Non-axisymmetric instability of shear-banded Taylor-Couette flow

Recent experiments show that shear-banded flows of semi-dilute worm-like micelles in Taylor-Couette geometry exhibit a flow instability in the form of Taylor-like vortices. Here we perform the non-axisymmetric linear stability analysis of the diffusive Johnson-Segalman model of shear banding and show that the nature of this instability depends on the applied shear rate. For the experimentally relevant parameters, we find that at the beginning of the stress plateau the instability is driven by the interface between the bands, while most of the stress plateau is occupied by the bulk instability of the high-shear-rate band. Our work significantly alters the recently proposed stability diagram of shear-banded flows based on axisymmetric analysis.Comment: 6 pages, 5 figures, main text and supplementary material; accepted to Phys. Rev. Let

SAtlas: Spherical Versions of the Atlas Stellar Atmosphere Program

Context: The current stellar atmosphere programs still cannot match some fundamental observations of the brightest stars, and with new techniques, such as optical interferometry, providing new data for these stars, additional development of stellar atmosphere codes is required. Aims: To modify the open-source model atmosphere program Atlas to treat spherical geometry, creating a test-bed stellar atmosphere code for stars with extended atmospheres. Methods: The plane-parallel Atlas has been changed by introducing the necessary spherical modifications in the pressure structure, in the radiative transfer and in the temperature correction. Results: Several test models show that the spherical program matches the plane-parallel models in the high surface gravity regime, and matches spherical models computed by Phoenix and by MARCS in the low gravity case.Comment: 10 pages, 10 figures, Accepted for publication in A&

Flow pattern transition accompanied with sudden growth of flow resistance in two-dimensional curvilinear viscoelastic flows

We find three types of steady solutions and remarkable flow pattern transitions between them in a two-dimensional wavy-walled channel for low to moderate Reynolds (Re) and Weissenberg (Wi) numbers using direct numerical simulations with spectral element method. The solutions are called "convective", "transition", and "elastic" in ascending order of Wi. In the convective region in the Re-Wi parameter space, the convective effect and the pressure gradient balance on average. As Wi increases, the elastic effect becomes suddenly comparable and the first transition sets in. Through the transition, a separation vortex disappears and a jet flow induced close to the wall by the viscoelasticity moves into the bulk; The viscous drag significantly drops and the elastic wall friction rises sharply. This transition is caused by an elastic force in the streamwise direction due to the competition of the convective and elastic effects. In the transition region, the convective and elastic effects balance. When the elastic effect dominates the convective effect, the second transition occurs but it is relatively moderate. The second one seems to be governed by so-called Weissenberg effect. These transitions are not sensitive to driving forces. By the scaling analysis, it is shown that the stress component is proportional to the Reynolds number on the boundary of the first transition in the Re-Wi space. This scaling coincides well with the numerical result.Comment: 33pages, 23figures, submitted to Physical Review

A streamwise-constant model of turbulent pipe flow

A streamwise-constant model is presented to investigate the basic mechanisms responsible for the change in mean flow occuring during pipe flow transition. Using a single forced momentum balance equation, we show that the shape of the velocity profile is robust to changes in the forcing profile and that both linear non-normal and nonlinear effects are required to capture the change in mean flow associated with transition to turbulence. The particularly simple form of the model allows for the study of the momentum transfer directly by inspection of the equations. The distribution of the high- and low-speed streaks over the cross-section of the pipe produced by our model is remarkably similar to one observed in the velocity field near the trailing edge of the puff structures present in pipe flow transition. Under stochastic forcing, the model exhibits a quasi-periodic self-sustaining cycle characterized by the creation and subsequent decay of "streamwise-constant puffs", so-called due to the good agreement between the temporal evolution of their velocity field and the projection of the velocity field associated with three-dimensional puffs in a frame of reference moving at the bulk velocity. We establish that the flow dynamics are relatively insensitive to the regeneration mechanisms invoked to produce near-wall streamwise vortices and that using small, unstructured background disturbances to regenerate the streamwise vortices is sufficient to capture the formation of the high- and low-speed streaks and their segregation leading to the blunting of the velocity profile characteristic of turbulent pipe flow

On statistically stationary homogeneous shear turbulence

A statistically stationary turbulence with a mean shear gradient is realized in a flow driven by suitable body forces. The flow domain is periodic in downstream and spanwise directions and bounded by stress free surfaces in the normal direction. Except for small layers near the surfaces the flow is homogeneous. The fluctuations in turbulent energy are less violent than in the simulations using remeshing, but the anisotropy on small scales as measured by the skewness of derivatives is similar and decays weakly with increasing Reynolds number.Comment: 4 pages, 5 figures (Figs. 3 and 4 as external JPG-Files

A Comparison of Measured Crab and Vela Glitch Healing Parameters with Predictions of Neutron Star Models

