1,584 research outputs found
A self-sustaining nonlinear dynamo process in Keplerian shear flows
A three-dimensional nonlinear dynamo process is identified in rotating plane
Couette flow in the Keplerian regime. It is analogous to the hydrodynamic
self-sustaining process in non-rotating shear flows and relies on the
magneto-rotational instability of a toroidal magnetic field. Steady nonlinear
solutions are computed numerically for a wide range of magnetic Reynolds
numbers but are restricted to low Reynolds numbers. This process may be
important to explain the sustenance of coherent fields and turbulent motions in
Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review
Letter
Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow
The magnetorotational (MRI) dynamo has long been considered one of the
possible drivers of turbulent angular momentum transport in astrophysical
accretion disks. However, various numerical results suggest that this dynamo
may be difficult to excite in the astrophysically relevant regime of magnetic
Prandtl number (Pm) significantly smaller than unity, for reasons currently not
well understood. The aim of this article is to present the first results of an
ongoing numerical investigation of the role of both linear and nonlinear
dissipative effects in this problem. Combining a parametric exploration and an
energy analysis of incompressible nonlinear MRI dynamo cycles representative of
the transitional dynamics in large aspect ratio shearing boxes, we find that
turbulent magnetic diffusion makes the excitation and sustainment of this
dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for
decreasing Pm. This results in an increase in the critical Rm of the dynamo for
increasing kinematic Reynolds number (Re), in agreement with earlier numerical
results. Given its very generic nature, we argue that turbulent magnetic
diffusion could be an important determinant of MRI dynamo excitation in disks,
and may also limit the efficiency of angular momentum transport by MRI
turbulence in low Pm regimes.Comment: 7 pages, 6 figure
Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?
In Keplerian accretion disks, turbulence and magnetic fields may be jointly
excited through a subcritical dynamo process involving the magnetorotational
instability (MRI). High-resolution simulations exhibit a tendency towards
statistical self-organization of MRI dynamo turbulence into large-scale cyclic
dynamics. Understanding the physical origin of these structures, and whether
they can be sustained and transport angular momentum efficiently in
astrophysical conditions, represents a significant theoretical challenge. The
discovery of simple periodic nonlinear MRI dynamo solutions has recently proven
useful in this respect, and has notably served to highlight the role of
turbulent magnetic diffusion in the seeming decay of the dynamics at low
magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a
common regime in accretion disks. The connection between these simple
structures and the statistical organization reported in turbulent simulations
remained elusive, though. Here, we report the numerical discovery in moderate
aspect ratio Keplerian shearing boxes of new periodic, incompressible,
three-dimensional nonlinear MRI dynamo solutions with a larger dynamical
complexity reminiscent of such simulations. These "chimera" cycles are
characterized by multiple MRI-unstable dynamical stages, but their basic
physical principles of self-sustainment are nevertheless identical to those of
simpler cycles found in azimuthally elongated boxes. In particular, we find
that they are not sustained at low Pm either due to subcritical turbulent
magnetic diffusion. These solutions offer a new perspective into the transition
from laminar to turbulent instability-driven dynamos, and may prove useful to
devise improved statistical models of turbulent accretion disk dynamos.Comment: 12 pages, 8 figures, submitted to A&
A Robust State Feedback Adaptive Controller with Improved Transient Tracking Error Bounds for Plants with Unknown Varying Control Gain
Corruption and Development: The Need for International Investigations with a Multijurisdictional Approach Involving Multilateral Development Banks and National Authorities
We argue that while Multilateral Development Banks (“MDBs”) and national governments have mechanisms to fight corruption, the objectives and outcomes of these enforcement mechanisms diverge. MDBs are interested in the causes and effects of corruption from a development perspective and, as such, tend to sanction small and medium enterprises and individuals, while national governments are focused on a more punitive outcome, targeting larger multinational corporations. This article examines the enforcement objectives articulated in national legislation, namely the US Foreign and Corrupt Practices Act and its Canadian counterpart, the Corruption of Foreign Public Officials Act, as well as several Canadian cases, on the one hand, and the tools and outcomes of MDBs’ sanctions systems on the other. We conclude that national enforcement efforts and MDBs’ sanctions outcomes intersect in their fight against international corruption in that their results are complementary; the former punishing large-scale offenders while the latter ensuring the integrity of development projects
Corruption and Development: The Need for International Investigations with a Multijurisdictional Approach Involving Multilateral Development Banks and National Authorities
We argue that while Multilateral Development Banks (“MDBs”) and national governments have mechanisms to fight corruption, the objectives and outcomes of these enforcement mechanisms diverge. MDBs are interested in the causes and effects of corruption from a development perspective and, as such, tend to sanction small and medium enterprises and individuals, while national governments are focused on a more punitive outcome, targeting larger multinational corporations. This article examines the enforcement objectives articulated in national legislation, namely the US Foreign and Corrupt Practices Act and its Canadian counterpart, the Corruption of Foreign Public Officials Act, as well as several Canadian cases, on the one hand, and the tools and outcomes of MDBs’ sanctions systems on the other. We conclude that national enforcement efforts and MDBs’ sanctions outcomes intersect in their fight against international corruption in that their results are complementary; the former punishing large-scale offenders while the latter ensuring the integrity of development projects
Low-Income Engineering Students: Considering Financial Aid and Differential Tuition
This paper explores the relationship between tuition differentials and low-income students in Engineering fields at two public, research intensive universities. Although current reports indicate the need for increased participation within the Science, Technology, Engineering, and Mathematics (STEM) fields, rising tuition prices at the university and program levels may deter low-income students to enroll and persist within STEM, specifically Engineering. The findings reveal that increased costs due to tuition differentials policies are initially offset by financial aid, but over time costs increase, particularly for low-income students. The results highlight the need for comprehensive, time-sensitive financial aid packages that provide students opportunities to complete their postsecondary degrees, particularly in fields with higher tuition rates
Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows
The nature of dynamo action in shear flows prone to magnetohydrodynamic
instabilities is investigated using the magnetorotational dynamo in Keplerian
shear flow as a prototype problem. Using direct numerical simulations and
Newton's method, we compute an exact time-periodic magnetorotational dynamo
solution to the three-dimensional dissipative incompressible
magnetohydrodynamic equations with rotation and shear. We discuss the physical
mechanism behind the cycle and show that it results from a combination of
linear and nonlinear interactions between a large-scale axisymmetric toroidal
magnetic field and non-axisymmetric perturbations amplified by the
magnetorotational instability. We demonstrate that this large scale dynamo
mechanism is overall intrinsically nonlinear and not reducible to the standard
mean-field dynamo formalism. Our results therefore provide clear evidence for a
generic nonlinear generation mechanism of time-dependent coherent large-scale
magnetic fields in shear flows and call for new theoretical dynamo models.
These findings may offer important clues to understand the transitional and
statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review
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