155 research outputs found
Realization of random-field dipolar Ising ferromagnetism in a molecular magnet
The longitudinal magnetic susceptibility of single crystals of the molecular
magnet Mn-acetate obeys a Curie-Weiss law, indicating a transition to a
ferromagnetic phase due to dipolar interactions. With increasing magnetic field
applied transverse to the easy axis, the transition temperature decreases
considerably more rapidly than predicted by mean field theory to a T=0 quantum
critical point. Our results are consistent with an effective Hamiltonian for a
random-field Ising ferromagnet in a transverse field, where the randomness is
induced by an external field applied to Mn-acetate crystals that are
known to have an intrinsic distribution of locally tilted magnetic easy axes.Comment: 4 pages, 4 figure
Final results from the EU project AVATAR: aerodynamic modelling of 10 MW wind turbines
This paper presents final results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Special attention is paid to the improvement of low fidelity engineering (BEM based) models with higher fidelity (CFD) models but also with intermediate fidelity free vortex wake (FVW) models. The latter methods were found to be a good basis for improvement of induction modelling in engineering methods amongst others for the prediction of yawed cases, which in AVATAR was found to be one of the most challenging subjects to model. FVW methods also helped to improve the prediction of tip losses. Aero-elastic calculations with BEM based and FVW based models showed that fatigue loads for normal production cases were over predicted with approximately 15% or even more. It should then be realised that the outcome of BEM based models does not only depend on the choice of engineering add-ons (as is often assumed) but it is also heavily dependent on the way the induced velocities are solved. To this end an annulus and element approach are discussed which are assessed with the aid of FVW methods. For the prediction of fatigue loads the so-called element approach is recommended but the derived yaw models rely on an annulus approach which pleads for a generalised solution method for the induced velocities
Experimental determination of the Weiss temperature of Mn-ac and Mn-ac-MeOH
We report measurements of the susceptibility in the temperature range from
K to K of a series of Mn-ac and Mn-ac-MeOH samples in
the shape of rectangular prisms of length and square cross-section of
side . The susceptibility obeys a Curie-Weiss Law, ,
where varies systematically with sample aspect ratio. Using published
demagnetization factors, we obtain for an infinitely long sample
corresponding to intrinsic ordering temperatures K and
K for Mn-ac and Mn-ac-MeOH, respectively. The
difference in for two materials that have nearly identical unit cell
volumes and lattice constant ratios suggests that, in addition to dipolar
interactions, there is a non-dipolar (exchange) contribution to the Weiss
temperature that differs in the two materials because of the difference in
ligand molecules.Comment: 4.5 page
A voxelized immersed boundary (VIB) finite element method for accurate and efficient blood flow simulation
We present an efficient and accurate immersed boundary (IB) finite element
(FE) method for internal flow problems with complex geometries (e.g., blood
flow in the vascular system). In this study, we use a voxelized flow domain
(discretized with hexahedral and tetrahedral elements) instead of a box domain,
which is frequently used in IB methods. The proposed method utilizes the
well-established incremental pressure correction scheme (IPCS) FE solver, and
the boundary condition-enforced IB (BCE-IB) method to numerically solve the
transient, incompressible Navier--Stokes flow equations. We verify the accuracy
of our numerical method using the analytical solution for the Poiseuille flow
in a cylinder, and the available experimental data (laser Doppler velocimetry)
for the flow in a three-dimensional 90{\deg} angle tube bend. We further
examine the accuracy and applicability of the proposed method by considering
flow within complex geometries, such as blood flow in aneurysmal vessels and
the aorta, flow configurations that would otherwise be difficult to solve by
most IB methods. Our method offers high accuracy, as demonstrated by the
verification examples, and high applicability, as demonstrated through the
solution of blood flow within complex geometry. The proposed method is
efficient, since it is as fast as the traditional finite element method used to
solve the Navier--Stokes flow equations, with a small overhead (not more than
5) due to the numerical solution of a linear system formulated for the IB
method.Comment: arXiv admin note: substantial text overlap with arXiv:2007.0208
The impact of the COVID-19 pandemic on the learning and wellbeing of secondary school students: a survey in Southern Europe
The transition from the traditional model of learning and teaching to full online mode had to be implemented in many countries, in an extremely short time, as the 2020-2021 school year was in mid-stream. Secondary education, which includes students in the age range of 12-18, faced many challenges in this rapid change, as many research studies have shown. Researchers raise questions regarding the readiness of the secondary education community to transition to fully online learning. The pilot study reported in this paper deals with the impact of the transition to online learning on secondary schools in southern European countries. More specifically, this paper presents the results of a literature survey and an empirical survey using an online questionnaire which captured non-traceable responses from secondary schools that, voluntarily and anonymously, completed the questionnaire. The questions were mainly closed, with some open-ended questions for students to fill in. The study also aims to capture data on the socio-economic dimension, accessibility/ availability of the necessary technologies that enable online learning, as well as the families’ employment status and their ability to support students. A total of 90 students participated (62% female, 28% male) from three Mediterranean countries. The students’ perspectives as seen by the students themselves along with the difficulties and the issues they faced are compared and contrasted. This investigation offers a pedagogical and socio-technical analysis and highlights the needs for wellbeing as well as quality learning and teaching in the new social distance reality
Transient Global Amnesia and Takosubo Syndrome: A Case Report
Broken heart syndrome, also called apical ballooning syndrome or Takotsubo cardiomyopathy, is characterized by an acute, transient systolic dysfunction of the apical and mid segments of the left ventricle. There have been reports showing an association with neurologic disorders. In this article, we present the case of a female patient, who suffered from an episode of transient global amnesia followed by takotsubo cardiomyopathy
Radio Interferometric Calibration Using The SAGE Algorithm
The aim of the new generation of radio synthesis arrays such as LOFAR and SKA
is to achieve much higher sensitivity, resolution and frequency coverage than
what is available now, especially at low frequencies. To accomplish this goal,
the accuracy of the calibration techniques used is of considerable importance.
