Analytic expression for Taylor-Couette stability boundary

We analyze the mechanism that determines the boundary of stability in Taylor-Couette flow. By simple physical argument we derive an analytic expression to approximate the stability line for all radius ratios and all speed ratios, for co- and counterrotating cylinders. The expression includes viscosity and so generalizes Rayleigh's criterion. We achieve agreement with linear stability theory and with experiments in the whole parameter space. Explicit formulae are given for limiting cases.Comment: 6 pages (LaTeX with REVTEX) including 4 figures (Postscript) Revised, discussion of two additional references. See also http://staff-www.uni-marburg.de/~esse

Mixing layer between two co-current Taylor-Couette flows

A new mixing layer can be generated if the rotation of either of the two cylinders in a Taylor--Couette apparatus varies discontinuously along the symmetry axis. The mixing zone between the two resulting co-current streams gives rise to radial vorticity in addition to the primary axial vorticity. An analytic solution for the azimuthal velocity has been derived from which we show that the width of the mixing zone varies only with radial position.Comment: 7 pages, 3 figures, accepted for publication in European Journal of Mechanics B/Fluid

Magnetized Ekman Layer and Stewartson Layer in a Magnetized Taylor-Couette Flow

In this paper we present axisymmetric nonlinear simulations of magnetized Ekman and Stewartson layers in a magnetized Taylor-Couette flow with a centrifugally stable angular-momemtum profile and with a magnetic Reynolds number below the threshold of magnetorotational instability. The magnetic field is found to inhibit the Ekman suction. The width of the Ekman layer is reduced with increased magnetic field normal to the end plate. A uniformly-rotating region forms near the outer cylinder. A strong magnetic field leads to a steady Stewartson layer emanating from the junction between differentially rotating rings at the endcaps. The Stewartson layer becomes thinner with larger Reynolds number and penetrates deeper into the bulk flow with stronger magnetic field and larger Reynolds number. However, at Reynolds number larger than a critical value $\sim 600$, axisymmetric, and perhaps also nonaxisymmetric, instabilities occur and result in a less prominent Stewartson layer that extends less far from the boundary.Comment: 24 pages, 12 figures, accepted by PRE, revision according to referee

The Ekman-Hartmann layer in MHD Taylor-Couette flow

We study magnetic effects induced by rigidly rotating plates enclosing a cylindrical MHD Taylor-Couette flow at the finite aspect ratio $H/D=10$. The fluid confined between the cylinders is assumed to be liquid metal characterized by small magnetic Prandtl number, the cylinders are perfectly conducting, an axial magnetic field is imposed \Ha \approx 10, the rotation rates correspond to \Rey of order $10^2-10^3$. We show that the end-plates introduce, besides the well known Ekman circulation, similar magnetic effects which arise for infinite, rotating plates, horizontally unbounded by any walls. In particular there exists the Hartmann current which penetrates the fluid, turns into the radial direction and together with the applied magnetic field gives rise to a force. Consequently the flow can be compared with a Taylor-Dean flow driven by an azimuthal pressure gradient. We analyze stability of such flows and show that the currents induced by the plates can give rise to instability for the considered parameters. When designing an MHD Taylor-Couette experiment, a special care must be taken concerning the vertical magnetic boundaries so they do not significantly alter the rotational profile.Comment: 9 pages, 6 figures; accepted to PR

Alien Registration- Couette, Alphonsine (Livermore Falls, Androscoggin County)

https://digitalmaine.com/alien_docs/27111/thumbnail.jp

The G-protein–gated K+ channel, IKACh, is required for regulation of pacemaker activity and recovery of resting heart rate after sympathetic stimulation

Parasympathetic regulation of sinoatrial node (SAN) pacemaker activity modulates multiple ion channels to temper heart rate. The functional role of the G-protein–activated K+ current (IKACh) in the control of SAN pacemaking and heart rate is not completely understood. We have investigated the functional consequences of loss of IKACh in cholinergic regulation of pacemaker activity of SAN cells and in heart rate control under physiological situations mimicking the fight or flight response. We used knockout mice with loss of function of the Girk4 (Kir3.4) gene (Girk4−/− mice), which codes for an integral subunit of the cardiac IKACh channel. SAN pacemaker cells from Girk4−/− mice completely lacked IKACh. Loss of IKACh strongly reduced cholinergic regulation of pacemaker activity of SAN cells and isolated intact hearts. Telemetric recordings of electrocardiograms of freely moving mice showed that heart rate measured over a 24-h recording period was moderately increased (10%) in Girk4−/− animals. Although the relative extent of heart rate regulation of Girk4−/− mice was similar to that of wild-type animals, recovery of resting heart rate after stress, physical exercise, or pharmacological β-adrenergic stimulation of SAN pacemaking was significantly delayed in Girk4−/− animals. We conclude that IKACh plays a critical role in the kinetics of heart rate recovery to resting levels after sympathetic stimulation or after direct β-adrenergic stimulation of pacemaker activity. Our study thus uncovers a novel role for IKACh in SAN physiology and heart rate regulation

