1,365 research outputs found
CFD predictions of near-field pressure signatures of a low-boom aircraft
A three dimensional Euler marching code has been utilized to predict near-field pressure signatures of an aircraft with low boom characteristics. Computations were extended to approximately six body lengths aft of the aircraft in order to obtain pressure data at three body lengths below the aircraft for a cruise Mach number of 1.6. The near-field pressure data were extrapolated to the ground using a Whitham based method. The distance below the aircraft where the pressure data are attained is defined in this paper as the 'separation distance.' The influences of separation distances and the still highly three-dimensional flow field on the predicted ground pressure signatures and boom loudness are presented in this paper
Robust Parameter Selection for Parallel Tempering
This paper describes an algorithm for selecting parameter values (e.g.
temperature values) at which to measure equilibrium properties with Parallel
Tempering Monte Carlo simulation. Simple approaches to choosing parameter
values can lead to poor equilibration of the simulation, especially for Ising
spin systems that undergo -order phase transitions. However, starting
from an initial set of parameter values, the careful, iterative respacing of
these values based on results with the previous set of values greatly improves
equilibration. Example spin systems presented here appear in the context of
Quantum Monte Carlo.Comment: Accepted in International Journal of Modern Physics C 2010,
http://www.worldscinet.com/ijmp
On form-preserving transformations for the time-dependent Schr\"odinger equation
In this paper we point out a close connection between the Darboux
transformation and the group of point transformations which preserve the form
of the time-dependent Schr\"odinger equation (TDSE). In our main result, we
prove that any pair of time-dependent real potentials related by a Darboux
transformation for the TDSE may be transformed by a suitable point
transformation into a pair of time-independent potentials related by a usual
Darboux transformation for the stationary Schr\"odinger equation. Thus, any
(real) potential solvable via a time-dependent Darboux transformation can
alternatively be solved by applying an appropriate form-preserving
transformation of the TDSE to a time-independent potential. The preeminent role
of the latter type of transformations in the solution of the TDSE is
illustrated with a family of quasi-exactly solvable time-dependent anharmonic
potentials.Comment: LaTeX2e (with amsmath, amssymb, amscd, cite packages), 11 page
Monosynaptic Functional Connectivity in Cerebral Cortex During Wakefulness and Under Graded Levels of Anesthesia
The balance between excitation and inhibition is considered to be of significant importance for neural computation and cognitive function. Excitatory and inhibitory functional connectivity in intact cortical neuronal networks in wakefulness and graded levels of anesthesia has not been systematically investigated. We compared monosynaptic excitatory and inhibitory spike transmission probabilities using pairwise cross-correlogram (CCG) analysis. Spikes were measured at 64 sites in the visual cortex of rats with chronically implanted microelectrode arrays during wakefulness and three levels of anesthesia produced by desflurane. Anesthesia decreased the number of active units, the number of functional connections, and the strength of excitatory connections. Connection probability (number of connections per number of active unit pairs) was unaffected until the deepest anesthesia level, at which a significant increase in the excitatory to inhibitory ratio of connection probabilities was observed. The results suggest that the excitatory–inhibitory balance is altered at an anesthetic depth associated with unconsciousness
Is my ODE a Painleve equation in disguise?
Painleve equations belong to the class y'' + a_1 {y'}^3 + 3 a_2 {y'}^2 + 3
a_3 y' + a_4 = 0, where a_i=a_i(x,y). This class of equations is invariant
under the general point transformation x=Phi(X,Y), y=Psi(X,Y) and it is
therefore very difficult to find out whether two equations in this class are
related. We describe R. Liouville's theory of invariants that can be used to
construct invariant characteristic expressions (syzygies), and in particular
present such a characterization for Painleve equations I-IV.Comment: 8 pages. Based on talks presented at NEEDS 2000, Gokova, Turkey, 29
June - 7 July, 2000, and at the AMS-HKMS joint meeting 13-16 December, 2000.
Submitted to J. Nonlin. Math. Phy
Size-scaling of clump instabilities in turbulent, feedback regulated disks
We explore the scaling between the size of star-forming clumps and rotational
support in massively star-forming galactic disks. The analysis relies on
simulations of a clumpy galaxy at and the observed DYNAMO sample of rare
clumpy analogs at to test a predictive clump size scaling
proposed by \citet{Fisher2017ApJ...839L...5F} in the context of the Violent
Disk Instability (VDI) theory. We here determine the clump sizes using a
recently presented 2-point estimator, which is robust against resolution/noise
effects, hierarchical clump substructure, clump-clump overlap and other
galactic substructure. After verifying Fisher's clump scaling relation for the
DYNAMO observations, we explore whether this relation remains characteristic of
the VDI theory, even if realistic physical processes, such as local asymetries
and stellar feedback, are included in the model. To this end, we rely on
hydrodynamic zoom-simulations of a Milky Way-mass galaxy with four different
feedback prescriptions. We find that, during its marginally stable epoch at
, this mock galaxy falls on the clump scaling relation, although its
position on this relation depends on the feedback model. This finding implies
that Toomre-like stability considerations approximately apply to large
() instabilities in marginally stable turbulent disks,
irrespective of the feedback model, but also emphasizes that the global clump
distribution of a turbulent disk depends strongly on feedback.Comment: Accepted by ApJ, no changes made. 11 pages, 4 figure
From Dimensional Reduction of 4d Spin Foam Model to Adding Non-Gravitational Fields to 3d Spin Foam Model
A Kaluza-Klein like approach for a 4d spin foam model is considered. By
applying this approach to a model based on group field theory in 4d (TOCY
model), and using the Peter-Weyl expansion of the gravitational field,
reconstruction of new non gravitational fields and interactions in the action
are found. The perturbative expansion of the partition function produces graphs
colored with su(2) algebraic data, from which one can reconstruct a 3d
simplicial complex representing space-time and its geometry; (like in the
Ponzano-Regge formulation of pure 3d quantum gravity), as well as the Feynman
graph for typical matter fields. Thus a mechanism for generation of matter and
construction of new dimensions are found from pure gravity.Comment: 11 pages, no figure, to be published in International Journal of
Geometric Methods in Modern Physic
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