420 research outputs found
Toward a reduction of mesh imprinting
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108641/1/fld3941.pd
Numerical simulation of spray coalescence in an eulerian framework : direct quadrature method of moments and multi-fluid method
The scope of the present study is Eulerian modeling and simulation of
polydisperse liquid sprays undergoing droplet coalescence and evaporation. The
fundamental mathematical description is the Williams spray equation governing
the joint number density function f(v, u; x, t) of droplet volume and velocity.
Eulerian multi-fluid models have already been rigorously derived from this
equation in Laurent et al. (2004). The first key feature of the paper is the
application of direct quadrature method of moments (DQMOM) introduced by
Marchisio and Fox (2005) to the Williams spray equation. Both the multi-fluid
method and DQMOM yield systems of Eulerian conservation equations with
complicated interaction terms representing coalescence. In order to validate
and compare these approaches, the chosen configuration is a self-similar 2D
axisymmetrical decelerating nozzle with sprays having various size
distributions, ranging from smooth ones up to Dirac delta functions. The second
key feature of the paper is a thorough comparison of the two approaches for
various test-cases to a reference solution obtained through a classical
stochastic Lagrangian solver. Both Eulerian models prove to describe adequately
spray coalescence and yield a very interesting alternative to the Lagrangian
solver
Implementation of the LANS-alpha turbulence model in a primitive equation ocean model
This paper presents the first numerical implementation and tests of the
Lagrangian-averaged Navier-Stokes-alpha (LANS-alpha) turbulence model in a
primitive equation ocean model. The ocean model in which we work is the Los
Alamos Parallel Ocean Program (POP); we refer to POP and our implementation of
LANS-alpha as POP-alpha. Two versions of POP-alpha are presented: the full
POP-alpha algorithm is derived from the LANS-alpha primitive equations, but
requires a nested iteration that makes it too slow for practical simulations; a
reduced POP-alpha algorithm is proposed, which lacks the nested iteration and
is two to three times faster than the full algorithm. The reduced algorithm
does not follow from a formal derivation of the LANS-alpha model equations.
Despite this, simulations of the reduced algorithm are nearly identical to the
full algorithm, as judged by globally averaged temperature and kinetic energy,
and snapshots of temperature and velocity fields. Both POP-alpha algorithms can
run stably with longer timesteps than standard POP.
Comparison of implementations of full and reduced POP-alpha algorithms are
made within an idealized test problem that captures some aspects of the
Antarctic Circumpolar Current, a problem in which baroclinic instability is
prominent. Both POP-alpha algorithms produce statistics that resemble
higher-resolution simulations of standard POP.
A linear stability analysis shows that both the full and reduced POP-alpha
algorithms benefit from the way the LANS-alpha equations take into account the
effects of the small scales on the large. Both algorithms (1) are stable; (2)
make the Rossby Radius effectively larger; and (3) slow down Rossby and gravity
waves.Comment: Submitted to J. Computational Physics March 21, 200
Some considerations for high‐order ‘incremental remap’‐based transport schemes: edges, reconstructions, and area integration
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96654/1/fld3703.pd
Formation of Subtropical Mode Water in a high-resolution ocean simulation of the Kuroshio Extension region
Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Ocean Modelling 17 (2007): 338-356, doi:10.1016/j.ocemod.2007.03.002.A high-resolution numerical model is used to examine the formation and variability of the North Pacific Subtropical
ModeWater (STMW) over a 3-year period. The STMW distribution is found to be highly variable in both
space and time, a characteristic often unexplored because of sparse observations or the use of coarse resolution
simulations. Its distribution is highly dependent on eddies, and where it was renewed during the previous winter.
Although the potential vorticity fluxes associated with down-front winds can be of the same order of magnitude
or even greater than the diabatic ones due to air-sea temperature differences, the latter dominate the potential
vorticity budget on regional and larger scales. Air-sea fluxes, however, are dominated by a few strong wind events,
emphasizing the importance of short time scales in the formation of mode waters. In the Kuroshio Extension
region, both advection and mixing play important roles to remove the STMW from the formation region.This work was sponsored by the National Science Foundation OCE-0220161 (S.J.) and OCE-0221781/0549225 (J.M.), the Office of Naval Research (J.M., M.M.), Department of Energy/CCPP (M.M.), and the Office of Science (BER), US Department of Energy, Grant No. DE-FG02-05ER64119 (J.M.)
Ignition and combustion development for high speed direct injection diesel engines under low temperature cold start conditions
Diesel engine cold start is an important issue for current technology at low (below 0 °C) temperatures and for future applications. The aim of this work is to develop a description of how, when and where does fuel spray ignition occur in a glow-plug assisted engine under simulated low temperature cold start conditions. In-cylinder pressure analysis is combined with high speed visualization in an optical engine. A pilot plus main injection strategy is used. Visualization results show that pilot ignition occurs in the vicinity of the glow plug, and strongly influences main combustion initiation. Main combustion starts from the pilot flame, and propagates to the rest of the combustion chamber showing a strong visible reaction zone. After end of main injection, the rapid leaning of the mixture suppresses the strong radiation, and OH radiation is observed to progress to the rest of the combustion chamber. The combustion process shows a strong scattering, which has been quantified by combustion parameters. At higher rail pressures scattering increases, which eventually inhibits combustion initiation. However, if ignition occurs at higher rail pressures, cycle performance is better.Authors thank the Spanish Ministry of Innovation and Science for the financial support through the project OPTICOMB (reference code: TRA2007-67961-C03-C01). Authors also thank Daniel Lerida Sanchez de las Heras for his outstanding work in the facility set-up and adaptation and for his support during the tests.Pastor Soriano, JV.; García Oliver, JM.; Pastor Enguídanos, JM.; Ramírez Hernández, JG. (2011). Ignition and combustion development for high speed direct injection diesel engines under low temperature cold start conditions. Fuel. 90(4):1556-1566. https://doi.org/10.1016/j.fuel.2011.01.008S1556156690
Investigation of the ignition and combustion processes of a dual-fuel spray under diesel-like conditions using computational fluid dynamics (CFD) modeling
Recent research activities in the field of diesel engines have shown the potential to reduce
pollutant emissions and improve the thermal efficiency by controlling the fuel reactivity.
