ALBERT

Description: An experimental study has been performed on the dynamics of a large turbulent buoyanthelium plume. Two-dimensional velocity fields were measured using particle image velocimetry (PIV) while helium mass fraction was determined by planar laser-induced fluorescence (PLIF). PIV and PLIF were performed simultaneously in order to obtain velocity and mass fraction data over a plane that encompassed the plume core, the near-field mixing zones and the surrounding air. The Rayleigh-Taylor instability at the base of the plume leads to the vortex that grows to dominate the flow. This process repeats in a cyclical manner. The temporally and spatially resolved data show a strong negative correlation between density and vertical velocity, as well as a strong 90° phase lag between peaks in the vertical and horizontal velocities throughout the flow field owing to large coherent structures associated with puffing of the turbulent plume. The joint velocity an mass fraction data are used to calculate Favre-averaged statistics in addition to Reynolds-(time) averaged statistics. Unexpectedly, the difference between both the Favre-averaged and Reynolds-averaged velocities and second-order turbulent statistics is less than the uncertainty in the data throughout the flow field. A simple analysis was performed to determine the expected differences between Favre and Reynolds statistics for flows with periodic fluctuations in which the density and velocity fields are perfectly correlated, but have the phase relations as suggested by the data. The analytical results agreewith the data, showing that the Favre and Reynolds statistics will be the same to lead order. The combination of observation and simple analysis suggests that for buoyancy-dominated flows in which it can be expected that density and velocity are strongly correlated,phase relations will result in only second-order differences between Favre- and Reynolds-averaged data in spite of strong fluctuations in both density and velocity. © 2005 Cambridge University Press.

Description: We present results for the average mass transfer to a spherical squirmer, a model micro-organism whose surface oscillates tangentially to itself. The surface motion drives a low-Reynolds-number flow which enables the squirmer either to swim relative to the fluid at infinity, at an average speed proportional to a streaming parameter, W, or to stir the fluid around it while remaining, on average, at rest (if W =0), as represented by a hovering parameter, b. We assume that the amplitude of the time-periodic surface distortions is scaled by a dimensionless small parameter ∈, and consider only high Péclet numbers P - a measure of convection versus diffusion - by setting P-1 = ∈2γ, where γ is a parameter of 0(1). It is shown that the average mass concentration distribution satisfies a steady convection-diffusion equation with an effective velocity field that is different from the actual mean velocity field. The model is used to calculate the mass transfer across the surface of the squirmer, measured by the mean Sherwood number Sh. We find asymptotic solutions for small and large γ and numerical results for the whole range of values. While the large-γ expansions are reproduced well by the numerical results, there is a discrepancy between the two at small γ. We believe this is due to very small recirculation regions, attached to the surface of the squirmer, which make boundary layer theory applicable only when 1/γ is immense. For the parameters chosen in this study, results indicate that both hovering and streaming contribute to the mass transfer, although streaming has a greater effect. Also, energy dissipation considerations show that an optimum swimming mode exists, at least at small and large γ, for any given uptake rate. However, other factors have still to be taken into account, and the model realism improved, if we want to make predictions for real aquatic micro-organisms. © 2005 Cambridge University Press.

Description: Observations from basal water-pressure sensors along the length of Bench Glacier, Alaska, USA, show that diurnal fluctuations of water pressure are seasonal and restricted to summer. Most notable about these fluctuations is their disappearance in the late summer and early autumn, long before the seasonal end of diurnal meltwater input. Here we present data documenting the end of diurnal water-pressure fluctuations during the 2002 and 2003 melt seasons. The end of diurnal fluctuations occurred abruptly in multiple boreholes spaced meters to kilometers apart. There was no obvious spatial progression of termination events, and a clear correlation with meteorological forcing or discharge in the outlet stream was not apparent. After diurnal pressure fluctuations ended, basal water pressure returned to a high, generally steady, value either in an irregular pattern or by a distinct increase. This high water pressure was interrupted by episodic, acyclic events throughout the autumn before becoming stable and high in winter.

Description: The objective was to explore, in lines derived from a bread wheat (Triticum aestivum L.) cultivar, the association of grain yield with carbon isotope discrimination (Δ) and ash content (Ash) determined in both flag leaf and kernel. Divergent selection within the cv. Nestos, based on individual plant yield under very low density (11547 plants/ha), produced 20 lines. Progeny evaluation was conducted in two sites of Northern Hellas (Greece) at both low (11547 plants/ha) and high (5000000 plants/ha) density. The results showed significant differences between lines for grain yield, Δ and Ash. However, only the conclusions on grain yield were similar in low and high density, perhaps because the selection criterion under low density was grain yield. This, combined with the lack of any strong relationship of grain yield with either Δ or Ash, failed to confirm the usefulness of these physiological traits as indirect selection criteria, when the within-cultivar variation for grain yield is exploited. The possible association of grain yield with Δ and Ash appeared to be influenced by various factors such as drought, level of grain yield and altitude. Results also showed that less plant-to-plant variability, as expressed by the coefficients of variation (CV) of single-plant yields, was associated with decreased drought response and higher yield stability. The less stressed lines were those that used water more conservatively till anthesis and maintained a high photosynthetic rate during grain filling.