Comment on "Minimal size of a barchan dune"

It is now an accepted fact that the size at which dunes form from a flat sand bed as well as their `minimal size' scales on the flux saturation length. This length is by definition the relaxation length of the slowest mode toward equilibrium transport. The model presented by Parteli, Duran and Herrmann [Phys. Rev. E 75, 011301 (2007)] predicts that the saturation length decreases to zero as the inverse of the wind shear stress far from the threshold. We first show that their model is not self-consistent: even under large wind, the relaxation rate is limited by grain inertia and thus can not decrease to zero. A key argument presented by these authors comes from the discussion of the typical dune wavelength on Mars (650 m) on the basis of which they refute the scaling of the dune size with the drag length evidenced by Claudin and Andreotti [Earth Pla. Sci. Lett. 252, 30 (2006)]. They instead propose that Martian dunes, composed of large grains (500 micrometers), were formed in the past under very strong winds. We show that this saltating grain size, estimated from thermal diffusion measurements, is not reliable. Moreover, the microscopic photographs taken by the rovers on Martian aeolian bedforms show a grain size of 87 plus or minus 25 micrometers together with hematite spherules at millimetre scale. As those so-called ``blueberries'' can not be entrained by reasonable winds, we conclude that the saltating grains on Mars are the small ones, which gives a second strong argument against the model of Parteli et al.Comment: A six page comment on ``Minimal size of a barchan dune'' by Parteli, Duran and Herrmann [Phys. Rev. E 75, 011301 (2007) arXiv:0705.1778

The Retention Problem: An Analysis of Enrolment Attrition at a Canadian College

A survey of returning and non-returning students at Scarborough College was undertaken to investigate the problem of declining retention rates at Ontario Universities. This study describes the opportunities available to and chosen by non-returning students, and then seeks to evaluate various hypotheses that have been proposed to account for enrolment attrition. Consistent with prior research, there was no evidence found to support the claim that students are being forced out of university by an inability to afford the costs of their education. Instead, the results suggested that enrolment attrition is produced by a combination of an attenuation of the student's commitment to obtain the degree and the student's awareness of and access to viable alternatives. As a result of a pervasive sense of uncertainty about the market value of a university degree, many students are willing to assay other opportunities. Specifically, those students who went to work appear to have been "pulled out" of university by the availability of a credible job. In addition, the contingencies affecting the commitment of the returning students and the students who transfer to other educational institutions are discussed. The conclusion addresses the relationship between financial circumstances and access to university, and raises questions about future enrolment patterns.Une enquête sur les étudiants qui continuent à fréquenter et ceux qui laissent le Collège de Scarborough fut entreprise dans le but d'étudier le problème des taux de rétention en déclin dans les universités de l'Ontario. La présente étude s'arrête à analyser les raisons d'abandon des étudiants et cherche ensuite à vérifier diverses hypothèses avancées pour expliquer la baisse des inscriptions. En conformité avec des recherches antérieures, aucune évidence n 'a pu être trouvée pour supporter l'idée que les étudiants soient forcés d'inter-rompre leurs études universitaires à cause d'une incapacité à supporter les coûts de l'en-seignement. Au contraire, les résultats de la recherche suggère que la baisse des inscriptions provient plutôt d'un manque d'intérêt pour l'obtention d'un diplôme de même que d'une meilleure connaissance de d'autres possibilités offertes sur le marché du travail. Etant donné le degré d'incertitude planant sur la valeur marchande d'un diplôme universitaire, de nombreux étudiants sont prêts à explorer d'autres avenues. Plus spécifiquement, les étudiants qui ont abandonné leurs études semblent avoir été attirés par la disponibilité d'un emploi satisfaisant. L'auteur discute également des circonstances entourant les étudiants qui retournent et ceux qui décident de transférer dans d'autres institutions d'enseignement. Les conclusions de cette étude touchent aussi au problème de l'accessi-bilité à l'université et des dépenses encourues; finalement, certaines questions sont posées au sujet des tendances futures des inscriptions universitaires

Design of a High Temperature Radiator for the Variable Specific Impulse Magnetoplasma Rocket

