Re-creation of site-specific multi-directional waves with non-collinear current

This paper was published in the Ocean Engineering journal in October 2017. It was a joint piece of work with fellow IDCORE researcher at FloWave, Sam Draycott, building on our separate areas of research. In it, we demonstrate the capability of the FloWave facility to generate complex wave-current conditions, and also highlight the importance of considering even relatively low tidal currents when testing wave-energy devices.

The capability of the FloWave facility is showcased, by re-creating a complex multi-directional sea state based on recorded site data from the EMEC[2]European Marine Energy Centre Billia-croo test site in Orkney. The waves were produced in the tank in combination with currents from various directions, representing the peak tidal flows. Two other sets of waves were also tested (with currents) to show the effects more clearly. These were regular waves and a simplified uni-directional parametric sea-state. The mean direction of all the wave conditions relative to the current was altered, which provided interesting non-collinear wave-current observations, i.e. with an oblique angle between the waves and current, on which little has been published.

Final non-parametric EMEC spectra following correction, at 5 relative angles to current. Top row shows spectral density S(f), middle row weighted mean directional spreading
function DSFmean, and bottom row directional spectra E(f,θ) for 0.1 m/s current only.

In the paper, we discuss the importance of measuring the current for wave-energy resource assessment, even though this is something that may not always be done. Even relatively slow flows (around 0.25m/s or 1 knot full scale) can have a measurable impact on the waves. Where the current is significant, it should be re-created in the test tank when conducting scale model tests. This is because the presence of a current alters the shape of the waves, changing both the height and length. It also affects the power available in the waves, which is critical for understanding the performance of devices designed to capture this power.

To produce the desired wave height in the centre of the FloWave tank, a correction process was required to account for the wave-current interaction. This is because the waves are generated in still water around the outside of the tank, then interact with the current in the central test area. A single step linear correction process was shown to be effective at the current velocities tested. This process can now be used to produce realistic combined wave-current  conditions for testing devices in the facility.

Hopefully this work will help developers of devices to capture renewable energy from the waves better understand the performance of their devices.

This was published in Ocean Engineering in October 2017, and is freely available with open access funded by the EPSRC.

S. Draycott, D.R. Noble, T. Davey, T. Bruce, D.M. Ingram, L. Johanning, H.C.M. Smith, A. Day, P. Kaklis, (2017) “Re-creation of site-specific multi-directional waves with non-collinear current”, Ocean Engineeringdoi:10.1016/j.oceaneng.2017.10.047