A framework for the design by optimization of hydrofoils under cavitating conditions

Hydrodynamic shape optimization based on CFD calculations can dramatically improve the design of marine devices (i.e. propellers, rudders and appendages) by simultaneously considering opposite objectives and by modeling phenomena that well-established and still widely adopted design approaches (i.e. lifting line and lifting surface) cannot accurately deal with. Cavitation on propellers, for instance and among the others, is one of the most dangerous phenomena. It causes vibration, erosion and it is a source of radiated noise, consequently resulting incompatible with modern propeller design, continuously aimed for higher efficiency, comfort and environmentally safe operations. An accurate selection, firstly, of the most appropriate blade sections is, consequently, of crucial importance at least to limit the side effects of cavitation. In the present work, therefore, a numerical framework for the design by optimization of marine hydrofoils under cavitating conditions is proposed. By combining a parametric description of the hydrofoil shape, the NSGA-II multi-objective genetic algorithm and appropriate flow solvers, new hydrofoil shapes are derived. Objectives of the design are blade sections with enlarged cavitation buckets to increase the cavitation inception speed and to reduce the cavity volume (under the constraint of unchanged delivered lift) with respect to widely accepted NACA66 profiles. Boundary element methods and RANSE solvers (a proprietary Hess & Smith implementation and the open-source RANSE solver OpenFOAM) are applied in succession in order to verify the influence of the inviscid/viscous nature of the flow on the final optimal hydrofoil shape and of the additional maximum lift/drag ratio objective required in the case of viscous calculations

Ship propeller side effects: pressure pulses and radiated noise

The present paper deals with the side effects of propellers cavitation, i.e. pressure pulses and radiated noise. These effects are gaining more and more importance for commercial ships for different reasons. Pressure pulses significantly affect comfort onboard, thus their reduction is of utmost importance for all ships carrying passengers. As regards the underwater radiated noise, in the last decade interest has shifted from navy applications to commercial ships, due to the concern for the rising background noise in the oceans. The propellers, generating noise directly in water, represent one of the main contributions to the overall underwater noise emitted from ships. Due to the complexity of the mechanisms of propeller noise generation, different complementary strategies have to be followed to properly analyze the problem, ranging from induced pressure pulses to broadband noise and cavitation. In the present work, part of the activities carried out in the framework of the collaborative EU FP7 project AQUO (Achieve QUieter Oceans by shipping noise footprint reduction, www.aquo.eu) are reported. The paper presents the investigations carried out on a specific test case represented by a single screw research vessel, which is analyzed with three different strategies: numerical calculations, model scale investigations and full scale measurement

Surrogate Model for Ship Resistance : a Sensitivity Analysis of Shape Deformation

Shape optimization and sensitivity can now be considered a standard to design efficient ships and new unconventional hull forms. There might be very different strategies to achieve such a process, depending on many circumstances, both operational (e.g. available time windows for computations) and methodological (e.g. available numerical methods). As in most of the engineering cases, the best trade-off between accuracy of the solution and the time required to achieve it is searched. When using medium-fidelity methods e.g. Boundary Element Methods (BEMs) for wave resistance and seakeeping, population based optimization algorithm can still be a viable way to achieve a design solution (see e.g. Vernengo et al., 2015 or Vernengo and Brizzolara, 2017). However, when the solution of the quantity of interest becomes more demanding, alternative methods of searching through the design space are needed. In this perspective, a surrogate model based approach for hull form sensitivity analysis is presented. The method relies on the features of a Kriging response surface (see for instance Forrester et al., 2008) to interpolate few computed solutions and to predict the same solution over the whole explored domain. The sensitivity analysis focuses on the effect of hull form variations with respect to calm water resistance at a given forward speed. The total ship resistance is computed by means of a high-fidelity viscous solver based on the openFOAM libraries (Jasak et al., 2007). The shape variations are achieved by a combined approach specifically developed to preserve the fairness of the hull surface based on Subdivision Surface and Free Form Deformation (FFD) (Copped\ue9 et al., 2018). The hydrodynamic solver has been preliminary validated by comparison against available experimental measurements on the KRISO Container Ship (KCS) hull. The variation of the Kriging response surface performance with respect to different sizes of the initial sampling have been studied and possible optimum hull shape have been detected and compared in terms of calm water resistance, wave patterns and pressure on the surface

EFD and CFD Design and Analysis of a Propeller in Decelerating Duct

Ducted propellers, in decelerating duct configuration, may represent a possible solution for the designer to reduce cavitation and its side effects, that is, induced pressures and radiated noise; however, their design still presents challenges, due to the complex evaluation of the decelerating duct effects and to the limited amount of available experimental information. In the present paper, a hybrid design approach, adopting a coupled lifting line/panel method solver and a successive refinement with panel solver and optimization techniques, is presented. In order to validate this procedure and provide information about these propulsors, experimental results at towing tank and cavitation tunnel are compared with numerical predictions. Moreover, additional results obtained by means of a commercial RANS solver, not directly adopted in the design loop, are also presented, allowing to stress the relative merits and shortcomings of the different numerical approaches

