The Lagrangian mass transport by non-dissipating surface gravity wavepackets consists of the Stokes drift and the wave-induced return flow. We examine how directional spreading and density stratification affect this mass transport for an isolated non-dissipating wavepacket in deep water using a perturbation expansion. For an unstratified ocean, we show that the net displacement by the return flow is finite, negative, the same at all vertical levels and inversely proportional to the depth for spanwise-infinite packets representing unidirectional (two-dimensional) seas, but zero for spanwise-localised packets representing directionally spread seas (three-dimensional). We resolve this difference by demonstrating that a transition between two-dimensional-like (finite) and three-dimensional-like (zero) displacement occurs on a time scale inversely proportional to the degree of directional spreading. For a stratified ocean, we show that in two dimensions the net displacement profile by the return flow oscillates slowly with depth, with a wavelength dependent on the ratio of buoyancy frequency to the surface wave group velocity, and infinite displacements are predicted when the surface wavepacket resonantly excites internal waves. In three dimensions, the net displacement remains zero in the presence of stratification, but finite-time displacements may be appreciably altered.

In order to investigate the early evolution of planetary nebulae (PNe) we solved numerically the hydrodynamical equations in cylindrical coordinates (r, z) assuming azimutal symmetry. The numerical method used is described in detail by Mair et al. (1988). Our simulations model the interaction of a fast, tenuous, spherical symmetrical central star wind with a slow, dense, aspherical Red Giant Envelope (RGE) expelled from the progenitor star. For the aspherical RGE with a polar/equatorial density contrast we used the initial model given by Mellema et al. (1991) in cylindrical coordinates. We have investigated the influence of each initial model parameter upon the evolution of PNe. Thereby we confirm that the polar/equatorial density contrast in the RGE and the thickness of the RGE-disk play an important role for the morphology of PNe. In agreement with the results from Mellema et al. (1991). The polar/equatorial density contrast in the RGE influences the ratio of the distances of the bright inner rim to the central star in z- and r-direction. This ratio increases with decreasing polar/equatorial density contrast. We find the thickness of the RGE-disk to be a key parameter for getting an elliptical or a butterfly PN: thin RGE-disks produce the first type of nebulae, thick disks the latter. We thank G. Mair, E. Müller and W. Hillebrandt for making available to us a copy of the SADIE code.

During the ORFEUS-SPAS II mission (November 19th - December 7th 1996, aboard the space shuttle Columbia), the elliptical planetary nebulae NGC 6543 and NGC 6826 have been studied with the Echelle spectrometer. The instrumental wavelength range and resolution are 900Å - 1400Å and 0.1Å, respectively (R = 10000), which provides detailed information about emission and absorption lines which are inaccessible for ground-based observations. We present a new analysis of the absorption lines imposed on the spectra of the central stars.

Since 1997, and following our detection of the first mm afterglow, we have followed-up 70 GRBs, mainly with the IRAMś Plateau de Bure Interferometer, what can be considered as the IRAM Legacy GRB Sample. 66 events were observed at 3 mm, with 19 of them being detected (with another 3 having marginal detections). 32 GRBs were followed up at 1 mm, with 6 of them being detected. Redshifts for the GRB afterglows lie in the range z = 0.03–8.3, with measured flux densities (at 3 mm) varying between 0.25 and 60 mJy (but usually <1.5 mJy) with first observations taking place around 1–2 days after the GRB. Forward shock emission expleains the observations with the exception of one particular case (GRB 090423 at z = 8.2) for which reverse shock emission is required.

An jet in cross-flow (JICF) of air is studied using three-dimensional direct numerical simulation with and without chemical reaction in order to investigate the role of the complex JICF turbulent flow field in the mechanism of fast fuel-oxidant mixing and of aerodynamic flame stabilization in the near field of the jet nozzle. Focus is on delineating the flow/mixing/chemistry conditions that are necessary and/or sufficient to achieve flame anchoring that ultimately enables the formulation of more reliable and precise guidelines for design of fuel injection nozzles. A mixture averaged diffusion formulation that includes the effect of thermal diffusion is used along with a detailed chemical kinetics mechanism for hydrogen–air combustion. A new parametrization technique is used to describe the jet trajectory: solution of Laplace’s equation upon, and then within, an opportune scalar surface anchored by Dirichlet boundary conditions at the jet nozzle and plume exit from the domain provides a smoothly varying field along the jet path. The surface is selected to describe the scalar mixing and reaction associated with a transverse jet. The derived field, , is used as a condition to mark the position along the natural jet trajectory when analysing the variation of relevant flow, mixing and reaction quantities in the present direct numerical simulation (DNS) datasets. Results indicate the presence of a correlation between the flame base location in parameter space and a region of low velocity magnitude, high enstrophy, high mixing rate and high equivalence ratio (flame root region). Instantaneously, a variety of vortical structures, well known from the literature as important contributors to fuel-oxidant mixing, are observed in both inert and reactive cases with a considerable span in length scales. Moreover, instantaneous plots from reactive cases illustrate that the most upstream flame tongues propagate close to the trailing edge of the fuel jet potential core near the jet shear layer vortex shedding position. Some degree of asymmetry with respect to the domain mid-plane in the spanwise direction is observed in the averaged fields, both for the inert and reactive cases.

The inclusion of winter cereals in spring-annual rotations in the northern Great Plains may reduce weed populations and herbicide requirements. A broad range of spring and winter cereals were compared for ability to suppress weeds and maximize grain yield at Lacombe (2002 to 2005) and Lethbridge (2003 to 2005), Alberta, Canada. High seeding rates (≥ 400 seeds/m2) were used in all years to maximize crop competitive ability. Spring cereals achieved high crop-plant densities (> 250 plants/m2) at most sites, but winter cereals had lower plant densities due to winterkill, particularly at Lethbridge in 2004. All winter cereals and spring barley were highly effective at reducing weed biomass at Lacombe for the first 3 yr of the study. Weed suppression was less consistently affected by winter cereals in the last year at Lacombe and at Lethbridge, primarily due to poor winter survival. Grain yields were highest for spring triticale and least for spring wheat at Lacombe, with winter cereals intermediate. At Lethbridge, winter cereals had higher grain yields in 2003 whereas spring cereals had higher yields in 2004 and 2005. Winter cereals were generally more effective at suppressing weed growth than spring cereals if a good crop stand was established, but overlap in weed-competitive ability among cultivars was considerable. This information will be used to enhance the sustainable production of winter and spring cereals in traditional and nontraditional agro-ecological zones.

Gas in galaxy clusters requires re-heating. We study the re-heating of the cool gas phases. Ionized and molecular gas is traced out to 20 kpc and found to be strongly coupled. The observed line emission may in part be explained by excitation due to hot, young stars.

Far ultraviolet (FUV) emission is observed in the central regions of cool-core clusters with the Hubble Space Telescope (HST). It is traced out to 20 kpc from the nuclei of the brightest cluster galaxies and found to be distributed in clumps and filaments, as shown in Figure 1. The FUV emission matches the global structure of the ionized gas nebulae. If produced by stars, this emission can account for the ionization but not the temperature of the gas (Voit & Donahue 1997; Oonk et al. in preparation).