ALBERT

Description: SUMMARYA reliable evaluation of crop nutritional status is crucial for supporting fertilization aiming at maximizing qualitative and quantitative aspects of production and reducing the environmental impact of cropping systems. Most of the available simulation models evaluate crop nutritional status according to the nitrogen (N) dilution law, which derives critical N concentration as a function of above-ground biomass. An alternative approach, developed during a project carried out with students of the Cropping Systems Masters course at the University of Milan, was tested and compared with existing models (N dilution law and approaches implemented in EPIC and DAISY models). The new model (MAZINGA) reproduces the effect of leaf self-shading in lowering plant N concentration (PNC) through an inverse of the fraction of radiation intercepted by the canopy. The models were tested using data collected in four rice (Oryza sativaL.) experiments carried out in Northern Italy under potential and N-limited conditions. MAZINGA was the most accurate in identifying the critical N concentration, and therefore in discriminating PNC of plants growing under N-limited and non-limited conditions, respectively. In addition, the present work proved the effectiveness of crop models when used as tools for supporting education.

Description: We study the shock-driven turbulent mixing that occurs when a perturbed planar density interface is impacted by a planar shock wave of moderate strength and subsequently reshocked. The present work is a systematic study of the influence of the relative molecular weights of the gases in the form of the initial Atwood ratio A. We investigate the cases A = ± 0.21, ±0.67 and ±0.87 that correspond to the realistic gas combinations air-CO 2, air-SF6 and H2-air. A canonical, three-dimensional numerical experiment, using the large-eddy simulation technique with an explicit subgrid model, reproduces the interaction within a shock tube with an endwall where the incident shock Mach number is ∼1.5 and the initial interface perturbation has a fixed dominant wavelength and a fixed amplitude-to-wavelength ratio ∼0.1. For positive Atwood configurations, the reshock is followed by secondary waves in the form of alternate expansion and compression waves travelling between the endwall and the mixing zone. These reverberations are shown to intensify turbulent kinetic energy and dissipation across the mixing zone. In contrast, negative Atwood number configurations produce multiple secondary reshocks following the primary reshock, and their effect on the mixing region is less pronounced. As the magnitude of A is increased, the mixing zone tends to evolve less symmetrically. The mixing zone growth rate following the primary reshock approaches a linear evolution prior to the secondary wave interactions. When considering the full range of examined Atwood numbers, measurements of this growth rate do not agree well with predictions of existing analytic reshock models such as the model by Mikaelian (Physica D, vol. 36, 1989, p. 343). Accordingly, we propose an empirical formula and also a semi-analytical, impulsive model based on a diffuse-interface approach to describe the A-dependence of the post-reshock growth rate. © 2011 Cambridge University Press.

Description: SUMMARYThe nodal distribution of free polyamines (important regulators of flower induction) in cotton (Gossypium hirsutum L.) ovaries was determined by high-performance liquid chromatography (HPLC). The objective of the study was to investigate the nodal distribution of putrescine (Put), spermidine (Spd) and spermine (Sp) of cotton lines (G. hirsutum L.), and to determine whether there are differences in ovarian polyamine content among three commercial cotton cultivars. A field study was conducted in 2005 and 2006 using the cultivars FM960BR, ST5599BR and DP444BR. Free polyamines Put, Spd and Sp were determined in ovarian tissue of first-position white flowers starting at the beginning of anthesis and collected from the 7th, 9th, 11th and 13th main-stem nodes for four consecutive weeks. There was no significant nodal position by cultivar interaction; thus, the main effects were tested. Put content decreased acropetally along the main stem of the cotton plant with the highest content observed at the 7th node and the lowest at the 13th node. Spd content decreased below and above the 9th node, with the 9th node showing the highest amount of Spd and the 13th node the lowest in both years of the study. Similarly, Sp content decreased below and above the 9th node. In general, the 7th and the 9th node had the highest titre of free polyamines. Among the cultivars tested, FM960BR showed higher polyamine content in one season; however, the observation was not consistent from year to year. The highest amounts of polyamines were observed at the 7th and the 9th node of cotton and this may be associated with the known yield distribution: almost 0·80 of the total yield of cotton is derived from these nodal positions.