Moreover, since these telescopes produce huge amounts of data, speed of
convergence of calibration is a major bottleneck. The errors in calibration are
due to system noise (sky and instrumental) as well as the estimation errors
introduced by the calibration technique itself, which we call solver noise. We
define solver noise as the distance between the optimal solution (the true
value of the unknowns, uncorrupted by the system noise) and the solution
obtained by calibration. We present the Space Alternating Generalized
Expectation Maximization (SAGE) calibration technique, which is a modification
of the Expectation Maximization algorithm, and compare its performance with the
traditional Least Squares calibration based on the level of solver noise
introduced by each technique. For this purpose, we develop statistical methods
that use the calibrated solutions to estimate the level of solver noise. The
SAGE calibration algorithm yields very promising results both in terms of
accuracy and speed of convergence. The comparison approaches we adopt introduce
a new framework for assessing the performance of different calibration schemes.Comment: 12 pages, 10 figures, Accepted for publication in MNRA
Analisis produktivitas dan kinerja lingkungan dengan menggunakan konsep green productivity (study kasus PT.ABC)
PT. ABC is a private company engaged in the provision of clean water services under the auspices of PT. Kaltim Prima Coal). Constraint occurs is in the process of drinking water treatment and wastewater sludge yield potential as a source of environmental pollution. Other problems also exist in the high BOD and Cod content of wastewater after measurement. As for the number of connections installed for corporate customers, there are only 2,895 connections, while from the data of PTABC, the company's connection target is 12,589, so from the total target of PT. ABC has only been able to obtain a connection of 23% of the target to be achieved. a need to increase water production by PT. ABC in order to increase the quantity of clean water production for all consumers. The concept of green productivity (GP) can be used in the search for solutions to increase productivity and protect the environment. The study begins by identifying the root cause of a lot of liquid waste and a low amount of water production, setting objectives and targets, choosing the resources and information available to arrange alternative green productivity. There are two alternatives, the selected alternative in this study are alternative 2 merge maximizing the raw water intake, repair Well Bore Tj.Bara , and the use of wastewater that is the use of waste as material raw returned by first applying multilevel aeration technology to reduce levels of COD and BOD5 in wastewater. based on the use of the cost benefit anlysis method the company's productivity and EPI increases if implementing this alternative will increase 40% from the initial production of 114% to 154%. And the initial EPI index total value of -20.8 increased to 18.8 so it rose 39.6 which indicates an increase in productivity and environmental performanc
Rapid migration from traditional or hybrid to fully virtual education in the age of the coronavirus pandemic: challenges, experiences and views of college and university students
The abrupt outbreak of the coronavirus pandemic throughout the world in March 2020 resulted in the sudden closure of all schools, colleges and universities, institutions, and an unprecedented pivot to remote learning. Students and teachers were confronted with the overwhelming challenge of migrating from the traditional face-to-face or hybrid mode of education to fully virtual learning and assessment environments within an extremely short amount of time. This migration was exceptionally difficult, as it took place halfway through the academic or school year in most countries. While pandemic restrictions currently vary across different regions, the 2020-2021 academic session continues to pose challenges despite the experience gained. In addition to a review of the current state-of-the-art in relation to the effects of COVID-19 on teaching and learning, this paper reports on an empirical study carried out in 26 countries (from Asia, Europe, Africa and America), by 36 academics from 29 academic institutions. Through an extensive global survey of college and university students, information was collected about the challenges (technological, economic, psychological) faced by them, as a result of the pandemic. We also asked the students’ to offer their ideas and suggestions for further improvements in teaching and learning, as we look toward a post-COVID world. In this paper, we address issues relating to the availability of, and accessibility to, necessary digital technologies (e.g., learning and communication platforms), isolation, disconnection, and loneliness among students, the overall impact of the pandemic on learning and academic performance, and the reliability of assessment methods., cybercrime dangers and fake information. A total of 1005 responses to the survey were received and analysed. The results are presented in this paper together with reflections of the authors. The paper concludes with a summary of suggestions for process improvements in distance education, and the need for preventive preparedness in the post-COVID period
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