A uni-directional focused wave event

Description

In this test case a focused wave event is generated and its propagation measured spatially up until, and beyond, the focal point. This case makes up part of a much larger experimental campaign, XMED (Hann et al. (2015)), the purpose of which was to assess the conditions responsible for the peak loads on wave energy converters (WECs). The wave in this case is a 1-in-50th scale, uni-directional, focused wave based on the NewWave design wave theory. A Pierson-Moskowitz spectrum was used as the input to the NewWave formulation with 100 year storm hindcast data for the Wave Hub site (fp = 0.356Hz, Tz = 14.1s, Hs = 14.4m, Halcrow, 2006 p.19). This gives a theoretical scaled crest amplitude of 0.268. In the experiment, measurements of the free-surface elevation were recorded by an array of resistive wave gauges.

Experimental Set-up

The experiments were performed in the COAST Laboratory Ocean Basin (35m long X 15.5m width). The water depth was 2.8m. Resitive wave gauges were positioned at the distances from the wave maker given in Table 1.

Table 1: Distances (in metres) of wave gauges upstream of the wavemaker
WG1 WG2 WG3 WG4 WG5 WG6 WG7 WG8 WG9 WG10 WG11 WG12 WG13 WG14 WG15 WG16
13.27 14.32 14.77 15.23 15.71 16.12 16.54 16.83 17.14 17.51 17.84 18.21 18.54 18.87 19.24 19.60

 

Test Program

The wave event, with a focal position at WG13 (18.54m) and a focus time of 60s, was generated in the COAST Laboratory Ocean Basin using the EDL paddle control software. The software is designed to reproduce the desired free-surface elevation by applying various corrections to account for the change in water depth in front of the wave paddles and the nonlinear propagation of the wave fronts. Each wave front is then transformed back to the position of the wave paddles by the control software. In this case, the wave was created using linear superposition of 244 wave fronts with frequencies evenly spaced between 0.101563Hz and 2Hz. Wave focusing was achieved using a trial and error process of adjusting the theoretical focus location (to within +-0.1m) to get the best possible focus. For this case a theoretical focus location, x0, 18.05m from the wave paddles was found to give the best focus event.

Physical Measurement Data

The surface elevation data at each of the wave gauge positions was recorded at 128Hz and can be found here. The data is arragenged as follows

  • The physical surface elevation data is all in one file (ka_015d.txt); the first row contains the column headers and begins with a '#' for post-processing purposes; the first column is the time in seconds and the proceeding columns are surface elevation in metres at each of the probes corresponding the wave gauge positions in Table 1.

NOTE: When using this data please state that 'the physical data is from the CCP-WSI Test Case 1' and be sure to cite Ransley et al. 2017 as the source of this data (see full citation in the 'Relevant References' section below).

Numerical Benchmarks

In this case we have the generation and propagation of a uni-directional focused wave event. The key physical phenomenon present in this case is the nonlinear evolution of the surface elevation, associated with the exchange of energy between frequency components, as the wave propagates. A true reproduction of the focused wave event therefore requires the surface elevation results to be reproduced at a number of positions in the wave basin. For this benchmarking case we propose the surface elevation at 5 positions (WG1, WG4, WG9, WG14, WG16) as defining the key phenomenon present.

This benchmarking case has been reproduced numerically by:

  • Ransley et al. (2017) - using OpenFOAM-2.3.0 and waves2Foam (RANS-VOF) [data available to download here].
  • Ransley (2015) - using OpenFOAM-2.2.1 and waves2Foam (r1984) (RANS-VOF) [data available to download here].

NOTE: When using this data please be sure to cite the original source of the data appropriately (see full citations in the 'Relevant References' section below).

Relevant References

Ransley, E., Greaves, D., Raby, A., Simmonds, D., Hann, M., 2017. "Survivability of Wave Energy Converters using CFD", Renewable Energy, 109: 235-247; doiPEARL.

Ransley, E., 2015. "Survivability of Wave Energy Converter and Mooring Coupled System using CFD", PhD thesis, Plymouth University, UK; PEARL.

Hann, M., Greaves, D. and Raby, A., 2015. "Snatch loading of a single taut moored floating wave energy converter due to focussed wave groups", Ocean Engineering, 96: 258-271; doi; PEARL.

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