Seakeeping analysis today is most often carried out using 3D potential flow codes which have proven quite valuable in assessing the behaviour of ships at the project stage. These potential flow codes improved significantly the capabilities for predicting seakeeping of ships. This improved notably the ability to deal with blunt, non-slender hulls, multi-hulls or platforms.
However, in spite of reasonable accuracy generally obtained from these methods, a number of problems that cannot be properly assessed due to assumptions supporting the theory, especially when dealing with response to high steep waves and severe seas conditions. In addition the linearization assumption in the large motions prediction is also a problem, in particular with regard to extreme roll response.
Some of these problems are tackled through model testing but these tests are rather onerous and cannot always be carried out due to physical limitations of model basins and scale effects. This holds in particular for roll damping predictions, large amplitude and non linear motions, slamming / whipping, flooding, sloshing.


Objectives:

CFD is now expected to provide the means for solving those problems in numerical simulations. However, the generic CFD codes lack many features and capabilities to address these problems which require specific tackling of maritime applications. The behaviour of the ship in relation to the constant motions of the free surface, the modeling of waves, specific effects as breaking waves, cushioning due to entrapped air, can only be taken into account while understanding these phenomena which are rather unique to the marine world.
There are great expectations that dedicated marine CFD packages will provide a significant improvement towards the solution of these problems. It is therefore the challenge facing the Virtual Seakeeping Tank to address those specific problems and solve them by incorporating CFD techniques in the ship design process, using automatic connection with CAD, grid generators and hydro-structure interfaces will drastically reduce the design timeframe. In addition, it is also planned to validate the numerical tools through extensive specific model tests as well as existing model tests available to the consortium.
Three main axes will be followed:

  • Compute hydrodynamic coefficients from viscous CFD to be used use in standard sea-keeping codes
  • Compute ship motions in critical conditions as severe seas (freak waves, green-water), slamming, coupling with sloshing, water flooding, dynamic instabilities in waves
  • Compute dynamic response as springing and whipping which are critical for hull design



Outline Work Plan:

Work Package 2 is subdivided into 5 functional tasks :

  • Tasks 2.1:Development of automatic pre and post-treatment of viscous CFD tools to derive hydrodynamic coefficients from viscous CFD and interfacing them with classical sea-keeping codes. This will enable a better handling of specific problems in such Seakeeping. Thus the tools will gain appreciable precision and reliability, especially when dealing with novel hullforms.


    Roll damping coefficient estimated through CFD

  • Task 2.2: Development of non linear waves modelling algorithms to implement in CFD codes with free surface tracking capability. They will be addressed in order to enable the simulation of ship motions in a seaway. This tasks will incorporate wave propagation into RANSE codes, mainly for irregular waves and waves grouping conditions
  • Task 2.3.: Improvement of predictions of large wave induced loads in CFD codes, which is an essential aspect of hydrodynamic simulations as they contribute to the development of a safe structural design.


    Non linear wave / ship hull interaction in a CFD code

  • Tasks 2.4.: Slamming is the leading hydro-elastic problems which keeps defying the designers understanding of ship behaviour at sea. It is just as difficult to measure this phenomenon in model tests as it is to simulate it properly. This task will aim at developing and validating slamming models using different free surface tracking methods such as Volume Of Fluid (VOF) and Smoothed Particle Hydrodynamics (SPH). 3D non linear panel method will be also developed as part as CFD codes.


    Slamming test to validate numerical models (MARIN)

  • Task 2.5.: Liquids sloshing in tanks or damaged compartments will be addressed, including coupling effects with ship motions. A series of numerical tools will be developed, ranging from simple and quick engineering models to full RANSE solutions. This will have applications in investigating damaged ship stability and in liquid tankers, in particular LNG carriers


    Liquid motion in LNG tank from CFD


THE PROJECT | WP 1 | WP 2 | WP 3 | WP 4 | WP 5