A Quasi-Spherical Gravitational Wave Solution in Kaluza-Klein Theory

An exact solution of the source-free Kaluza-Klein field equations is presented. It is a 5D generalization of the Robinson-Trautman quasi-spherical gravitational wave with a cosmological constant. The properties of the 5D solution are briefly described.Comment: 10 pages Latex, Revtex, submitted to GR

Comment on "Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2"

The microscopic origin of the recently observed helical structure in LiCu2O2 [1] is considered. It is shown that the frustrated antiferromagnetic double chain scenario adopted in Ref.1 is unrealistic. It should be replaced by a frustrated single-chain (CuO2) scenario proposed in Ref. 2 with ferromagnetic nearest neighbor and antiferromagnetic next nearest neighbor exchange integrals \. [1] T. Masuda et al. Phys. Rev. Lett. 92 (2004), 177201. [2] A. Gippius et al. Phys. Rev. B 70 (2004), R01426; cond-mat/0312576.Comment: one page, one figur

"Swarm relaxation": Equilibrating a large ensemble of computer simulations

It is common practice in molecular dynamics and Monte Carlo computer simulations to run multiple, separately-initialized simulations in order to improve the sampling of independent microstates. Here we examine the utility of an extreme case of this strategy, in which we run a large ensemble of $M$ independent simulations (a "swarm"), each of which is relaxed to equilibrium. We show that if $M$ is of order $10^3$, we can monitor the swarm's relaxation to equilibrium, and confirm its attainment, within $\sim 10\bar\tau$, where $\bar\tau$ is the equilibrium relaxation time. As soon as a swarm of this size attains equilibrium, the ensemble of $M$ final microstates from each run is sufficient for the evaluation of most equilibrium properties without further sampling. This approach dramatically reduces the wall-clock time required, compared to a single long simulation, by a factor of several hundred, at the cost of an increase in the total computational effort by a small factor. It is also well-suited to modern computing systems having thousands of processors, and is a viable strategy for simulation studies that need to produce high-precision results in a minimum of wall-clock time. We present results obtained by applying this approach to several test cases.Comment: 12 pages. To appear in Eur. Phy. J. E, 201

Effect of feed salinity on the performance of humidification dehumidification desalination

Humidification dehumidification (HDH) is a thermal desalination technology that imitates the rain cycle in an engineered setting. It can be advantageous is small-scale, decentralized applications. In addition, the components used in HDH systems are fairly robust, and can treat highly saline water. The technology has recently been commercialized in order to treat highly saline produced water from hydraulically fractured oil and gas wells. That plant has proved HDH’s ability to treat water that most current seawater desalination technologies are unable to treat. The major disadvantage of HDH is its low energy efficiency compared to other desalination technologies when treating seawater. Previous studies have shown that the system’s energy efficiency can be improved greatly by varying the water-to-air mass flow rate ratio within the system. This translates into operating two or more adjacent stages at different mass flow rate ratios, which is done by extracting an air stream from an intermediate location in the humidifier and injecting it at an intermediate location in the dehumidifier. Previous models have used fixed effectiveness or fixed pinch approaches to evaluate the benefits of multi-staging, but these do not take account of the size of the system. In physical systems, what remains constant when going from a single-stage to a multi-stage system is the total size of the system and not the effectiveness or the pinch. Therefore, comparing systems with the same total heat exchanger area is the best way to understand the effect of extraction/injection and whether its implementation is beneficial or not. In this paper, a numerical heat and mass transfer model is used to simulate the operation of HDH at various feed salinities. For each case, the performance of the single-stage system is compared to that of a two-stage system of the same size at different values of feed salinity. The ability of HDH to treat feeds with varying salinity is also studied.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Entropy generation analysis of electrodialysis

