POLLUTION DISPERSION PREDICTION FOR THE MUST WIND TUNNEL EXPERIMENT WITH ANISOTROPIC ALGEBRAIC MODELS FOR TURBULENT SCALAR FLUXES

The numerical prediction of pollution dispersion in urban environments by means of solution of the statistically steady Reynolds Averaged Navier Stokes (RANS) equations is known to be strongly dependent on the turbulence models. In the case of pollution dispersion turbulence models do not only have to be used for the Reynolds stresses but also for the turbulent scalar fluxes. While the influence of several turbulence models for the Reynolds stresses on the dispersion in urban environments has been examined already several times, the turbulent scalar fluxes were exclusively modelled by the simple gradient diffusion assumption. In the present work therefore the influence of more advanced, anisotropic algebraic models for the turbulent scalar fluxes on the dispersion in the MUST wind tunnel experiment is examined. To that end, three anisotropic algebraic flux models were implemented in the commercial software FLUENT 6.3. All these models together with the simple gradient diffusion model (with two turbulent Schmidt numbers) are performed and compared using statistical performance measures to assess their predictive capability

Ion hopping in crystalline and glassy spodumene LiAlSi2O6: Li7 spin-lattice relaxation and Li7 echo NMR spectroscopy

Nuclear magnetic resonance spectroscopy was used to study polycrystalline β-spodumene (β−LiAlSi2O6) as well as glassy specimens with the same chemical composition. 7Li spin-lattice relaxation measurements were carried out in a broad temperature range and for several Larmor frequencies. In addition to a pronounced rate maximum at high temperatures, stemming from the long-range Li motion in these aluminosilicates, we found a weak maximum in the crystalline modification near 120K. The latter result confirms the existence of a local double-well structure in which the Li ions reside. The ionic motion was also monitored by solid- and stimulated-echo spectra as well as by the decay of the Jeener-Broekaert echo. Under conditions which are discussed in detail, the latter is a direct measure of the hopping correlation function. For the glass this function was found to decay faster and more stretched than that of the crystal at a given temperature. Furthermore, the relevant barriers against the high-temperature long-range Li motion are larger in the crystal as compared to the glass. © 2005 The American Physical Society

Increasing the dimension in high-dimensional two-photon orbital angular momentum entanglement

Any practical experiment utilising the innate D-dimensional entanglement of the orbital angular momentum (OAM) state space of photons is subject to the modal capacity of the detection system. We show that given such a constraint, the number of measured, entangled OAM modes in photon pairs generated by spontaneous parametric down-conversion (SPDC) can be maximised by tuning the phase-matching conditions in the SPDC process. We demonstrate a factor of 2 increase on the half-width of the OAM-correlation spectrum, from 10 to 20, the latter implying \approx 50 -dimensional two-photon OAM entanglement. Exploiting correlations in the conjugate variable, angular position, we measure concurrence values 0.96 and 0.90 for two phase-matching conditions, indicating bipartite, D-dimensional entanglement where D is tuneable

Comparison of quantum field perturbation theory for the light front with the theory in lorentz coordinates

The relationship between the perturbation theory in light-front coordinates and Lorentz-covariant perturbation theory is investigated. A method for finding the difference between separate terms of the corresponding series without their explicit evaluation is proposed. A procedure of constructing additional counter-terms to the canonical Hamiltonian that compensate this difference at any finite order is proposed. For the Yukawa model, the light-front Hamiltonian with all of these counter-terms is obtained in a closed form. Possible application of this approach to gauge theories is discussed.Comment: LaTex 2.09, 20 pages, 5 figure

Ferromagnetic coupling of mononuclear Fe centers in a self-assembled metal-organic network on Au(111)

The magnetic state and magnetic coupling of individual atoms in nanoscale structures relies on a delicate balance between different interactions with the atomic-scale surrounding. Using scanning tunneling microscopy, we resolve the self-assembled formation of highly ordered bilayer structures of Fe atoms and organic linker molecules (T4PT) when deposited on a Au(111) surface. The Fe atoms are encaged in a three-dimensional coordination motif by three T4PT molecules in the surface plane and an additional T4PT unit on top. Within this crystal field, the Fe atoms retain a magnetic ground state with easy-axis anisotropy, as evidenced by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The magnetization curves reveal the existence of ferromagnetic coupling between the Fe centers