Network asymptotics for high contrast impedance tomography

Fluid contaminant plumes underground are often electrically conducting and, therefore, can be imaged using electrical impedance tomography. The authors introduce an output least-squares method for impedance tomography problems that have regions of high conductivity surrounded by regions of lower conductivity. The high conductivity is modeled on network approximation results from an asymptotic analysis and its recovery is based on this model. The smoothly varying part of the conductivity is recovered by a linearization process as is usual. The authors present the results of several numerical experiments that illustrate the performance of the method

Quasi Two-dimensional Transfer of Elastic Waves

A theory for multiple scattering of elastic waves is presented in a random medium bounded by two ideal free surfaces, whose horizontal size is infinite and whose transverse size is smaller than the mean free path of the waves. This geometry is relevant for seismic wave propagation in the Earth crust. We derive a time-dependent, quasi-2D radiative transfer equation, that describes the coupling of the eigenmodes of the layer (surface Rayleigh waves, SH waves, and Lamb waves). Expressions are found that relate the small-scale fluctuations to the life time of the modes and to their coupling rates. We discuss a diffusion approximation that simplifies the mathematics of this model significantly, and which should apply at large lapse times. Finally, coherent backscattering is studied within the quasi-2D radiative transfer equation for different source and detection configurations.Comment: REVTeX, 36 pages with 10 figures. Submitted to Phys. Rev.

Adiabatic elimination in quantum stochastic models

We consider a physical system with a coupling to bosonic reservoirs via a quantum stochastic differential equation. We study the limit of this model as the coupling strength tends to infinity. We show that in this limit the solution to the quantum stochastic differential equation converges strongly to the solution of a limit quantum stochastic differential equation. In the limiting dynamics the excited states are removed and the ground states couple directly to the reservoirs.Comment: 17 pages, no figures, corrected mistake

Three-dimensional flow effects on forced convection heat transfer in a channel with stepwise-varying width

A numerical investigation based on the finite volume methodology of the three-dimensional laminar flow and conjugate heat transfer inside a plate-fin heat sink with stepwise-varying channel width is presented. Results are obtained for the three-dimensional flow developed in the vicinity of the flow contraction as manifested by the existence of two pairs of contra-rotating longitudinal vortices, one at the corners of the fin leading edge and the other at the flow separated regions. The topology of the flow is thoroughly examined and the contributions of the endwalls and the front surface of the flow-contracting fin to the mechanisms that cause the emergence of the three-dimensionality are determined. It is established that the corner longitudinal vortices are in fact horseshoe vortices, which emerge due to the deceleration effect of the fin front surface on the oncoming fluid. Furthermore, the interaction between the corner longitudinal vortices and the downstream recirculation bubble formed over the fin tip is also elucidated. The analysis covers a wide range of flow conditions within the laminar region in order to monitor the effect of the Reynolds number on the flow topology and to verify that the flow remains symmetrical in this region. From the numerical results, it is deduced that the recirculation downstream of the flow contraction has a beneficial impact on the heat sink thermal performance, a fact quantified through the local Nusselt number distributions

Vertical round buoyant jets and fountains in a linearly, density-stratified fluid

Turbulent round buoyant jets and fountains issuing vertically into a linearly density-stratified calm ambient have been investigated in a series of laboratory experiments. The terminal (steady-state) height of rise and the mean elevation of subsequent horizontal spreading have been measured in positively buoyant jets (at source level), including pure momentum jets and plumes, as well in momentum-driven negatively buoyant jets (fountains). The results from experiments confirmed the asymptotic analysis that was based on dimensional arguments. The normalized terminal height and spreading elevation with respect to the elevation of injection of momentum-driven (positively) buoyant jets and fountains attained the same asymptotic values. The numerical results from the solution of entrainment equations, using an improved entrainment coefficient function, confirmed the results related to buoyancy dominant flows (plumes), while their predictions in momentum-driven flows were quite low if compared to measurements. © 2020 by the authors

Manufacturing technology and dynamic mechanical investigation of moisture-resistant Luffa cylindrica-reinforced epoxy composites

Natural fiber (NF)-reinforced thermosetting composites exhibit excellent properties but, due to the usually hygroscopic nature of the fibers, these composites may suffer from the effects of moisture absorption. Luffa cylindrica is a NF, potentially useful in the manufacture of composite materials, but it is highly hygroscopic. A technology is developed for the fabrication of moisture-resistant L. cylindrica-reinforced epoxy composites and the various parameters that affect the process are investigated using long-term moisture absorption experiments and dynamic mechanical analysis of dry and moist specimens. The technology concludes to a L. cylindrica-reinforced bilayer matrix made of a first linseed oil-modified precured epoxy and a final unmodified epoxy layer. This composite offers a reduced water absorbance by 33% and an almost identical Tg compared to the non-modified epoxy reinforced with L. cylindrica. The same technology is used to manufacture water-resistant L. cylindrica-reinforced composites from both, environmentally friendly, matrix and reinforcement. © 2021 Wiley Periodicals LLC