112 research outputs found
A robust method for calculating interface curvature and normal vectors using an extracted local level set
The level-set method is a popular interface tracking method in two-phase flow
simulations. An often-cited reason for using it is that the method naturally
handles topological changes in the interface, e.g. merging drops, due to the
implicit formulation. It is also said that the interface curvature and normal
vectors are easily calculated. This last point is not, however, the case in the
moments during a topological change, as several authors have already pointed
out. Various methods have been employed to circumvent the problem. In this
paper, we present a new such method which retains the implicit level-set
representation of the surface and handles general interface configurations. It
is demonstrated that the method extends easily to 3D. The method is validated
on static interface configurations, and then applied to two-phase flow
simulations where the method outperforms the standard method and the results
agree well with experiments.Comment: 31 pages, 18 figure
A Volume of Fluid Method for Three Dimensional Direct Numerical Simulations of Immiscible Droplet Collisions
An advanced Volume of Fluid (VOF) method is presented that enables performant
three-dimensional Direct Numerical Simulations (DNS) of the interaction of two
immiscible fluids in a gaseous environment with large topology changes, e.g.,
binary droplet collisions. One of the challenges associated with the
introduction of a third immiscible phase into the VOF method is the
reconstruction of the phase boundaries near the triple line in arbitrary
arrangements. For this purpose, an efficient method based on a Piecewise Linear
Interface Calculation (PLIC) is shown. Moreover, the surface force modeling
with the robust Continuous Surface Stress (CSS) model was enhanced to treat
such three-phase situations with large topology changes and thin films. A
consistent scaling of the fluid properties at the interfaces ensures energy
conservation. The implementation of these methods in the multi-phase flow
solver Free Surface 3D (FS3D) allowed a successful validation. A qualitative
comparison of the morphology in binary collisions of immiscible droplets as
well as a quantitative comparison regarding the threshold velocities that
distinguish different collision regimes shows excellent agreement with
experimental results. These simulations enable the evaluation of experimentally
inaccessible data like the contributions of the kinetic, surface and
dissipative energy of both immiscible liquids during the collision process.
Furthermore, the comparison with binary collisions of the same liquids
highlights similarities and differences between the collisions. Both can
support the modeling of the immiscible liquid interaction in the future.Comment: dataset on https://doi.org/10.18419/darus-3557; Changes in v2: VOF in
title written out as Volume of Fluid, a handful of typos removed and some
sentences language polished, keywords added, new citation styl
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Moving Polygon Methods for Incompressible Fluid Dynamics
Hybrid particle-mesh numerical approaches are proposed to solve incompressible fluid flows. The methods discussed in this work consist of a collection of particles each wrapped in their own polygon mesh cell, which then move through the domain as the flow evolves. Variables such as pressure, velocity, mass, and momentum are located either on the mesh or on the particles themselves, depending on the specific algorithm described, and each will be shown to have its own advantages and disadvantages. This work explores what is required to obtain local conservation of mass, momentum, and convergence for the velocity and pressure in a particle-mesh CFD simulation method. Current particle methods are explored and analyzed for their benefits and deficiencies, and newly developed methods are described with results and analysis.
A new method for generating locally orthogonal polygonal meshes from a set of generator points is presented in which polygon areas are a constraint. The area constraint property is particularly useful for particle methods where moving polygons track a discrete portion of material. Voronoi polygon meshes have some very attractive mathematical and numerical properties for numerical computation, so a generalization of Voronoi polygon meshes is formulated that enforces a polygon area constraint. Area constrained moving polygonal meshes allow one to develop hybrid particle-mesh numerical methods that display some of the most attractive features of each approach. It is shown that this mesh construction method can continuously reconnect a moving, unstructured polygonal mesh in a pseudo-Lagrangian fashion without change in cell area/volume, and the method\u27s ability to simulate various physical scenarios is shown. The advantages are identified for incompressible fluid flow calculations, with demonstration cases that include material discontinuities of all three phases of matter and large density jumps
Numerical Modelling of Transport in Complex Porous Media: Metal Foams to the Human Lung
Transport in porous media has many practical applications in science and engineering. This work focuses on the development of numerical methods for analyzing porous media flows and uses two major applications, metal foams and the human lung, to demonstrate the capabilities of the methods. Both of these systems involve complex pore geometries and typically involve porous domains of complex shape. Such geometric complexities make the characterization of the relevant effective properties of the porous medium as well as the solution of the governing equations in conjugate fluid-porous domains challenging. In porous domains, there are typically too many individual pores to consider transport processes directly; instead the governing equations are volume-averaged to obtain a new sets of governing equations describing the conservation laws in a bulk sense. There are, however, unknown pore-level terms remaining in the volume-averaged equations that must be characterized using effective properties that account for the effects of processes at the pore level. Once closed, the volume-averaged equations can be solved numerically, however currently available numerical methods for conjugate domains do not perform well at fluid-porous interfaces when using unstructured grids.
