Abstract

The last decades there is a strong interest in predicting cavitation dynamics as it is a prerequisite in order to predict cavitation erosion. Industrial applications require accurate results in an acceptable time span and as a result there is a focus on large scale dynamics. In this paper the RANS equations are used to investigate the shedding frequency of sheet cavities in two-dimensional simulations. First a verification study is made for the NACA 0015 in 6 degrees angle of incidence. A grid sensitivity study is conducted in wetted flow and in steady (non-shedding) cavitating condition (σ=1.6). Then an investigation is conducted in order to capture the shedding frequency. The results show that only when a correction for turbulent viscosity at the cavity-water interface is used it was possible to capture the shedding frequency as found in other numerical studies. Furthermore, a validation study is conducted on a NACA66-312 α=0.8 for two different angles of attack. The obtained results are compared and validated with the experimental data from Leroux ''et al''. They indicate that the 2D shedding frequency predicted by the numerical simulations is in good agreement with the frequency obtained in the experiment.

Abstract

Autonomous underwater navigation remains, as of today, a challenging task. The marine environment limits the number of sensors available for precise localization, hence Autonomous Underwater Vehicles (AUVs) usually rely on inertial and velocity sensors to obtain an estimate of their position either through dead reckoning or by means of more sophisticated navigation lters (such as Kalman lters and its extensions). On the other hand, acoustic localization makes possible the determination of a reliable vehicles pose estimate exploiting suitable acoustic modems; such estimate can even be integrated within the navigation lter of the vehicle in order to increase its accuracy. In this paper, the authors discuss the development and the performance of an Ultra-Short BaseLine (USBL) buoy to aid the navigation of AUVs. At first, the components and the physical realization of the buoy will be discussed; then, the procedure to compute the position of the target will be analyzed. The following part of the paper will be focused on the development of a recursive state estimation algorithm to process the measurements computed by the buoy; speci cally, Extended Kalman Filter has been adopted to deal with the nonlinearities of the sensor housed on the buoy. A validation of the measurement ltering with data obtained from experimental tests is also proposed.

Abstract

Much research has been done on the fracture of ship hulls due to collision or grounding; especially over the past two decades, where emphasis has been on advancing relevant nonlinear finite element analysis techniques. These simulations typically involve prediction of hull fracture/rupture, and may be validated against laboratory or field trials experiments ranging in complexity from uniaxial tensile tests to large-scale grillage fracture. Generally, validation efforts ignore the sliding motion of the “struck object;” with the notable exception of Rodd [8], who mounted 1/5th scale hull side-shell modules to a sled and impacted them against a cone-shaped “rock” composed of steel. For the case of steady-state plate cutting, which is typical of stranding, grounding, and oblique collision events, the sliding motion is intrinsic to the nature of the structural response (i.e. without sliding motion, there is no plate cutting), and is captured by existing analysis tools. There has been, however, comparatively little focus (except [1], [5], [6], and [7]) on the case of sliding hull loads resulting from grounding on a soft/wide bottom, or due to hull impact with ice. Both scenarios do not implicitly assume that fracture occurs, and the development of the hull structural response from initial impact to the (potential) point of fracture is of great interest. Typically, these scenarios – particularly impacts with ice features – are simplified to exclude tangential load motion. [1] and [6] predicted numerically that this simplification is unrealistic and unconservative, and [7] confirmed these predictions experimentally. Numerically, the development of fracture due to sliding loads depends on the damage history from the sliding load, the fracture model chosen, and the method of implementation of that fracture model. Quinton [7], using laboratory experiments, showed that nonlinear hull response due to sliding loads (without fracture) exhibits a significantly reduced hull capacity when compared with stationary loads of similar magnitude. This “capacity loss” increased with increasing plastic damage on the trailing side of the sliding load (i.e. increasing damage history). Regarding the fracture model, it is presently common in nonlinear finite element analysis to assume that the ductile fracture of steel (aluminum and some other marine materials) occurs at some equivalent strain that is: 1. a constant (i.e. a point), 2. a function of stress triaxiality (i.e. a line) [3], or 3. a function of stress triaxiality and Lode parameter (i.e. a surface) [2]. Additionally, considering temperature and/or strain-rate effects changes the point, line or surface (as appropriate to the fracture model). Implementation of a fracture model in finite element simulations is non-trivial, and numerous methods are available (e.g. the GISSMO model, the cohesive element approach [4], and the phenomenological approach (e.g. [9]). The application of these approaches to sliding induced fracture, however, is well beyond the scope of this paper; which instead focuses on the development of (i.e. changes in) stress triaxiality and Lode parameter in plates and frames subject to sliding loads; and hence the development of the point of onset of fracture based on the initial choice of fracture model. REFERENCES [1]    Alsos, Hagbart S. "Ship Grounding - Analysis of Ductile Fracture, Bottom Damage and Hull Girder Response." PhD Norwegian University of Science and Technology (NTNU), 2008. [2]    Bai, Yuanli, and Tomasz Wierzbicki. "A New Model of Metal Plasticity and Fracture with Pressure and Lode Dependence." International Journal of Plasticity 24.6 (2008). [3]    Bao, Yingbin, and Tomasz Wierzbicki. "On Fracture Locus in the Equivalent Strain and Stress Triaxiality Space." International Journal of Mechanical Sciences 46.1 (2004). [4]    Cirak, Fehmi, Michael Ortiz, and Anna Pandolfi. "A Cohesive Approach to Thin-Shell Fracture and Fragmentation." Computer Methods in Applied Mechanics and Engineering 194.21–24 (2005). [5]    Hong, L., and J. Amdahl. "Rapid Assessment of Ship Grounding Over Large Contact Surfaces." Ships and Offshore Structures 7.1 (2012). [6]    Quinton, B. W. T. "Progressive Damage to a Ship’s Structure due to Ice Loading." Master of Engineering Memorial University of Newfoundland, 2008. Print.St. John's, Newfoundland. [7]    Quinton, B. W. T. "Experimental and Numerical Investigation of Moving Loads on Hull Structures." PhD Memorial University of Newfoundland, 2015. St. John's, NL.  [8]    Rodd, James L. "Large Scale Tanker Grounding Experiments". Proceedings of the Sixth (1996) International Offshore and Polar Engineering Conference. May 26-31, 1996, Los Angeles.  [9]    Woelke, Pawel B., and Najib N. Abboud. "Modeling Fracture in Large Scale Shell Structures." Journal of the Mechanics and Physics of Solids 60.12 (2012).

