21,552 research outputs found
Linear Transmission of Composite Gaussian Measurements over a Fading Channel under Delay Constraints
Delay constrained linear transmission (LT) strategies are considered for the transmission of composite Gaussian measurements over an additive white Gaussian noise fading channel under an average power constraint. If the channel state information (CSI) is known by both the encoder and decoder, the optimal LT scheme in terms of the average mean-square error distortion is characterized under a strict delay constraint, and a graphical interpretation of the optimal power allocation strategy is presented. Then, for general delay constraints, two LT strategies are proposed based on the solution to a particular multiple measurements-parallel channels scenario. It is shown that the distortion decreases as the delay constraint is relaxed, and when the delay constraint is completely removed, both strategies achieve the optimal performance under certain matching conditions. If the CSI is known only by the decoder, the optimal LT strategy is derived under a strict delay constraint. The extension to general delay constraints is elusive. As a first step towards understanding the structure of the optimal scheme in this case, it is shown that for the multiple measurementsparallel channels scenario, any LT scheme that uses only a oneto-one linear mapping between measurements and channels is suboptimal in general
Kelvin-Helmholtz versus Hall Magneto-shear instability in astrophysical flows
We study the stability of shear flows in a fully ionized plasma.
Kelvin-Helmholtz is a well known, macroscopic and ideal shear-driven
instability. In sufficiently low density plasmas, also the microscopic Hall
magneto-shear instability can take place. We performed three-dimensional
simulations of the Hall-MHD equations where these two instabilities are
present, and carried out a comparative study. We find that when the shear flow
is so intense that its vorticity surpasses the ion-cyclotron frequency of the
plasma, the Hall magneto-shear instability is not only non-negligible, but it
actually displays growth rates larger than those of the Kelvin-Helmholtz
instability
Energy spectrum, dissipation and spatial structures in reduced Hall magnetohydrodynamic
We analyze the effect of the Hall term in the magnetohydrodynamic turbulence
under a strong externally supported magnetic field, seeing how this changes the
energy cascade, the characteristic scales of the flow and the dynamics of
global magnitudes, with particular interest in the dissipation.
Numerical simulations of freely evolving three-dimensional reduced
magnetohydrodynamics (RHMHD) are performed, for different values of the Hall
parameter (the ratio of the ion skin depth to the macroscopic scale of the
turbulence) controlling the impact of the Hall term. The Hall effect modifies
the transfer of energy across scales, slowing down the transfer of energy from
the large scales up to the Hall scale (ion skin depth) and carrying faster the
energy from the Hall scale to smaller scales. The final outcome is an effective
shift of the dissipation scale to larger scales but also a development of
smaller scales. Current sheets (fundamental structures for energy dissipation)
are affected in two ways by increasing the Hall effect, with a widening but at
the same time generating an internal structure within them. In the case where
the Hall term is sufficiently intense, the current sheet is fully delocalized.
The effect appears to reduce impulsive effects in the flow, making it less
intermittent.Comment: 17 pages, 10 figure
Simulations of the Kelvin-Helmholtz instability driven by coronal mass ejections in the turbulent corona
Recent high resolution AIA/SDO images show evidence of the development of the
Kelvin-Helmholtz instability, as coronal mass ejections (CMEs) expand in the
ambient corona. A large-scale magnetic field mostly tangential to the interface
is inferred, both on the CME and on the background sides. However, the magnetic
field component along the shear flow is not strong enough to quench the
instability. There is also observational evidence that the ambient corona is in
a turbulent regime, and therefore the criteria for the development of the
instability are a-priori expected to differ from the laminar case. To study the
evolution of the Kelvin-Helmholtz instability with a turbulent background, we
perform three-dimensional simulations of the incompressible magnetohydrodynamic
equations. The instability is driven by a velocity profile tangential to the
CME-corona interface, which we simulate through a hyperbolic tangent profile.
