530 research outputs found

    Application of unstrained flamelet SGS closure for multi-regime premixed combustion

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    Large eddy simulation of turbulent premixed combustion behind a bluff-body is performed using unstrained flamelet model with a presumed probability density function to calculate filtered reaction rate. The subgrid variance of the progress variable required in this approach is calculated using its transport equation to include contributions from reaction, turbulence and molecular diffusive and dissipative processes at sub-grid scales. The dissipation rate of the variance is obtained using an algebraic closure, which maintains physical consistency among turbulence, reaction and molecular diffusion. Various quantities such as mean velocity, temperature and species mass fractions computed for two bluff-body flames experiencing 2% and 24% turbulence intensities are compared to their respective measurements. These comparisons are very good suggesting that the unstrained flamelet SGS closure works well for multi-regime combustion. The demonstrated success of this modelling framework is explained on a physical basis.Engineering and Physical Sciences Research Council (Grant ID: EP/I027556/1), Siemens, Rolls-Royc

    Pygmy Dipol Resonances as a Manifestation of the Structure of the Neutron-Rich Nuclei

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    Dipole excitations in neutron-rich nuclei below the neutron threshold are investigated. The method is based on Hartree-Fock-Bogoliubov (HFB) and Quasiparticle-Phonon Model (QPM) theory. Of our special interest are the properties of the low-lying 1- Pygmy Resonance and the two-phonon quadrupole-octupole 1- states in Sn-isotopes including exploratory investigations for the experimentally unknown mass regions. In particular we investigate the evolution of the dipole strength function with the neutron excess. The use of HFB mean-field potentials and s.p. energies is found to provide a reliable extrapolation into the region off stability.Comment: 8 pages, 3 figures, Proceedings of the International Conference on Collective Motion in Nuclei Under Extreme Conditions (COMEX1), Paris, France, 10-13 June 200

    Comparison between four published definitions of hyposmia in Parkinson's disease

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    Objectives: Hyposmia is a common feature of Parkinson's disease (PD), yet there is no standard method to define it. A comparison of four published methods was performed to explore and highlight differences. // Materials and methods: Olfactory testing was performed in 2097 cases of early PD in two prospective studies. Olfaction was assessed using various cut-offs, usually corrected by age and/or gender. Control data were simulated based on the age and gender structure of the PD cases and published normal ranges. Association with age, gender, and disease duration was explored by method and study cohort. Prevalence of hyposmia was compared with the age and gender-matched simulated controls. Between method agreement was measured using Cohen's kappa and Gwet's AC1. // Results: Hyposmia was present in between 69.1% and 97.9% of cases in Tracking Parkinson's cases, and between 62.2% and 90.8% of cases in the Parkinson's Progression Marker Initiative, depending on the method. Between-method agreement varied (kappa 0.09–0.80, AC1 0.55–0.86). The absolute difference between PD cases and simulated controls was similar for men and women across methods. Age and male gender were positively associated with hyposmia (p < .001, all methods). Odds of having hyposmia increased with advancing age (OR:1.06, 95% CI:1.03, 1.10, p < .001). Longer disease duration had a negative impact on overall olfactory performance. // Conclusions: Different definitions of hyposmia give different results using the same dataset. A standardized definition of hyposmia in PD is required, adjusting for age and gender, to account for the background decline in olfactory performance with ageing, especially in men

    Orbital M1 versus E2 strength in deformed nuclei: A new energy weighted sum rule

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    Within the unified model of Bohr and Mottelson we derive the following linear energy weighted sum rule for low energy orbital 1+^+ excitations in even-even deformed nuclei S_{\rm LE}^{\rm lew} (M_1^{\rm orb}) \cong (6/5) \epsilon (B(E2; 0^+_1 \rightarrow 2_1^+ K=0)/Z e^2^2) \mu^2_N with B(E2) the E2 strength for the transition from the ground state to the first excited state in the ground state rotational band, the charge r.m.s. radius squared and ϵ\epsilon the binding energy per nucleon in the nuclear ground state. It is shown that this energy weighted sum rule is in good agreement with available experimental data. The sum rule is derived using a simple ansatz for the intrinsic ground state wave function that predicts also high energy 1+^+ strength at 2ω\hbar \omega carrying 50\% of the total m1m_1 moment of the orbital M1 operator.Comment: REVTEX (3.0), 9 pages, RU924

