Georgios Fourtakas

Bio: Georgios Fourtakas is an academic researcher from University of Manchester. The author has contributed to research in topics: Smoothed-particle hydrodynamics & Boundary value problem. The author has an hindex of 10, co-authored 26 publications receiving 339 citations.

Modelling multi-phase liquid-sediment scour and resuspension induced by rapid flows using Smoothed Particle Hydrodynamics (SPH) accelerated with a Graphics Processing Unit (GPU)

Abstract: A two-phase numerical model using Smoothed Particle Hydrodynamics (SPH) is applied to two-phase liquid-sediments flows. The absence of a mesh in SPH is ideal for interfacial and highly non-linear flows with changing fragmentation of the interface, mixing and resuspension. The rheology of sediment induced under rapid flows undergoes several states which are only partially described by previous research in SPH. This paper attempts to bridge the gap between the geotechnics, non-Newtonian and Newtonian flows by proposing a model that combines the yielding, shear and suspension layer which are needed to predict accurately the global erosion phenomena, from a hydrodynamics prospective. The numerical SPH scheme is based on the explicit treatment of both phases using Newtonian and the non-Newtonian Bingham-type Herschel-Bulkley-Papanastasiou constitutive model. This is supplemented by the Drucker-Prager yield criterion to predict the onset of yielding of the sediment surface and a concentration suspension model. The multi-phase model has been compared with experimental and 2-D reference numerical models for scour following a dry-bed dam break yielding satisfactory results and improvements over well-known SPH multi-phase models. With 3-D simulations requiring a large number of particles, the code is accelerated with a graphics processing unit (GPU) in the open-source DualSPHysics code. The implementation and optimisation of the code achieved a speed up of x58 over an optimised single thread serial code. A 3-D dam break over a non-cohesive erodible bed simulation with over 4 million particles yields close agreement with experimental scour and water surface profiles.

Local Uniform Stencil (LUST) boundary condition for arbitrary 3-D boundaries in parallel smoothed particle hydrodynamics (SPH) models

Abstract: This paper presents the development of a new boundary treatment for free-surface hydrodynamics using the smoothed particle hydrodynamics (SPH) method accelerated with a graphics processing unit (GPU). The new solid boundary formulation uses a local uniform stencil (LUST) of fictitious particles that surround and move with each fluid particle and are only activated when they are located inside a boundary. This addresses the issues currently affecting boundary conditions in SPH, namely the accuracy, robustness and applicability while being amenable to easy parallelization such as on a GPU. In 3-D, the methodology uses triangles to represent the geometry with a ray tracing procedure to identify when the LUST particles are activated. A new correction is proposed to the popular density diffusion term treatment to correct for pressure errors at the boundary. The methodology is applicable to complex arbitrary geometries without the need of special treatments for corners and curvature is presented. The paper presents the results from 2-D and 3-D Poiseuille flows showing convergence rates typical for weakly compressible SPH. Still water in a complex 3-D geometry with a pyramid demonstrates the robustness of the technique with excellent agreement for the pressure distributions. The method is finally applied to the SPHERIC benchmark of a dry-bed dam-break impacting an obstacle showing satisfactory agreement and convergence for a violent flow.

State-of-the-art SPH solver DualSPHysics: from fluid dynamics to multiphysics problems.

Abstract: DualSPHysics is a weakly compressible smoothed particle hydrodynamics (SPH) Navier-Stokes solver initially conceived to deal with coastal engineering problems, especially those related to wave impact with coastal structures. Since the first release back in 2011, DualSPHysics has shown to be robust and accurate for simulating extreme wave events along with a continuous improvement in efficiency thanks to the exploitation of hardware such as graphics processing units (GPUs) for scientific computing or the coupling with wave propagating models such as SWASH and OceanWave3D. Numerous additional functionalities have also been included in the DualSPHysics package over the last few years which allow the simulation of fluid-driven objects. The use of the discrete element method (DEM) has allowed the solver to simulate the interaction among different bodies (sliding rocks, for example), which provides a unique tool to analyse debris flows. In addition, the recent coupling with other solvers like Project Chrono or MoorDyn has been a milestone in the development of the solver. Project Chrono allows the simulation of articulated structures with joints, hinges, sliders and springs and MoorDyn allows simulating moored structures. Both functionalities make DualSPHysics one of the meshless model world leaders in the simulation of offshore energy harvesting devices. Lately, the present state of maturity of the solver goes beyond single phase simulations, allowing multi-phase simulations with gas-liquid and a combination of Newtonian and non-Newtonian models expanding further the capabilities and range of applications for the DualSPHysics solver. These advances and functionalities make DualSPHysics a state-of-the-art meshless solver with emphasis on free-surface flow modelling.

