Onset of collective and cohesive motion

We study the onset of collective motion, with and without cohesion, of groups of noisy self-propelled particles interacting locally. We find that this phase transition, in two space dimensions, is always discontinuous, including for the minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for which a non-trivial critical point was previously advocated. We also show that cohesion is always lost near onset, as a result of the interplay of density, velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let

Factors in the management of the statutory financial, aimed at developing the bidding process

El presente artículo, enmarca la importancia y los factores determinantes en la gestión de la Revisoría Fiscal en los procesos licitatorios identificando las principales características, lineamientos y desarrollo de cada proceso en donde interviene esta figura creada por el estado Colombiano para ejercer control y vigilancia a operaciones económicas y financieras privadas que de cierta manera afectan el interés público, la figura de la revisoría fiscal se fortaleció con el Código de Comercio Colombiano el cual le delega funciones y facultades muy importantes en las que se le establecen responsabilidades muy únicas del estado, sin embargo este también brinda herramientas para que la Revisoría Fiscal puede cumplir a cabalidad con su rol, actuaciones éticas y legales, establece un código de ética para el Contador Público y también genera leyes que aportan en este caso al correcto manejo, se encuentra la Ley 80 de 1993, ley que tiene por objeto disponer las reglas y principios que rigen los contratos de las entidades estatales, también el estado expide normas como la ley 1474 de 2011 por medio de la cual buscan fortalecer los mecanismos de prevención, investigación y sanción de actos de corrupción y la efectividad del control de la gestión pública. A través de este documento se identifican las principales características que enmarcan la gestión de la revisoría fiscal en los procesos licitatorios, se logran evaluar los factores relevantes de la gestión en la revisoría fiscal en el desarrollo de los procesos licitatorios y se logra determinar una línea base en la gestión de la revisoría fiscal, encaminados al desarrollo de los procesos licitatorios.This article frames this article , framed the importance and determinants in the management of the Statutory Auditor in bidding processes by identifying the main features , guidelines and development of each process in which intervenes this figure created by the Colombian state exercise control and surveillance to private economic and financial operations that in some way affect the public interest , the figure of the tax inspection was strengthened by the Colombian Commercial Code which delegated important functions and powers will establish responsibilities that very only the state , but this also provides tools for the statutory Auditor may faithfully fulfill their role , ethical and legal actions, establishes a code of ethics for public accountant and also generates laws that provide in this case the correct handling , It is Law 80 of 1993, law that is to stipulate the rules and principles governing contracts of state agencies , also the state issued rules as the law 1474 of 2011 by which seek to strengthen mechanisms for the prevention, investigation and punishment of acts of corruption and control effectiveness of public administration . Through this document identifies the main features that frame the management of the tax inspection and bidding processes are achieved assess the relevant factors of management in tax inspection in the development of the bidding process and is able to determine a baseline in the management of tax inspection, aimed at developing the bidding process

Quantum Dimer Model on the triangular lattice: Semiclassical and variational approaches to vison dispersion and condensation

After reviewing the concept of vison excitations in Z_2 dimer liquids, we study the liquid-crystal transition of the Quantum Dimer Model on the triangular lattice by means of a semiclassical spin-wave approximation to the dispersion of visons in the context of a "soft-dimer" version of the model. This approach captures some important qualitative features of the transition: continuous nature of the transition, linear dispersion at the critical point, and \sqrt{12}x\sqrt{12} symmetry-breaking pattern. In a second part, we present a variational calculation of the vison dispersion relation at the RK point which reproduces the qualitative shape of the dispersion relation and the order of magnitude of the gap. This approach provides a simple but reliable approximation of the vison wave functions at the RK point.Comment: 12 pages, 10 figures. v2: minor changes, to appear in Phys. Rev.

Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence

We present a new experimental set-up that creates a shear flow with zero mean advection velocity achieved by counterbalancing the nonzero streamwise pressure gradient by moving boundaries, which generates plane Couette-Poiseuille flow. We carry out the first experimental results in the transitional regime for this flow. Using flow visualization we characterize the subcritical transition to turbulence in Couette-Poiseuille flow and show the existence of turbulent spots generated by a permanent perturbation. Due to the zero mean advection velocity of the base profile, these turbulent structures are nearly stationary. We distinguish two regions of the turbulent spot: the active, turbulent core, which is characterized by waviness of the streaks similar to traveling waves, and the surrounding region, which includes in addition the weak undisturbed streaks and oblique waves at the laminar-turbulent interface. We also study the dependence of the size of these two regions on Reynolds number. Finally, we show that the traveling waves move in the downstream (Poiseuille).Comment: 17 pages, 15 figure

Irreducible Hamiltonian BRST-anti-BRST symmetry for reducible systems

An irreducible Hamiltonian BRST-anti-BRST treatment of reducible first-class systems based on homological arguments is proposed. The general formalism is exemplified on the Freedman-Townsend model.Comment: LaTeX 2.09, 35 page