    Transition Rates between Mixed Symmetry States: First Measurement in 94Mo

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    The nucleus 94Mo was investigated using a powerful combination of gamma-singles photon scattering experiments and gamma-gamma-coincidence studies following the beta-decay of 94mTc. The data survey short-lived J^pi=1+,2+ states and include branching ratios, E2/M1 mixing ratios, lifetimes, and transition strengths. The mixed-symmetry (MS) 1+ scissors mode and the 2+ MS state are identified from M1 strengths. A gamma transition between MS states was observed and its rate was measured. Nine M1 and E2 strengths involving MS states agree with the O(6) limit of the interacting boson model-2 using the proton boson E2 charge as the only free parameter.Comment: 9 pages, 3 PostScript figures included, ReVTeX, accepted for publication in Physical Review Letters, tentatively scheduled for August 9, 199

    Strong fragmentation of low-energy electromagnetic excitation strength in 117^{117}Sn

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    Results of nuclear resonance fluorescence experiments on 117^{117}Sn are reported. More than 50 γ\gamma transitions with Eγ<4E_{\gamma} < 4 MeV were detected indicating a strong fragmentation of the electromagnetic excitation strength. For the first time microscopic calculations making use of a complete configuration space for low-lying states are performed in heavy odd-mass spherical nuclei. The theoretical predictions are in good agreement with the data. It is concluded that although the E1 transitions are the strongest ones also M1 and E2 decays contribute substantially to the observed spectra. In contrast to the neighboring even 116124^{116-124}Sn, in 117^{117}Sn the 11^- component of the two-phonon [21+31][2^+_1 \otimes 3^-_1] quintuplet built on top of the 1/2+^+ ground state is proved to be strongly fragmented.Comment: 4 pages, 3 figure

    Nuclear Scissors Mode with Pairing

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    The coupled dynamics of the scissors mode and the isovector giant quadrupole resonance are studied using a generalized Wigner function moments method taking into account pair correlations. Equations of motion for angular momentum, quadrupole moment and other relevant collective variables are derived on the basis of the time dependent Hartree-Fock-Bogoliubov equations. Analytical expressions for energy centroids and transitions probabilities are found for the harmonic oscillator model with the quadrupole-quadrupole residual interaction and monopole pairing force. Deformation dependences of energies and B(M1)B(M1) values are correctly reproduced. The inclusion of pair correlations leads to a drastic improvement in the description of qualitative and quantitative characteristics of the scissors mode.Comment: 36 pages, 5 figures, the results of calculation by another method and the section concerning currents are adde

    High-energy scissors mode

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    All the orbital M1 excitations, at both low and high energies, obtained from a rotationally invariant QRPA, represent the fragmented scissors mode. The high-energy M1 strength is almost purely orbital and resides in the region of the isovector giant quadrupole resonance. In heavy deformed nuclei the high-energy scissors mode is strongly fragmented between 17 and 25 MeV (with uncertainties arising from the poor knowledge of the isovector potential). The coherent scissors motion is hindered by the fragmentation and B(M1)<0.25  μN2B(M1) < 0.25 \; \mu^2_N for single transitions in this region. The (e,e)(e,e^{\prime}) cross sections for excitations above 17 MeV are one order of magnitude larger for E2 than for M1 excitations even at backward angles.Comment: 20 pages in RevTEX, 5 figures (uuencoded,put with 'figures') accepted for publication in Phys.Rev.
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