Stability and energy budget of pressure-driven collapsible channel flows

Although self-excited oscillations in collapsible channel flows have been extensively studied, our understanding of their origins and mechanisms is still far from complete. In the present paper, we focus on the stability and energy budget of collapsible channel flows using a fluid–beam model with the pressure-driven (inlet pressure specified) condition, and highlight its differences to the flow-driven (i.e. inlet flow specified) system. The numerical finite element scheme used is a spine-based arbitrary Lagrangian–Eulerian method, which is shown to satisfy the geometric conservation law exactly. We find that the stability structure for the pressure-driven system is not a cascade as in the flow-driven case, and the mode-2 instability is no longer the primary onset of the self-excited oscillations. Instead, mode-1 instability becomes the dominating unstable mode. The mode-2 neutral curve is found to be completely enclosed by the mode-1 neutral curve in the pressure drop and wall stiffness space; hence no purely mode-2 unstable solutions exist in the parameter space investigated. By analysing the energy budgets at the neutrally stable points, we can confirm that in the high-wall-tension region (on the upper branch of the mode-1 neutral curve), the stability mechanism is the same as proposed by Jensen and Heil. Namely, self-excited oscillations can grow by extracting kinetic energy from the mean flow, with exactly two-thirds of the net kinetic energy flux dissipated by the oscillations and the remainder balanced by increased dissipation in the mean flow. However, this mechanism cannot explain the energy budget for solutions along the lower branch of the mode-1 neutral curve where greater wall deformation occurs. Nor can it explain the energy budget for the mode-2 neutral oscillations, where the unsteady pressure drop is strongly influenced by the severely collapsed wall, with stronger Bernoulli effects and flow separations. It is clear that more work is required to understand the physical mechanisms operating in different regions of the parameter space, and for different boundary conditions

On the creep fatigue behavior of Metal Matrix Composites

The mechanical behaviour of Metal Matrix Composites (MMCs) subjected to a high temperature and cyclic load condition is difficult to understand. The significantly differing coefficients of thermal expansion between ceramic and metal give rise to micro thermal stresses. Their performance under varying load and high temperature is complex and inconsistent, where fatigue and creep damages become the main failures of MMCs. To improve current understanding of the relationship between creep fatigue interaction of MMCs, the history of thermal and mechanical loading, and the creep dwell period, a highly accurate but robust direct simulation technique on the basis of the Linear Matching Method (LMM) framework has been proposed in this paper, and been applied to model the fatigue and creep behaviour of MMCs. A homogenised FE model is considered in all analyses, which consist of continuous silicon carbide fibres embedded in a square 2024T3 aluminium alloy matrix array. Various factors that affect creep and fatigue behaviours of composites are analysed and discussed, including effects of the applied load level, dwell period and temperature on the MMC's performance. The effects of reversed plasticity on stress relaxation and creep deformation of MMC are investigated, and the behaviours of cyclically enhanced creep and elastic follow-up are presented. The applicability and accuracy of the proposed direct method has also been verified by the detailed step-by-step analysis via Abaqus

Landscape-level vegetation conversion and biodiversity improvement after 33 years of restoration management in the Drentsche Aa brook valley

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Impacts of cover crops and crop residues on phosphorus losses in cold climates: a review

Non-Peer ReviewedThe use of plants in riparian buffers or cover crops is widely proposed as a strategy to mitigate sediment and nutrient losses from land to water. In cold climates, concerns may arise with regard to potentially elevated phosphorus (P) losses associated with freeze-thaw of plant materials. Here, we review the impacts of cover crops and crop residues on P loss in cold climates, and explore linkages between water extractable P in the plant materials and P loss in surface runoff and subsurface drainage from cropped soils. Water extractable P in plants is greatly affected by crop species and hardiness, as well as freezing regimes including both freezing temperature and the number of freeze-thaw cycles. Although controls on water extractable P in plant tissues and residues are relatively well understood, impacts on P runoff and leaching are inconsistent across studies due to the influences of soil, climate, and management factors. This review sheds light on improving winter crop cover management to minimize P losses from land to water in cold climates and points to future research needs. Specifically, more research is needed to understand interactions between soil, plant, hydrology, and management in influencing P loss, and to improve the assessment of crop contributions to P loss in field settings of cold climates. Further, the trade-offs between the concern over P and the control of sediment loss and nitrogen leaching should be acknowledged, as should the uncertainties of freezing and crop adaptability under future climate regimes

On fitting the Pareto-Levy distribution to stock market index data: selecting a suitable cutoff value

The so-called Pareto-Levy or power-law distribution has been successfully used as a model to describe probabilities associated to extreme variations of worldwide stock markets indexes data and it has the form $Pr(X>x) ~ x**(-alpha) for gamma< x <infinity. The selection of the threshold parameter gamma$ from empirical data and consequently, the determination of the exponent alpha, is often is done by using a simple graphical method based on a log-log scale, where a power-law probability plot shows a straight line with slope equal to the exponent of the power-law distribution. This procedure can be considered subjective, particularly with regard to the choice of the threshold or cutoff parameter gamma. In this work is presented a more objective procedure, based on a statistical measure of discrepancy between the empirical and the Pareto-Levy distribution. The technique is illustrated for data sets from the New York Stock Exchange Index and the Mexican Stock Market Index (IPC).Comment: Econophysics paper. 5 pages 9 figure

