The proposed research will focus on the study of the generated wave and current fields in 3D (Coastal & Ocean Basin) and 2D (Ghent University wave flume) laboratories. The interaction of waves and currents is highly complex and rarely studied and barely understood. Measurements from the 3D and 2D laboratories will be conducted and analysed to better describe the quality of the generated waves and currents and their interaction. 

Laboratories

As 3D facility the Coastal & Ocean Basin (COB) is proposed for the study. The COB wave tank facility is located in the Flanders Maritime Laboratory at the Ostend Science Park, Belgium. The COB and its associated testing services are designed (Troch et al. 2016) to facilitate the needs of the offshore renewable energy sector, coastal and offshore engineering community and offer the opportunity to academia, companies and government agencies, to test scale models under combined action of waves and currents in any relative direction, to develop innovative designs. The COB is operational since March 2023 and has since then successfully completed its first projects on wave energy converters (WECFarm), wave diffraction study around a monopile (PhairywinD) and floating offshore photovoltaic islands (MarineSpots). A dedicated study on the generated wave field was carried out in 2023 and the data will be available for the proposed research line.   

As 2D facility the wave flume at Ghent University and the newly constructed Saltwater flume at Ostend Science Park are proposed.

Wave and current field generation 

The wave fields in the 3D facility (Coastal & Ocean Basin) are generated by an L-shaped, piston type and wet-back wave generator spanning two full sides of the basin, in combination with a parabolic double-layer rubble mound passive absorber. The wave generator features generation of 1st and 2nd order, regular waves and irregular sea-states, as well as short-crested wave fields and extra’s such as cnoidal, bichromatic, focused and solitary waves for a range of operational water depths between 0.3 and 1.4 m. A maximum wave height of 0.55 m can be generated for the design water depth of 1.1 m, together with wave periods physically limited by wave steepness and machine stroke. The wave generator is equipped with active absorption.  

Along the other two sides of the wave tank, a passive absorber is installed. A rubble mound passive absorber, with parabolic shape, decreasing porosity towards its core and sharp-edged stones with low chalk contents is selected, to increase the wave dissipation rate and limit the amount of reflected waves from the boundaries ,thereby increasing the quality of the waves in the measurement volume. The design of the passive absorber is based on the comprehensive overview by Ouellet et al. (1986) and fine-tuned based on experimental pre-studies carried out in a wave flume facility.  

The currents are generated using 56 individual pumps in the basement of the facility, which drive the current upwards towards a turbulence mixing and absorption screen to establish a homogeneous current field with realistic turbulence levels (10%). The current is then bended using guiding veines in horizontal direction and moving through the basin until it leaves downwards again at the other side of the tank, creating a vertical current system loop. 

The wave and current fields in the 2D facility (Ghent University wave flume and newly constructed saltwater flume at Ostend Science Park) are generated similarly but with only 1 paddle generating long-crested unidirectional waves and one pump generating co-directional waves and currents. 

Quality of the generated wave and current field 

The quality of the generated wave field and the assessment of wave tank specific uncertainties related to model effects are key to interpreting the measurements from a laboratory facility (Hughes, 1993). Designers rely heavily on the outcome of wave tank tests and the prediction ability of design guidelines, such as EurOtop (2018) or testing standards for wave conversion systems (e.g. Holmes, 2009) or the ITTC is typically based on wave parameters derived from scale model testing and therefore strongly dependant on an accurate and reproducible representation of the wave field within a defined error margin. 

The wave generation capabilities are described in terms of steepness, water depth and stroke limitation curves to encompass the possible generated wave fields. Further,  to investigate the performance of the passive absorber by decomposing the generated wave fields into reflected and incident components. The accuracy assessment of the wave fields is then based on the comparison of regular wave time series and irregular wave spectral sea state representations to theoretical solutions. The homogeneity of the obtained wave accuracy across the basin and the stability of the wave crest are further investigated. Since the L-shaped wave generator set-up enables the generation of highly oblique waves with an operational direction of 45 ° (diagonal waves), the quality of these waves is assessed in terms of spurious wave content (Lykke Andersen et al., 2020). Finally, the effectiveness of the active absorption at the wave generator and the capability to generate short-crested waves are studied.  Regarding the currents, a similar approach is proposed and the quality of the current described in terms of flow homogeneity over the depth, development and size of the boundary layer as well as characteristic turbulence scales and intensities in the measurement volume. Once, waves and currents are characterised individually, the interaction of both is studied. 

Method

Two dedicated experimental campaigns have been carried out in COB, in March 2023 and in October 2023, to assess the quality of the generated wave field, by mapping the basin with resistive wave probes (accuracy 0.1% FS) and measuring the obtained wave fields for the operational water depths and wave conditions. Furthermore, in Q1 2025 a dedicated campaign is planned to commission the current generation system in COB and characterise the generated current field. Similar studies have been and will be performed in the 2D facilities, to compile the required data for the proposed research. Besides physical model testing and the analysis of the obtained data-sets, numerical models (SWASH) will be set-up, validated and operated based on the experimental data.