State-of-art analysis: wave basin measurement and numerical simulation come together for better designs at lower costs

Extreme wave loading… the future is hybrid!

Undeniably, the future in hydrodynamic research will see an ever- growing role for numerical simulations. But what does this mean for the challenging field of extreme wave loading? Joop Helder,

Advanced numerical tools are increasingly being used as design and verification tools. For some application areas, such tools are expected to replace model tests completely. For many others, a more complementary role is foreseen.

Predicting extreme wave loads numerically is considered one of the most challenging tasks for any simulation tool. The strong nonlinearities in the incoming wave conditions, the interaction of the structure with its environment and the associated diffraction and radiation all together make a very complex mix of ingredients that is difficult to capture numerically. Nevertheless, there has been significant progress made.

Using CFD, step-by-step and validated with dedicated experiments, reliable numerical

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predictions of impact loads have become a reality. However, predicting these loads when an extreme event is given is only half the challenge. Identifying the occurrence of extreme events and thereby obtaining the right boundary conditions for detailed CFD predictions is the next challenge.

In a model test, statistics on extreme events are gathered by performing many realisations of a critical 3-hour sea-state, in a fraction of real time. In CFD however, the ‘scales are reversed’, as it will cost many CPU hours to simulate just a fraction of a real-time event. The solution therefore lies in a hybrid approach, allowing CFD, model testing and fast numerical tools to work together.

Interface To make such an approach possible, there needs to be an appropriate

interface between the different models used. At MARIN, an interface -from model test results to CFD - is already in place and being used to numerically optimise designs for extreme wave loading, based on realistic extreme event scenarios obtained from model tests. This interface is currently being extended to allow us to go back and forth from a model test result to a numerical simulation. By including appropriate lower order numerical simulation tools in the hybrid loop, even long-term statistical data on extreme events can be included in the assessment.

We are convinced that the hybrid approach will eventually result in improved designs with less uncertainty and lower costs, justifying the title of this article…the future in wave impact loading is indeed hybrid!

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