44,906 research outputs found
Interface Tracking and Solid-Fluid Coupling Techniques with Coastal Engineering Applications
Multi-material physics arise in an innumerable amount of engineering problems. A broadly
scoped numerical model is developed and described in this thesis to simulate the dynamic interaction
of multi-fluid and solid systems. It is particularly aimed at modelling the interaction
of two immiscible fluids with solid structures in a coastal engineering context; however it can
be extended to other similar areas of research. The Navier Stokes equations governing the
fluids are solved using a combination of finite element (FEM) and control volume finite element
(CVFE) discretisations. The sharp interface between the fluids is obtained through the
compressive transport of material properties (e.g. material concentration). This behaviour is
achieved through the CVFE method and a conveniently limited flux calculation scheme based
on the Hyper-C method by Leonard (1991). Analytical and validation test cases are provided,
consisting of steady and unsteady flows. To further enhance the method, improve accuracy, and
exploit Lagrangian benefits, a novel moving mesh method is also introduced and tested. It is
essentially an Arbitrary Lagrangian Eulerian method in which the grid velocity is defined by
semi-explicitly solving an iterative functional minimisation problem.
A multi-phase approach is used to introduce solid structure modelling. In this approach,
solution of the velocity field for the fluid phase is obtained using Model B as explained by
Gidaspow (1994, page 151). Interaction between the fluid phase and the solids is achieved
through the means of a source term included in the fluid momentum equations. The interacting
force is calculated through integration of this source term and adding a buoyancy contribution.
The resulting force is passed to an external solid-dynamics model such as the Discrete Element
Method (DEM), or the combined Finite Discrete Element Method (FEMDEM).
The versatility and novelty of this combined modelling approach stems from its ability to
capture the fluid interaction with particles of random size and shape. Each of the three main
components of this thesis: the advection scheme, the moving mesh method, and the solid interaction
are individually validated, and examples of randomly shaped and sized particles are
shown. To conclude the work, the methods are combined together in the context of coastal engineering
applications, where the complex coupled problem of waves impacting on breakwater
amour units is chosen to demonstrate the simulation possibilities. The three components developed
in this thesis significantly extend the application range of already powerful tools, such
as Fluidity, for fluids-modelling and finite discrete element solids-modelling tools by bringing
them together for the first time
Computational analysis of performance deterioration of a wind turbine blade strip subjected to environmental erosion
Wind-turbine blade rain and sand erosion, over long periods of time, can degrade the aerodynamic performance and therefore the power production. Computational analysis of the erosion can help engineers have a better understanding of the maintenance and protection requirements. We present an integrated method for this class of computational analysis. The main components of the method are the streamline-upwind/Petrov–Galerkin (SUPG) and pressure-stabilizing/Petrov–Galerkin (PSPG) stabilizations, a finite element particle-cloud tracking method, an erosion model based on two time scales, and the solid-extension mesh moving technique (SEMMT). The turbulent-flow nature of the analysis is handled with a Reynolds-averaged Navier–Stokes model and SUPG/PSPG stabilization, the particle-cloud trajectories are calculated based on the computed flow field and closure models defined for the turbulent dispersion of particles, and one-way dependence is assumed between the flow and particle dynamics. Because the geometry update due to the erosion has a very long time scale compared to the fluid–particle dynamics, the update takes place in a sequence of “evolution steps” representing the impact of the erosion. A scale-up factor, calculated in different ways depending on the update threshold criterion, relates the erosions and particle counts in the evolution steps to those in the fluid–particle simulation. As the blade geometry evolves, the mesh is updated with the SEMMT. We present computational analysis of rain and sand erosion for a wind-turbine blade strip, including a case with actual rainfall data and experimental aerodynamic data for eroded airfoil geometries
Spectral/hp element methods for plane Newtonian extrudate swell
Spectral/hp element methods and an arbitrary Lagrangian-Eulerian (ALE)
moving-boundary technique are used to investigate planar Newtonian extrudate
swell. Newtonian extrudate swell arises when viscous liquids exit long die
slits. The problem is characterised by a stress singularity at the end of the
slit which is inherently difficult to capture and strongly influences the
predicted swelling of the fluid. The impact of inertia (0 <Re < 100) and slip
along the die wall on the free surface profile and the velocity and pressure
values in the domain and around the singularity are investigated. The high
order method is shown to provide high resolution of the steep pressure profile
at the singularity. The swelling ratio and exit pressure loss are compared with
existing results in the literature and the ability of high-order methods to
capture these values using significantly fewer degrees of freedom is
demonstrated
A full Eulerian finite difference approach for solving fluid-structure coupling problems
A new simulation method for solving fluid-structure coupling problems has
been developed. All the basic equations are numerically solved on a fixed
Cartesian grid using a finite difference scheme. A volume-of-fluid formulation
(Hirt and Nichols (1981, J. Comput. Phys., 39, 201)), which has been widely
used for multiphase flow simulations, is applied to describing the
multi-component geometry. The temporal change in the solid deformation is
described in the Eulerian frame by updating a left Cauchy-Green deformation
tensor, which is used to express constitutive equations for nonlinear
Mooney-Rivlin materials. In this paper, various verifications and validations
of the present full Eulerian method, which solves the fluid and solid motions
on a fixed grid, are demonstrated, and the numerical accuracy involved in the
fluid-structure coupling problems is examined.Comment: 38 pages, 27 figures, accepted for publication in J. Comput. Phy
Adaptive finite element simulation of three-dimensional surface tension dominated free-surface flow problems
An arbitrary Lagrangian--Eulerian finite element method is described for the solution of time-dependent, three-dimensional, free-surface flow problems. Many flows of practical significance involve contact lines, where the free surface meets a solid boundary. This contact line may be pinned to a particular part of the solid but is more typically free to slide in a manner that is characterised by the dynamic contact angle formed by the fluid. We focus on the latter case and use a model that admits spatial variation of the contact angle: thus permitting variable wetting properties to be simulated.
The problems are driven by the motion of the fluid free surface (under the action of surface tension and external forces such as gravity) hence the geometry evolves as part of the solution, and mesh adaptivity is required to maintain the quality of the computational mesh for the physical domain. Continuous mesh adaptivity, in the form of a pseudo-elastic mesh movement scheme, is used to move the interior mesh nodes in response to the motion of the fluid's free surface. Periodic, discrete remeshing stages are also used for cases in which the fluid volume has grown, or is sufficiently distorted, by the free-surface motion. Examples are given of a droplet sliding on an inclined uniform plane and of a droplet spreading on a surface with variable wetting properties
Diagnosing numerical Cherenkov instabilities in relativistic plasma simulations based on general meshes
Numerical Cherenkov radiation (NCR) or instability is a detrimental effect
frequently found in electromagnetic particle-in-cell (EM-PIC) simulations
involving relativistic plasma beams. NCR is caused by spurious coupling between
electromagnetic-field modes and multiple beam resonances. This coupling may
result from the slow down of poorly-resolved waves due to numerical (grid)
dispersion and from aliasing mechanisms. NCR has been studied in the past for
finite-difference-based EM-PIC algorithms on regular (structured) meshes with
rectangular elements. In this work, we extend the analysis of NCR to
finite-element-based EM-PIC algorithms implemented on unstructured meshes. The
influence of different mesh element shapes and mesh layouts on NCR is studied.
Analytic predictions are compared against results from finite-element-based
EM-PIC simulations of relativistic plasma beams on various mesh types.Comment: 31 pages, 20 figure
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