Electrodialysis (ED) is a desalination technology with many applications. In order to better understand how the energetic performance of this technology can be improved, the various losses in the system should be quantified and characterized. This can be done by looking at the entropy generation in ED systems. In this paper, we implement an ED model based on the Maxwell-Stefan transport model, which is the closest model to fundamental equations. We study the sources of entropy generation at different salinities, and locate areas where possible improvements need to be made under different operating conditions. In addition, we study the effect of the channel height, membrane thickness, and cell-pair voltage on the specific rate of entropy generation. We express the second-law efficiency of ED as the product of current and voltage utilization rates, and study its variation with current density. Further, we define the useful voltage that is used beneficially for separation. We derive the rate of entropy generation that is due to the passage of ions through a voltage drop, and we investigate whether voltage drops themselves can provide a good estimate of entropy generation.Kuwait Foundation for the Advancement of Sciences (KFAS) (Project No. P31475EC01

How will land degradation neutrality change future land system patterns?:A scenario simulation study

Land degradation is a major global issue and achieving a land degradation-neutral world is one of the Sustainable Development Goals. However, striving for land degradation neutrality (LDN) is challenged by increasing claims on land resources and could result in major land use conflicts. The aim of this study is to demonstrate how LDN can be implemented in land system modelling and how achieving LDN alongside sufficient supplies of food, timber and shelter could affect future land system patterns, using the Republic of Turkey as a case study. We developed a LDN scenario with full implementation of the guidelines and a business-as-usual scenario without pursuing LDN, and compared the resulting differences in land system changes. Additionally, the influence of different elements of the LDN framework on the land use projections was tested. Our results show that although it is possible to achieve LDN in the context of increasing demands for resources and housing, it might require a considerable re-organization of the land systems. Intensification of annual cropland systems was the main driver of new land degradation, which was in the LDN scenario primarily counterbalanced by large areas of afforestation, while other land improvement options only played a minor role. To achieve a no-net-loss, about 20% of Turkey’s territory was afforested in our scenario, mainly claiming extensively used annual cropland (~70%) and grassland (~30%). All individual LDN principles had a substantial impact on the final land system patterns meaning that the final outcome is not the result of just one of the principles, it is affected by all. Our findings suggest that pursuing LDN under growing demands for land-based products could stimulate a land sparing approach which might have trade-offs with other sustainability dimensions. This highlights the need for local support and new solutions for rural areas, thereby avoiding poverty, migration and illegal use of restoration areas

Controllability and stability of 3D heat conduction equation in a submicroscale thin film

We obtain a closed form analytic solution for the Dual Phase Lagging equation. This equation is a linear, time-independent partial differential equation modeling the heat distribution in a thin film. The spatial domain is of micrometer and nanometer geometries. We show that the solution is described by a semigroup, and obtain a basis of eigenfunctions. The closure of the set of eigenvalues contains an interval, and so the theory on Riesz spectral operator of Curtain and Zwart cannot be applied directly. The exponential stability and the approximate controllability is shown

Incremental expansions for Hubbard-Peierls systems

The ground state energies of infinite half-filled Hubbard-Peierls chains are investigated combining incremental expansion with exact diagonalization of finite chain segments. The ground state energy of equidistant infinite Hubbard (Heisenberg) chains is calculated with a relative error of less than $3 \cdot 10^{-3}$ for all values of $U$ using diagonalizations of 12-site (20-site) chain segm ents. For dimerized chains the dimerization order parameter $d$ as a function of the onsite repulsion interaction $U$ has a maximum at nonzero values of $U$, if the electron-phonon coupling $g$ is lower than a critical value $g_c$. The critical value $g_c$ is found with high accuracy to be $g_c=0.69$. For smaller values of $g$ the position of the maximum of $d(U)$ is approximately $3t$, and rapidly tends to zero as $g$ approaches $g_c$ from below. We show how our method can be applied to calculate breathers for the problem of phonon dynamics in Hubbard-Peierls systems.Comment: 4 Pages, 3 Figures, REVTE