Stability and turbulent transport in Taylor–Couette flow from analysis of experimental data

International audienceThis paper provides a prescription for the turbulent viscosity in rotating shear flows for use e.g. in geophysical and astrophysical contexts. This prescription is the result of the detailed analysis of the experimental data obtained in several studies of the transition to turbulence and turbulent transport in Taylor-Couette flow. We first introduce a new set of control parameters, based on dynamical rather than geometrical considerations, so that the analysis applies more naturally to rotating shear flows in general and not only to Taylor-Couette flow. We then investigate the transition thresholds in the supercritical and the subcritical regime in order to extract their general dependencies on the control parameters. The inspection of the mean profiles provides us with some general hints on the mean to laminar shear ratio. Then the examination of the torque data allows us to propose a decomposition of the torque dependence on the control parameters in two terms, one completely given by measurements in the case where the outer cylinder is at rest, the other one being a universal function provided here from experimental fits. As a result, we obtain a general expression for the turbulent viscosity and compare it to existing prescription in the literature. Finally, throughout all the paper we discuss the influence of additional effects such as stratification or magnetic fields

La propriété intellectuelle, la santé publique et l'influence des experts

L'interface entre la gouvernance mondiale des brevets et de la santé publique est très complexe et sujette aux tensions. Doté de flexibilités, le régime mondial des brevets devait être en mesure d'évoluer pour répondre aux préoccupations de santé publique. Malgré une réactivité initiale au début du millénaire, le régime s'est néanmoins depuis figé. Un quart de siècle après sa mise en œuvre, la pandémie de COVID-19 dirige le feu des projecteurs sur une gouvernance mondiale hésitante reposant sur un régime en apparence immuable. Si le droit international explique en partie la complexité de la régulation de cette interface, l'économie politique, quant à elle, offre des pistes d'explication à sa remarquable immuabilité. Empruntant une conception élargie des communautés épistémiques, cette recherche indique que l'interface entre la gouvernance des brevets et de la santé publique est plongée dans une compétition épistémique entre deux réseaux d'experts : l'un plus conservateur et prônant le statu quo de la gouvernance des brevets, et l'autre plus réformiste prônant plus de flexibilité face aux préoccupations de santé publique. Cette compétition persistante a accentué la polarisation entre les défenseurs de chaque perspective et, conséquemment, contribué à la stagnation des initiatives de réforme multilatérales. Le présent travail s'appuie sur des analyses de réseaux sociaux retraçant d'abord l'activité de chacune des communautés épistémiques et l'interactivité entre elles. Des entrevues semi-dirigées permettent ensuite d'interroger la manière et le degré selon lesquels la compétition épistémique est vécue par les membres de ces communautés et affecte les débats multilatéraux internationaux. L'assemblage théorique et méthodologique de cette recherche permet de jeter un regard novateur sur les communautés épistémiques et sur leur influence dans une interface de gouvernance. Plus encore, cette recherche contribue aux discussions nouvellement ravivées sur l'avenir de l'interface de gouvernance entre les brevets et la santé publique.The governance interface between patents and public health is highly complex and is prone to tensions. Equipped with flexibilities, the global patent regime was meant to be able to evolve in response to public health concerns. Despite its initial reactiveness, the regime has since then frozen. A quarter of a century later, the COVID-19 pandemic has thrusted into the spotlight a hesitant global governance relying on an apparently immutable regime. If international law partly explains the regulatory complexity of this interface, international political economy offers explanation for its remarkable continuity. Adopting a broader conceptualization of epistemic communities, this research shows that the interface between the governance of patents and of public health has been plunged into an epistemic competition between two experts' networks: one more conservative and arguing in favor of the status quo in patent governance and another more reformist arguing for greater regulation flexibility for public health concerns. This ongoing competition has increased polarization between defendants of each perspective and, consequently, contributed to the stagnation of multilateral initiatives. This research builds on social network analyses to observe the activity of each epistemic community and the interactions between them. Semi-structured interviews then investigate how the epistemic competition is felt by the members of each epistemic community and to what extent it has affected the international multilateral debates. The theoretical and methodological framework of this research takes an innovative approach of epistemic communities and of their influence in a governance interface. Moreover, this research informs the newly revived discussions on the future of the governance interface between patents and public health