183 research outputs found
Turbulent Details Simulation for SPH Fluids via Vorticity Refinement
A major issue in Smoothed Particle Hydrodynamics (SPH) approaches is the
numerical dissipation during the projection process, especially under coarse
discretizations. High-frequency details, such as turbulence and vortices, are
smoothed out, leading to unrealistic results. To address this issue, we
introduce a Vorticity Refinement (VR) solver for SPH fluids with negligible
computational overhead. In this method, the numerical dissipation of the
vorticity field is recovered by the difference between the theoretical and the
actual vorticity, so as to enhance turbulence details. Instead of solving the
Biot-Savart integrals, a stream function, which is easier and more efficient to
solve, is used to relate the vorticity field to the velocity field. We obtain
turbulence effects of different intensity levels by changing an adjustable
parameter. Since the vorticity field is enhanced according to the curl field,
our method can not only amplify existing vortices, but also capture additional
turbulence. Our VR solver is straightforward to implement and can be easily
integrated into existing SPH methods
A SPH model for incompressible turbulence
A coarse-grained particle model for incompressible Navier-Stokes (NS)
equation is proposed based on spatial filtering by utilizing smoothed particle
hydrodynamics (SPH) approximations. This model is similar to our previous
developed SPH discretization of NS equation ({\it Hu X.Y. & N.A. Adams, J.
Comput. Physics}, 227: 264-278, 2007 and 228: 2082-2091, 2009) and the
Lagrangian averaged NS (LANS-) turbulence model. Other than using
smoothing approaches, this model obtains particle transport velocity by
imposing constant which is associated with the particle density, and
is called SPH- model. Numerical tests on two-dimensional decay and
forced turbulences with high Reynolds number suggest that the model is able to
reproduce both the inverse energy cascade and direct enstrophy cascade of the
kinetic energy spectrum, the time scaling of enstrophy decay and the
non-Guassian probability density function (PDF) of particle acceleration.Comment: 23 pages. 7 figure
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Study on Actuator Line Modeling of Two NREL 5-MW Wind Turbine Wakes
The wind turbine wakes impact the efficiency and lifespan of the wind farm. Therefore, to improve the wind plant performance, research on wind plant control is essential. The actuator line model (ALM) is proposed to simulate the wind turbine efficiently. This research investigates the National Renewable Energy Laboratory 5 Million Watts (NREL 5-MW) wind turbine wakes with Open Field Operation and Manipulation (OpenFOAM) using ALM. Firstly, a single NREL 5-MW turbine is simulated. The comparison of the power and thrust with Fatigue, Aerodynamics, Structures, and Turbulence (FAST) shows a good agreement below the rated wind speed. The information relating to wind turbine wakes is given in detail. The top working status is proved at the wind speed of 8 m/s and the downstream distance of more than 5 rotor diameters (5D). Secondly, another case with two NREL 5-MW wind turbines aligned is also carried out, in which 7D is validated as the optimum distance between the two turbines. The result also shows that the upstream wind turbine has an obvious influence on the downstream one
GIZMO: A New Class of Accurate, Mesh-Free Hydrodynamic Simulation Methods
We present two new Lagrangian methods for hydrodynamics, in a systematic
comparison with moving-mesh, SPH, and stationary (non-moving) grid methods. The
new methods are designed to simultaneously capture advantages of both
smoothed-particle hydrodynamics (SPH) and grid-based/adaptive mesh refinement
(AMR) schemes. They are based on a kernel discretization of the volume coupled
to a high-order matrix gradient estimator and a Riemann solver acting over the
volume 'overlap.' We implement and test a parallel, second-order version of the
method with self-gravity & cosmological integration, in the code GIZMO: this
maintains exact mass, energy and momentum conservation; exhibits superior
angular momentum conservation compared to all other methods we study; does not
require 'artificial diffusion' terms; and allows the fluid elements to move
with the flow so resolution is automatically adaptive. We consider a large
suite of test problems, and find that on all problems the new methods appear
competitive with moving-mesh schemes, with some advantages (particularly in
angular momentum conservation), at the cost of enhanced noise. The new methods
have many advantages vs. SPH: proper convergence, good capturing of
fluid-mixing instabilities, dramatically reduced 'particle noise' & numerical
viscosity, more accurate sub-sonic flow evolution, & sharp shock-capturing.