The turbulent background is generated by the application of a stationary
stirring force. We compute the instability growth-rate for different values of
the turbulence intensity, and find that the role of turbulence is to attenuate
the growth. The fact that the Kelvin-Helmholtz instability is observed, sets an
upper limit to the correlation length of the coronal background turbulence
Child education and work choices in the presence of a conditional cash transfer programme in rural Colombia
This research is part of a large evaluation effort, undertaken by a consortium formed by IFS, Econometria and SEI, which has considered the effects of Familias en Accion on a variety of outcomes one year after its implementation. In early reports, we focussed on the effects of the programme on school enrolment. In this paper, we both expand those results, by carefully analysing anticipation effects along with other issues, and complement them with an analysis of child labour - both paid and unpaid (including domestic) work. The child labour analysis is made possible due to a rich time use module of the surveys that has not previously been analysed. We find that the programme increased the school participation rates of 14 to 17 year old children quite substantially, by between 5 and 7 percentage points, and had lower, but non-negligible effects on the enrolment of younger children of between 1.4 and 2.4 percentage points. In terms of work, the effects are generally largest for younger children whose participation in domestic work decreased by around 10 to 12 percentage points after the programme but whose participation in income-generating work remained largely unaffected by the programme. We also find evidence of school and work time not being fully substitutable, suggesting that some, but not all, of the increased time at school may be drawn from children's leisure time
On the inverse cascade of magnetic helicity
We study the inverse cascade of magnetic helicity in conducting fluids by
investigating the detailed transfer of helicity between different spherical
shells in Fourier space in direct numerical simulations of three-dimensional
magnetohydrodynamics (MHD). Two different numerical simulations are used, one
where the system is forced with an electromotive force in the induction
equation, and one in which the system is forced mechanically with an ABC flow
and the magnetic field is solely sustained by a dynamo action. The magnetic
helicity cascade at the initial stages of both simulations is observed to be
inverse and local (in scale space) in the large scales, and direct and local in
the small scales. When saturation is approached most of the helicity is
concentrated in the large scales and the cascade is non-local. Helicity is
transfered directly from the forced scales to the largest scales. At the same
time, a smaller in amplitude direct cascade is observed from the largest scale
to small scales.Comment: Submitted to PR
When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy
Quenching, the cessation of star formation, is one of the most significant
events in the life cycle of galaxies. We show here the first evidence that the
Milky Way experienced a generalised quenching of its star formation at the end
of its thick disk formation 9 Gyr ago. Elemental abundances of stars
studied as part of the APOGEE survey reveal indeed that in less than 2
Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude.
Because of the tight correlation between age and alpha abundance, this event
reflects in the dearth of stars along the inner disk sequence in the
[Fe/H]-[/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the
Milky Way was actively forming stars. Afterwards, the star formation resumed at
a much lower level to form the thin disk. These events are very well matched by
the latest observation of MW-type progenitors at high redshifts. In late type
galaxies, quenching is believed to be related to a long and secular exhaustion
of gas. In our Galaxy, it occurred on a much shorter time scale, while the
chemical continuity before and after the quenching indicates that it was not
due to the exhaustion of the gas. While quenching is generally associated with
spheroids, our results show that it also occurs in galaxies like the Milky Way,
possibly when they are undergoing a morphological transition from thick to thin
disks. Given the demographics of late type galaxies in the local universe, in
which classical bulges are rare, we suggest further that this may hold true
generally in galaxies with mass lower than or approximately , where
quenching could be directly a consequence of thick disk formation. We emphasize
that the quenching phase in the Milky Way could be contemporaneous with, and
related to, the formation of the bar. We sketch a scenario on how a strong bar
may inhibit star formation.Comment: 17 pages, 8 figures. Published versio
Privacy-cost trade-offs in demand-side management with storage
Demand-side energy management (EM) is studied from a privacy-cost trade-off perspective, considering time-of-use pricing and the presence of an energy storage unit. Privacy i s measured as the variation of the power withdrawn from the gri d from a fixed target value. Assuming non-causal knowledge of t he household’s aggregate power demand profile and the electric ity prices at the energy management unit (EMU), the privacy-cos t trade-off is formulated as a convex optimization problem, a nd a low-complexity backward water-filling algorithm is proposed to compute the optimal EM policy. The problem is studied also in the online setting assuming that the power demand profile is known to the EMU only causally, and the optimal EM policy is obtained numerically through dynamic programming (DP). Du e to the high computational cost of DP, a low-complexity heuri stic EM policy with a performance close to the optimal online solu tion is also proposed, exploiting the water-filling algorithm ob tained in the offline setting. As an alternative, information theor etic leakage rate is also evaluated, and shown to follow a similar trend as the load variance, which supports the validity of th e load variance as a measure of privacy. Finally, the privacy- cost trade-off, and the impact of the size of the storage unit on th is trade-off are studied through numerical simulations using real smart meter data in both the offline and online settings
EDROOM: a free tool for the UML2 component based design and automatic code generation of tiny embedded real time system
International audienceThe use of tiny real time kernels to develop embedded systems is broadly extended. They offer basic services with small overhead footprints in the final product. Usually, these kind of kernels are compliant with the POSIX 1003.13 specification. The use of graphical modelling and automatic code generation tools for developing these kind of small software embedded system if often not considered for several reasons: they are expensive, the learning curve to obtain benefits is often large and finally the generated code usually does not fit well with the platform or exceed the desired size. In this paper we present the adaptation of a free tool, known as EDROOM, to develop this kind of real time software system. EDROOM is inspired on the ROOM modelling language and provides graphical modelling and automatic Embedded C++ code generation. EDROOM is compliant with the new UML2 graphical notation for component based system design and hierarchical behaviour. The new version of EDROOM is a cross development multiplatform generation tool and includes facilities for static control of all resources in order to completely avoid the use of dynamic memory. Our tool has been used in the software development of a small satellite (NANOSAT-01) which is fully functional nowadays. The tool is free distributed in conjunction with a group of code test bench that can be used to validate any port to another architecture
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