28,041 research outputs found
Fiscal responsibility laws for subnational discipline : the Latin American experience
This paper discusses fiscal responsibility laws in Latin America, with special attention to their provisions for fiscal discipline by subnational governments. It discusses why and when such laws might be useful-to help resolve the coordination problem in getting diverse governments to avoid overusing the common national credit market and to help individual governments make a time-consistent commitment for fiscal prudence. It examines the cases of Brazil, Colombia, Peru, and Argentina, as well as the case of Mexico where other types of laws and regulations aim to achieve the same objectives of solidifying incentives for fiscal discipline at all levels of government. Fiscal responsibility laws are found to be useful in some cases, although the experience is not long enough to be certain, but they are clearly not necessary in every case, nor always sufficient to assure fiscal stability.Urban Economics,Banks&Banking Reform,Public&Municipal Finance,Public Sector Economics&Finance,National Governance,National Governance,Banks&Banking Reform,Public Sector Economics&Finance,Urban Economics,Public&Municipal Finance
Structural relaxation in silicate melts and non-Newtonian melt rheology in geologic processes
The timescale of structural relaxation in a silicate melt defines the transition from liquid (relaxed) to glassy (unrelaxed) behavior. Structural relaxation in silicate melts can be described by a relaxation time, , consistent with the observation that the timescales of both volume and shear relaxation are of the same order of magnitude. The onset of significantly unrelaxed behavior occurs 2 log10 units of time above . In the case of shear relaxation, the relaxation time can be quantified using the Maxwell relationship for a viscoelastic material; S = S/G (where S is the shear relaxation time, G is the shear modulus at infinite frequency and S is the zero frequency shear viscosity). The value of G known for SiO2 and several other silicate glasses. The shear modulus, G , and the bulk modulus, K , are similar in magnitude for every glass, with both moduli being relatively insensitive to changes in temperature and composition. In contrast, the shear viscosity of silicate melts ranges over at least ten orders of magnitude, with composition at fixed temperature, and with temperature at fixed composition. Therefore, relative to S, G may be considered a constant (independent of composition and temperature) and the value of S, the relaxation time, may be estimated directly for the large number of silicate melts for which the shear viscosity is known.
For silicate melts, the relaxation times calculated from the Maxwell relationship agree well with available data for the onset of the frequency-dependence (dispersion) of acoustic velocities, the onset of non-Newtonian viscosities, the scan-rate dependence of the calorimetric glass transition, with the timescale of an oxygen diffusive jump and with the Si-O bond exchange frequency obtained from 29Si NMR studies.
Using data obtained over a range of frequencies and strain-rates we illustrate the significance of relaxed versus unrelaxed behavior in laboratory experiments on silicate melts. Similarly, using strain-rate estimates for magmatic processes we evaluate the significance of the liquid-glass transition in igneous petrogenesis.
Dedicated to the memory of Chris Scarf
Isospectral Graph Reductions and Improved Estimates of Matrices' Spectra
Via the process of isospectral graph reduction the adjacency matrix of a
graph can be reduced to a smaller matrix while its spectrum is preserved up to
some known set. It is then possible to estimate the spectrum of the original
matrix by considering Gershgorin-type estimates associated with the reduced
matrix. The main result of this paper is that eigenvalue estimates associated
with Gershgorin, Brauer, Brualdi, and Varga improve as the matrix size is
reduced. Moreover, given that such estimates improve with each successive
reduction, it is also possible to estimate the eigenvalues of a matrix with
increasing accuracy by repeated use of this process.Comment: 32 page
Non-Newtonian Rheology of Igneous Melts at High Stresses and Strain Rates: Experimental Results for Rhyolite, Andesite, Basalt, and Nephelinite
The stress-strain rate relationships of four silicate melt compositions (high-silica rhyolite, andesite, tholeiitic basalt, and nephelinite) have been studied using the fiber elongation method. Measurements were conducted in a stress range of 10–400 MPa and a strain rate range of 10−6 to 10−3 s−1. The stress-strain rate relationships for all the melts exhibit Newtonian behavior at low strain rates, but non-Newtonian (nonlinear stress-strain rate) behavior at higher strain rates, with strain rate increasing faster than the applied stress. The decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt. The decrease in viscosity observed at the high strain rates of the present study ranges from 0.25 to 2.54 log10 Pa s. The shear relaxation times τ of these melts have been estimated from the low strain rate, Newtonian, shear viscosity, using the Maxwell relationship τ = η s /G ∞. Non-Newtonian shear viscosity is observed at strain rates ( ɛ ˙ = time - 1 ) equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time. Brittle failure of the fibers occurs 2 log10 units of time above the relaxation time. This study illustrates that the occurrence of non-Newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate. High-silica rhyolite melts involved in ash flow eruptions are expected to undergo a non-Newtonian phase of deformation immediately prior to brittle failure
Evaluating the New Automatic Method for the Analysis of Absorption Spectra Using Synthetic Spectra
We recently presented a new "artificial intelligence" method for the analysis
of high-resolution absorption spectra (Bainbridge and Webb, Mon. Not. R.
