group in the Hawaii Natural Energy Institute (HNEI) at the University of Hawaii conducts research on Wave Energy Converters (WECs). The Halona WEC is being developed with NAVY and DOE funds at the University of Hawaii. The initial design and construction were conceived by co-author, Mr. Nicholas Ulm. The device is currently being leveraged as a platform for Powering the Blue Economy (PBE) applications. The device itself is a floating Oscillating Water Column (OWC) -See Fig.1- which is expected to be fitted with an Impulse turbine. Development of this device continues on several fronts.
Free decay tests are less onerous to carry out compared to experiments that include waves. In addition to generating waves -and the additional instrumentation and data analysis requirement that comes with it- the facilities (or wave basins) must also minimize reflections using some mechanism such as a beach. These decay tests can also shed light on hydrodynamic characteristics of the device, such as resonance.
Nascimento et al. (2020), implement system identification using a Physics Informed Neural Network (PINN) and calibrate coefficients of Ordinary Differential Equations (ODE). In section 3.3.1 of their paper, the authors solve forced vibration of a 2-degree-of-freedom system, where two masses are linked together with springs and dashpots. These ODE formulations follow the well-known simple harmonic oscillator (SHO) equations in the field of mechanics. The damping coefficients - more than 1- of this rather complicated equation/model are calibrated based on experimental data of the system response. In the field of ocean engineering also, motions of marine structures can be modeled with equations resembling SHO equations (Ruehl, K. et al. 2019, Chakrabarti S., K., 2002). Although simplified, they provide a quick method to study and appreciate device response in ocean waves. In this paper the MATLAB function lsqcurvefit is utilized for calibrating coefficients of ODEs formulated for the device motions.
