Abstract
Measuring waves in laboratories is one of the most fundamental parameters in physical model testing of wave propagation and interaction of fixed and floating marine structures. Waves represent the driving forces for design, as well as the input variable to assess performance of wave energy converters. It's stochastic character and inherent space and time variability, require detailed measurements both, in the far field as well as in the vicinity of the device.
Typically, waves are measured with instruments located at a fixed location, and with the adequate sampling rate and duration, a time series is obtained of an indirect variable that can be used to derive the free surface and, thus, after performing a time- or frequency-domain analysis, the wave field can be determined. Instruments suited for this measurement include surface piercing wave gauges (WG), ultrasonic wave gauges (USWG), acoustic Doppler velocimeters (ADV), acoustic Doppler current profilers (ADCP), or pressure gauges (PG). Attempts to capture the free surface using digital and infrared cameras, as well as to implement PIV or PTV techniques using floating seeding has been marginally successful in laboratory environments due to the precision and high sampling rates required.
Additionally, standard free surface measuring techniques are sometimes inadequate or incompatible with marine structures, particularly if the device contains moving parts responding to the wave action, floating structures with a relatively ample watch circle, or when supporting the gauges requires a structure that may introduce additional disturbances in the wave field.
Motion capture, or motion tracking, is the process of recording the movement of objects (or people), typically by means of a series of cameras to calculate 3D positions. Motion tracking is widely used in physical modeling of marine structures, including wave energy devices, tracking the position of individual points or the position and orientation of an object with a number of degrees of freedom. One of the most common mocap systems in coastal and ocean laboratories is Qualisys, which uses infrared emitting cameras to track the position of (almost) weightless reflective markers. Hence, the system is non-intrusive in a large sampling volume and reconstructs the position of points in space at a high sampling rate.
The OH Hinsdale Wave Research Laboratory, Oregon State University, have implemented the use of small floating markers to measure the free surface and enable the mapping of the wave field in large areas with a high resolution in space and time. The markers were glued to small foam pieces, becoming an almost weightless free surface tracker. To avoid drifting and clustering, the markers had a slack fishing line attached to a small nut resting on the wave tank bottom.
Different wave conditions (wave height and period) as well as wave types (regular and irregular waves) have been recorded successfully, providing a new insight on the behavior of water particles. Post-processing of the time series requires the implementation of new techniques, since the measurement changes in space and time. Comparison with standard measuring techniques yielded excellent match on wave properties.