MS4.3 - Computational Fluid-structure Interaction

The analysis of wave-structure interaction in practical coastal and ocean engineering problems requires large-domains and complex geometries. A study using single-physics numerical models can be computationally prohibitive or have limited accuracy. It is the need of the hour to develop models which are accurate, but should also have a reasonable run-time. Hybrid modelling is an approach that can combine the strengths of different models by applying them only within specific regions of the domain. In the context of ocean engineering problems, the potential flow assumption may be sufficient for capturing wave refraction, reflection and diffraction over the majority of the domain. However, an accurate viscous flow model is required in the local sub-domain around fixed and floating structures for simulating the complex fluid-structure interaction. The manuscript presents fluid-fluid coupling between a mesh-based potential flow model named FEBOUSS and a particle-based viscous flow model MLPG_R to develop a hybrid model for simulating wave-structure interaction in 3D. It combines the computational efficiency of FEBOUSS with the accuracy of MLPG_R to reduce computation time, allow subjecting structures to realistic waves and enable the use of particle-based methods for real-domain problems. FEBOUSS is an in-house finite element model for depth-integration potential flow equations. It is a weakly non-linear 2D free-surface model capable of simulating 3D flows under a wave and is hence computationally efficient. It is proven for simulating wave refraction, diffraction and reflection over large domains with complex bathymetries in shallow and intermediate water depths. MLPG_R is the in-house particle-based model for solving the Navier-Stokes equations in a 3D domain. It solves the pressure Poisson equation in weak form using a Petrov-Galerkin approach resulting in smooth and accurate calculation of pressure. Particle-based methods allow a freely moving free-surface and hence are ideal for capturing the interaction of waves with fixed, floating or moving structures, but have a considerable computational cost. This work presents the application of weak coupling between the two models. FEBOUSS will be used for wave generation and propagation in the realistic large domain of a harbour. A small 3D MLPG_R domain will be generated in the vicinity of a coastal structure within this harbour, to expose the structure to realistic wave conditions from FEBOUSS. This work is a demonstration of this hybrid approach, which can simulate realistic wave loadings with reasonable accuracy and at a moderate computational cost. The techniques developed in this work can be applied to fluid-fluid coupling with other high-fidelity hydro-elastic models for studying the response of flexible wave-energy converters and floating solar farms when exposed to directional wave-spectrums in near-shore conditions.

Computational efficiency has remained one of the major barriers constraining the development of computational approaches for simulating many industrial problems involving fluid-structure interaction. The most efficient algorithms are the partitioned staggered solution approaches. These methods require one execution of the solid and fluid sub-solvers per time step. However, such strategies usually lack robustness and efficiency. In this work, we propose robust second-order accurate staggered solution schemes for fluid-structure interaction. These approaches are based on linear combinations of higher-order predictors and are based on Dirichlet–Neumann coupling. The schemes are unconditionally stable to a high amount of added mass and are suitable for code coupling. These schemes are developed and tested in the setting of one-dimensional model problems. The performance of the methods in the context of multi-axial finite element computations is demonstrated using numerical evaluations and benchmark problems.

To transition towards a carbon neutral future, we are required to look for new sources of renewable energy. The potential of the sea for capturing renewable energy is immense, but it requires us to move towards deeper waters. Therefore, there is an increased demand for research into floating renewable energy capturing devices which are accompanied by their own unique challenges. One driver in further increasing the feasibility of floating energy capturing devices is an increase of fidelity and efficiency in modeling of Fluid-Structure Interaction (FSI) problems involving free surface flows and floating bodies. One of the main issues in computational FSI for floating renewables is the need to deal with complex structural geometries. This is especially relevant in the design optimization phase, where an optimal structural solution must be defined for site-specific conditions in a limited time. Unfitted Finite Element (FE) methods are convenient for these situations, avoiding the need of ad-hoc mesh generation. The challenge here is how to deal with an unfitted structure that interacts with free surface flows. In this talk we will present a single-phase FE approach for free surface flows, where only the wave-structure interaction is accounted for, in combination with an unfitted floating structure with arbitrary geometry. In this work we propose a monolithic method with block preconditioning, ensuring robustness and efficiency of the solution. We will demonstrate the capabilities of the proposed framework with a series of tests for wave-structure interaction problems, assessing accuracy and conservation properties.

We consider methods for dynamic coupled problems, in particular partitioned solvers for fluid structure interaction where different sub-solvers are used for the fluid and solid domain. More specifically we want to create a detailed simulation of a jackdaw's feather to investigate how feathers generate lift and ultimately answer the question of how birds evolved flight. This requires a fast and robust solver. Specifically, we want a solver that uses few coupling iterations, allows for high order in both space and time, is time adaptive and allows for different timesteps in the sub-solvers. Using so called waveform iterations these goals have been achieved for thermal heat transfer problems. The waveform iteration with adaptive time steps consists of nested solvers that each have their termination criteria and tolerance. Despite its success for simulating thermal heat transfer problems, it is still an open question how one should select the tolerances and the termination criteria within the nested solvers. Additionally, there is a problem with convergence of the waveform iteration which does not exist for fixed time steps. For adaptive ones, it can happen that the time grids do not converge and thus neither the waveform iteration. For fluid structure interaction, waveform iterations have previously been combined with interface Quasi-Newton acceleration to achieve an efficient partitioned solver in the case where both sub-solvers use different fixed time steps. In this talk we present the general framework of waveform iterations and their implementation in the open source coupling library PreCICE. We present some insight into how to select the termination criteria and tolerance within the nested solvers in the waveform iteration. We also extend the Quasi-Newton waveform iterations to the time adaptive case, where both of the sub-solvers use an adaptive time stepping scheme. Lastly, we also show that using a time adaptive solver results in faster run times for the simulation of our bird feather.