Roll damping
by Christian Klimt Nielsen, Project Manager at FORCE Technology
Roll dampening plays an important role for ships in connection with sea keeping and manoeuvering. Today the Naval Architect’s standard numerical tools are not capable of predicting roll dampening. Yet the tools for seakeeping and manvoeuvering calculations need as input information on how the vessel behaves with regards to roll dampening. Therefore such information is often either estimated from empirical methods or directly taken from experiments. But empirical methods are relatively inaccurate and experiments are costly.
In the present project the aim is to address this problem by introducing RANS-CFD as a tool for numerically obtaining roll dampening data for a given vessel. A free surface RANS-CFD solver coupled with a 6DOF solver will in principle be able to match the physical situation of a ship in roll motion, including viscous and turbulent flow effects. Now, the present project is a study of the possibility of doing this by adopting a commercial RANS-CFD/6DOF solver Starccm+ to the problem.
Objective
The objective of the present project is to investigate the possibility of using RANS-CFD to quantify roll motion of a vessel in still water, and to study in more detail the physics of the phenomenon. This includes determination of roll dampening coefficients for given conditions and utilisation of RANS-CFD to visualize complex flow phenomena.
In order to validate the RANS-CFD calculations, data from previously performed tank tests will be adopted as reference for the calculations - no additional tank tests are planned. The project will be closely linked to the “Applied CFD” DCMT project which aims at building a complete CFD model for hull, propeller and appendages to make it possible to calculate the flow field and the phenomena related to the mutual interaction between the components.
Depending on the success of the project, directions shall be given on how to use RANS-CFD for practical problems in connection with evaluating roll characteristics of a vessel in still water.
Project activities
The present study will mainly focus on the CFD tools’ capability to predict roll decay motion and forces. This is done by validating results obtained through CFD work against results obtained through tank testing.
The below-mentioned activities state how the numerical work will be sequenced, so that at the end of the project it can be demonstrated to what degree it is possible to model the roll motion of a ship with acceptable accuracy:
- Definition of benchmark cases for CFD
- CFD model tests and settings (meshes, models, temporal resolution, …)
- CFD production runs for cases
- Analysis of CFD results: Flow field/physics, roll period, roll dampening coefficients
- Comparisons with existing tank test
- Documentation and sharing of results
All computations will be made in model scale as it is common practice with RANS-based CFD. There is no solid experience base for full-scale calculations for ships, as these demand very large numerical models to be able to resolve the flow in the near wall boundary layer. However, the few examples in literature indicate that the actual calculation can be run at relatively coarse grids, but as there are no full-scale data available for validation of the methods, it is difficult to estimate the quality of such simulations.
For further information on this project, please contact Christian Klimt Nielsen (ckn@force.dk), project manager, FORCE Technology.