17,398 research outputs found
Central Schemes for Porous Media Flows
We are concerned with central differencing schemes for solving scalar
hyperbolic conservation laws arising in the simulation of multiphase flows in
heterogeneous porous media. We compare the Kurganov-Tadmor, 2000 semi-discrete
central scheme with the Nessyahu-Tadmor, 1990 central scheme. The KT scheme
uses more precise information about the local speeds of propagation together
with integration over nonuniform control volumes, which contain the Riemann
fans. These methods can accurately resolve sharp fronts in the fluid
saturations without introducing spurious oscillations or excessive numerical
diffusion. We first discuss the coupling of these methods with velocity fields
approximated by mixed finite elements. Then, numerical simulations are
presented for two-phase, two-dimensional flow problems in multi-scale
heterogeneous petroleum reservoirs. We find the KT scheme to be considerably
less diffusive, particularly in the presence of high permeability flow
channels, which lead to strong restrictions on the time step selection;
however, the KT scheme may produce incorrect boundary behavior
Mesonic states in the generalised Nambu-Jona-Lasinio theories
For any Nambu-Jona-Lasinio model of QCD with arbitrary nonlocal,
instantaneous, quark current-current confining kernels, we use a generalised
Bogoliubov technique to go beyond BCS level (in the large-Nc limit) so as to
explicitly build quark-antiquark compound operators for creating/annihilating
mesons. In the Hamiltonian approach, the mesonic bound-state equations appear
(from the generalised Bogoliubov transformation) as mass-gap-like equations
which, in turn, ensure the absence, in the Hamiltonian, of mesonic Bogoliubov
anomalous terms. We go further to demonstrate the one-to-one correspondence
between Hamiltonian and Bethe-Salpeter approaches to non-local NJL-type models
for QCD and give the corresponding "dictionary" necessary to "translate" the
amplitudes built using the graphical Feynman rules to the terms of the
Hamiltonian, and vice versa. We comment on the problem of multiple vacua
existence in such type of models and argue that mesonic states in the theory
should be prescribed to have an extra index - the index of the replica in which
they are created. Then the completely diagonalised Hamiltonian should contain a
sum over this new index. The method is proved to be general and valid for any
instantaneous quark kernel.Comment: LaTeX2e, uses aipproc class, Talk given at the conference "Quark
Confinement and the Hadron Spectrum VI", 21-25 September 2004, Sardinia,
Italy, to appear in Proceeding
Quantum field theory approach to the vacuum replica in QCD
Quantum field theory is used to describe the contribution of possible new QCD
vacuum replica to hadronic processes. This sigma-like new state has been
recently shown to be likely to appear for any realistic four-quark interaction
kernel as a consequence of chiral symmetry. The local operator creating the
replica vacuum state is constructed explicitly. Applications to physical
processes are outlined.Comment: LaTeX2e, 2 EPS figures, uses ws-procs9x6 (included) and epsfig
classes, Talk given at the conference "Quark Confinement and the Hadron
Spectrum V", 10-14 September 2002, Gargnano, Italy, to appear in Proceeding
DNS of Laminar to Turbulent Transition on NACA 0012 Airfoil with Sand Grain Roughness
The Lattice-Boltzmann-based solver PowerFLOW is used to perform direct numerical simulations of the transitional flow over an airfoil at Reynolds number equal to 0.657 million. The leading edge of the airfoil is covered with sand particles, represented by polyhedra, to mimic the grit used in experiments. The sensitivity of the laminar to turbulent transition to the size of these particles, grid resolution, spanwise length is evaluated and rectangular trips are also tested
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