138,392 research outputs found
Intermittency in Hall-magnetohydrodynamics with a strong guide field
We present a detailed study of intermittency in the velocity and magnetic
field fluctuations of compressible Hall-magnetohydrodynamic turbulence with an
external guide field. To solve the equations numerically, a reduced model valid
when a strong guide field is present is used. Different values for the ion skin
depth are considered in the simulations. The resulting data is analyzed
computing field increments in several directions perpendicular to the guide
field, and building structure functions and probability density functions. In
the magnetohydrodynamic limit we recover the usual results with the magnetic
field being more intermittent than the velocity field. In the presence of the
Hall effect, field fluctuations at scales smaller than the ion skin depth show
a substantial decrease in the level of intermittency, with close to monofractal
scaling.Comment: 10 pages, 8 figure
Problems of the rotating-torsion-balance limit on the photon mass
We discuss the problems (and the promise) of the ingenious method introduced
by Lakes, and recently improved on by Luo, to detect a possible small photon
mass by measuring the ambient magnetic vector potential from large scale
magnetic fields. We also point out how an improved ``indirect'' limit can be
obtained using modern measurements of astrophysical magnetic fields and plasmas
and that a good ``direct'' limit exists using properties of the solar wind.Comment: 4 pages, revised title and content
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
Generic guide concepts for the European Spallation Source
The construction of the European Spallation Source (ESS) faces many
challenges from the neutron beam transport point of view: The spallation source
is specified as being driven by a 5 MW beam of protons, each with 2 GeV energy,
and yet the requirements in instrument background suppression relative to
measured signal vary between 10 and 10. The energetic particles,
particularly above 20 MeV, which are expected to be produced in abundance in
the target, have to be filtered in order to make the beamlines safe,
operational and provide good quality measurements with low background.
We present generic neutron guides of short and medium length instruments
which are optimized for good performance at minimal cost. Direct line of sight
to the source is avoided twice, with either the first point out of line of
sight or both being inside the bunker (20\,m) to minimize shielding costs.
These guide geometries are regarded as a baseline to define standards for
instruments to be constructed at ESS. They are used to find commonalities and
develop principles and solutions for common problems. Lastly, we report the
impact of employing the over-illumination concept to mitigate losses from
random misalignment passively, and that over-illumination should be used
sparingly in key locations to be effective. For more widespread alignment
issues, a more direct, active approach is likely to be needed
VSAERO analysis of tip planforms for the free-tip rotor
The results of a numerical analysis of two interacting lifting surfaces separated in the spanwise direction by a narrow gap are presented. The configuration consists of a semispan wing with the last 32 percent of the span structurally separated from the inboard section. The angle of attack of the outboard section is set independently from that of the inboard section. In the present study, the three-dimensional panel code VSAERO is used to perform the analysis. Computed values of tip surface lift and pitching moment coefficients are correlated with experimental data to determine the proper approach to model the gap region between the surfaces. Pitching moment data for various tip planforms are also presented to show how the variation of tip pitching moment with angle of attack may be increased easily in incompressible flow. Calculated three-dimensional characteristics in compressible flow at Mach numbers of 0.5 and 0.7 are presented for new tip planform designs. An analysis of sectional aerodynamic center shift as a function of Mach number is also included for a representative tip planform. It is also shown that the induced drag of the tip surface is reduced for negative incidence angles relative to the inboard section. The results indicate that this local drag reduction overcomes the associated increase in wing induced drag at high wing lift coefficients
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