Probing the shell valence structure underlying the B(E2: 0+->2+) for N,Z~40: preponderance of the p-n interaction over the sub-shell closures

The very simple product of the number of particles by the number of holes appearing in the expression of the reduced B(E2:0+->2+) transition probability of even-even nuclei obtained from the extension of the seniority scheme is used to analyze the experimental B(E2:0+->2+) values in the Cr up to Se isotopes. A new interpretation is given to the B(E2:0+->2+) measured in 68Ni and 70Zn. The B(E2:0+->2+) features of the even-even nuclei between Ni and Se with neutron number ranging from 28 up to 50 fit in with a global scenario involving p-n interaction. The evolution of the B(E2:0+->2+) curves presenting very large values is amazingly reproduced by very schematic binomial calculations.Comment: submitted to PRC the 14th May 2002, resubmitted to PRC the 24th June 200

Seniority scenario for the 68-72Zn and 66-68Ni B(E2)^ difference

The very simple product of the number of particles by the number of holes appearing in the expression of the reduced B(E2: 0+1 -> 2+1) (B(E2)^) transition probability of even-even nuclei obtained from the extension of the seniority scheme is used to analyze in a same time the experimental B(E2)^ values of 56-68 Ni and those of 62-72Zn. The evolution of these B(E2)^ values with neutron number fits in with a scenario involving p-n interaction

Three-dimensional stability of a horizontally sheared flow in a stably stratified fluid

International audienceThis paper investigates the three-dimensional stability of a horizontal flow sheared horizontally, the hyperbolic tangent velocity profile, in a stably stratified fluid. In an homogeneous fluid, the Squire theorem states that the most unstable perturbation is two-dimensional. When the flow is stably stratified, this theorem does not apply and we have performed a numerical study to investigate the three-dimensional stability characteristics of the flow. When the Froude number, Fh, is varied from 8 to 0.05, the most unstable mode remains two-dimensional. However, the range of unstable vertical wavenumbers widens proportionally to the inverse of the Froude number for Fh " 1. This means that the stronger the stratification, the smaller the vertical scales that can be destabilized. This loss of selectivity of the two-dimensional mode in horizontal shear flows stratified vertically may explain the layering observed numerically and experimentally. © 2007 Cambridge University Press

Three-dimensional stability of vortex arrays in a stratified and rotating fluid

International audienceThis paper investigates numerically and through an asymptotic approach the three-dimensional stability of steady vertical vortex arrays in a stratified and rotating fluid. Three classical vortex arrays are studied: the Kármán vortex street, the symmetric double row and the single row of co-rotating vortices. The asymptotic analysis assumes well-separated vortices and long-wavelength bending perturbations following Billant (J. Fluid Mech., vol. 660, 2010, p. 354) and Robinson & Saffman (J. Fluid Mech., vol. 125, 1982, p. 411). Very good agreement with the numerical stability analysis is found even for finite wavelength and relatively close vortices. For a horizontal Froude number Fh = 1 and for a non-rotating fluid, it is found that the Kármán vortex street for a street spacing ratio (the distance h between the rows divided by the distance b between vortices in the same row) ? = 0.41 and the symmetric double row for any spacing ratio are most unstable to a three-dimensional instability of zigzag type that vertically bends the vortices. The most amplified vertical wavenumber scales like 1/(bFh) and the growth rate scales with the strain (2pb2), where is the vortex circulation. For the Kármán vortex street, the zigzag instability is symmetric with respect to the midplane between the two rows while it is antisymmetric for the symmetric double row. For the Kármán vortex street with well-separated vortex rows ? > 0.41 and the single row, the dominant instability is two-dimensional and corresponds to a pairing of adjacent vortices of the same row. The main differences between stratified and homogeneous fluids are the opposite symmetry of the dominant three-dimensional instabilities and the scaling of their most amplified wavenumber. When Fh > 1, three-dimensional instabilities are damped by a viscous critical layer. In the presence of background rotation in addition to the stratification, symmetric and antisymmetric modes no longer decouple and cyclonic vortices are less bent than anticyclonic vortices. However, the dominant instability remains qualitatively the same for the three vortex arrays, i.e. quasi-symmetric or quasi-antisymmetric and three-dimensional or two-dimensional. The growth rate continues to scale with the strain but the most unstable wavenumber of three-dimensional instabilities decreases with rotation and scales like Ro/(bFh) for small Rossby number Ro, in agreement with quasi-geostrophic scaling laws. © 2011 Cambridge University Press

Nonlinear evolution of the zigzag instability in stratified fluids: A shortcut on the route to dissipation

