CFD methods have long been used to investigate and understand the steady-state fluid dynamics of partly and fully submerged bodies, i.e. surface ships and underwater vehicles, as a part of the design process or as a means of generating input data for various performance models. In recent years, however, both compute resources and unsteady methods have advanced to a level where unsteady problems are tractable propositions for many researchers. Three recent NATO-AVT RTGs have looked at this class of problem: the sea facet of AVT-161 computed the MOERI KVLCC2 tanker and the DTMB 5415M cases, amongst others, in order to assess the predictive capability of computational methods to predict manoeuvrability and course-keeping performance; the follow-on AVT-216 RTG extended the complexity of the manoeuvres being simulated, including aspects such as active control surfaces and irregular waves; AVT-183 shifted the focus from prediction of the overall manoeuvring characteristics to the quality of the computed flow-field itself. Whilst all three activities have been of great value and generated a huge amount of valuable data and understanding, the breadth and complexity of the test cases hitherto computed has, conversely, left many questions unanswered. Subsequently, a generic submarine geometry and a limited set of manoeuvring test cases were identified in NATO AVT-ET-155 to allow a much narrower focus. With this test case, more thorough understanding of the fluid dynamics processes and the relative capabilities of the computational methods can be achieved, with deeper and more effective interchange between the participating researchers.
Importantly, unlike all previous AVT fluid dynamics work in the sea domain, using a submarine geometry will enable free-surface modelling to be avoided. Aspects that will be considered include, amongst others: the use of higher fidelity CFD methods for the prediction of the flow around the submarine at straight flight, at a steady drift angle and at steady rotation. Validation of the predictions for straight flight and steady drift will be based on flow field measurements and forces and moments. For the steady rotation case, force and moment data acquired from rotating-arm tests will be available.
Within recent AVT work, e.g. that performed within AVT-183, it was found that some computational procedures performed better than other used. Since different codes, grid setups, grid densities and turbulence models were used, it is hard to identify the best approach to accurately predict the flow around ships. The proposed RTG will address this matter more thoroughly by exchanging grids and conducting in-depth comparisons between solutions. Some of the benefits of the outcome of the RTG would include:
• Reduced development risks through advances in reliable prediction of flow and loads on naval vessels;
• Reduced development costs through improved computational methods and techniques;
• Improved tools and techniques to address anomalous flow features.
The new RTG will assess the ability the maturity and applicability of state-of-the-art viscous-flow solvers to predict submarine hydrodynamics of interest to NATO. Specifically, the following activities will be undertaken:
• Predict the flow and loads on a submarine under straight ahead conditions;
• Predict the flow and loads on a submarine at static drift;
• Predict the flow and loads on a submarine in steady turning motion;
• Make recommendations on the selection of grid densities, turbulence models and other numerical settings for predictions of submarine hydrodynamics.
Solutions produced by the various participants will be compared and grids will be exchanged to better understand any numerical uncertainties in the results and/or limitations of methods and approaches used.