Abstract

There is no question about the relevancy of the blue growth concept, as a long term strategy to support sustainable growth in the marine sector. One of the sectors that has a higher potential for sustainable growth in this field is the aquaculture. This work is focused on the development of a time domain hydrodynamic solver for analysis and assessment of floating fishing farms. The paper also presents a demonstration case based on typical configuration of fishing farms used in the Mediterranean Sea. It is composed by a set of individual circular fishing cages. Several arrangements of fishing cages are also analysed subject to wind waves and currents. Different fishing cages are used in the performed simulations: circular, square and hexagonal. The influence of several configurations on fishing farm mooring behaviour in different sea states is studied. The analysis is focused in the dynamic behaviour of fishing cages. In particular mean drift and slow drift motions, including the second order-effects, will be studied, since they have important role on the behaviour of fishing farm in seaway. The results lead to final discussion about the best performance in fishing farm layout.

Abstract

The aim of this work is to be able to cope with complex sloshing-seakeeping problems in an effective manner. For this purpose, two different solvers will be integrated into one coupled tool to take advantage of their characteristics. The Particle Finite Element Method is a versatile framework for the analysis of fluid-structure interaction problems. The latest development within the framework of the PFEM is the X-IVAS (eXplicit Integration along the Velocity and Acceleration Streamlines) scheme. It is a semi-implicit scheme built over a Semi-Lagrangian (SL) formulation. This new scheme was named PFEM-2 and will be used in this work to solve the fluid dynamics (sloshing) inside the tanks. The PFEM-2 will be coupled in the time domain with SeaFEM, a solver developed for seakeeping problems. SeaFEM solves the second-order diffraction-radiation equations by using the Finite Element Method (FEM) and will be used in this work to simulate the interaction between waves and a floating body. The coupling of the two tools will be accomplished by using an effective coupling algorithm and a communication technique that allows simulations to be computed without affecting the global compute time and the accuracy of the solvers. This new tool has been validated against experimental benchmarks carried out in a model basin at model scale.

Abstract

A finite element method for the solution of the up-to-second-order wave diffraction-radiation problem in the time-domain is proposed. The solver has been validated against experimental data available for the HiPRWind semisubmersible platform (designed for floating wind turbines). To perform the validation, the wave diffraction-radiation solver is coupled to a body dynamics and mooring solvers in the time-domain. The HiPRWind movements and mooring forces have been compared for a large number of test cases, including decay tests, bichromatic, and irregular waves. Good agreement has been found for both, body movements and mooring forces.