235,135 research outputs found
Phase fluctuations of s-wave superconductors on a lattice
Based on an attractive Hubbard model on a lattice with up to second
neighbor hopping we derive an effective Hamiltonian for phase fluctuations. The
superconducting gap is assumed to have s-wave symmetry. The effective
Hamiltonian we finally arrive at is of the extended XY type. While it correctly
reduces to a simple XY in the continuum limit, in the general case, it contains
higher neighbor interaction in spin space. An important feature of our
Hamiltonian is that it gives a much larger fluctuation region between the
Berezinskii-Kosterlitz-Thouless transition temperature identified with
for superconducting and the mean field transition temperature identified with
the pseudogap temperature.Comment: 3 figure
Large eddy simulation of turbulent channel flow: ILLIAC 4 calculation
The three-dimensional time dependent equations of motion were numerically integrated for fully-developed turbulent channel flow. A large scale flow field was obtained directly from the solution of these equations, and small scale field motions were simulated through an eddy viscosity model. The calculations were carried out on the ILLIAC 4 computer. The computed flow patterns show that the wall layer consists of coherent structures of low speed and high speed streaks alternating in the spanwise direction. These structures were absent in the regions away from the wall. Hot spots, small localized regions of very large turbulent shear stress, were frequently observed. The profiles of the pressure velocity-gradient correlations show a significant transfer of energy from the normal to the spanwise component of turbulent kinetic energy in the immediate neighborhood of the wall ('the splatting effect')
The structure of the vorticity field in turbulent channel flow. Part 1: Analysis of instantaneous fields and statistical correlations
An investigation into the existence of hairpin vortices in turbulent channel flow is conducted using a database generated by the large eddy simulation technique. It is shown that away from the wall the distribution of the inclination angle of vorticity vector attains its maximum at about 45 deg to the wall. Two point correlations of velocity and vorticity fluctuations strongly support a flow model consisting of vortical structures inclined at 45 deg to the wall. The instantaneous vorticity vectors plotted in planes inclined at 45 deg show that the flow contains an appreciable number of hairpins. Vortex lines are used to display the three dimensional structure of hairpins, which are shown to be generated from deformation of transverse vortex filaments
Numerical investigation of turbulent channel flow
Fully developed turbulent channel flow was simulated numerically at Reynolds number 13800, based on centerline velocity and channel halt width. The large-scale flow field was obtained by directly integrating the filtered, three dimensional, time dependent, Navier-Stokes equations. The small-scale field motions were simulated through an eddy viscosity model. The calculations were carried out on the ILLIAC IV computer with up to 516,096 grid points. The computed flow field was used to study the statistical properties of the flow as well as its time dependent features. The agreement of the computed mean velocity profile, turbulence statistics, and detailed flow structures with experimental data is good. The resolvable portion of the statistical correlations appearing in the Reynolds stress equations are calculated. Particular attention is given to the examination of the flow structure in the vicinity of the wall
The structure of the vorticity field in turbulent channel flow. Part 2: Study of ensemble-averaged fields
Several conditional sampling techniques are applied to a data base generated by large-eddy simulation of turbulent channel flow. It is shown that the bursting process is associated with well-organized horseshoe vortices inclined at about 45 deg. to the wall. These vortical structures are identified by examining the vortex lines of three-dimensional, ensemble averaged vorticity fields. Two distinct horseshoe-shaped vortices corresponding to the sweep and ejection events are detected. These vortices are associated with high Reynolds shear stress and hence make a significant contribution to turbulent energy production. The dependency of the ensemble averaged vortical structures on the detection criteria, and the question of whether this ensemble-averaged structure is an artifact of the ensemble averaging process are examined. The ensemble-averaged pattern of these vortical structures that emerge from the analysis could provide the basis for a hypothetical model of the organized structures of wall-bounded shear flows
The Backreacted K\"ahler Geometry of Wrapped Branes
For supersymmetric solutions of D3(M2) branes with AdS3(AdS2) factor, it is
known that the internal space is expressible as U(1) fibration over K\"ahler
space which satisfies a specific partial differential equation involving the
Ricci tensor. In this paper we study the wrapped brane solutions of D3 and
M2-branes which were originally constructed using gauged supergravity and
uplifted to D=10 and D=11. We rewrite the solutions in canonical form, identify
the backreacted K\"ahler geometry, and present a class of solutions which
satisfy the Killing spinor equation.Comment: v2: 13 pages, refs adde
Validation of a model of regulation in the tryptophan operon against multiple experiment data using global optimisation
This paper is concerned with validating a mathematical model of regulation in the tryptophan operon using global optimization. Although a number of models for this biochemical network are proposed, in many cases only qualitative agreement between the model output and experimental data was demonstrated, since very little information is currently available to guide the selection of parameter values for the models. This paper presents a model validating method using both multiple experimental data and global optimization
Demonstration of dispersive rarefaction shocks in hollow elliptical cylinder chains
We report an experimental and numerical demonstration of dispersive
rarefaction shocks (DRS) in a 3D-printed soft chain of hollow elliptical
cylinders. We find that, in contrast to conventional nonlinear waves, these DRS
have their lower amplitude components travel faster, while the higher amplitude
ones propagate slower. This results in the backward-tilted shape of the front
of the wave (the rarefaction segment) and the breakage of wave tails into a
modulated waveform (the dispersive shock segment). Examining the DRS under
various impact conditions, we find the counter-intuitive feature that the
higher striker velocity causes the slower propagation of the DRS. These unique
features can be useful for mitigating impact controllably and efficiently
without relying on material damping or plasticity effects
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