Description: The interactions between shear-free turbulence in two regions (denoted as + and on either side of a nearly flat horizontal interface are shown here to be controlled by several mechanisms, which depend on the magnitudes of the ratios of the densities, +/, and kinematic viscosities of the fluids, +/, and the root mean square (r.m.s.) velocities of the turbulence, u0+/u0, above and below the interface. This study focuses on gasliquid interfaces so that+/ 1 and also on where turbulence is generated either above or below the interface so that u0+/u0 is either very large or very small. It is assumed that vertical buoyancy forces across the interface are much larger than internal forces so that the interface is nearly flat, and coupling between turbulence on either side of the interface is determined by viscous stresses. A formal linearized rapid-distortion analysis with viscous effects is developed by extending the previous study by Hunt & Graham (J. Fluid Mech., vol. 84, 1978, pp. 209235) of shear-free turbulence near rigid plane boundaries. The physical processes accounted for in our model include both the blocking effect of the interface on normal components of the turbulence and the viscous coupling of the horizontal field across thin interfacial viscous boundary layers. The horizontal divergence in the perturbation velocity field in the viscous layer drives weak inviscid irrotational velocity fluctuations outside the viscous boundary layers in a mechanism analogous to Ekman pumping. The analysis shows the following. (i) The blocking effects are similar to those near rigid boundaries on each side of the interface, but through the action of the thin viscous layers above and below the interface, the horizontal and vertical velocity components differ from those near a rigid surface and are correlated or anti-correlated respectively. (ii) Because of the growth of the viscous layers on either side of the interface, the ratio uI/u0, where uI is the r.m.s. of the interfacial velocity fluctuations and u0 the r.m.s. of the homogeneous turbulence far from the interface, does not vary with time. If the turbulence is driven in the lower layer with +/1 and u0+/u0 1, then uI/u0 ~ 1 when Re (=u0L/v) 1 and R = (/+)(v/v+)1/2 1. If the turbulence is driven in the upper layer with +/1 and u0+/u0 1, then uI/u0+ ~ 1/(1 + R). (iii) Nonlinear effects become significant over periods greater than Lagrangian time scales. When turbulence is generated in the lower layer, and the Reynolds number is high enough, motions in the upper viscous layer are turbulent. The horizontal vorticity tends to decrease, and the vertical vorticity of the eddies dominates their asymptotic structure. When turbulence is generated in the upper layer, and the Reynolds number is less than about 106107, the fluctuations in the viscous layer do not become turbulent. Nonlinear processes at the interface increase the ratio uI/u0+ for sheared or shear-free turbulence in the gas above its linear value of uI/u0+ ∼1/(1 + R) to (+)1/2 ∼ 1/30 for airwater interfaces. This estimate agrees with the direct numerical simulation results from Lombardi, De Angelis & Bannerjee (Phys. Fluids, vol. 8, no. 6, 1996, pp. 16431665). Because the linear viscousinertial coupling mechanism is still significant, the eddy motions on either side of the interface have a similar horizontal structure, although their vertical structure differs. © 2011 Cambridge University Press.

Description: In the present study we investigate three-scalar mixing in a turbulent coaxial jet. In this flow a centre jet and an annular flow, consisting of acetone-doped air and ethylene respectively, are mixed with the co-flow air. A unique aspect of this study compared to previous studies of three-scalar mixing is that two of the scalars (the centre jet and air) are separated by the third (annular flow); therefore, this flow better approximates the mixing process in a non-premixed turbulent reactive flow. Planar laser-induced fluorescence and Rayleigh scattering are employed to measure the mass fractions of the acetone-doped air and ethylene. The results show that the most unique aspects of the three-scalar mixing occur in the near field of the flow. The mixing process in this part of the flow are analysed in detail using the scalar means, variances, correlation coefficient, joint probability density function (JPDF), conditional diffusion, conditional dissipation rates and conditional cross-dissipation rate. The diffusion velocity streamlines in scalar space representing the conditional diffusion generally converge quickly to a manifold along which they continue at a lower rate. A widely used mixing model, interaction through exchange with mean, does not exhibit such a trend. The approach to the manifold is generally in the direction of the ethylene mass fraction. The difference in the magnitudes of the diffusion velocity components for the two scalars cannot be accounted for by the difference in their dissipation time scales. The mixing processes during the approach to the manifold, therefore, cannot be modelled by using different dissipation time scales alone. While the three scalars in this flow have similar distances in scalar space, mixing between two of the scalars can occur only through the third, forcing a detour of the manifold (mixing path) in scalar space. This mixing path presents a challenging test for mixing models since most mixing models use only scalar-space variables and do not take into account the spatial (physical-space) scalar structure. The scalar JPDF and the conditional dissipation rates obtained in the present study have similarities to those of mixture fraction and temperature in turbulent flames. The results in the present study provide a basis for understanding and modelling multiscalar mixing in reactive flows. © 2011 Cambridge University Press.

