Computational Fluid Dynamics (CFD) simulations are increasingly exploited within all fields of mechanical engineering. For example, when designing offshore wind turbine foundations and other marine structures exposed to a harsh water environment, it is of great value to be able to calculate peak loads from breaking waves prior to construction and installation. In a recent DHI Tech Talk, we explain how we use CFD simulations for the benefit of our clients within marine and coastal engineering:
CFD simulations involving tracking of free surfaces, like the water surface of a breaking wave, are particularly challenging because of the complex surface motion. In the past 2 ½ years, DHI scientist Johan Roenby from the Department of Ports and Offshore Technology has led the project ‘Breaking the Code of Breaking Waves’ with the aim of inventing new improved computer algorithms for simulating complex water surfaces. The project was conducted in close collaboration with Associate Professor Henrik Bredmose from DTU Wind Energy and Professor Hrvoje Jasak from Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb.
In an article just published in Royal Society Open Science, the scientists present a novel water surface simulation algorithm, which they have dubbed isoAdvector. By rethinking the numerical representation of the surface motion and exploiting a number of clever geometric tricks, they have demonstrated the ability to produce accurate, fast and robust solutions with their new algorithm.
‘One of the main advantages of isoAdvector is its ability to move water surfaces across computational meshes, where the cells are not just cubes, but may have any shape. The ability to work with general cell shapes makes the tedious process of generating meshes around complex structures such as offshore wind turbine foundations or wave energy converters much faster and easier for the engineers. This in turn makes the whole simulation process cheaper and more agile.’
– Johan Roenby, Research Scientist, Ports and Offshore Technology
A simple standard method for assessing the accuracy of this kind of algorithm is to distort a spherical water droplet in a known flow, and then reverse the flow and check if the droplet flows back into its spherical shape as it should. An example of such a standard test case on a mesh consisting of tetrahedral cells is shown here:
Available to anyone to download, use and modify
The new algorithm is implemented in the open source CFD coding framework, OpenFOAM®, and is released as open source. This means that anyone can download it, use it and modify it to their needs. It is our hope that our contribution will increase the usage and reliability of computer aided engineering in the large range of areas, where accurate prediction of complex fluid surface motion is paramount.
Acknowledgement
The project ‘Breaking The Code of Breaking Waves’ was funded by a Sapere Aude Research Talent grant from The Danish Council for Independent Research (Grant ID: 1337-00118B – FTP) to Johan Roenby and by the GTS grant to DHI from the Danish Agency for Science, Technology and Innovation.