Advantages vs. non-moving meshes include: automatic adaptivity, dramatically
reduced advection errors & numerical overmixing, velocity-independent errors,
accurate coupling to gravity, good angular momentum conservation and
elimination of 'grid alignment' effects. We can, for example, follow hundreds
of orbits of gaseous disks, while AMR and SPH methods break down in a few
orbits. However, fixed meshes minimize 'grid noise.' These differences are
important for a range of astrophysical problems.Comment: 57 pages, 33 figures. MNRAS. A public version of the GIZMO code,
user's guide, test problem setups, and movies are available at
http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.htm
Numerical modelling of local scour with computational methods
Evaluating bed morphological evolution (specifically the scoured bed level) accurately using computational modelling is critical for analyses
of the stability of many marine and coastal structures, such as piers, groynes, breakwaters, submarine pipelines and even telecommunication cables.
This thesis considers the coupled hydrodynamic and morphodynamic modelling of the local scour around hydraulic structures, such as near
a vertical pile or near a horizontal pipe. The focus in this study is on applying a fluid-structure interaction (FSI) approach to simulate the
morphodynamical behaviour of the bed deformation, replacing the structural (i.e. solid mechanics) equation by the sediment continuity equation
or Exner equation. Specifically, this works presents a novel method of mesh movement with anisotropic mesh adaptivity based on optimization
for simulating local scour near structures with discontinuous Garlerkin (DG) discretisation methods for solving the flow field. Amongst the
other goals of this work is the validation of the proposed procedure with previously performed laboratory as well as two- and three-dimensional
numerical experiments.
Additionally, performance is considered using an implementation of the methodology within Fluidity (http://fluidityproject.github.io/),
an open-source, multi-physics, computational fluid dynamics (CFD) code, capable of handling arbitrary multi-scale unstructured tetrahedral meshes
and including algorithms to perform dynamic anisotropic mesh adaptivity and mesh movement. The flexibility over mesh structure and
resolution that these optimisation capabilities provide makes it potentially highly suitable for accounting the extreme bed morphological evolution close to a fixed solid structure under the action of hydrodynamics.
Galerkin-based finite element methods have been used for the hydrodynamics (including discontinuous Galerkin discretisations) and morphological calculations, and automatic mesh deformation has been utilised to account for bed evolution changes while preserving the validity and quality of the mesh.
Finally, the work extends the scope in regards of computational methods and considers scour modelling with pure Lagrangian and meshless
methods such as smoothed particle hydrodynamics (SPH), which have also become of interest in the analysis and modelling of coastal sediment
transport, particularly in scour-related processes. The SPH modelling is considered in a two-phase, flow-sediment fully Lagrangian scour simulation where the discrete-particle interaction forces between phases are resolved at the interface and continuous changes in the bed profile are obtained naturally.Open Acces
Development of the Distributed Points Method with Application to Cavitating Flow
A mesh-less method for solving incompressible, multi-phase flow problems has been developed and is discussed along with the presentation of benchmark results showing good agreement with theoretical and experimental results. Results of a systematic, parametric study of the single phase flow around a 2D circular cylinder at Reynolds numbers up to 1000 are presented and discussed. Simulation results show good agreement with experimental results. Extension of the method to deal with multiphase flow including liquid-to-vapor phase transition along with applications to cavitating flow are discussed. Insight gleaned from numerical experiments of the cavity closure problem are discussed along with recommendations for additional research. Several conclusions regarding the use of the method are made
Enhanced SPH modeling of free-surface flows with large deformations
The subject of the present thesis is the development of a numerical solver to
study the violent interaction of marine flows with rigid structures.
