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International audience

International audience

International audience

International audience

This paper presents an efficient screening tool, based on the 2D strip method and the CFD-based approach. The proposed method allows the study of the dynamics of the vortex-induced vibration (VIV) phenomenon with almost the same accuracy as a fully 3D Fluid-Structure interaction (FSI) model, but with a reduction in computational times by a factor of nearly 20 compared to the 3D approach. Such a large reduction makes the use of CFD-based analysis feasible on a workstation within a reasonable time frame and opens the possibility of CFD approaches in a subsea pipeline design process.

International audience

International audience; Surrogate models are simplified models for the behaviour of a complex system, based on a limited number of operating points where the system behaviour is simulated accurately. In hydrofoil design for sailing yachts, surrogate models can be used both for automatic shape optimisation of the foil and for performance evaluation of the entire yacht in the context of a VPP, as a response surface. We present an adaptive method for the construction of surrogate models which can be used for both these objectives, with a simple change of parameters. A Gaussian process regression (GPR) is used for the data fitting, while the adaptive choice of sample points is based either on the GPR variance or on a combination with a cross-validation error estimation. A first test on analytical functions shows that the cross-validation approach is superior for response surface creation, while both adaptation methods are equally suited for shape optimisation. A second test on the shape optimisation of a two-dimensional hydrofoil indicates that for moderate immersion depths, the optimum shape is not sensible to the distance to the free surface.

International audience

International audience; Most engineering design processes rely on physical or numerical experiments to evaluate the candidate design performances. When the design space is large or when the model response is complex, the possibly high computational expense poses a challenge since the number of simulations is usually limited by time or financial cost. This is even more of a challenge when performing global optimization as many evaluations are required. Hydrofoil optimisation for naval applications is typically facing these limitations. One way to address this limitation is to build a simplified model of the actual model from responses of experiments. Meta-modelling is the process of generating such models of models or meta-models. This process is also referred in engineering as surrogate modelling. Strategies involving surrogate models are used in a wide range of engineering fields to efficiently address computational cost limitations. Evaluations of a surrogate model are cheap and can be used for visualization, trade-off analysis and optimization. Nevertheless, building a surrogate requires evaluating the original model at specified design points and gathering the corresponding responses. The amount of points needed to approximate the behaviour of a numerical model depends on the complexity of the response, which is not necessarily known beforehand. The chosen sampling strategy plays an important role in the quality of the surrogate. Under-sampling might not allow capturing the complexity of the phenomenon and over-sampling could lead to expensive computation times. Adaptive sampling or response-adaptive design uses the responses to the simulation for adjusting the sampling design in a sequential way while the surrogate is being constructed. This allows for determining and minimizing the required number of points for efficient global surrogate construction. Hydrofoil optimisation can be performed in different ways. Hydrofoils can be either optimized independently from the rest of the ship they will be fitted to or, in a more robust way, by taking into account the whole ship and simulating its behaviour in a third party simulating code (VPP). We usually switch from one approach to another according to the design stage. Surrogate models can be used for both approaches but, if build in adaptive way, will be sampled quite differently. If the goal is response surface creation, exploration (sampling from areas of high uncertainty) will be favoured whereas exploitation (sampling areas likely to offer improvement over the current best observation) will lead to sampling areas close to the optimum. In this study we investigate the possibility of both creating responses surface and perform shape optimisation with the same tool. We chose Gaussian process regression (GPR), also known as Kriging, to create the surrogates. After defining an initial set of points using a traditional design of experiment method, the following points are chosen in a sequential and adaptive way. An appropriate acquisition function (infill sampling criterion) has been investigated to access the exploration/exploitation trade-off. The efficiency is tested on analytical function (Branin function) and then applied both to an optimisation and response surface creation of a 2D hydrofoil NACA profile.

An overview is presented of the activities conducted within the NATO STO Task Group AVT-204 to “Assess the Ability to Optimize Hull Forms of Sea Vehicles for the Best Per- formance in a Sea Environment.” The objective is the development of a greater understanding of the potential and limitations of the hydrodynamic optimization tools. These include low- and high-fidelity solvers, automatic shape modification methods, and multi-objective optimiza- tion algorithms, and are limited here to a deterministic application. The approach includes simulation-based design optimization methods from different research teams. Analysis tools include potential flow and Reynolds-averaged Navier-Stokes equation solvers. Design modifica- tion tools include global modification functions, control point based methods, and parametric modelling by hull sections and basic curves. Optimization algorithms include particle swarm optimization, sequential quadratic programming, genetic and evolutionary algorithms. The ap- plication is the hull-form and propeller optimization of the DTMB 5415 model for significant conditions, based on actual missions at sea.

International audience

International audience; This paper is devoted to the verification and validation exercises with the ISIS-CFD code conducted for the Tokyo 2015 workshop. In addition to numerical uncertainty estimation, we also address issues such as the effect of turbulence modelization, wall resolved versus wall modeled approach, propeller resolved versus propeller modeled approach for self-propulsion simulation, etc.