on the hydrodynamic performance is now possible from full-scale to concept design.

CFD simulation of flow around a submarine at the surface

Has all this CFD been easy? The advan- tages of CFD have been widely known: the ability to predict the total forces acting on (or generated by) the submarine without a supporting strut between model and carriage, and the ability to generate very detailed flow field information without undesired disturbances. But, the big question was whether CFD was accurate and complete enough. Luckily, after over a decade of studies, there is now plenty of experience about the proper CFD settings and grid setup. Therefore, the outcome of CFD studies can now be used to derive hydrodynamic coefficients that are subsequently used in simulations concerning the turning ability and control- lability of a submarine. The results of the simulations can be validated using the tests with MARIN’s free running submarine model.

CFD simulation of a powering prediction of a full-scale submarine

Modern submarines are one of the most complex structures to design, build and operate. Many hydrodynamic aspects come together in an optimised submarine design. To be able to answer all questions, the hydro- dynamic assessment methodologies are taken to their limits. Luckily, in the last decade the quality and quantity of the tools at the disposal of the hydrodynamicists have grown significantly.

Concept design tools up to full-scale measurement equipment In the early design stage, the preliminary location and sizing of control surfaces takes place. Powering and manoeuvring are key aspects. Empirical tools still play a prominent role due to the short turnaround time in which a large number of designs can be assessed, but CFD is becoming increasingly important. This is largely due to the increase in cluster capacity, which enables results to be calculated quickly and extremely accurately.

CFD has become the tool of choice to determine hydrodynamic manoeuvring coefficients, which are used in simulations. These simulations are used for safety analysis, manoeuvring

predictions, scenario evaluation, Monte Carlo and full mission simulations. In the full mission simulations, the crew is trained regarding various operational scenarios. MARIN’s eXtensible Modelling Framework (XMF) for time domain simulation facilitates easy switching from fast-time, to real-time simu- lations (see page 11).

Experimental techniques have also evolved. A new test rig has been made for captive underwater tests. Its design is such that little set-up time is needed, enabling the submarine to be tested underwater, near surface and in surfaced condition. And finally, fascinating experiments with free running models are an excellent opportunity to perform a final check on manoeuvrability, to study behaviour while sailing under waves, to test the control strategies and the selected autopilot design.

Final validation takes place at sea. The under- water, full-scale measurement techniques are certainly more complex than they are for surface ships. But continuous developments in accurate, on board measurement systems has grown significantly. Constant feedback

report 9

New developments in simulations, CFD and experiments Recent challenges were the prediction of the resistance of submarines while sailing at the surface. This is challenging due to the pronounced breaking wave at the bow caused by the very blunt nose. Knowledge developed in this research is used to enhance empirical tools and design submarines that are more efficient during transit. At first, model scale experiments were performed and these were followed by free surface ReFRESCO computations.

Further model-scale experimental and computational research will help move towards the prediction of behaviour and control under waves using free running underwater models, non-cavitating noise, performance at periscope depth, signatures, more accurate prediction of separated flow around submarines (including hybrid LES models) and to the simulation of free running manoeuvres of submarines, as well as sea- keeping and manoeuvring behaviour at the surface. The results of these studies will ultimately not only benefit the CFD-based, hydrodynamic optimisation of submarines, but also navy surface ships and merchant vessels.

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