International audienceWe present high-resolution direct numerical simulations of the nonlinear evolution of a pair of counter-rotating vertical vortices in a stratified fluid for various high Reynolds numbers Re and low Froude numbers Fh. The vortices are bent by the zigzag instability producing high vertical shear. There is no nonlinear saturation so that the exponential growth is stopped only when the viscous dissipation by vertical shear is of the same order as the horizontal transport, i.e. when Zmaxh/Re = O(1) where Zmaxh is the maximum horizontal enstrophy non-dimensionalized by the vortex turnover frequency. The zigzag instability therefore directly transfers the energy from large scales to the small dissipative vertical scales. However, for high Reynolds number, the vertical shear created by the zigzag instability is so intense that the minimum local Richardson number Ri decreases below a threshold of around 1/4 and small-scale Kelvin-Helmholtz instabilities develop. We show that this can only occur when ReFh2 is above a threshold estimated as 340. Movies are available with the online version of the paper. © 2008 Cambridge University Press

Zigzag instability of vortex pairs in stratified and rotating fluids. Part 2. Analytical and numerical analyses

International audienceThe three-dimensional stability of vertical vortex pairs in stratified and rotating fluids is investigated using the analytical approach established in Part 1 and the predictions are compared to the results of previous direct numerical stability analyses for pairs of co-rotating equal-strength Lamb-Oseen vortices and to new numerical analyses for equal-strength counter-rotating vortex pairs. A very good agreement between theoretical and numerical results is generally found, thereby providing a comprehensive description of the zigzag instability. Co-rotating and counter-rotating vortex pairs are most unstable to the zigzag instability when the Froude number Fh =/(2pR 2N) (where is the vortex circulation, R the vortex radius and N the Brunt-Väiäsälä frequency) is lower than unity independently of the Rossby number Ro =/(4pR2?b) (?b is the planetary rotation rate). In this range, the maximum growth rate is proportional to the strain/(2pb2) (b is the separation distance between the vortices) and is almost independent of F h and Ro. The most amplified wavelength scales like Fhb when the Rossby number is large and like Fhb/|Ro| when |Ro| 1, in agreement with previous results. While the zigzag instability always bends equal-strength co-rotating vortex pairs in a symmetric way, the instability is only quasi-antisymmetric for finite Ro for equal-strength counter-rotating vortex pairs because the cyclonic vortex is less bent than the anticyclonic vortex. The theory is less accurate for co-rotating vortex pairs around Ro 2 because the bending waves rotate very slowly for long wavelength. The discrepancy can be fully resolved by taking into account higher-order three-dimensional effects. When Fh is increased above unity, the growth rate of the zigzag instability is strongly reduced because the bending waves of each vortex are damped by a critical layer at the radius where the angular velocity of the vortex is equal to the Brunt-Visl frequency. The zigzag instability, however, continues to exist and is dominant up to a critical Froude number, which mostly depends on the Rossby number. Above this threshold, equal-strength co-rotating vortex pairs are stable with respect to long-wavelength bending disturbances whereas equal-strength counter-rotating vortex pairs become unstable to a quasi-symmetric instability resembling the Crow instability in homogeneous fluids. However, its growth rate is lower than in homogeneous fluids because of the damping by the critical layer. The structure of the critical layer obtained in the computations is in excellent agreement with the theoretical solution. Physically, the different stability properties of vortex pairs in stratified and rotating fluids compared to homogeneous fluids are shown to come from the reversal of the direction of the self-induced motion of bent vortices. © 2010 Cambridge University Press

High-spin structures of 88Kr and 89Rb: Evolution from collective to single-particle behaviors

The high-spin states of the two neutron-rich nuclei, 88Kr and 89R have been studied from the 18O + 208Pb fusion-fission reaction. Their level schemes were built from triple gamma-ray coincidence data and gamma-gamma angular correlations were analyzed in order to assign spin and parity values to most of the observed states. The two levels schemes evolve from collective structures to single-particle excitations as a function of the excitation energy. Comparison with results of shell-model calculations gives the specific proton and neutron configurations which are involved to generate the angular momentum along the yrast lines.Comment: 12 pages, 9 figures, Physical Review C (2013) in pres

Collective quadrupole excitations in the 50<Z,N<82 nuclei with the generalized Bohr Hamiltonian

The generalized Bohr Hamiltonian is applied to a description of low-lying collective excitations in even-even isotopes of Te, Xe, Ba, Ce, Nd and Sm. The collective potential and inertial functions are determined by means of the Strutinsky method and the cranking model, respectively. A shell-dependent parametrization of the Nilsson potential is used. An approximate particle-number projection is performed in treatment of pairing correlations. The effect of coupling with the pairing vibrations is taken into account approximately when determining the inertial functions. The calculation does not contain any free parameter.Comment: Latex2e source, 20 pages, 14 figures in EPS format, tar gzipped fil

Ground-gamma band mixing and evolution of collectivity in even-even neutron-rich nuclei with 40<Z<50

We propose an extended band mixing formalism capable of describing the ground-gamma band interaction in a wide range of collective spectra beyond the regions of well deformed nuclei. On this basis we explain the staggering effects observed in the gamma bands of Mo, Ru and Pd nuclei providing a consistent interpretation of new experimental data in the neutron rich region. As a result the systematic behavior of the odd-even staggering effect and some general characteristics of the spectrum such as the mutual disposition of the bands, the interaction strength and the band structures is explained as the manifestation of respective changes in collective dynamics of the system.Comment: 17 pages, 6 figures, 4 table