Research for people : IDRC's experience in forestry research

Abstract in Frenc

Viscoelasticity of crystal- and bubble-bearing rhyolite melts

The effect of non-deformable inclusions on the frequency-dependent rheology of a rhyolite melt plus crystals has been investigated using a sinusoidal torsion deformation device for measurements of shear viscosity and modulus in the frequency range of 5 mHz to 20 Hz at temperatures of 750–1050°C. The relaxed shear viscosity and unrelaxed shear modulus of rhyolite magma (rhyolite melt plus crystals plus bubbles) decreases with increasing bubble content and increases with the addition of crystals. At a crystal concentration of about 45% a relaxed value of the shear viscosity is not attainable. The presence of rigid inclusions results in an imaginary component of the shear modulus that becomes more symmetrical and shifted to the low-frequency—high-temperature range with respect to that for a crystal-free melt. The slope of log(Q−1) (internal friction) as a function of the dimensionless variable log(ωτ), is unaffected in the low-temperature—high-frequency range of crystals, with Q−1 ≈ 1/(ωτ)0.5 (the same as for bubble- and crystal-free rhyolite). For the present type of suspension, the internal friction is practically constant and independent of log(ωτ) in the high-temperature—low-frequency limit (ωτ 1). The shape of the Cole-Cole diagram becomes symmetrical and can be described as a Caputo body with parameter γ ≈ 0.45, whereas for bubble-bearing and inclusion-free rhyolite melts the shape of diagram relates to the β-relaxation exponent with β ≈ 0.5. The present work demonstrates that magma may or may not follow a power-law rheology depending on the relative volume proportion between crystals and bubbles

Guide to species selection for tropical and sub-tropical plantations

2d ed. rev

Forestación en zonas áridas y semiáridas : actas

Reunión: Encuentro Regional CIID América Latina y el Caribe, 2o, 13-17 mayo 1985, Santiago, C

Optimised mixing and flow resistance during shear flow over a rib roughened boundary

A series of numerical investigations has been performed to study the effect of lower boundary roughness on turbulent flow in a two-dimensional channel. The roughness spacing to height ratio, w/k, has been investigated over the range 0.12 to 402 by varying the horizontal rib spacing. The square roughness elements each have a cross-sectional area of (0.05 H)2, where H is the full channel height. The Reynolds number, Reτ is fixed based on the value of the imposed pressure gradient, dp/dx, and is in the range 6.3 × 103 − 4.5 × 104. A Reynolds Averaged Navier–Stokes (RANS) based turbulence modelling approach is adopted using a commercial CFD code, ANSYS-CFX 14.0. Measurements of eddy viscosity and friction factor have been made over this range to establish the optimum spacings to produce maximum turbulence enhancement, mixing and resistance to flow. These occur when w/k is approximately 7. It is found that this value is only weakly dependent on Reynolds number, and the decay rate of turbulence enhancement as a function of w/k ratio beyond this optimum spacing is slow. The implications for heat transfer design optimisation and particle transport are considered

Sum rules and energy scales in the high-temperature superconductor YBa2Cu3O6+x

The Ferrell-Glover-Tinkham (FGT) sum rule has been applied to the temperature dependence of the in-plane optical conductivity of optimally-doped YBa_2Cu_3O_{6.95} and underdoped YBa_2Cu_3O_{6.60}. Within the accuracy of the experiment, the sum rule is obeyed in both materials. However, the energy scale \omega_c required to recover the full strength of the superfluid \rho_s in the two materials is dramatically different; \omega_c \simeq 800 cm^{-1} in the optimally doped system (close to twice the maximum of the superconducting gap, 2\Delta_0), but \omega_c \gtrsim 5000 cm^{-1} in the underdoped system. In both materials, the normal-state scattering rate close to the critical temperature is small, \Gamma < 2\Delta_0, so that the materials are not in the dirty limit and the relevant energy scale for \rho_s in a BCS material should be twice the energy gap. The FGT sum rule in the optimally-doped material suggests that the majority of the spectral weight of the condensate comes from energies below 2\Delta_0, which is consistent with a BCS material in which the condensate originates from a Fermi liquid normal state. In the underdoped material the larger energy scale may be a result of the non-Fermi liquid nature of the normal state. The dramatically different energy scales suggest that the nature of the normal state creates specific conditions for observing the different aspects of what is presumably a central mechanism for superconductivity in these materials.Comment: RevTeX 4 file, 9 pages with 7 embedded eps figure

Dimensionless cosmology

Although it is well known that any consideration of the variations of fundamental constants should be restricted to their dimensionless combinations, the literature on variations of the gravitational constant $G$ is entirely dimensionful. To illustrate applications of this to cosmology, we explicitly give a dimensionless version of the parameters of the standard cosmological model, and describe the physics of Big Bang Neucleosynthesis and recombination in a dimensionless manner. The issue that appears to have been missed in many studies is that in cosmology the strength of gravity is bound up in the cosmological equations, and the epoch at which we live is a crucial part of the model. We argue that it is useful to consider the hypothetical situation of communicating with another civilization (with entirely different units), comparing only dimensionless constants, in order to decide if we live in a Universe governed by precisely the same physical laws. In this thought experiment, we would also have to compare epochs, which can be defined by giving the value of any {\it one} of the evolving cosmological parameters. By setting things up carefully in this way one can avoid inconsistent results when considering variable constants, caused by effectively fixing more than one parameter today. We show examples of this effect by considering microwave background anisotropies, being careful to maintain dimensionlessness throughout. We present Fisher matrix calculations to estimate how well the fine structure constants for electromagnetism and gravity can be determined with future microwave background experiments. We highlight how one can be misled by simply adding $G$ to the usual cosmological parameter set

Cosmic Microwave Background constraint on residual annihilations of relic particles

Energy injected into the Cosmic Microwave Background at redshifts z<10^6 will distort its spectrum permanently. In this paper we discuss the distortion caused by annihilations of relic particles. We use the observational bounds on deviations from a Planck spectrum to constrain a combination of annihilation cross section, mass, and abundance. For particles with (s-wave) annihilation cross section, =\sigma_0, the bound is f[(\sigma_0/6e-27cm^3/s)(\Omega_{X\bar{X}}h^2)^2]/(m_X/MeV)<0.2, where m_X is the particle mass, \Omega_{X\bar{X}} is the fraction of the critical density the particle and its antiparticle contribute if they survive to the present time, h=H_0/(100km/s/Mpc), H_0 is the Hubble constant, and f is the fraction of the annihilation energy that interacts electromagnetically. We also compute the less stringent limits for p-wave annihilation. We update other bounds on residual annihilations and compare them to our CMB bound.Comment: submitted to Phys. Rev.

Fermi acceleration in astrophysical jets

We consider the acceleration of energetic particles by Fermi processes (i.e., diffusive shock acceleration, second order Fermi acceleration, and gradual shear acceleration) in relativistic astrophysical jets, with particular attention given to recent progress in the field of viscous shear acceleration. We analyze the associated acceleration timescales and the resulting particle distributions, and discuss the relevance of these processes for the acceleration of charged particles in the jets of AGNs, GRBs and microquasars, showing that multi-component powerlaw-type particle distributions are likely to occur.Comment: 6 pages, one figure; based on talk at "The multimessenger approach to unidentified gamma-ray sources", Barcelona/Spain, July 2006; accepted for publication in Astrophysics and Space Scienc