Astron. Soc. 2017, 468,1639-1670). This new method unifies three established
numerical methods: a genetic algorithm (GVPFIT); non-linear least-squares
optimisation with parameter constraints (VPFIT); and Bayesian Model Averaging
(BMA). In this work, we investigate the performance of GVPFIT and BMA over a
broad range of velocity structures using synthetic spectra. We found that this
new method recovers the velocity structures of the absorption systems and
accurately estimates variation in the fine structure constant. Studies such as
this one are required to evaluate this new method before it can be applied to
the analysis of large sets of absorption spectra. This is the first time that a
sample of synthetic spectra has been utilised to investigate the analysis of
absorption spectra. Probing the variation of nature's fundamental constants
(such as the fine structure constant), through the analysis of absorption
spectra, is one of the most direct ways of testing the universality of physical
laws. This "artificial intelligence" method provides a way to avoid the main
limiting factor, i.e., human interaction, in the analysis of absorption
spectra.Comment: 9 pages, 5 figures, published on 5 April 2017 in Univers
The fluxing effect of fluorine at magmatic temperatures (600-800 °C): A scanning calorimetric study
The effect of F on the glass transition behavior of albite, diopside, and four other silicate
melts has been investigated using scanning calorimetry. The addition of F to all silicate
melts investigated results in a strong, nonlinear decrease of the glass transition temperature
(Z' as recorded by the peak temperatures of heat capacity). The decreases observed extrapolate
consistently to published fluoride glass transition temperatures. The largest Z,
decrease is observed for albite-FrO-, melts (AT = 250 °C at 6 wt%F ). The effect of F is
similar to that previously observed for HrO (Taniguchi, 1981).
Physical properties of low-temperature silicate liquids are a valuable constraint on lowtemperature
petrogenetic processes in granite and pegmatite petrogenesis. Low-temperature
wiscosities can be estimated from the glass transition data. These data are combined
with previously published high-temperature, concentric-cylinder viscosity data to obtain
a much more complete description of the temperature dependence of viscosity for these
melts.
The present data, obtained on supercooled liquids close to the glass transition, are of
special significance because it is at the glass transition that silicate glass structures are
frozen. A separate multinuclear NMR study of glasses quenched from these experiments
has shown that the predominant coordination of F in albite glass is octahedral to Al. The
coordination state of F does not appear to be concentration dependent, and thus the
structural origin of the nonlinear Z, decrease does not arise from such a mechanism
The onset of non-Newtonian rheology of silicate melts
The viscoelastic behavior of silicate melts has been measured for a range of compositions (NaAlSi3O8, NaCaAlSi2O7, CaMgSi2O6, Li2Si4O9, Na2Si4O9, K2Si4O9, Na2Si3O7, K2Si3O7 and Na2Si2O5) using the fiber elongation method. A1l compositions exhibit Newtonian behavior at low strain-rates, but non-Newtonian behavior at higher strain-rates, with strain-rate increasing faster than the applied stress. The decrease in shear viscosity observed at the high strain-rates ranges from 0.3 to 1.6 log10 units (Pa s). The relaxation strain-rates, relax, of these melts have been estimated from the low strain-rate, Newtonian, shear viscosity, using the Maxwell relationship; relax= –1=(s/G)–1. For all compositions investigated, the onset of non-Newtonian rheology is observed at strain-rates 2.5+0.5 orders of magnitude less than the calculated relaxation strain-rate. This difference between the non-Newtonian onset and the relaxation strain-rate is larger than that predicted by the single relaxation time Maxwell model. Normalization of the experimental strain-rates to the relaxation strain-rate predicted from the Maxwell relation, eliminates the composition. and temperature-dependence of the onset of non-Newtonian behavior. The distribution of relaxation in the viscoelastic region appears to be unrelated to melt chemistry. This conclusion is consistent with the torsional, frequency domain study of Mills (1974) which illustrated a composition-invariance of the distribution of the imaginary component of the shear modulus in melts on the Na2O-SiO2 join. The present, time domain study of viscoelasticity contrasts with frequency domain studies in terms of the absolute strains employed. The present study employs relatively large total strains (up to 2). This compares with typical strains of 10–8 in ultrasonic (frequency domain) studies. The stresses used to achieve the strain-rates required to observe viscoelastic behavior in this study approach the tensile strength of the fibers with the result that some of our experiments were terminated by fiber breakage. Although the breakage is unrelated to the observation of non-Newtonian viscosity, their close proximity in this and earlier studies suggests that brittle failure of igneous melts, may, in general, be preceded by a period of non-Newtonian rheology
- …