State-of-the-Art Simulation
THE work of classification societies on design assessment increasingly relies on the support of computer based numerical simulations. This review has been prepared by Ould el Moctar, Karsten Fach, Christian Cabos, Holger Mumm, of Germanischer Lloyd, and Volker Bertram, Dept Mech Eng, Stellenbosch University.
A
LTHOUGH design assessments based on advanced finite-element analyses have long
been part of the services of a classification society, the scope and depth of development of applied simulation methods has been so rapid that it is time to take stock. Shipyards frequently outsource the extensive
analysis associated with simulations in the design process of ships. The trend of modern classification society work also tends towards simulation-based decisions, both to assess the ship’s design and to evaluate its operational aspects. To assess the safety of modern ships, it is vital for
Germanischer Lloyd to have available numerical tools to investigate the dynamic stability of intact and damaged ships in a seaway. Large amplitude motions may lead to high accelerations. In severe seas, ships may also be subject to phenomena like pure loss of stability, broaching to, and parametric rolling. Linear seakeeping methods are unsuited to
predict such phenomena, mainly because they do not account for stability changes caused by passing waves. Furthermore, linear methods are restricted to small amplitude ship motions, and hydrodynamic pressures are only integrated up to the undeformed water surface. Two simulation tools, ROLLSS and GL SIMBEL,
are available at GL to simulate large amplitude ship motions. Depending on the extent of the nonlinearities accounted for, simulation methods tend to be cumbersome to handle and unsuitable for routine application. Therefore, the numerically more efficient method ROLLSS is used to identify regions of large amplitude ship motions quickly, while the fully nonlinear method GL SIMBEL is then employed to yield more accurate motion predictions. To validate these tools and to demonstrate their
practical application, extensive simulations were carried out to predict parametrically induced roll motions that were then compared to model test measurements performed at the Hamburg Ship Model Basin.
Propulsion and movement Diagrams to estimate rudder forces were customary in classical rudder design. These diagrams either extrapolate model test results from wind tunnel tests, or they are based on potential flow computations. However, the maximum lift is determined by viscous flow phenomena, namely, flow separation (stall). Potential flow models are not capable of
predicting stall, and model tests predict stall at too small angles. CFD is by now the most appropriate tool to support practical rudder design. The same approach for propeller and rudder
interaction can be applied for podded drives. RANSE solvers also allow the treatment of cavitating flow.
THE NAVAL ARCHITECT FEBRUARY 2007
Simulations were carried out to predict parametrically induced roll motions that were then compared against model test measurements performed at the Hamburg Ship Model Basin.
The extensive experience gathered in the last five years resulted in a GL guideline for rudder design procedures. Aerodynamic issues are also increasingly of
interest for ships and offshore platforms. Potential applications include smoke and exhaust tracing, operational conditions for take-off and landing of helicopters and wind resistance and drift forces. The traditional approach to study aerodynamic flows around ships employs model tests in wind tunnels. These tests are a proven tool supporting design and relatively fast and cheap. Forces are quite easy to measure, but insight into local flow details can be difficult in some spaces. Computational fluid dynamics (CFD) is
increasingly used in related fields to investigate aerodynamic flows e.g. around buildings or cars. CFD offers some advantages over wind tunnel tests: The complete flow field can be stored and allow evaluation at any time in the future. There is more control over what to view and what to block out. CFD can capture more flow details. CFD also allows full- scale simulations. Despite these advantages, CFD has so far rarely been employed for aerodynamic
analyses of ships. This is due to a combination of obstacles: the complex geometry of superstructures makes grid generation labour-intensive. The flows are turbulent and often require unsteady simulations due to large-scale vortex generation. Recent progress in available hardware and grid
generation techniques has allowed a re-evaluation of CFD for aerodynamic flows around ship superstructures. Hybrid grids with tetrahedral and prism elements near the ship allow partially automatic grid generation for complex domain boundaries. The resulting higher cell count is acceptable for aerodynamic flows because the Reynolds numbers are lower than for hydrodynamic ship flows and thus there are fewer elements needed. In 2002, GL performed RANSE simulations for a ship superstructure to investigate aerodynamic problems and smoke propagation.
Fire Simulation SOLAS regulation allows the consideration of alternative designs and alternative arrangements concerning fire safety. The requirement is to prove (by engineering analysis) that the safety level
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