Lifetime of the 4D 3/2 and 4D 5/2 metastable states in Sr II

Sr⁺ ions were confined in a r.f. quadrupole trap for times of the order of 30 min. The metastable 4D states were populated via laser excitation of the 5P states. The weak quadrupole transition rate into the 5S _1/2 ground state at 674 and 687 nm was deduced from observation of the exponential decay. At background pressures above 10^?7 mbar the radiative decay is dominated by collisional quenching. Extrapolation of the observed decay rate to zero background pressure yields the radiative lifetimes. At pressures around 10^?6 mbar fine structure mixing collisions between the 4D states have been observed, which lead to corrections of the extrapolated lifetimes. As the final result we obtain 395±38 ms for 4D _3/2 and 345±33 ms for 4D _5/2. These results are somewhat higher than theoretical predictions.     

A possible explanation of countergradient fluxes in homogeneous turbulence

This study addresses the phenomenon of persistent countergradient (PCG) fluxes of momentum and heat (density) as observed in homogeneous turbulence forced by shear and stratification. Countergradient fluxes may occur at large scales when stratification is strong. However, they always occur at small scales, independently of stratification. A conceptional model is introduced to explain PCG fluxes at small scales as the result of the collision of large-scale fluid parcels. The large parcels collide under the driving force of inclined vortex structures (in a shear-dominated flow) or of buoyancy (in a strongly stratified shear flow). This collision model also explains the PCG heat flux in an unsheared stratified flow with zero average momentum flux. It is found that the energy of the small-scale PCG motions is provided (i) by quick transport of kinetic energy from the scales of production to relatively slowly dissipating scales if the flow is shear-driven and (ii) by conversion of available potential energy to kinetic energy at small scales when the flow is stratified. The collision mechanism is an inherent property of the turbulence dynamics. Therefore, the PCG fluxes at small scales reflect a universal character of homogeneous turbulence, and are found over a large range of Reynolds numbers. The Prandtl (or Schmidt) number influences the rate of dissipation of temperature (or density) variance but not the dissipation rate of the velocity variance. In stratified flows, therefore, the number directly affects the strength of the PCG heat flux at small scales. It is found, however, that the PCG momentum flux is also altered slightly when the Prandtl number is large enough to sustain small buoyantly moving parcels after collision.     

Aircraft wake vortices – prediction and mitigation

The current abstract presents selected topics investigated within the wake-vortex research program of DLR. Two approaches are addressed that both aim at increasing airport capacity without compromising safety. One approach is to directly alleviate wake vortex strength and stability by constructive measures at the aircraft wings. The other approach utilizes the dominant influence of meteorological parameters like turbulence, wind shear, and temperature stratification on wake vortex fate. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurement of the4He-D2 mass difference

A Penning ion trap spectrometer has been used to measure the modified cyclotron frequencies of trapped⁴He⁺ and D ₂ ⁺ ions. The resonances were detected by a time of flight method after ejection of the ions from the trap. We obtain a mass differenceΔm (D₂-⁴He) = 25600331(5)·10^?9 amu in acordance with published values but with a reduced uncertainty.     

Lifetime and collisional depopulation of the metastable 5D 3/2-state of Yb+

The lifetime and collisional depopulation rates of the metastable 5D _3/2 state of Yb⁺ have been determined in a radiofrequency ion trap by observation of the fluorescence count rate after ion excitation by a short laser pulse. From measurements using He, N₂ and H₂ as buffer gases between 10^?8 and 10^?6 mbar pressure and linear extrapolation to zero pressure we obtain a lifetime of τ=52.15±1.00 ms and rate constants ofR(H₂)=(1.02±0.10)×10^?9 cm³/s andR(N₂)=(1.78±0.19)×10^?10 cm³/s. The lifetime is in fair agreement with a calculated value of 74 ms.     

Turbulent breaking of overturning gravity waves below a critical level

The interaction of an internal gravity wave with its evolving critical layer and the subsequent generation of turbulence by overturning waves are studied by three-dimensional numerical simulations. The simulation describes the flow of a stably stratified Boussinesq fluid between a bottom wavy surface and a top flat surface, both without friction and adiabatic. The amplitude of the surface wave amounts to about 0.03 of the layer depth. The horizontal flow velocity is negative near the lower surface, positive near the top surface with uniform shear and zero mean value. The bulk Richardson number is one. The flow over the wavy surface induces a standing gravity wave causing a critical layer at mid altitude. After a successful comparison of a two-dimensional version of the model with experimental observations (Thorpe [21]), results obtained with two different models of viscosity are discussed: a direct numerical simulation (DNS) with constant viscosity and a large-eddy simulation (LES) where the subgrid scales are modelled by a stability-dependent first-order closure. Both simulations are similar in the build-up of a primary overturning roll and show the expected early stage of the interaction between wave and critical level. Afterwards, the flows become nonlinear and evolve differently in both cases: the flow structure in the DNS consists of coherent smaller-scale secondary rolls with increasing vertical depth. On the other hand, in the LES the convectively unstable primary roll collapses into three-dimensional turbulence. The results show that convectively overturning regions are always formed but the details of breaking and the resulting structure of the mixed layer depend on the effective Reynolds number of the flow. With sufficient viscous damping, three-dimensional turbulent convective instabilities are more easily suppressed than two-dimensional laminar overturning.     

Interaction of Two-Dimensional Trailing Vortex Pair with a Shear Layer

The basic laws governing the interaction of a two-dimensional vortex pair with a shear layer of constant thickness are considered. The main idea of the study is to develop and adapt a simplified representation of a hydrodynamic flow based on a point-vortex model simulating the actual interaction of full-scale vortex patterns over the ground surface. It is shown that vortices with vorticity opposite in sign to the shear layer may stop or even ricochet from this layer, while the other vortex may penetrate through the layer. Numerical results are presented as plots and analyzed