Regular Wave Interaction with a 2D Floating Structure


This test case considers a 2D box-shaped floating object with a superstructure under a rather steep regular wave condition. The purpose is to examine the performance of a numerical model for problems involving nonlinear interaction between waves and floating structures.

Experimental Set-up

The experiments were conducted in a narrow wave flume at Research Institute for Applied Mechanics (RIAM), Kyushu University. The wave flume is 18m long, 0.3m wide and 0.7m high. The water depth was set to be 0.4m. At the wave-maker side, it was equipped with a plunge-type wave generator, and at the other end, a wave absorbing device was used. The incident wave height is H = 0.062m and its period is T = 1s. A box-shaped floating object is placed at 7m from the wavemaker side. It is connected with a heaving rod through a rotational joint, and the heaving rod is set in between the slider mechanism installed in a carriage on guide rails. Therefore, it is allowed to heave and roll freely, but sway is restrained. A sketch on the setup of the experiments is given in Fig. 10, where the dimension of the floating body is presented. The mass of the body is measured to be 15kg and roll moment of inertia was 0.3417kgm2. Note that the centre of mass and the centre of rotation are not at the same position, since a rotational joint was installed to force the body roll around it. Readers are referred to Zhao and Hu (2012) for further details on the experiments.

Fig. 1 Sketch of the experiment set-up (unit: m)

Physical Measurement Data

In the experiment, wave elevation at a distance of 5.1m from the wavemaker side is measured with a 1000Hz sampling frequency, and both the heave displacement and roll motion are also recorded. A high-speed video camera is used to record the wave structure interaction process and to help reveal the water-on-deck occurrence. In addition, a pressure gauge is installed on the seaward face of the superstructure at a height of 0.01m from the deck to record the green water impact pressure.

Numerical Benchmarks

Two sets of the numerical simulations using OpenFOAM have been carried out for the flow problem. In the first simulation (please see Martinez-Ferrer et al. 2018 for more details of the numerical set-up), the dynamic mesh functionality within OpenFOAM was applied to represent both the moving wavemaker and the floating object. Fig. 2 compares the numerical results for the wave elevation and heave and roll motions of the floating structure with the experiments. In Fig.3, the time history of the pressure at the point on the superstructure is also compared with the numerical measurement and other numerical results.

Fig. 2 Time history of (a) water elevation; (b) heave and (c) rotational angle of the floating body

Fig. 3 Time history of the pressure measured above the deck of the floating body

In the second simulation (please see Chen et al. 2019 for more details), the new overset functionality in OpenFOAM has been used to represent the motion of the floating object and a static boundary condition is used to generate the required waves. In Fig. 4, the wave profiles around the floating object are compared with the experimental results. In Fig.5 wave elevation at the point in front of the body and both the heave and roll motions are also compared.

Fig. 4 Comparison of the free surface and body positions between the numerical simulations with different overlapping domain sizes and the experimental results within one wave period. Form the left to right: simulation results with small, medium and large overlapping domains and the experimental results.

Fig. 5 Comparison of the heave and roll motion of the floating body, and the surface elevation at 1.9m in front of the body between the numerical results with three different overlapping domain sizes and the experimental data. The heave, roll and surface elevation were non-dimensionalized where k is the wave number and H is the wave height.

Relevant References

Zhao, X. and Hu, C., 2012. “Numerical and experimental study on a 2-D floating body under extreme wave conditions”, Applied Ocean Research, 35, 1-13.

Martinez-Ferrer, P.J., Qian, L., Ma, Z., Causon, D.M. and Mingham, C.G., 2018. “Improved numerical wave generation for modelling ocean and coastal engineering problems”, Ocean Engineering, 152, 257-272.

Chen, H., Qian, L., Ma. Z., Bai, W., Li, Y., Causon, D.M. and Mingham, C.G., 2019. “Application of an overset mesh based numerical wave tank for modelling realistic free-surface hydrodynamic problems”, Ocean Engineering, 176, 97-117.