In light of the preceding discussion, the goals of this work are: (i) to develop a finite-volume-based numerical method for solving fluid flow and non-equilibrium heat transfer problems in conjugate fluid-porous domains that is compatible with general unstructured grids, (ii) to characterize the relevant flow and thermal properties of an idealized graphite foam, (iii) to determine the permeability of an alveolated duct, which is considered as a representative element of the respiratory region of the human lung, and (iv) to conduct simulations of airflow in the human lung using a novel fluid-porous description of the domain. Results show that the numerical method that has been developed for conjugate fluid-porous systems is able to maintain accuracy on all grid types, flow directions, and flow speeds considered. This work also introduces a comprehensive set of correlations for the effective properties of graphite foam, which will be useful for studying the performance of devices incorporating this new material. In order to model air flow in the lung as a porous medium, the permeability of an alveolated duct is obtained using direct pore-level simulations. Finally, simulations of air flow in the lung are presented which use a novel fluid-porous approach wherein the upper airways are considered as a pure fluid region and the smaller airways and alveoli are considered as a porous domain
A generalised immersed boundary method for flows of dense suspension of solid particles
Immersed boundary method (IBM) provides computational advantages in approximating moving solid surfaces on fixed numerical meshes. It has been widely used for fully-resolved simulations of particulate flows. This thesis proposes a generalised formulation of IBM with improved applicability to flows with dense concentrations of particles and unstructured meshes. The new IBM formulation, which is based on the smooth-interface direct forcing approach, directly uses the algebraic discretised terms of the momentum equations in the evaluation of the forces on Lagrangian immersed boundary (IB) points, and evaluate the integral Lagrangian volumes based on these forces. Appropriate reconstructions of the boundary forces are adopted to ensure the compatibility with the momentum-weighted interpolation used for the finite-volume discretisation with a collocated mesh arrangement. A modified direct forcing formulation is also proposed, which results in an efficiency gain of a devised segregated flow-particle coupling scheme. The novel framework is applied to flows with stationary and moving IBs on both Cartesian and arbitrary triangular/tetrahedral meshes, and the results are similar or better than other related methods that are mostly developed for Cartesian meshes. Accurate and stable enforcement of the no-slip condition on the IB at every time-step is demonstrated, even for flows with strong transient behaviour and high velocity and pressure gradients. Local continuity in the vicinity of the IB is also preserved, ensuring local and global mass conservation alongside the local no-slip condition. Adaptations devised for unstructured meshes results in an accuracy close to that obtained on Cartesian meshes. The framework is successfully applied in the simulations of fluidisation of dense particle bed and a rising pack of light particles, showing robust stability. The issues related to the interfering regularised forces of different particle surfaces are not significant using the present formulation, hence eliminate unphysical flow patterns between aggregated particles.Open Acces
Stellar Wind Confinement of Evaporating Exoplanet Atmospheres and Its Signatures in 1083 nm Observations
Atmospheric escape from close-in exoplanets is thought to be crucial in
shaping observed planetary populations. Recently, significant progress has been
made in observing this process in action through excess absorption in transit
spectra and narrowband light curves. We model the escape of
initially-homogeneous planetary winds interacting with a stellar wind. The ram
pressure balance of the two winds governs this interaction. When the
impingement of the stellar wind on the planetary outflow is mild or moderate,
the planetary outflow expands nearly spherically through its sonic surface
before forming a shocked boundary layer. When the confinement is strong, the
planetary outflow is redirected into a cometary tail before it expands to its
sonic radius. The resultant transmission spectra at the He 1083 nm line are
accurately represented by a 1D spherical wind solution in cases of mild to
moderate stellar wind interaction. In cases of strong stellar wind interaction,
the degree of absorption is enhanced and the cometary tail leads to an extended
egress from transit. The crucial features of the wind--wind interaction are,
therefore, encapsulated in the light curve of He 1083 nm equivalent width as a
function of time. The possibility of extended He 1083 nm absorption well beyond
the optical transit carries important implications for planning
"out-of-transit" observations that serve as a baseline for in-transit data.Comment: Accepted for publication in AAS Journals. Associated data and
software at: https://zenodo.org/record/5750747 and
https://zenodo.org/record/575077
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