Description: The nonlinear deformation and break-up of a bubble or drop immersed in a uniaxial extensional flow of an incompressible viscous fluid is analysed by means of viscous potential flow. In this approximation, the flow field is irrotational and viscosity enters through the balance of normal stresses at the interface. The governing equations are solved numerically to track the motion of the interface by coupling a boundary-element method with a time-integration routine. When break-up occurs, the break-up time computed here is compared with results obtained elsewhere from numerical simulations of the Navier-Stokes equations (Revuelta, Rodríguez-Rodríguez & Martínez-Bazán J. Fluid Mech., vol. 551, 2006, p. 175), which thus keeps vorticity in the analysis, for several combinations of the relevant dimensionless parameters of the problem. For the bubble, for Weber numbers 3 ≤ We ≤ 6, predictions from viscous potential flow shows good agreement with the results from the Navier-Stokes equations for the bubble break-up time, whereas for larger We, the former underpredicts the results given by the latter. When viscosity is included, larger break-up times are predicted with respect to the inviscid case for the same We. For the drop, and considering moderate Reynolds numbers, Re, increasing the viscous effects of the irrotational motion produces large, elongated drops that take longer to break up in comparison with results for inviscid fluids. For larger Re, it comes as a surprise that break-up times smaller than the inviscid limit are obtained. Unfortunately, results from numerical analyses of the incompressible, unsteady Navier-Stokes equations for the case of a drop have not been presented in the literature, to the best of the authors knowledge; hence, comparison with the viscous irrotational analysis is not possible. © 2011 Cambridge University Press.

Description: Although amphitheatre-shaped valley heads can be cut by groundwater flows emerging from springs, recent geological evidence suggests that other processes may also produce similar features, thus confounding the interpretations of such valley heads on Earth and Mars. To better understand the origin of this topographic form, we combine field observations, laboratory experiments, analysis of a high-resolution topographic map and mathematical theory to quantitatively characterize a class of physical phenomena that produce amphitheatre-shaped heads. The resulting geometric growth equation accurately predicts the shape of decimetre-wide channels in laboratory experiments, 100 m-wide valleys in Florida and Idaho, and kilometre-wide valleys on Mars. We find that, whenever the processes shaping a landscape favour the growth of sharply protruding features, channels develop amphitheatre-shaped heads with an aspect ratio of . Copyright © Cambridge University Press 2011.

Description: The effects of viscosity on Kelvin-Helmholtz instability in a channel are studied using three different theories; a purely irrotational theory based on the dissipation method, an exact rotational theory and a hybrid irrotational-rotational theory. These new results are compared with previous results from a viscous irrotational theory. An analysis of the neutral state is conducted and its predictions are compared with experimental results related to the transition from a stratified-smooth to a stratified-wavy or slug flow. For values of the gas fraction greater than about 0.20, there is an interval of velocity differences for which the flow is unstable for an interval of wavenumbers between two cutoff wavenumbers, kâ̂' and k+. For unstable flows with a velocity difference above that interval or with gas fractions less than 0.20, kâ̂' = 0. The maximum critical relative velocity that determines the onset of instability can be found when the kinematic viscosity of the gas and liquid are the same. This critical value is surprisingly achieved when both fluids are inviscid. The neutral curves from the analyses of potential flow of viscous fluids and the hybrid method, the only theories that account for the viscosity of both fluids in this work, indicate that the critical velocity does not change with the viscosity ratio when the kinematic viscosity of the liquid is greater than a critical value. For smaller liquid viscosities, the critical relative velocity decreases. © 2011 Cambridge University Press.

Description: Lower slopes of the Sandia Mountains are characterized by granitic corestone topography and weathering-limited slopes with thin grusy colluvium and weakly developed soils. In contrast, thick soils with illuvial clay and pedogenic carbonate have developed below aplite outcrops. Aplite is resistant to chemical decomposition, but physically weathers to blocky clasts that enhance surface roughness and erosional resistance of colluvium, promoting accumulation of eolian fines. Thick B horizons on aplite slopes indicate limited erosion and prolonged periods of stability and soil development. Accretion of eolian material limits runoff and prevents attainment of a steady-state balance between soil production and downslope transport.

Description: Two interstadial tree ring-width chronologies from Geikie Inlet, Glacier Bay Southeast, Alaska were built from 40 logs. One of these chronologies has been calendar dated to AD 224–999 (775 yr) crossdating with a living ring-width chronology from Prince William Sound, Alaska. Trees in this chronology were likely killed through inundation by sediments and meltwater from the advancing Geikie Glacier and its tributaries ca. AD 850. The earlier tree-ring chronology spans 545 yr and is a floating ring-width series tied to radiocarbon ages of about 3000 cal yr BP. This tree-ring work indicates two intervals of glacial expansion by the Geikie Glacier system toward the main trunk glacier in Glacier Bay between 3400 and 3000 cal yr BP and again about AD 850. The timing of both expansions is consistent with patterns of ice advance at tidewater glaciers in other parts of Alaska and British Columbia about the same time, and with a relative sea-level history from just outside Glacier Bay in Icy Strait. This emerging tree-ring dated history builds on previous radiocarbon-based glacial histories and is the first study to use tree-ring dating to assign calendar dates to glacial activity for Glacier Bay.