There are currently two well-accepted models that explain how pulsars exhibit glitches, sudden changes in their regular rotational spin-down. According to the starquake model, the glitch healing parameter, Q, which is measurable in some cases from pulsar timing, should be equal to the ratio of the moment of inertia of the superfluid core of a neutron star (NS) to its total moment of inertia. Measured values of the healing parameter from pulsar glitches can therefore be used in combination with realistic NS structure models as one test of the feasibility of the starquake model as a glitch mechanism. We have constructed NS models using seven representative equations of state of superdense matter to test whether starquakes can account for glitches observed in the Crab and Vela pulsars, for which the most extensive and accurate glitch data are available. We also present a compilation of all measured values of Q for Crab and Vela glitches to date which have been separately published in the literature. We have computed the fractional core moment of inertia for stellar models covering a range of NS masses and find that for stable NSs in the realistic mass range 1.4 +/- 0.2 solar masses, the fraction is greater than 0.55 in all cases. This range is not consistent with the observational restriction Q < 0.2 for Vela if starquakes are the cause of its glitches. This confirms results of previous studies of the Vela pulsar which have suggested that starquakes are not a feasible mechanism for Vela glitches. The much larger values of Q observed for Crab glitches (Q > 0.7) are consistent with the starquake model predictions and support previous conclusions that starquakes can be the cause of Crab glitches.Comment: 8 pages, including 3 figures and 1 table. Accepted for publication in Ap

Facilitators and barriers to physical activity participation experienced by Aboriginal and Torres Strait Islander adults: A mixed methods systematic review protocol

Objective:To synthesize the existing research about physical activity and sport facilitators and barriers experienced by Aboriginal and Torres Strait Islander adults in Australia.Introduction:Physical activity and sport have cultural importance for First Nations peoples. Achieving health and broader benefits from physical activity and sport is impacted by experiences of both facilitators and barriers to participation. Identifying how to facilitate participation and overcome barriers to physical activity and sport is important to develop strategies to increase physical activity levels and sport participation among Aboriginal and Torres Strait Islander adults. Several studies have examined physical activity and sport facilitators and barriers experienced by Aboriginal and Torres Strait Islander adults, and collective synthesis of these studies can provide a more comprehensive understanding of their findings.Inclusion criteria:This mixed methods systematic review will consider studies that include Aboriginal and Torres Strait Islander peoples aged 18 years and over from any setting or region of Australia. Studies will be considered if they report on facilitators and barriers to physical activity and/or sport participation.Methods:Eleven databases will be searched, as well as gray literature sources, and a selection of websites containing resources relevant to physical activity participation for Aboriginal and Torres Strait Islander adults. Studies published in English will be included. No date limits will be set. After screening the titles and abstracts of identified citations, potentially relevant studies will be retrieved in full. Study selection, critical appraisal, data extraction, and data synthesis will be undertaken according to the convergent integrated approach to mixed methods reviews.Systematic review registration number:PROSPERO CRD42020162134

Numerical simulations of two dimensional magnetic domain patterns

I show that a model for the interaction of magnetic domains that includes a short range ferromagnetic and a long range dipolar anti-ferromagnetic interaction reproduces very well many characteristic features of two-dimensional magnetic domain patterns. In particular bubble and stripe phases are obtained, along with polygonal and labyrinthine morphologies. In addition, two puzzling phenomena, namely the so called `memory effect' and the `topological melting' observed experimentally are also qualitatively described. Very similar phenomenology is found in the case in which the model is changed to be represented by the Swift-Hohenberg equation driven by an external orienting field.Comment: 8 pages, 8 figures. Version to appear in Phys. Rev.

Theoretical values of convective turnover times and Rossby numbers for solar-like, pre-main sequence stars

Magnetic fields are at the heart of the observed stellar activity in late-type stars, and they are presumably generated by a dynamo mechanism at the interface layer between the radiative and the convective stellar regions. Since dynamo models are based on the interaction between differential rotation and convective motions, the introduction of rotation in the ATON 2.3 stellar code allows for explorations regarding a physically consistent treatment of magnetic effects in stellar structure and evolution, even though there are formidable mathematical and numerical challenges involved. As examples, we present theoretical estimates for both the local (tau_c) and global (tau_g) convective turnover times for rotating pre-main sequence solar-type stars, based on up-to-date input physics for stellar models. Our theoretical predictions are compared with the previous ones available in the literature. In addition, we investigate the dependence of the convective turnover time on convection regimes, the presence of rotation and atmospheric treatment. Those estimates, this quantities can be used to calculate the Rossby number, Ro, which is related to the magnetic activity strength in dynamo theories and, at least for main-sequence stars, shows an observational correlation with stellar activity. More important, they can also contribute for testing stellar models against observations. Our theoretical values of tau_c, tau_g and Ro qualitatively agree with those published by Kim & Demarque (1996). By increasing the convection efficiency, tau_g decreases for a given mass. FST models show still lower values. The presence of rotation shifts tau_g towards slightly higher values when compared with non-rotating models. The use of non-gray boundary conditions in the models yields values of tau_g smaller than in the gray approximation.Comment: 10 pages, 14 figures, accepted for publication in A&