On a theorem by Treves

According to a theorem of Treves, the conserved functionals of the KdV equation vanish on each formal Laurent series 1/x^2 + u0 + u2 x^2 + u3 x^3 + >... . We propose a new, very simple geometrical proof for this statement.Comment: 7 page

Acceleration statistics in thermally driven superfluid turbulence

New methods of flow visualization near absolute zero have opened the way to directly compare quantum turbulence (in superfluid helium) to classical turbulence (in ordinary fluids such as air or water) and explore analogies and differences. We present results of numerical simulations in which we examine the statistics of the superfluid acceleration in thermal counterflow. We find that, unlike the velocity, the acceleration obeys scaling laws similar to classical turbulence, in agreement with a recent quantum turbulence experiment of La Mantia et al.Comment: 5 pages, 5 figures, to appear in PR

Hydromagnetic Taylor--Couette flow: wavy modes

We investigate magnetic Taylor--Couette flow in the presence of an imposed axial magnetic field. First we calculate nonlinear steady axisymmetric solutions and determine how their strength depends on the applied magnetic field. Then we perturb these solutions to find the critical Reynolds numbers for the appearance of wavy modes, and the related wavespeeds, at increasing magnetic field strength. We find that values of imposed magnetic field which alter only slightly the transition from circular--Couette flow to Taylor--vortex flow, can shift the transition from Taylor--vortex flow to wavy modes by a substantial amount. The results are compared against onset in the absence of a magnetic field.Comment: 12 pages, 8 figures. To appear in J. Fluid Mech. To appear in J. Fluid Mec

A primer on quantum fluids

This book introduces the theoretical description and properties of quantum fluids. The focus is on gaseous atomic Bose-Einstein condensates and, to a minor extent, superfluid helium, but the underlying concepts are relevant to other forms of quantum fluids such as polariton and photonic condensates. The book is pitched at the level of advanced undergraduates and early postgraduate students, aiming to provide the reader with the knowledge and skills to develop their own research project on quantum fluids. Indeed, the content for this book grew from introductory notes provided to our own research students. It is assumed that the reader has prior knowledge of undergraduate mathematics and/or physics; otherwise, the concepts are introduced from scratch, often with references for directed further reading.Comment: 132 pages. Published as SpringerBriefs in Physics book. Typos corrected in this versio

Silver-palladium braze alloy recovered from masking materials

Method for recovering powdered silver-palladium braze alloy from an acrylic spray binder and rubber masking adhesive used in spray brazing is devised. The process involves agitation and dissolution of masking materials and recovery of suspended precious metal particles on a filter

Coupled normal fluid and superfluid profiles of turbulent helium II in channels

We perform fully coupled two--dimensional numerical simulations of plane channel helium II counterflows with vortex--line density typical of experiments. The main features of our approach are the inclusion of the back reaction of the superfluid vortices on the normal fluid and the presence of solid boundaries. Despite the reduced dimensionality, our model is realistic enough to reproduce vortex density distributions across the channel recently calculated in three--dimensions. We focus on the coarse--grained superfluid and normal fluid velocity profiles, recovering the normal fluid profile recently observed employing a technique based on laser--induced fluorescence of metastable helium molecules.Comment: 26 pages, 8 Figures, accepted for publication in Phys. Rev.

A revised estimate of the distance to the clouds in the Chamaeleon complex using the Tycho-Gaia Astrometric Solution

The determination of the distance to dark star-forming clouds is a key parameter to derive the properties of the cloud itself, and of its stellar content. This parameter is still loosely constrained even in nearby star-forming regions. We want to determine the distances to the clouds in the Chamaeleon-Musca complex and to explore the connection between these clouds and the large scale cloud structures in the galaxy. We use the newly estimated distances obtained from the parallaxes measured by the Gaia satellite and included in the Tycho-Gaia Astrometric Solution catalog. When known members of a region are included in this catalog we use their parallaxes to infer the distance to the cloud. Otherwise, we analyze the dependence of the color excess on the distance of the stars and look for a turn-on of this excess, which is a proxy of the position of the front-edge of the star-forming cloud. We are able to measure the distance to the three Chamaeleon clouds. The distance to Chamaeleon I is 179 pc, 20 pc further away than previously assumed. The Chamaeleon II cloud is located at the distance of 181 pc, which agrees with previous estimates. We are able to measure for the first time a distance to the Chamaeleon III cloud of 199 pc. Finally, the distance of the Musca cloud is smaller than 603 pc. These estimates do not allow us to distinguish between the possibility that the Chamaeleon clouds are part of a sheet of clouds parallel to the galactic plane, or perpendicular to it. Gaia Data Release 2 will allow us to put more stringent constraints on the distances to these clouds by giving us access to parallax measurements for a larger number of members of these regions.Comment: Accepted for publication on A&A. Abstract shortened for arxiv constraint

Predicting loss in magnetic steels under arbitrary induction waveform and with minor hysteresis loops

We have studied ways of predicting power losses in soft magnetic laminations for generic time dependence of the periodic magnetic polarization J(t). We found that, whatever the frequency and the induction waveform, the loss behavior can be quantitatively assessed within the theoretical framework of the statistical loss model. The prediction requires a limited set of preemptive experimental data, depending on whether or not the arbitrary J(t) waveform is endowed with local slope inversions (i.e., minor hysteresis loops) in its periodic time behavior. In the absence of minor loops, such data reduce, for any peak polarization value Jp, to the loss figures obtained under sinusoidal J(t) at two different frequency values. In the presence of minor loops of semiamplitude Jm, the two-frequency loss experiment should be carried out for both peak polarization values Jp and Jm. Additional knowledge of the quasi-static major loop, to be used for modeling hysteresis loss, does improve the accuracy of the prediction method. A more general approach to loss in soft magnetic laminations is obtained in this way, the only limitation apparently being the onset of skin effect at high frequencie