Instabilities Research Papers - Academia.edu (original) (raw)

If a wheel rolling over a rail transmits a tangential traction, frictional microslip occurs in part of the contact area, resulting in energy dissipation and localized wear. If the applied forces oscillate in time, the resulting wear will... more

Aims. We investigate the linear stability properties of the plane interface separating two relativistic magnetized flows in motion with respect to each other. The two flows are governed by the (special) relativistic equations for a... more

Aims. We investigate the linear stability properties of the plane interface separating two relativistic magnetized flows in motion with respect to each other. The two flows are governed by the (special) relativistic equations for a magnetized perfect gas in the infinite conductivity approximation. Methods. By adopting the vortex-sheet approximation, the relativistic magnetohydrodynamics equations are linearized around the equilibrium state and the corresponding dispersion relation is derived and discussed. The behavior of the configuration and the regimes of instability are investigated following the effects of four physical parameters: the flow velocity, the relativistic and Alfvénic Mach numbers, and the inclination of the wave vector on the plane of the interface. Results. From the numerical solution of the dispersion relation, we find in general two separate regions of instability, associated with the slow and fast magnetosonic modes respectively. Modes parallel to the flow velocity are destabilized only for sufficiently low magnetization. For the latter case, stabilization is attained, in addition, at sufficiently large relativistic velocities between the two flows in relative motion. Conclusions. We briefly comment the relevance of these results to the study of the stability of astrophysical jets.

In the present work, a study of the mixed convection flow of low Prandtl number fluid ( 015 . 0 Pr= ) confined in a cylindrical container having an aspect ratio equal to 2, with and without magnetic fields, has been considered. The... more

In the present work, a study of the mixed convection flow of low Prandtl number fluid ( 015 . 0 Pr= ) confined in a cylindrical container having an aspect ratio equal to 2, with and without magnetic fields, has been considered. The finite volumes method has been used to resolve the equations of continuity, momentum (or Navier-Stokes), energy and electric potential. In the absence of magnetic field, the numerical results obtained show the appearance of oscillator y instabilities for the values of the critical Reynolds number , 802 , 924 , 2575 Re = cr and 606 , corresponding respectively to the values of the Richardson number = Ri 0, 0.5, 1.0 and 2.0. However, in the presence of the vertical magnetic field, the fluid continues its stable flow until the values of Reynolds number greater than those predictable to have oscillatory instabilities. Stability diagrams have been established according to the numerical results of this investigation. These diagrams put in evidence the depend...

An effect of a mean velocity shear on a turbulence and on the effective force which is determined by the gradient of the Reynolds stresses is studied. Generation of a mean vorticity in a homogeneous incompressible nonhelical turbulent... more

An effect of a mean velocity shear on a turbulence and on the effective force which is determined by the gradient of the Reynolds stresses is studied. Generation of a mean vorticity in a homogeneous incompressible nonhelical turbulent flow with an imposed mean velocity shear due to an excitation of a large-scale instability is found. The instability is caused by a combined effect of the large-scale shear motions ''skew-induced'' deflection of equilibrium mean vorticity and ''Reynolds stress-induced'' generation of perturbations of mean vorticity. Spatial characteristics of the instability, such as the minimum size of the growing perturbations and the size of perturbations with the maximum growth rate, are determined. This instability and the dynamics of the mean vorticity are associated with Prandtl's turbulent secondary flows.

This paper is about the free surface instabilities of granular flows, usually called roll waves. A shallow layer of shear-thickening fluid (τ = a(∂ u/∂ y)n with n = 2) is considered to study finite-amplitude permanent roll waves... more

This paper is about the free surface instabilities of granular flows, usually called roll waves. A shallow layer of shear-thickening fluid (τ = a(∂ u/∂ y)n with n = 2) is considered to study finite-amplitude permanent roll waves down a slope, simplified by Karman’s momentum integral approach. The existence of conditions of a periodic discontinuous solution is derived, as smooth profiles with depth increasing monotonically between periodic shocks. Energy dissipation in the body of the stream and in the discontinuity is analysed and discussed. Two conditions are derived. The first is related to the physically acceptable shape of the smooth profiles, and the second is related to positive energy loss across the shock. These conditions can be converted into a limiting discharge, viewed in the fixed frame, and in a limiting flow thickness (or limiting Froude number), for the permanent periodic roll wave to exist without further conditions. A minimum-length roll wave (MLRW) is defined as the periodic permanent roll waves with zero energy dissipation in the shock. The MLRW also requires a limiting value of the Froude number to exist.

Using a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean... more

Using a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 1448 and 1548E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual vari-abilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.

Aims. We consider relativistic electrons in magnetospheric flows close to a supermassive black hole and study the mechanism of quasi-linear diffusion (QLD) to investigate the correlation between γ-ray and radio emission in active galactic... more

Aims. We consider relativistic electrons in magnetospheric flows close to a supermassive black hole and study the mechanism of quasi-linear diffusion (QLD) to investigate the correlation between γ-ray and radio emission in active galactic nuclei. Methods. Moving in the nonuniform magnetic field, the particles experience a force that is responsible for the conservation of the adiabatic invariant. This force, together with the radiation reaction force, tends to decrease the pitch angles. Contrary to this, the QLD attempts to increase the pitch angle, and this maintains the synchrotron emission regime. To examine the balance between the QLD and the aforementioned dissipative factors we investigate the quasi-stationary state by applying the kinetic equation. Results. Considering the magnetospheric plasma close to the supermassive black hole, we examined the efficiency of the QLD for different parameters. By examining the cyclotron instability, we show that despite the efficient dissipative factors, the cyclotron modes excite transverse and longitudinal-transversal waves, which lead to the QLD. We find that the QLD provides a connection of emission in the γ-ray and radio domains. We show that under favourable conditions the radio emission from 22 MHz to 9 GHz on the mpc scale is associated with γ-ray emission from 900 GeV down to 9 GeV on the same scale.