Systematic analysis of mesh and meshless CFD methods for water impact problems

Two types of numerical simulations for the hydrodynamic solution of water entry problems are performed and systematically compared in order to highlight their peculiarities: a viscous Unsteady Reynolds Averaged Navier Stokes (URANS) based on Volume of Fluid (VoF) approach and a meshless Smooth Particle Hydrodynamics (SPH) solver. In both cases open-source software have been chosen. The numerical solutions from the two proposed CFD methods are verified against experimental measurements. The systematic analysis is performed considering a prescribed motion of the wedge aiming to a better understanding of the effect of the model set-up on the prediction of both local and global field variables. Even if a both codes shows a high ability to capture the global physics of the problem, due to the strongly non- linear dynamic of the body-flow interactions involved in impact problems, the local pressure peaks can be hardly predicted if the numerical method is not suitably tuned for the specific problem. Finally the two approaches are performed also for a free falling simulation, comparing the numerical predictions to the available experimental drop tests results

Free Surface Hydrodynamics of Submarine Masts Configurations

Sailing at snorkel depth is a necessary but dangerous operating scenario for submarines. The main and straightforward reason is that such an operating condition represents a time of possible vulnerability of the vessel. From a design perspective this condition affects the so-called indiscretion rate, that is exactly the ratio between this time of greater vulnerability and the total operating time. Moreover, when the vessel operates at snorkel depths there are some relevant operations that might be accomplished related to both snorkeling, communications and threats detection. These operations are typically carried out by using a certain number of masts, of slightly different shapes and sizes, that might be used in various configurations. The proposed study aims at providing some insights into the unsteady, turbulent, hydrodynamics of several submarine masts configurations. The analysis is carried out in terms of behaviors of the developed free surface, considering the nonlinear interactions rising among the masts considered. The maximum height and length of the breaking wave generated at the bow of each mast is analyzed. Both the near field and the far field unsteady wave patterns are discussed comparing several configurations and focusing on the interaction effects. The computational study is carried out by using an open-source Smoothed Particle Hydrodynamic solver called DualSPHysics, able to exploit the computational acceleration provided by GP-GPU cards

Propeller geometry optimization for pressure pulses reduction: an analysis of the influence of the rake distribution

The evaluation of pressure pulses is a current issue for any high-performance propeller design. It has been addressed experimentally, by means of model tests, and numerically but in most cases the analysis has been limited to the verification of a given geometry (or, at least of few configurations) identified at the end of a traditional design loop. A more direct inclusion of pressure pulses evaluation in the design procedure, for instance by very attractive multi-objective optimization approaches, could be beneficial, especially if more accurate codes may be exploited. Among the others, BEM represent an acceptable compromise between computational costs and accuracy with the further advantage, with respect to lower fidelity approaches, to account for effects of geometrical haracteristics (such as rake distribution) which are often defined only according to designer experience and special needs. However, if the ability of the BEM methods to predict propeller performance and cavitation extension is well documented, the direct computation of pressure pulses may be less reliable, especially in correspondence to heavy cavitating conditions, requiring further validations in particular when the influence of characteristics such as rake distribution, hardly addressed in literature also from the experimental point of view, are considered. Cavitation tunnel test, BEM and RANS calculations have been consequently carried out for two propellers, designed for the same functioning conditions with different rake distributions, in order to stress the capabilities and the limitations of the numerical approaches in dealing with cavitation, pressure pulses predictions and the capability to discriminate between slightly different geometries in the light of their possible application in a design by optimization procedure

Black holes, gravitational waves and fundamental physics: a roadmap

The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'

A Study on the Wake Evolution of a Set of RIM-Driven Thrusters

In the current paper, high-fidelity improved delayed detached-eddy simulations (IDDES) using the OpenFOAM library are employed to characterize the performances and the wake dynamics of RIM-driven thrusters. Despite their relatively old conception, this type of propulsors has gained attention only recently thanks to innovative manufacturing technologies and materials, which finally have made possible their practical implementation. Fostered by stringent regulations on radiated noise and emissions enforced in protected areas, they are also replacing conventional thrusters and main propulsors. By mitigating the tip vortex cavitation and, more in general, by reducing the strength of tip vortices, indeed, these propulsors may grant a reduction in the induced pressure pulses and of the radiated noise without excessively sacrificing the efficiency of the equivalent ducted propellers they replace. To provide proof of this, three different RIM-driven thrusters (one four-bladed, two six-bladed) are analyzed and compared to a reference decelerating ducted propeller delivering the same thrust at identical functioning conditions. The evolution of the trailing wakes of the propulsors, the role of the leakage vortex, and the nozzle wake destabilizing effects, are highlighted, and the superior performances of RIM-driven thrusters, in terms of less intense tip vortices, are discussed. Near-field pressure pulses, as a measure of the radiated noise, are compared, showing a reduction in the sound pressure levels of the selected RIM propulsors up to 15 dB (non-cavitating case) with respect to the reference ducted propeller