However, understanding the impact of blending fuels with different physical and especially
chemical properties on diesel-like spray mixing and combustion processes is still a
challenge. Since the experimental techniques are still far from providing detailed temporal
and spatial information about local spray conditions, computational fluid dynamics
(CFD) modeling tools have become the key source of information for investigating the
characteristics of these dual-fuel sprays. In this frame, the present research focuses on
modeling a dual-fuel spray in diesel-like conditions, comparing different gasoline and
diesel blends in terms of ignition characteristics and flame structure. The results confirm
the suitability of the state of the art computational CFD modeling tools for reproducing
the complex phenomena associated to dual-fuel sprays. Moreover, the important benefits
provided by dual-fuel blends, considering the expected reduction in pollutant emissions as
a consequence of the differences observed in terms of flame structure, are confirmed.The authors thank Dr. Jose Manuel Pastor for his support during this work and for sharing his profound knowledge and experience. Support for this research was provided by the Universitat Politecnica de Valencia inside the program Programas de Apoyo a la I + D + I, Primeros proyectos de investigacion (reference PAID-06-11 2033) and by the Ministerio de Ciencia e Innovacion inside the VeLoSoot project (TRA 2008_06448), which is gratefully acknowledged.López Sánchez, JJ.; Novella Rosa, R.; García Martínez, A.; Winklinger, JF. (2011). Investigation of the ignition and combustion processes of a dual-fuel spray under diesel-like conditions using computational fluid dynamics (CFD) modeling. Mathematical and Computer Modelling. 57:1897-1906. https://doi.org/10.1016/j.mcm.2011.12.030S189719065
Numerical wave propagation on the hexagonal C-grid
Copyright © 2008 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Journal of Computational Physics. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Computational Physics, Vol. 227, Issue 11 (2008), DOI: 10.1016/j.jcp.2008.02.010Inertio-gravity mode and Rossby mode dispersion properties are examined for discretizations of the linearized rotating shallow water equations on a regular hexagonal C-grid in planar geometry. It is shown that spurious non-zero Rossby mode frequencies found by previous authors in the f-plane case can be avoided by an appropriate discretization of the Coriolis terms. Three generalizations of this discretization that conserve energy even for non-constant Coriolis parameter are presented. A quasigeostrophic ββ-plane analysis is carried out to investigate the Rossby mode dispersion properties of these three schemes. The Rossby mode dispersion relation is found to have two branches. The primary branch modes are good approximations, in terms of both structure and frequency, to corresponding modes of the continuous governing equations, and offer some improvements over a quadrilateral C-grid scheme. The secondary branch modes have vorticity structures approximating those of small-scale modes of the continuous governing equations, suggesting that the hexagonal C-grid might have an advantage in terms of resolving extra Rossby modes; however, the frequencies of the secondary branch Rossby modes are much smaller than those of the corresponding continuous modes, so this potential advantage is not fully realized
Multi-Dimensional, Compressible Viscous Flow on a Moving Voronoi Mesh
Numerous formulations of finite volume schemes for the Euler and
Navier-Stokes equations exist, but in the majority of cases they have been
developed for structured and stationary meshes. In many applications, more
flexible mesh geometries that can dynamically adjust to the problem at hand and
move with the flow in a (quasi) Lagrangian fashion would, however, be highly
desirable, as this can allow a significant reduction of advection errors and an
accurate realization of curved and moving boundary conditions. Here we describe
a novel formulation of viscous continuum hydrodynamics that solves the
equations of motion on a Voronoi mesh created by a set of mesh-generating
points. The points can move in an arbitrary manner, but the most natural motion
is that given by the fluid velocity itself, such that the mesh dynamically
adjusts to the flow. Owing to the mathematical properties of the Voronoi
tessellation, pathological mesh-twisting effects are avoided. Our
implementation considers the full Navier-Stokes equations and has been realized
in the AREPO code both in 2D and 3D. We propose a new approach to compute
accurate viscous fluxes for a dynamic Voronoi mesh, and use this to formulate a
finite volume solver of the Navier-Stokes equations. Through a number of test
problems, including circular Couette flow and flow past a cylindrical obstacle,
we show that our new scheme combines good accuracy with geometric flexibility,
and hence promises to be competitive with other highly refined Eulerian
methods. This will in particular allow astrophysical applications of the AREPO
code where physical viscosity is important, such as in the hot plasma in galaxy
clusters, or for viscous accretion disk models.Comment: 26 pages, 21 figures. Submitted to MNRA
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