Among the many numerical models available, the Smoothed Particle
Hydrodynamics (SPH) has been chosen as it proved
appropriate in dealing with violent free-surface flows. Due to its
Lagrangian and meshless character it can naturally handle breaking waves and
fragmentation that generally are not easily treated by standard methods. On
the other hand, some consolidated features of mesh-based methods, such as
the solid boundary treatment, still remain unsolved issues in the SPH
context.
In the present work a great part of the research activity has been devoted
to tackle some of the bottlenecks of the method. Firstly, an enhanced SPH
model, called delta-SPH, has been proposed. In this model, a proper numerical diffusive
term has been added in the continuity equation in order to remove the spurious
numerical noise in the pressure field which typically affects the weakly-compressible SPH
models. Then, particular attention has been paid to the development of suitable
techniques for the enforcement of the boundary conditions. As for the free-surface, a
specific algorithm has been designed to detect free-surface particles and
to define a related level-set function with two main targets: to allow the
imposition of peculiar conditions on the free-surface and to analyse and
visualize more easily the simulation outcome (especially in 3D cases).
Concerning the solid boundary treatment, much effort has been spent to
devise new techniques for handling generic body geometries with an adequate
accuracy in both 2D and 3D problems. Two different techniques have been
described: in the first one the standard ghost fluid method has been
extended in order to treat complex solid geometries. Both free-slip and
no-slip boundary conditions have been implemented, the latter being a quite
complex matter in the SPH context. The proposed boundary treatment proved
to be robust and accurate in evaluating local and global loads, though it
is not easy to extend to generic 3D surfaces.
The second technique has been adopted for these cases.
Such a technique has been developed in the context of Riemann-SPH methods
and in the present work is reformulated in the context of the standard SPH scheme.
The method proved to be robust in treating complex 3D
solid surfaces though less accurate than the former.
Finally, an algorithm to correctly initialize the SPH simulation in the case of generic
geometries has been described. It forces a resettlement of the fluid particles
to achieve a regular and uniform spacing even in complex configurations. This
pre-processing procedure avoids the generation of spurious currents due to
local defects in the particle distribution at the beginning of the simulation.
The delta-SPH model has been validated against several problems
concerning fluid-structure interactions. Firstly, the capability of the
solver in dealing with water impacts has been tested by simulating a
jet impinging on a flat plate and a dam-break flow against a vertical
wall. In this cases, the accuracy in the prediction of local loads and of
the pressure field have been the main focus. Then, the viscous flow around
a cylinder, in both steady and unsteady conditions, has been simulated
comparing the results with reference solutions. Finally, the generation
and propagation of 2D gravity waves has been simulated. Several
regimes of propagation have been tested and the results
compared against a potential flow solver.
The developed numerical solver has been applied to several cases of
free-surface flows striking rigid structures and to the problem of the
generation and evolution of ship generated waves. In the former case, the
robustness of the solver has been challenged by simulating 2D and 3D water impacts
against complex solid surfaces. The numerical outcome have been compared
with analytical solutions, experimental data and other numerical results
and the limits of the model have been discussed.
As for the ship generated waves, the problem has been firstly studied
within the 2D+t approximation, focusing
on the occurrence and features of the breaking bow waves. Then, a
dedicated 3D SPH parallel solver has been developed to tackle the simulation
of the entire ship in constant forward motion. This simulation is quite demanding in
terms of complexities of the boundary geometry and computational resources
required. The wave pattern obtained has been compared against experimental
data and results from other numerical methods, showing in both the cases a fair
and promising agreement
Development of the Distributed Points Method with Application to Cavitating Flow
A mesh-less method for solving incompressible, multi-phase flow problems has been developed and is discussed along with the presentation of benchmark results showing good agreement with theoretical and experimental results. Results of a systematic, parametric study of the single phase flow around a 2D circular cylinder at Reynolds numbers up to 1000 are presented and discussed. Simulation results show good agreement with experimental results. Extension of the method to deal with multiphase flow including liquid-to-vapor phase transition along with applications to cavitating flow are discussed. Insight gleaned from numerical experiments of the cavity closure problem are discussed along with recommendations for additional research. Several conclusions regarding the use of the method are made
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