Formation of giant planets around intermediate-mass stars

To understand giant planet formation, we need to focus on host stars close to ⁠, where the occurrence rate of these planets is the highest. In this initial study, we carry out pebble-driven core accretion planet formation modelling to investigate the trends and optimal conditions for the formation of giant planets around host stars in the range of ⁠. We find that giant planets are more likely to form in systems with a larger initial disc radius; higher disc gas accretion rate; pebbles of ∼millimeter in size; and birth location of the embryo at a moderate radial distance of ∼10 au. We also conduct a population synthesis study of our model and find that the frequency of giant planets and super-Earths decreases with increasing stellar mass. This contrasts the observational peak at ⁠, stressing the need for strong assumptions on stellar mass dependencies in this range. Investigating the combined effect of stellar mass dependent disc masses, sizes, and lifetimes in the context of planet population synthesis studies is a promising avenue to alleviate this discrepancy. The hot-Jupiter occurrence rate in our models is around – similar to RV observations around Sun-like stars, but drastically decreases for higher mass stars

Doping Dependence of the Magnetic Resonance Peak in YBa_2 Cu_3 O_{6+x}

We report inelastic neutron scattering experiments on the doping dependence of the energy and spectral weight of the sharp magnetic resonance peak in YBa_2 Cu_3 O_{6+x}. These measurements also shed light on the relationship between the magnetic excitations in the normal and superconducting states.Comment: (mostly corrections to figures and minor change to caption in Fig.5) 5 figures. It will be published in the proceedings of M2S-HTSC-V. It needs the elsevier style file (not included) to convert to postscrip

The Fire and Smoke Model Evaluation Experiment—A Plan for Integrated, Large Fire–Atmosphere Field Campaigns

The Fire and Smoke Model Evaluation Experiment (FASMEE) is designed to collect integrated observations from large wildland fires and provide evaluation datasets for new models and operational systems. Wildland fire, smoke dispersion, and atmospheric chemistry models have become more sophisticated, and next-generation operational models will require evaluation datasets that are coordinated and comprehensive for their evaluation and advancement. Integrated measurements are required, including ground-based observations of fuels and fire behavior, estimates of fire-emitted heat and emissions fluxes, and observations of near-source micrometeorology, plume properties, smoke dispersion, and atmospheric chemistry. To address these requirements the FASMEE campaign design includes a study plan to guide the suite of required measurements in forested sites representative of many prescribed burning programs in the southeastern United States and increasingly common high-intensity fires in the western United States. Here we provide an overview of the proposed experiment and recommendations for key measurements. The FASMEE study provides a template for additional large-scale experimental campaigns to advance fire science and operational fire and smoke models

In-plane magnetic anisotropy of bcc Co on GaAs 001

Y. Z. Wu, H. F. Ding, C. Jing, D. Wu, G. L. Liu, V. Gordon (currently with UT Austin), G. S. Dong, and X. F. Jin are with Fudan T. D. Lee Physics Laboratory and Surface Physics Laboratory, Fudan University, Shanghai 200433, China -- S. Zhu and K. Sun are with the Beijing Laboratory of Electron Microscopy, Chinese Academy of Science, and Department of Material Engineering, Dalian University of Technology, Dalian, ChinaEpitaxial growth of Co on GaAs(001) and its in-plane magnetic anisotropy are studied using reflection high-energy electron diffraction, a high-resolution transmission electron microscope, and the magneto-optical Kerr effect. In the initial and final stages of growth, Co exists in single-crystalline body-centered-cubic (bcc) and hexagonal-closed-packed (hcp) phases, respectively, while in the middle stage the coexistence of the bcc and hcp structures is observed. For the bcc Co thin films on GaAs(001), a fourfold in-plane magnetic anisotropy with easy axes along the directions is realized and discussed.Chemistr

Doping dependence of the resonance peak and incommensuration in high-TcT_{c} superconductors

The doping and frequency evolutions of the incommensurate spin response and the resonance mode are studied based on the scenario of the Fermi surface topology. We use the slave-boson mean-field approach to the $t-t^{\prime}-J$ model and including the antiferromagnetic fluctuation correction in the random-phase approximation. We find that the equality between the incommensurability and the hole concentration is reproduced at low frequencies in the underdoped regime. This equality observed in experiments was explained {\it only} based on the stripe model before. We also obtain the downward dispersion for the spin response and predict its doping dependence for further experimental testing, as well as a proportionality between the low-energy incommensurability and the resonance energy. Our results suggest a common origin for the incommensuration and the resonance peak based on the Fermi surface topology and the d-wave symmetry.Comment: 5 pages, 4 PS figure