132,549 research outputs found
VIRTUE : integrating CFD ship design
Novel ship concepts, increasing size and speed, and strong competition in the global maritime market require that a ship's hydrodynamic performance be studied at the highest level of sophistication. All hydrodynamic aspects need to be considered so as to optimize trade-offs between resistance, propulsion (and cavitation), seakeeping or manoeuvring. VIRTUE takes a holistic approach to hydrodynamic design and focuses on integrating advanced CFD tools in a software platform that can control and launch multi-objective hydrodynamic design projects. In this paper current practice, future requirements and a potential software integration platform are presented. The necessity of parametric modelling as a means of effectively generating and efficiently varying geometry, and the added-value of advanced visualization, is discussed. An illustrating example is given as a test case, a container carrier investigation, and the requirements and a proposed architecture for the platform are outlined
Digraph Complexity Measures and Applications in Formal Language Theory
We investigate structural complexity measures on digraphs, in particular the
cycle rank. This concept is intimately related to a classical topic in formal
language theory, namely the star height of regular languages. We explore this
connection, and obtain several new algorithmic insights regarding both cycle
rank and star height. Among other results, we show that computing the cycle
rank is NP-complete, even for sparse digraphs of maximum outdegree 2.
Notwithstanding, we provide both a polynomial-time approximation algorithm and
an exponential-time exact algorithm for this problem. The former algorithm
yields an O((log n)^(3/2))- approximation in polynomial time, whereas the
latter yields the optimum solution, and runs in time and space O*(1.9129^n) on
digraphs of maximum outdegree at most two. Regarding the star height problem,
we identify a subclass of the regular languages for which we can precisely
determine the computational complexity of the star height problem. Namely, the
star height problem for bideterministic languages is NP-complete, and this
holds already for binary alphabets. Then we translate the algorithmic results
concerning cycle rank to the bideterministic star height problem, thus giving a
polynomial-time approximation as well as a reasonably fast exact exponential
algorithm for bideterministic star height.Comment: 19 pages, 1 figur
No Pulsar Kicks from Deformed Neutrinospheres
In a supernova core, magnetic fields cause a directional variation of the
neutrino refractive index so that resonant flavor oscillations would lead to a
deformation of the "neutrinosphere" for, say, tau neutrinos. The associated
anisotropic neutrino emission was proposed as a possible origin of the observed
pulsar proper motions. We argue that this effect was vastly overestimated
because the variation of the temperature over the deformed neutrinosphere is
not an adequate measure for the anisotropy of neutrino emission. The neutrino
flux is generated inside the neutron star core and is transported through the
atmosphere at a constant luminosity, forcing the temperature gradient in the
atmosphere to adjust to the inflow of energy from below. Therefore, no emission
anisotropy is caused by a deformation of the neutrinosphere to lowest order. An
estimate of the higher-order corrections must take into account the modified
atmospheric temperature profile in response to the deformation of the
neutrinosphere and the corresponding feedback on the core. We go through this
exercise in the framework of a simplified model which can be solved
analytically.Comment: Final version with minor corrections, to be published in PRD.
