51,484 research outputs found
Three-boson problem at low energy and Implications for dilute Bose-Einstein condensates
It is shown that the effective interaction strength of three bosons at small
collision energies can be extracted from their wave function at zero energy. An
asymptotic expansion of this wave function at large interparticle distances is
derived, from which is defined a quantity named three-body scattering
hypervolume, which is an analog of the two-body scattering length. Given any
finite-range interaction potentials, one can thus predict the effective
three-body force from a numerical solution of the Schr\"{o}dinger equation. In
this way the constant for hard-sphere bosons is computed, leading to the
complete result for the ground state energy per particle of a dilute
Bose-Einstein condensate (BEC) of hard spheres to order , where
is the number density. Effects of are also demonstrated in the three-body
energy in a finite box of size , which is expanded to the order ,
and in the three-body scattering amplitude in vacuum. Another key prediction is
that there is a violation of the effective field theory (EFT) in the condensate
fraction in dilute BECs, caused by short-range physics. EFT predictions for the
ground state energy and few-body scattering amplitudes, however, are
corroborated.Comment: 24 pages, no figur
Simulation of Unsteady Aerdynamic Load for Rigid Coaxial Rotor in Forward Flight with Vortex Particle Method
Co-axial rotor systems are frequently used for high-speed helicopters. Nevertheless, issues related to rotor-head drag,
aerodynamic performance and vibration should also be considered. Simulating the unsteady aerodynamic loads for a rigid
coaxial rotor, including the aerodynamic interactions between rotors and rotor blades, is an essential part of analyzing
their vibration characteristics. In this paper, an unsteady aerodynamic analysis based on the vortex-lattice method is
presented. In this method, a reversed flow model on the retreating side of the coaxial rotor is proposed based on the
unsteady panel method. To account for reversed flow, shedding a vortex from the leading-edge is used rather than from
the trailing-edge. Moreover, vortex-blade aerodynamic interactions are modelled. The model considers the unsteady
pressure term induced on a blade by tip vortices of other blades, and thus accounts for the aerodynamic interaction
between the rotors and its contribution to the unsteady airloads. Coupling the reversed flow model and the vortex-blade
aerodynamic interaction model with a viscous vortex particle method is used to simulate the complex wake of the coaxial
rotor, closing the loop in modelling aerodynamic interactions of coaxial rotors. Following this, the unsteady aerodynamic
loads on the X2 coaxial rotor are simulated in forward flight, and compared with the results of PRASADUM (Parallelized
Rotorcraft Analysis for Simulation And Design, developed at the University of Maryland) and CFD/CSD computations with
the OVERFLOW and the CREATE-AV Helios tools. The results of the present method agree with the results of the
CFD/CSD method, and compare better than the PRASADUM solutions. Furthermore, the influence of the aerodynamic
interaction between the coaxial rotors on the unsteady airloads, frequency, wake structure, induced flow and force
distributions are analyzed. Additionally, the results are also compared against computation for a single rotor case,
simulated at similar conditions as the coaxial rotor. It is shown that the effect of tip vortex interaction plays a significant role
in unsteady airloads of coaxial rotors at low-speeds, while the rotor blade passing effect is obvious strengthened at
high-speed
Evaluating the application of neural networks and fundamental analysis in the Australian Stockmarket
Magnetic monopole loop for the Yang-Mills instanton
We investigate 't Hooft-Mandelstam monopoles in QCD in the presence of a
single classical instanton configuration. The solution to the Maximal Abelian
projection is found to be a circular monopole trajectory with radius
centered on the instanton. At zero loop radius, there is a marginally stable
(or flat) direction for loop formation to . We argue that loops
will form, in the semi-classical limit, due to small perturbations such as the
dipole interaction between instanton anti-instanton pairs. As the instanton gas
becomes a liquid, the percolation of the monopole loops may therefore provide a
semi-classical precursor to the confinement mechanism.Comment: 19 pages, ReVTeX, 5 Encaptulated Postscript figure
Joint perception: gaze and beliefs about social context
The way that we look at images is influenced by social context. Previously we demonstrated this phenomenon of joint perception. If lone participants believed that an unseen other person was also looking at the images they saw, it shifted the balance of their gaze between negative and positive images. The direction of this shift depended upon whether participants thought that later they would be compared against the other person or would be collaborating with them. Here we examined whether the joint perception is caused by beliefs about shared experience (looking at the same images) or beliefs about joint action (being engaged in the same task with the images). We place our results in the context of the emerging field of joint action, and discuss their connection to notions of group emotion and situated cognition. Such findings reveal the persuasive and subtle effect of social context upon cognitive and perceptual processes
Fundamental constants and tests of theory in Rydberg states of one-electron ions
The nature of the theory of circular Rydberg states of hydrogenlike ions
allows highly-accurate predictions to be made for energy levels. In particular,
uncertainties arising from the problematic nuclear size correction which beset
low angular-momentum states are negligibly small for the high angular-momentum
states. The largest remaining source of uncertainty can be addressed with the
help of quantum electrodynamics (QED) calculations, including a new
nonperturbative result reported here. More stringent tests of theory and an
improved determination of the Rydberg constant may be possible if predictions
can be compared with precision frequency measurements in this regime. The
diversity of information can be increased by utilizing a variety of
combinations of ions and Ryberg states to determine fundamental constants and
test theory.Comment: 10 pages; LaTe
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