Multi-phase SPH model for simulation of erosion and scouring by means of the shields and Drucker–Prager criteria.

Abstract: A two-phase numerical model using Smoothed Particle Hydrodynamics (SPH) is developed to model the scouring of two-phase liquid-sediments flows with large deformation. The rheology of sediment scouring due to flows with slow kinematics and high shear forces presents a challenge in terms of spurious numerical fluctuations. This paper bridges the gap between the non-Newtonian and Newtonian flows by proposing a model that combines the yielding, shear and suspension layer mechanics which are needed to predict accurately the local erosion phenomena. A critical bed-mobility condition based on the Shields criterion is imposed to the particles located at the sediment surface. Thus, the onset of the erosion process is independent on the pressure field and eliminates the numerical problem of pressure dependant erosion at the interface. This is combined with the Drucker–Prager yield criterion to predict the onset of yielding of the sediment surface and a concentration suspension model. The multi-phase model has been implemented in the open-source DualSPHysics code accelerated with a graphics processing unit (GPU). The multi-phase model has been compared with 2-D reference numerical models and new experimental data for scour with convergent results. Numerical results for a dry-bed dam break over an erodible bed shows improved agreement with experimental scour and water surface profiles compared to well-known SPH multi-phase models.

An Eulerian-Lagrangian incompressible SPH formulation (ELI-SPH) connected with a sharp interface

Abstract: An Eulerian–Lagrangian incompressible SPH (ELI-SPH) formulation is proposed that improves accuracy over a fully Lagrangian formulation for many problems. This develops the original formulation of Lind and Stansby (2016) by providing a sharp interface rather than a transition zone. This is generally convenient and avoids any need for Arbitrary Lagrangian Eulerian (ALE) particle mass correction. It also enables a simple, accurate solid boundary condition in a Lagrangian formulation by having the interface close to the solid boundary with an Eulerian fluid domain typically three particles thick with mirror particles. Particle regularisation is necessary in a Lagrangian domain and we apply a general form based on Fick’s shifting which is modified at the interface by ignoring Eulerian particles and using mirror particles to give zero concentration gradient and hence zero shifting across the interface, avoiding spurious migration. Continuity is enforced at the interface as part of the combined Eulerian–Lagrangian domain. The formulation is validated against the analytical solution for Taylor–Green vortices, vortex spin down in a box, and propagating waves. The use of mixed kernel order in the Eulerian domain is also demonstrated.

DualSPHysics: Open-source parallel CFD solver based on Smoothed Particle Hydrodynamics (SPH)

Abstract: DualSPHysics is a hardware accelerated Smoothed Particle Hydrodynamics code developed to solve free-surface flow problems. DualSPHysics is an open-source code developed and released under the terms of GNU General Public License (GPLv3). Along with the source code, a complete documentation that makes easy the compilation and execution of the source files is also distributed. The code has been shown to be efficient and reliable. The parallel power computing of Graphics Computing Units (GPUs) is used to accelerate DualSPHysics by up to two orders of magnitude compared to the performance of the serial version. Program summary Program title: DualSPHysics Catalogue identifier: AEUS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEUS_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: GNU General Public License No. of lines in distributed program, including test data, etc.: 121,399 No. of bytes in distributed program, including test data, etc.: 12,324,308 Distribution format: tar.gz Programming language: C++ and CUDA. Computer: Tested on CPU Intel X5500 and GPUs: GTX 480, GTX 680, Tesla K20 and GTX Titan. Operating system: Any system with a C++ and NVCC compiler, tested on Linux distribution Centos 6.5 CUDA: Tested on versions 4.0, 4.1, 4.2, 5.0 and 5.5 with driver version 331.38. Has the code been vectorised or parallelised?: Different threads of CPU or number of cores of GPU. RAM: Tens of MB to several GB, depending on problem Classification: 4.12. Nature of problem: The DualSPHysics code has been developed to study free-surface flows requiring high computational cost. Solution method: DualSPHysics is an implementation of Smoothed Particle Hydrodynamics, which is a Lagrangian meshless particle method. Running time: 6 h on 8 processors of Intel X5500 (15 min on GTX Titan) for the dam-break case with 1 million particles simulating 1.5 s of physical time (more than 26,000 steps).

