1,300 research outputs found
Superfluidity of metastable bulk glass para-hydrogen at low temperature
Molecular para-hydrogen has been proposed theoretically as a possible
candidate for superfluidity, but the eventual superfluid transition is hindered
by its crystallization. In this work, we study a metastable non crystalline
phase of bulk p-H2 by means of the Path Integral Monte Carlo method in order to
investigate at which temperature this system can support superfluidity. By
choosing accurately the initial configuration and using a non commensurate
simulation box, we have been able to frustrate the formation of the crystal in
the simulated system and to calculate the temperature dependence of the
one-body density matrix and of the superfluid fraction. We observe a transition
to a superfluid phase at temperatures around 1 K. The limit of zero temperature
is also studied using the diffusion Monte Carlo method. Results for the energy,
condensate fraction, and structure of the metastable liquid phase at T=0 are
reported and compared with the ones obtained for the stable solid phase.Comment: 10 pages, accepted for publication in Phys. Rev.
Definition of a shortcut methodology for assessing flood-related Na-Tech risk
Abstract. In this paper a qualitative methodology for the initial assessment of flood-related Na-Tech risk was developed as a screening tool to identify which situations require a much more expensive quantitative risk analysis (QRA). Through the definition of some suitable key hazard indicators (KHIs), the proposed methodology allows the identification of the Na-Tech risk level associated with a given situation; the analytical hierarchy process (AHP) was used as a multi-criteria decision tool for the evaluation of such qualitative KHIs. The developed methodology was validated through two case studies by comparing the predicted risk levels with the results of much more detailed QRAs previously presented in literature and then applied to the real flood happened at Spolana a.s., Neratovice, Czech Republic in August 2002.</p
Safe optimization of potentially runaway processes using topology based tools and software
In chemical industries, fast and strongly exothermic reactions are often to be carried out to
synthesize a number of intermediates and final desired products. Such processes can exhibit a phenomenon
known as \u201cthermal runaway\u201d that consists in a reactor temperature loss of control.
During the course of the years, lots of methods, aimed to detect the set of operating parameters (e.g., dosing
times, initial reactor temperature, coolant temperature, etc..) at which such a dangerous phenomenon can
occur, have been developed. Moreover, in the last few years, the attention has been posed on safe process
optimization, that is how to compute the set of operating parameters able to ensure high reactor productivity
and, contextually, safe conditions.
To achieve this goal, with particular reference to industrial semibatch synthesis carried out using both
isothermal and isoperibolic temperature control mode, a dedicated optimization software has been
implemented. Such a software identifies the optimum set of operating parameters using a topological
criterion able to bind the so-called \u201cQFS region\u201d (where reactants accumulation is low and all the heat
released is readily removed by the cooling equipment) and, then, iteratively searching for the constrained
system optimum. To manage the software, only a few experimental parameters are needed; essentially:
heat(s) of reaction, apparent system kinetics (Arrhenius law), threshold temperature(s) above which
unwanted side reactions, decompositions or boiling phenomena are triggered, heat transfer coefficients and
reactants heat capacities. Such parameters can be obtained using simple calorimetric techniques (DSC, ARC,
RC1, etc..). Over the optimization section, the software posses a simulation section where both normal and
upset operating conditions (such as pumps failure and external fire) can be tested
A relaxation scheme for computation of the joint spectral radius of matrix sets
The problem of computation of the joint (generalized) spectral radius of
matrix sets has been discussed in a number of publications. In the paper an
iteration procedure is considered that allows to build numerically Barabanov
norms for the irreducible matrix sets and simultaneously to compute the joint
spectral radius of these sets.Comment: 16 pages, 2 figures, corrected typos, accepted for publication in
JDE
Walking on a split-belt treadmill induces a higher power output and a shorter step length from the faster leg in healthy subjects, with opposite (after)-effect lasting less than 5 minutes after exercise
