MS4.4 - Computational Fluid-structure Interaction

Floating offshore structures are of great interest for many applications. A particular type of floating structures are the so called Very Large Floating Structures (VLFS). One can find several examples of VLFS, such as floating airports, floating solar energy installations, floating breakwaters, or even futuristic floating modular cities. The study of the behavior of VLFS is, therefore, relevant for a wide variety of industries and scientific disciplines. In this work we present a novel monolithic Finite Element Method (FEM) for the hydroelastic analysis of VLFS with arbitrary shapes that is stable, energy conserving and overcomes the need of an iterative algorithm. The new formulation [1] is general in the sense that solutions can be found in the frequency and time domains, and overcomes the need of using elements with C1 continuity by employing a continuous/discontinuous Galerkin (C/DG) approach. In this talk we will show how this formulated can efficiently be used in large scale computing facilities to assess the realistic behavior of VLFS structures. We will assess the hydroelastic phenomena of VLFS with a variety of tests, including structures with elastic joints, variable bathymetry and arbitrary strucutral shapes. [1] O. Colomés, F. Verdugo, and I. Akkerman. "A monolithic Finite Element formulation for the hydroelastic analysis of Very Large Floating Structures." International Journal of Numerical Methods in Engineering (2022), In press.

Modern ships can easily exceed 400m in length. At this size, the flexural response of the ship can be significant, and the natural frequencies can be dangerously close to the energetic region of typical ocean waves. Container ships with relatively little deck stiffening are particularly flexible, especially in torsion. This makes them vulnerable to both whipping and springing responses. Whipping is the term used to describe a transient, resonant, structural response of the hull caused by the sudden impact of a large wave. This is a nonlinear process which cannot be captured by strictly linear models but can be an important component of the Ultimate Limit State. Springing is the term used to describe resonant hull vibrations induced by continuous wave forcing, and this can be induced by both linear and nonlinear wave forcing. Springing is important for the prediction of fatigue loading. This talk will describe some of our recent work on numerical modelling of the hydro-elastic response of ships. We apply potential flow theory to describe the interaction between the waves and the ship. Several levels of modelling are applied, starting with a linear method where the hydrodynamic loading is computed using the concept of generalized modes, and the ship structure is approximated as a slender Euler or Timoshenko beam. Hydrodynamic calculations are performed using our in-house high-order finite difference-based open-source tool OceanWave3D-Seakeeping. The linearized problem for the flexural response can be formulated either in the global coordinates (Newman’s method) or in a local coordinate system fixed to the instantaneous rigid-body position (Malenica’s method). We show that the two formulations result in different inertia and hydrostatic contributions but give the same response, and are thus both consistent and equally valid. Several examples will be given to illustrate this for simple geometries. Examples will also be given comparing calculations to experimental measurements for several geometries at both zero and non-zero forward speed. Work is also in progress on more accurate, nonlinear models for the structural response of the ship and extending the hydrodynamic analysis to nonlinear wave loading.

Understanding the mechanics of fish propulsion has been an active field of research for decades and continues to inspire novel propulsion systems. Biologists and engineers have identified that a combination of kinematics, active as well as passive morphing, exceptional control and sensory perception gives aquatic swimmers their fascinating abilities [1]. Small-scale pro-pulsion solutions that resemble fishtails or flukes, such as the MIT RoboTuna [2], have been proposed for energy-efficient autonomous underwater vehicles (AUVs). At a larger-scaler biomimetic thrusters are also being studied as auxiliary ship propulsion systems and energy-saving devices that capture wave energy and reduce ship motions; see, e.g., the symbiotic ship/engine and propulsion innovation [3]. However, to mimic the high propulsive efficiency of marine mammals it is essential to investigate effects of elasticity in flapping-foil thruster designs [4]. The response of passively morphing wings is implicitly nonlinear, because deformations affects the hydrodynamic load excitation and vise-versa. Thus, fluid-structure interaction coupling is essential for accurate predictions of the wing response [5]. In the present work, a coupled boundary element/finite element method (BEM-FEM) is proposed for the hydro-elastic analysis of a flapping-foil thruster operating as the main propulsion system of an autonomous underwater vehicle (AUV). The structural dynamics are modelled using plate theory while the fluid dynamics of the lifting surface are modelled using an ideal-flow BEM solver using GPU-acceleration techniques to reduce the overall computational cost [6]. The verification of the present method is accomplished by means of comparisons against experimental da-ta and other numerical methods found in the literature. Results concerning the hydro-elastic response of the flapping-foil thruster for a realistic propulsion scenario are also presented, indicating that incorporating flexibility when tuned properly, can enhance the propulsive performance of the thruster and thus extend the operational capabilities of the AUV. The developed computational tool is able to predict the response of the passively morphing dynamic wings with acceptable accuracy, thus enabling the cost-effective optimization of such devices. References: [1] M. Sfakiotakis, D. Lane, J. Davies, Review of fish swimming modes for aquatic loco-motion, IEEE Journal of Oceanic Engineering 24 (2), 237–252, 1999. [2] D. S. Barrett, Propulsive efficiency of a flexible hull underwater vehicle, Ph.D. thesis, Department of Ocean Engineering MIT (1996). [3] D. Ntouras, G. Papadakis, K. Belibassakis, Ship bow wings with application to trim and resistance control in calm water and in waves, Ocean Engineering 10, 492, 2022. [4] Richards, A.J.; Oshkai, P. Effects of the stiffness, inertia and oscillation kinematics on the thrust and efficiency of an oscillating-foil propulsion system. J. Fluids Struct. 57, 357–374, 2015. [5] Zhu, Q. Numerical simulation of a flapping foil with chordwise and spanwise flexibility. AIAA J., 45, 2448–2457, 2007. [6] E. Filippas, K. Belibassakis, A nonlinear time-domain bem for the performance of 3d flapping-wing thrusters in directional waves, Ocean Engineering 245, 110157, 2022.