Towards Interpretable Deep Learning Models for Knowledge Tracing

As an important technique for modeling the knowledge states of learners, the traditional knowledge tracing (KT) models have been widely used to support intelligent tutoring systems and MOOC platforms. Driven by the fast advancements of deep learning techniques, deep neural network has been recently adopted to design new KT models for achieving better prediction performance. However, the lack of interpretability of these models has painfully impeded their practical applications, as their outputs and working mechanisms suffer from the intransparent decision process and complex inner structures. We thus propose to adopt the post-hoc method to tackle the interpretability issue for deep learning based knowledge tracing (DLKT) models. Specifically, we focus on applying the layer-wise relevance propagation (LRP) method to interpret RNN-based DLKT model by backpropagating the relevance from the model's output layer to its input layer. The experiment results show the feasibility using the LRP method for interpreting the DLKT model's predictions, and partially validate the computed relevance scores from both question level and concept level. We believe it can be a solid step towards fully interpreting the DLKT models and promote their practical applications in the education domain

Criterion for purely elastic Taylor-Couette instability in the flows of shear-banding fluids

In the past twenty years, shear-banding flows have been probed by various techniques, such as rheometry, velocimetry and flow birefringence. In micellar solutions, many of the data collected exhibit unexplained spatio-temporal fluctuations. Recently, it has been suggested that those fluctuations originate from a purely elastic instability of the flow. In cylindrical Couette geometry, the instability is reminiscent of the Taylor-like instability observed in viscoelastic polymer solutions. In this letter, we describe how the criterion for purely elastic Taylor-Couette instability should be adapted to shear-banding flows. We derive three categories of shear-banding flows with curved streamlines, depending on their stability.Comment: 6 pages, 3 figure

Potential "ways of thinking" about the shear-banding phenomenon

Shear-banding is a curious but ubiquitous phenomenon occurring in soft matter. The phenomenological similarities between the shear-banding transition and phase transitions has pushed some researchers to adopt a 'thermodynamical' approach, in opposition to the more classical 'mechanical' approach to fluid flows. In this heuristic review, we describe why the apparent dichotomy between those approaches has slowly faded away over the years. To support our discussion, we give an overview of different interpretations of a single equation, the diffusive Johnson-Segalman (dJS) equation, in the context of shear-banding. We restrict ourselves to dJS, but we show that the equation can be written in various equivalent forms usually associated with opposite approaches. We first review briefly the origin of the dJS model and its initial rheological interpretation in the context of shear-banding. Then we describe the analogy between dJS and reaction-diffusion equations. In the case of anisotropic diffusion, we show how the dJS governing equations for steady shear flow are analogous to the equations of the dynamics of a particle in a quartic potential. Going beyond the existing literature, we then draw on the Lagrangian formalism to describe how the boundary conditions can have a key impact on the banding state. Finally, we reinterpret the dJS equation again and we show that a rigorous effective free energy can be constructed, in the spirit of early thermodynamic interpretations or in terms of more recent approaches exploiting the language of irreversible thermodynamics.Comment: 14 pages, 6 figures, tutorial revie

Efficacité d'un seuil artificiel sur l'oxygénation de l'eau et l'élimination de CH4 contenu dans l'eau évacuée par la barrage hydroélectrique de Petit Saut (Guyane française)