Walking on a split-belt treadmill has been claimed as a possible treatment of pathologic step asymmetries: in particular, the step lengthening on the affected side [1]. Placing the paretic limb on the slower belt would increase this asymmetry, reverting to long-lasting symmetry after exposure (after-effect). These studies neglected the underlying dynamics. Recently, it has been demonstrated that this paradigm entails an opposite spatial and dynamic asymmetry in healthy subjects. The stance on the faster belt is shortened, thus mimicking the paretic step temporally. On the contrary, the step is shorter and more muscle power is produced [2]. This challenges the rationale of the previous researches. The present study aims at extending these findings by investigating the after-effect both on spatiotemporal step parameters and power output from the plantar flexors on either belt. METHODS Ten healthy adults (21-34 years, 1.61-1.91 m tall, 5 women) participated in the study. After a brief familiarization, participants walked on a force-sensorized split-belt treadmill with one belt running at 0.4 m s-1 and the other belt running at 1.2 m s-1 (split condition) for 15 minutes and then, with no interruption, with the belts running at the same velocity (0.4 m s-1, tied condition) for other 5 minutes. The dominant lower limb was assigned to the faster belt. Kinematic data were recorded through an optoelectronic system as per the Davis anthropometric model. Joint sagittal power was computed by multiplying the moment generated by the ground reaction forces at the joints, times the rotation speed. All signals were simultaneously recorded [2]. The study was approved by the Local Ethics Committee. RESULTS Consistently with previous studies [3], during the split condition, the step length on the slower belt was longer, reaching gradually about 130% of the opposite step length. Ankle peak power attained about 15% of that observed on the opposite side. During the following tied condition, the step length on the formerly slower belt initially shortened by about 65% (after-effect), compared to the opposite step, and returned to values similar to that of the opposite side within 5 minutes. During this transition phase, ankle peak power gradually increased by up to 50% compared to baseline. On the formerly faster belt, step length did not change, while ankle peak power suddenly dropped to the contralateral level (Figure 1). Figure 1 Stride by stride plots (moving average, time-window 30 strides) of step length (upper panel) and ankle power (lower panel) from one representative subject (woman, 21 years, 1.65 m tall, body mass 60 kg) walking on a split-belt treadmill with the dominant lower limb on the faster belt (red) and the nondominant lower limb on the slower belt (blue). Strides from 1 to 867 refer to the split condition, and stride from 868 to 1025 refer to the following tied condition. DISCUSSION The increase in plantar flexor power on the faster belt, despite the shorter stance period and length, may reflect the priority need to counteract the backward drag from the faster belt, with respect to the slower one. This adaptation does not seem to lead to substantial learning, given that an after-effect, both on step length and ankle peak power, is only seen during the 5 minutes following split walking. In pathologic claudication, placing the affected lower limb on the faster belt might represent an effective form of \u201cforced-use\u201d [4], as far as enhanced power is requested. Long term effects remain questionable
The liaison between respiratory failure and high blood pressure: evidence from COVID-19 patients
Topological representations of matroid maps
The Topological Representation Theorem for (oriented) matroids states that
every (oriented) matroid can be realized as the intersection lattice of an
arrangement of codimension one homotopy spheres on a homotopy sphere. In this
paper, we use a construction of Engstr\"om to show that structure-preserving
maps between matroids induce topological mappings between their
representations; a result previously known only in the oriented case.
Specifically, we show that weak maps induce continuous maps and that the
process is a functor from the category of matroids with weak maps to the
homotopy category of topological spaces. We also give a new and conceptual
proof of a result regarding the Whitney numbers of the first kind of a matroid.Comment: Final version, 21 pages, 8 figures; Journal of Algebraic
Combinatorics, 201
The Epstein-Glaser approach to pQFT: graphs and Hopf algebras
The paper aims at investigating perturbative quantum field theory (pQFT) in
the approach of Epstein and Glaser (EG) and, in particular, its formulation in
the language of graphs and Hopf algebras (HAs). Various HAs are encountered,
each one associated with a special combination of physical concepts such as
normalization, localization, pseudo-unitarity, causality and an associated
regularization, and renormalization. The algebraic structures, representing the
perturbative expansion of the S-matrix, are imposed on the operator-valued
distributions which are equipped with appropriate graph indices. Translation
invariance ensures the algebras to be analytically well-defined and graded
total symmetry allows to formulate bialgebras. The algebraic results are given
embedded in the physical framework, which covers the two recent EG versions by
Fredenhagen and Scharf that differ with respect to the concrete recursive
implementation of causality. Besides, the ultraviolet divergences occuring in
Feynman's representation are mathematically reasoned. As a final result, the
change of the renormalization scheme in the EG framework is modeled via a HA
which can be seen as the EG-analog of Kreimer's HA.Comment: 52 pages, 5 figure
Composition-Diamond lemma for -differential associative algebras with multiple operators
In this paper, we establish the Composition-Diamond lemma for
-differential associative algebras over a field with multiple
operators. As applications, we obtain Gr\"{o}bner-Shirshov bases of free
-differential Rota-Baxter algebras. In particular, linear bases of
free -differential Rota-Baxter algebras are obtained and consequently,
the free -differential Rota-Baxter algebras are constructed by words
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