Nonlinear, single-mode, two-dimensional Rayleigh-Taylor instability in ideal media.

A model for the single mode, two-dimensional Rayleigh-Taylor instability in ideal, incompressible, immiscible, and inviscid fluids is developed as an extension of a previous linear model based on the Newton's second law [A. R. Piriz et al., Am. J. Phys. 74, 1095 (2006)0002-950510.1119/1.2358158]. It describes the transition from linear to nonlinear regimes and takes into account the mass of fluids participating in the motion during the instability evolution, including the laterally displaced mass. This latter feature naturally leads to the bubble and spike velocity saturation without requiring the usual drag term necessary in the well-known buoyancy-drag model (BDM). In addition, it also provides an explanation to the latter phase of bubble reacceleration without appealing to the vorticity generation due to the Kelvin-Helmholtz instability. The model is in perfect agreement with the BDM buoyancy-drag model, but, apart from extending its range of application, it solves many of its issues of concern and provides a more consistent physical picture.

Two-dimensional simulations of Rayleigh-Taylor instability in elastic-plastic media.

Two-dimensional numerical simulations for the Rayleigh-Taylor instability in an elastic-plastic medium are presented. Recent predictions of the theory regarding the asymmetric growth of peaks and valleys during the linear phase of the instability evolution are confirmed. Extension to the nonlinear regime reveals singular features, such as the long delay in achieving the nonlinear saturation and an intermediate phase with growth rate larger than the classical one.

A comprehensive study of thiol reduction gene expression under stress conditions in Arabidopsis thaliana.

Thiol reduction proteins are key regulators of the redox state of the cell, managing development and stress response programs. In plants, thiol reduction proteins, namely thioredoxin (TRX), glutaredoxin (GRX), and their respective reducers glutathione reductase (GR) and thioredoxin reductase (TR), are organized in complex multigene families. In order to decipher the function of the different proteins, it is necessary to have a clear picture of their respective expression profiles. By collecting information from gene expression databases, we have performed a comprehensive in silico study of the expression of all members of different classes of thiol reduction genes (TRX, GRX) in Arabidopsis thaliana. Tissue expression profiles and response to many biotic and abiotic stress conditions have been studied systematically. Altogether, the significance of our data is discussed with respect to published biochemical and genetic studies.

Dynamic stabilization of Rayleigh-Taylor instability in newtonian fluids.

Dynamic stabilization of incompressible and immiscible newtonian fluids is studied by means of an approximate analytical model that considers the vertical vibration of the interface between the fluids. The force driving the vibration is modeled by periodic sequences of Dirac deltas. The model shows the roles played by surface tension and viscosity in determining the stability boundaries and the relevant similarity parameters are found. The results are compared with previous theoretical and experimental studies that used a sinusoidal vibration and they are found to present the same qualitative features provided a symmetric sequence of Dirac deltas is considered. Instead, important differences are observed when an asymmetric driving is used.

Linear analysis of incompressible Rayleigh-Taylor instability in solids.

The study of the linear stage of the incompressible Rayleigh-Taylor instability in elastic-plastic solids is performed by considering thick plates under a constant acceleration that is also uniform except for a small sinusoidal ripple in the horizontal plane. The analysis is carried out by using an analytical model based on the Newton second law and it is complemented with extensive two-dimensional numerical simulations. The conditions for marginal stability that determine the instability threshold are derived. Besides, the boundary for the transition from the elastic to the plastic regime is obtained and it is demonstrated that such a transition is not a sufficient condition for instability. The model yields complete analytical solutions for the perturbation amplitude evolution and reveals the main physical process that governs the instability. The theory is in general agreement with the numerical simulations and provides useful quantitative results. Implications for high-energy-density-physics experiments are also discussed.

Richtmyer-Meshkov instability in elastic-plastic media.

An analytical model for the linear Richtmyer-Meshkov instability in solids under conditions of high-energy density is presented, in order to describe the evolution of small perturbations at the solid-vacuum interface. The model shows that plasticity determines the maximum perturbation amplitude and provides simple scaling laws for it as well as for the time when it is reached. After the maximum amplitude is reached, the interface remains oscillating with a period that is determined by the elastic shear modulus. Extensive two-dimensional simulations are presented that show excellent agreement with the analytical model. The results suggest the possibility to experimentally evaluate the yield strength of solids under dynamic conditions by using a Richtmyer-Meshkov-instability-based technique.

Richtmyer-Meshkov flow in elastic solids.

Richtmyer-Meshkov flow is studied by means of an analytical model which describes the asymptotic oscillations of a corrugated interface between two perfectly elastic solids after the interaction with a shock wave. The model shows that the flow stability is due to the restoring effect of the elastic force. It provides a simple approximate but still very accurate formula for the oscillation period. It also shows that as it is observed in numerical simulations, the amplitude oscillates around a mean value equal to the post-shock amplitude, and that this is a consequence of the stress free conditions of the material immediately after the shock interaction. Extensive numerical simulations are presented to validate the model results.

Rayleigh-Taylor instability in elastic solids.

We present an analytical model for the Rayleigh-Taylor instability that allows for an approximate but still very accurate and appealing description of the instability physics in the linear regime. The model is based on the second law of Newton and it has been developed with the aim of dealing with the instability of accelerated elastic solids. It yields the asymptotic instability growth rate but also describes the initial transient phase determined by the initial conditions. We have applied the model to solid/solid and solid/fluid interfaces with arbitrary Atwood numbers. The results are in excellent agreement with previous models that yield exact solutions but which are of more limited validity. Our model allows for including more complex physics. In particular, the present approach is expected to lead to a more general theory of the instability that would allow for describing the transition to the plastic regime.

[With the things on the back: estate remodelling and inter-urban mobility]. / Con los trastes en la espalda: remodelacion estatal y movilidad intraurbana.

PIP: This work argues that intraurban mobility resulting from urban renewal in Santo Domingo is one of the city's major problems. Santo Domingo's growth from 30,000 in 1920 to 1,800,000 in the late 1980s was unplanned, and the expansion of urban infrastructure generally followed demographic expansion. The Dominican government has regularly provoked massive dislocations in the course of urban renewal, especially at the end of the Trujillo era and under the governments of Balaguer. More than 50,000 families have been forced to move in the past 30 years, representing 20% of the population. Among the consequences of this policy are a form of rootlessness of the population resulting from lack of title to the land and the constant threat of having to move again. The feeling of impermanence has psychological and cultural effects and also discourages residents from making significant improvements to their housing. The disorder of the city is increased because most of the families forced to move from areas undergoing renewal establish themselves in other marginal zones, which then undergo chaotic growth. From the fall of Trujillo to the disturbances of 1965-66, rural migration to the periphery of the city was massive, and it resumed again with the return of political stability. The massive construction projects beginning in 1967 were intended to beautify areas of high visibility, improve traffic flow, develop aesthetic elements, and remove low income populations from the center for the city. Around one-third of the national budget was devoted to construction in these years. Thousands of families were relocated in new areas belonging to the government at great distances from the city center and with few or no public services. The urban policy encouraged speculation and deterioration of urban management due to confusion between government institutions and private interests. Between 1978-86, government construction policy changed, with the amount of investment reduced and low cost housing emphasized. But massive construction began again with Balaguer's return to power. Some 20,000 families have been displaced and another 14,000 have been told to move. The forced removals have involved around 10% of the population. Their effects have included rapid expansion of the marginal urban periphery, artificially high rents, aggravation of problems in providing public services, loss of identity of neighborhoods, reinforcement of individual strategies at the expense of communal welfare, and weakening of survival resources of the most impoverished sectors.^ieng