Quelques mois après le début de la mise en eau du barrage de Petit Saut, la mise en service normale de l'usine conduisait à une désoxygénation de l'eau du tronçon de rivière aval, le rendant incompatible avec la vie aquatique. La solution retenue a été la construction d'un seuil, afin d'apporter de l'oxygène et d'éliminer les gaz réducteurs produits au fond de la retenue, notamment le méthane, consommateur potentiel d'oxygène dissous.Un seuil métallique à deux lames déversantes successives a été construit ; sa configuration prend en compte les principaux critères physiques jouant un rôle significatif sur l'oxygénation de l'eau (hauteur de chute, épaisseur de la lame déversante, le dimensionnement du bassin de réception des chutes, la présence de dispositifs favorisant l'éclatement de la lame d'eau). Placé dans le canal de fuite de l'usine, à une centaine de mètres à l'aval du barrage principal, il est à l'abri des crues et ne crée pas d'obstacle supplémentaire en rivière.L'article chiffre l'effet d'aération de ce seuil pour les deux gaz O2 et CH4 dans deux configurations : celles consécutives à l'abaissement partiel de la chute amont réalisé en deux étapes. Après décembre 2001, pour le débit moyen turbiné (près de 200 m3 /s), l'efficacité d'aération du seuil a baissé de près de 10 % (gain de 80 % en oxygène dissous et élimination de 70 % et 75 % du méthane dissous). Après février 2003, pour un débit de 100 m3/s, 75 % du déficit amont en oxygène dissous est comblé et près de 70 % du méthane dissous éliminé.From the moment tropical reservoirs are impounded, climatic conditions cause rapid (within several weeks) and marked thermal stratification, especially during the dry season. This phenomenon is further exacerbated by the chemical and biochemical processes taking place in the reservoir due to the decomposition of submerged organic matter. In dense tropical forests, the overhead biomass is estimated at roughly 170 t(C)/ha, and the carbon contained in the soil is also not negligible since it is on the order of 100 t(C)/ha. The degree of biodegradability of the different compounds in the flooded biomass is variable, ranging from a few weeks for bacteria to several centuries for tree trunks.The studies carried out at Petit Saut (French Guiana) show that, immediately after impoundment, only the epilimnion (a few dozen centimetres thick) was oxygenated whereas the hypolimnion was characterized by complete anoxia and a very high methane content (about 15 mg/L). Water quality in the river downstream from the reservoir was of course strongly linked to variations in the water quality in the reservoir as well as to its operating mode. The waters passing through the turbines, coming from the bottom layers, were anoxic and loaded with fixed or volatile reducing compounds (e.g., CH4, H2 S), and were responsible for a high immediate or progressive oxygen demand. At Petit Saut, despite an inflow of good quality water, there has been a progressive deoxygenation in the river downstream due to the high methane content (roughly 8 mg/L) of the turbined water. Thus, 40 km downstream from the dam, the oxygen content was less than 2 mg/L and therefore incompatible with most aquatic life. To solve this problem, it was necessary to build an aerating weir capable of reoxygenating the turbined waters and, more importantly, eliminating reducing gases such as methane at the same time. The function of the overflow weir was to entrain air bubbles into the water and to give these bubbles a sufficiently long immersion time to ensure that they dissolve. At the time of its installation, only three examples of oxygenating weirs existed in the entire world, all located in the United States. The weir configuration was tested using a physical model to qualitatively examine the form of the flow both across the weir and downstream from it. The degree to which air bubbles were entrained in the water was also tested, but not the question of evaluating the flux of gaseous exchanges between the air and the water.The system that was finally designed by EDF, in October 1994, was a metallic weir with two consecutive falls, the configuration of which respected the main physical criteria that play a significant role in the oxygenation of water, i.e.:- the height of the falls (roughly 5.40 m, depending on the flow rate);- the thickness of the water stream, the function of which is to entrain air bubbles and keep them in the water for a sufficiently long period of time for the oxygen to dissolve (between 12 and 25 seconds, depending on the flow rate); - the dimensions of the receiving basin of the first waterfall where the air bubbles are held (5 hexagonal alveoli); and- systems to promote the fragmentation of the flow. This structure was placed in the tailrace channel of the plant, approximately 100 m downstream from the main dam. This location protected it from floods and did not create an extra obstacle in the river. In addition, it allowed the water to be re-oxygenated as soon as it left the reservoir.The efficiency of the two waterfalls of the Petit Saut re-aerating weir was tested at two different turbine flow rates: 80 m3 /s and 230 m3 /s. In 1996, the results of the measurements showed that for a flow rate of 230 m3 /s, upstream of the weir the concentrations of CH4 were around 5 mg/L and dissolved oxygen was 0.8 mg/L. Downstream from the weir CH4 concentrations were 1.3 mg/L and dissolved oxygen concentrations were 6.8 mg/L. The dissolved methane elimination rate was approximately 75 per cent. At a flow rate of 80 m3/s, upstream of the weir the concentration of CH4 was 5.5 mg/L and the dissolved oxygen concentration was 0.7 mg/L. Downstream from the weir concentrations of CH4 and dissolved oxygen were 1.0 mg/L and 7.1 mg/L, respectively. The dissolved methane elimination rate was around 80%. The efficiency of the re-oxygenation was always greater than 90%. These data prove that the efficiency of the Petit Saut weir installation was higher when the turbine flow rate was lower. This could be due to a greater waterfall height, the better entrainment of air bubbles per unit volume and/or a longer air bubble residence time in the downstream flow.Between December 2001 and February 2003, for a flow rate of 200 m3 /s, the efficiency of the weir decreased by 10%, with the dissolved methane elimination rate at around 70-75%. The level of re-oxygenation was around 80%. Since February 2003, for a flow rate of 100 m3 /s, the efficiency of the weir has decreased by 10%, the dissolved methane elimination rate was around 70% and the level of re-oxygenation was around 75%.On a local scale, the effect on the quality of the river water has been very positive, as aquatic life has been maintained. Without the weir, the methane contained in the turbined water would have been progressively transformed, along the course of the river, into carbon dioxide. In the absence of significant additions of good quality water and without the weir, a large part of the course of the river would have a dissolved oxygen content of less than 2 mg/L, the critical threshold for the maintenance of aquatic life.At present time, the results of the current ecological survey are used to support studies on biogeochemical processes

Involving the Public in Public Health Genomics: A Review of Guidelines and Policy Statements

[À l'origine dans / Was originally part of : CRDP - Droit, biotechnologie et rapport au milieu]Public health genomics raises exciting possibilities for preventing or reducing the occurrence of both rare and common disease. However, this area of research raises challenging ethical, legal and social issues that must be addressed. One way of addressing these issues is through public involvement in the policy-making process. This GenEdit reviews how international guidelines and policy statements related to public health genomics address the issue of public involvement. Key areas of discussion are the values and goals justifying public involvement, the proposed activities to increase public involvement, who is / who represents "the public", and the projected outcomes of public involvement