Smoothed particle hydrodynamics (SPH) for free-surface flows: past, present and future

Abstract: This paper assesses some recent trends in the novel numerical meshless method smoothed particle hydrodynamics, with particular focus on its potential use in modelling free-surface flows. Due to its Lagrangian nature, smoothed particle hydrodynamics (SPH) appears to be effective in solving diverse fluid-dynamic problems with highly nonlinear deformation such as wave breaking and impact, multi-phase mixing processes, jet impact, sloshing, flooding and tsunami inundation, and fluid–structure interactions. The paper considers the key areas of rapid progress and development, including the numerical formulations, SPH operators, remedies to problems within the classical formulations, novel methodologies to improve the stability and robustness of the method, boundary conditions, multi-fluid approaches, particle adaptivity, and hardware acceleration. The key ongoing challenges in SPH that must be addressed by academic research and industrial users are identified and discussed. Finally, a roadmap is propose...

On the state-of-the-art of particle methods for coastal and ocean engineering

Abstract: The article aims at providing an up-to-date review on several latest advancements related to particle methods with applications in coastal and ocean engineering. The latest advancements corresponding to accuracy, stability, conservation properties, multiphase multi-physics multi-scale simulations, fluid-structure interactions, exclusive coastal/ocean engineering applications and computational efficiency are reviewed. The future perspectives for further enhancement of applicability and reliability of particle methods for coastal/ocean engineering applications are also highlighted.

Particle methods in ocean and coastal engineering

Abstract: This paper aims at providing a state-of-the-art review on the applications of particle methods in hydrodynamics-related problems in ocean and coastal engineering. The problems are placed into three categories according to their physical characteristics, namely, wave hydrodynamics and corresponding mass (air, oil, etc.) transport, wave-structure interaction, and wave-current-sediment interaction. For the first category, particle-based simulations of wave generation, propagation, breaking, as well as the associated turbulence production and dissipation, air entrainment, and mass transport, are reviewed. For wave-structure interaction, extensive structural types are considered that include fixed and moving (floating) structures, rigid and deformable structures, impermeable and porous structures, etc. For the third category, the latest advances of particle methods in wave/current interaction with sediments, i.e., sediment transport and coastal morphological changes, are outlined. This article also reviews the latest developments of particle methods with respect to enhancement of numerical stability, accuracy, efficiency and consistency in order to handle the multi-physics and multi-scale problems emerging from coastal and ocean engineering practices. Finally, the future perspectives of extending particle methods to a wider range of ocean and coastal engineering applications are highlighted.

Modelling multi-phase liquid-sediment scour and resuspension induced by rapid flows using Smoothed Particle Hydrodynamics (SPH) accelerated with a Graphics Processing Unit (GPU)

Abstract: A two-phase numerical model using Smoothed Particle Hydrodynamics (SPH) is applied to two-phase liquid-sediments flows. The absence of a mesh in SPH is ideal for interfacial and highly non-linear flows with changing fragmentation of the interface, mixing and resuspension. The rheology of sediment induced under rapid flows undergoes several states which are only partially described by previous research in SPH. This paper attempts to bridge the gap between the geotechnics, non-Newtonian and Newtonian flows by proposing a model that combines the yielding, shear and suspension layer which are needed to predict accurately the global erosion phenomena, from a hydrodynamics prospective. The numerical SPH scheme is based on the explicit treatment of both phases using Newtonian and the non-Newtonian Bingham-type Herschel-Bulkley-Papanastasiou constitutive model. This is supplemented by the Drucker-Prager yield criterion to predict the onset of yielding of the sediment surface and a concentration suspension model. The multi-phase model has been compared with experimental and 2-D reference numerical models for scour following a dry-bed dam break yielding satisfactory results and improvements over well-known SPH multi-phase models. With 3-D simulations requiring a large number of particles, the code is accelerated with a graphics processing unit (GPU) in the open-source DualSPHysics code. The implementation and optimisation of the code achieved a speed up of x58 over an optimised single thread serial code. A 3-D dam break over a non-cohesive erodible bed simulation with over 4 million particles yields close agreement with experimental scour and water surface profiles.