As part of its ongoing programme into investigating Computational Fluid Dynamics (CFD) tools and the contribution they can make to accelerating the design process while at the same time cutting costs, Damen has been undertaking a project designated ‘Gone with the Wind’ (GWTW). This is looking specifically at the aerodynamic forces that act upon vessels above the waterline, as a separate analysis to those CFD studies that focus on the interactions that occur below the waterline.
The search for new tools to assess transverse stability
The issue being tackled here is meeting the requirements of the IMO regulation 749.18, which addresses criteria relating to severe wind and resultant rolling. Its objective is to ensure that vessels have sufficient transversal stability to resist over-rolling in severe side winds. By necessity the regulation generally proposed by the Classification Societies errs on the side of caution so that it covers the full spectrum of vessel designs, however, as a result it is difficult for long, slender vessels to satisfy the empirical requirements of the regulation. This can only be achieved through expensive and extensive experimentation, and so this has a direct impact on both the time needed and the cost of gaining certification for vessels such as Damen’s monohull Fast Crew Suppliers (FCS) and their variants.
Developing a new methodology
Traditionally, satisfying the classification societies that vessels of this type comply with the demands of regulation 749.18 has required data gathered from physical assessments using scale models in towing tanks and wind tunnels. However, CFD has the potential to replace this by combining applied mathematics, physics and computational software to visualise in a simulated 3D environment how the vessels will behave in a wide range of wind and wave conditions. The objective of project GWTW is to develop a CFD methodology to replace the physical assessments for vessels like its FCS range, and to demonstrate that the tool is capable of demonstrating compliance to the satisfaction of the classification societies. At the same time, for Damen and its clients a successful result must deliver both time and cost savings, so the methodology must also minimise computation costs and the turnaround time.
Physical vs. Virtual
GWTW is comprised of two main elements. Damen is developing the CFD methodology in partnership with CFD code supplier Numeca International while at the same time conducting the physical tests that are needed to validate and verify the CFD calculations. The tests related to the aerodynamic part of the study have been taking place at the UK’s Southampton University using a 1:18 scale model of Damen’s FCS 3307. The model has been evaluated in a wind tunnel using various heading and heeling angles, and also with scaled containers both on and off the rear deck. Meanwhile back in the Netherlands the computational software has been configured to reproduce the internal structure and conditions in the Southampton wind tunnel to a high degree of accuracy. Simulations using the FCS 3307 were then run using two separate CFD mathematical models; URANS and DES (click here for more information – see page 3) to determine their efficacy and compare them with the findings of the wind tunnel testing programme.
The results so far
In summary, the CFD model reproduced the wind tunnel experiment and tunnel apparatus to a high degree of accuracy across a range of simulated conditions and achieved good correlation for forces and moments at various heading and heeling angles. Work is now underway to adapt this methodology to full-scale prediction, with only minor changes anticipated.
Meanwhile, parallel activities are currently taking place to carry out the CFD validation of the hydrodynamic related part of the study, which will complete the demonstration of how vessels of this type achieve the transversal stability required to resist over-rolling in severe side winds, and so satisfy the demands of the regulation.
The concluding objective of the project will be to present the fully virtualised assessment procedure to the Classification Societies in order to have this generally accepted for compliance purposes.
To view the published scientific paper that details both the methodology and results, click here.