OC6 Fixed platform simulation in irregular waves

This benchmark problem uses the two-phase ExaWind solver, where water and air are simulated using the volume-of-fluid approach in both Nalu-Wind and AMR-Wind. The setup conforms to load case 3.3 from Phase Ia of the OC6 project (the definition document can be accessed here: https://www.nrel.gov/docs/fy22osti/80803.pdf). This load case involves a fixed platform within an irregular wave field, where the simulation outputs are forces and moments on the platform. The reference data for this case are from experiments performed in the Concept Basin of the Maritime Research Institute Netherlands (MARIN) using a 1:50 scale platform model. The numerical simulations are also performed in model scale, whereas the results have been scaled up to full scale using Froude scaling, as is typical for the OC6 project.

The AMR-Wind computational domain extends from -10m to 10m in the x direction, which is only a portion of the length of the original wave tank, while it extends from -2m to 2m in y, the full width of the tank, and from -3.6m to 2.4m in z, reaching the full depth of the tank and providing enough room around the vertical extent of the platform. Two levels of refinement are added, and these are targeted at the position of the air-water interface and the platform geometry. The refinements reach a resolution of 31.25mm in the x and y directions and 15.625mm in the z direction. The inlet in x uses a wave generation boundary condition, and the waves are forced into the domain with a relaxation zone. The outlet in x has a pressure outflow condition and the waves are absorbed using a numerical beach. The other boundaries are represented as slip walls.

The Nalu-Wind computational mesh is tetrahedral and adheres to the mesh resolution recommendations in section 3.2.3 of this paper: https://www.sciencedirect.com/science/article/pii/S0029801822013580. The mesh file is available at https://github.com/Exawind/exawind-benchmarks/releases/download/mesh_assets/oc5-pointwise-delaunay.exo.

The irregular wave field forced into the AMR-Wind domain is first generated by the high-order spectral solver HOS-NWT, which produces a file containing a time series of wave modes. These are read into AMR-Wind, transformed, and interpolated onto the computational mesh during the ExaWind simulation. When selecting the wave setup for HOS-NWT, a variety of approaches were evaluated, including iterative approaches and multi-wave decompositions, to reach similarity with the observed spectrum from the MARIN experiment. Ultimately, the most similar wave case was produced by using the built-in capability of HOS-NWT to produce waves using the JONSWAP spectrum with \(\gamma = 3.3\), as listed in the OC6 case 3.3 description. These waves have a significant wave height of 0.148m and a peak period of 1.6971s. In HOS-NWT, the wave simulation was performed in two dimensions (2D), eliminating the spanwise direction, which is incorporated only in the context of the ExaWind simulation. The resulting modes file is available at https://github.com/Exawind/exawind-benchmarks/releases/download/mesh_assets/modes_HOS_SWENSE_2.dat.bz2.

This case has been run for over 10 minutes. After removing the initial 40 seconds of transient data, we analyzed the following 9 minutes by computing the power spectral density of surge force, heave force, and pitching moment. Integrated over a middle range of wave frequencies and a range of low frequencies, we found the integrated quantities to be within predetermined acceptance thresholds from the experimental data. This target range was 15% for the wave frequencies and 25% for the low frequencies. An exact match is not expected because the wave time series is not identical. Data from these runs is currently being prepared for publication.

The subdirectory ‘run_info’ contains the headers of the log files for AMR-Wind and Nalu-Wind to show the git hashes and an excerpt from the ExaWind log file to show the time per timestep, which is about 6 seconds. This set of runs used 576 CPU cores on Kestrel, with 252 cores devoted to AMR-Wind and 324 devoted to Nalu-Wind.

Below is a video showing the evolution of the air-water interface within the Nalu-Wind portion of the domain. The platform is colored by the pressure field.