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR), currently under development by Ad Astra Rocket Company (Webster, TX), is a unique propulsion system that could change the way space propulsion is performed. VASIMR's efficiency, when compared to that of a conventional chemical rocket, reduces the propellant needed for exploration missions by a factor of 10. Currently plans include flight tests of a 200 kW VASIMR system, titled VF-200, on the International Space Station (ISS). The VF-200 will consist of two 100 kW thruster units packaged together in one engine bus. Each thruster core generates 27 kW of waste heat during its 15 minute firing time. The rocket core will be maintained between 283 and 573 K by a pumped thermal control loop. The design of a high temperature radiator is a unique challenge for the vehicle design. This paper will discuss the path taken to develop a steady state and transient-based radiator design. The paper will describe the radiator design option selected for the VASIMR thermal control system for use on ISS, and how the system relates to future exploration vehicles

Geometric observation for the Bures fidelity between two states of a qubit

In this Brief Report, we present a geometric observation for the Bures fidelity between two states of a qubit.Comment: 4 pages, 1 figure, RevTex, Accepted by Phys. Rev.

Building more accurate decision trees with the additive tree.

The expansion of machine learning to high-stakes application domains such as medicine, finance, and criminal justice, where making informed decisions requires clear understanding of the model, has increased the interest in interpretable machine learning. The widely used Classification and Regression Trees (CART) have played a major role in health sciences, due to their simple and intuitive explanation of predictions. Ensemble methods like gradient boosting can improve the accuracy of decision trees, but at the expense of the interpretability of the generated model. Additive models, such as those produced by gradient boosting, and full interaction models, such as CART, have been investigated largely in isolation. We show that these models exist along a spectrum, revealing previously unseen connections between these approaches. This paper introduces a rigorous formalization for the additive tree, an empirically validated learning technique for creating a single decision tree, and shows that this method can produce models equivalent to CART or gradient boosted stumps at the extremes by varying a single parameter. Although the additive tree is designed primarily to provide both the model interpretability and predictive performance needed for high-stakes applications like medicine, it also can produce decision trees represented by hybrid models between CART and boosted stumps that can outperform either of these approaches

Trigonal columnar self-assembly of bent phasmid mesogens

Three compounds with a bent rod-like aromatic core and with three alkoxy chains at each end were synthesised by click reaction. The compounds form a columnar liquid crystal phase with non-centrosymmetric trigonal p31m symmetry, the columns having a 3-arm star-like cross-section

Ionic Switch Induced by a Rectangular-Hexagonal Phase Transition in Benzenammonium Columnar Liquid Crystals.

We demonstrate switching of ionic conductivities in wedge-shaped liquid-crystalline (LC) ammonium salts. A thermoreversible phase transition between the rectangular columnar (Colr) and hexagonal columnar (Colh) phases is used for the switch. The ionic conductivities in the Colh phase are about four orders of magnitude higher than those in the Colr phase. The switching behavior of conductivity can be ascribed to the structural change of assembled ionic channels. X-ray experiments reveal a highly ordered packing of the ions in the Colr phase, which prevents the ion transport

Pattern formation in directional solidification under shear flow. I: Linear stability analysis and basic patterns

An asymptotic interface equation for directional solidification near the absolute stabiliy limit is extended by a nonlocal term describing a shear flow parallel to the interface. In the long-wave limit considered, the flow acts destabilizing on a planar interface. Moreover, linear stability analysis suggests that the morphology diagram is modified by the flow near the onset of the Mullins-Sekerka instability. Via numerical analysis, the bifurcation structure of the system is shown to change. Besides the known hexagonal cells, structures consisting of stripes arise. Due to its symmetry-breaking properties, the flow term induces a lateral drift of the whole pattern, once the instability has become active. The drift velocity is measured numerically and described analytically in the framework of a linear analysis. At large flow strength, the linear description breaks down, which is accompanied by a transition to flow-dominated morphologies, described in a companion paper. Small and intermediate flows lead to increased order in the lattice structure of the pattern, facilitating the elimination of defects. Locally oscillating structures appear closer to the instability threshold with flow than without.Comment: 20 pages, Latex, accepted for Physical Review