engleski

Numerical method for the evaluation of the structural response of ship appendices

Numerical framework for the assessment of hydrodynamic loads and structural responses of ship appendages is developed in this work. The ship entire life cycle is considered, and the expected long-term maximum loads are evaluated emphasizing the statistical wave environment and appropriate loading parameters. Different existing numerical models are employed, and hydrostructure interaction models are developed and verified. The study can be generally divided into two parts: definition of the relevant load cases for the ship appendage and development of the applicable hydro-structural model capturing the necessary physical phenomena relevant for accurate structural response calculation. First part relates to the input for the numerical simulation model, while the second part provides the structural stresses essential for the appendage design. Both steps combined make the basis for the direct approach in the design of ship structures. The numerical method proposed in this work is applied on the design of Pre-Swirl Stator (PSS) ship appendage. PSS is an Energy Saving Device (ESD) which aims to reduce the amount of the rotational kinetic energy in the propeller slipstream, thus resulting in lower power delivered to the propeller shaft and decreasing the on-board total fuel consumption. Although majority of the current research is dedicated to the performance and efficiency gains of ESDs, this study is primarily related to the structural issues exhibited by such appendages. Due to the position of the PSS in the ship stern wake near the propeller, the obvious and foremost load case on PSS is related to the propeller rotation. For these simulations to reach a satisfying level of accuracy, Computational Fluid Dynamics (CFD) solvers are necessary because the inclusion of viscosity, turbulence and non-linearity is mandatory. However, propeller induced loads cover only one segment of the PSS life cycle i.e., when the ship operates in calm-water. On the contrary, design life wave loads are much harder to approximate, and their quantification requires stochastic methods to reach a representative load-case for the variety of sea-states the ship may encounter. Usually, the sea-states are represented through multiple design conditions or the so-called Equivalent Design Waves (EDWs) corresponding to different levels of probability and this methodology is developed and used in the current work for the case of PSS. In order to analyse a vast number of possible sea-states and directions, a linear potential flow code in frequency domain is deemed sufficient given its incomparable CPU efficiency. For non-linear simulations on defined EDWs, two hydro-structural models are developed in the scope of the study: quasi-static and dynamic (or so-called hydroelastic). The former presumes the independence of the fluid force on the structural nodal displacements and the problem deduces to the pressure interpolation from fluid to structural sharing interface. The latter requires solution of the structural dynamics inside the fluid domain and subsequent influence of nodal displacement on the fluid field is taken into account. The structure is modelled with a wellestablished Finite Element Method (FEM) which follows a need for the FEM-CFD interaction model. The quasi-static model is implemented as the projection method and the dynamic model is realized with the modal superposition which greatly adds to the efficiency of the coupling. The developments in this study are performed mainly on the interface between the CFD and FEM computational tools for which OpenFOAM and NASTRAN software is applied, respectively. The linear potential solution is obtained by means of Bureau Veritas software HydroStar.

Numerical simulation ; CFD-FEM ; fluid-structure interaction ; quasi-static response ; hydroelastic response ; Pre-Swirl Stator ; Equivalent Design Wave ; long-term wave statistics

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