Includes a "Note Added" in response to astro-ph/981114
Chiral Fermions on the Lattice
A recently proposed method for regularizing chiral gauge theories
non-perturbatively is discussed in detail. The result is an effective action
which can be computed from the lattice gauge field, and which is suited for
numerical simulations.Comment: Talk given by G. Schierholz at Yukawa International Seminar on
Non-Perturbative QCD: Structure of the QCD Vacuum (YKIS97), Kyoto, December
1997; typos correcte
Critical Unmixing of Polymer Solutions
We present Monte Carlo simulations of semidilute solutions of long
self-attracting chain polymers near their Ising type critical point. The
polymers are modeled as monodisperse self-avoiding walks on the simple cubic
lattice with attraction between non-bonded nearest neighbors. Chain lengths are
up to N=2048, system sizes are up to lattice sites and monomers. These simulations used the recently introduced pruned-enriched
Rosenbluth method which proved extremely efficient, together with a histogram
method for estimating finite size corrections. Our most clear result is that
chains at the critical point are Gaussian for large , having end-to-end
distances . Also the distance (where ) scales with the mean field exponent, . The critical density seems to scale with a non-trivial
exponent similar to that observed in experiments. But we argue that this is due
to large logarithmic corrections. These corrections are similar to the very
large corrections to scaling seen in recent analyses of -polymers, and
qualitatively predicted by the field theoretic renormalization group. The only
serious deviation from this simple global picture concerns the N-dependence of
the order parameter amplitudes which disagrees with a minimalistic ansatz of de
Gennes. But this might be due to problems with finite size scaling. We find
that the finite size dependence of the density of states (where is
the total energy and is the number of chains) is slightly but significantly
different from that proposed recently by several authors.Comment: minor changes; Latex, 22 pages, submitted to J. Chem. Phy
A non-chiral extension of the standard model with mirror fermions
The difficulties of defining chiral gauge theories non-perturbatively suggest
a vector-like extension of the standard model with three mirror fermion
families. Some phenomenological implications of such an extension are
discussed.Comment: latex, 6 pages, 1 figure with epsfig. Talk given at the workshop
"Beyond the Standard Model V", Balholm, Norway, May 199
Numerical simulation with light Wilson-quarks
The computational cost of numerical simulations of QCD with light dynamical
Wilson-quarks is estimated by determining the autocorrelation of various
quantities. In test runs the expected qualitative behaviour of the pion mass
and coupling at small quark masses is observed.Comment: 5 pages, 3 figures, to appear in the Proceedings of SEWM, Heidelberg,
200
Numerical simulation tests with light dynamical quarks
Two degenerate flavours of quarks are simulated with small masses down to
about one fifth of the strange quark mass by using the two-step multi-boson
(TSMB) algorithm. The lattice size is 8^3 x 16 with lattice spacing about
0.27fm which is not far from the N_t=4 thermodynamical cross-over line.
Autocorrelations of different physical quantities are estimated as a function
of the quark mass. The eigenvalue spectra of the Wilson-Dirac operator are
investigated.Comment: 14 pages, 7 figures, uses svjour.cls; mistake in the autocorrelation
of the pion mass corrected, version accepted for publication on Eur. Phys. J.
Mass estimation in the outer regions of galaxy clusters
We present a technique for estimating the mass in the outskirts of galaxy
clusters where the usual assumption of dynamical equilibrium is not valid. The
method assumes that clusters form through hierarchical clustering and requires
only galaxy redshifts and positions on the sky. We apply the method to
dissipationless cosmological N-body simulations where galaxies form and evolve
according to semi-analytic modelling. The method recovers the actual cluster
mass profile within a factor of two to several megaparsecs from the cluster
centre. This error originates from projection effects, sparse sampling, and
contamination by foreground and background galaxies. In the absence of velocity
biases, this method can provide an estimate of the mass-to-light ratio on
scales ~1-10 Mpc/h where this quantity is still poorly known.Comment: 14 pages, 7 figures, MN LaTeX style, MNRAS, in pres
The vacuum preserving Lie algebra of a classical W-algebra
We simplify and generalize an argument due to Bowcock and Watts showing that
one can associate a finite Lie algebra (the `classical vacuum preserving
algebra') containing the M\"obius subalgebra to any classical
\W-algebra. Our construction is based on a kinematical analysis of the
Poisson brackets of quasi-primary fields. In the case of the \W_\S^\G-algebra
constructed through the Drinfeld-Sokolov reduction based on an arbitrary
subalgebra of a simple Lie algebra \G, we exhibit a natural
isomorphism between this finite Lie algebra and \G whereby the M\"obius
is identified with .Comment: 11 pages, BONN-HE-93-25, DIAS-STP-93-13. Some typos had been removed,
no change in formula
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