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A note on the modelling of hyper-inflations
In time series macroeconometric models, the first difference in the logarithm of a variable is routinely used to represent the rate of change of that variable. It is often overlooked that the assumed approximation is accurate only if the rates of change are small. Models of hyper-inflation are a case in point, since in these models, by definition, changes in price are large. In this letter, Caganâs model is applied to Hungarian hyper-inflation data. It is then demonstrated that use of the approximation in the formation of the price inflation variable is causing an upward bias in the modelâs key parameter, and therefore an exaggeration of the effect postulated by Cagan
Large Scale Structures a Gradient Lines: the case of the Trkal Flow
A specific asymptotic expansion at large Reynolds numbers (R)for the long
wavelength perturbation of a non stationary anisotropic helical solution of the
force less Navier-Stokes equations (Trkal solutions) is effectively constructed
of the Beltrami type terms through multi scaling analysis. The asymptotic
procedure is proved to be valid for one specific value of the scaling
parameter,namely for the square root of the Reynolds number (R).As a result
large scale structures arise as gradient lines of the energy determined by the
initial conditions for two anisotropic Beltrami flows of the same helicity.The
same intitial conditions determine the boundaries of the vortex-velocity tubes,
containing both streamlines and vortex linesComment: 27 pages, 2 figure
Creation and evolution of magnetic helicity
Projecting a non-Abelian SU(2) vacuum gauge field - a pure gauge constructed
from the group element U - onto a fixed (electromagnetic) direction in isospace
gives rise to a nontrivial magnetic field, with nonvanishing magnetic helicity,
which coincides with the winding number of U. Although the helicity is not
conserved under Maxwell (vacuum) evolution, it retains one-half its initial
value at infinite time.Comment: Clarifying remarks and references added; 12 pages, 1 figure using
BoxedEPSF, REVTeX macros; submitted to Phys Rev D; email to
[email protected]
A numerical model of the VKS experiment
We present numerical simulations of the magnetic field generated by the flow
of liquid sodium driven by two counter-rotating impellers (VKS experiment).
Using a dynamo kinematic code in cylindrical geometry, it is shown that
different magnetic modes can be generated depending on the flow configuration.
While the time averaged axisymmetric mean flow generates an equatorial dipole,
our simulations show that an axial field of either dipolar or quadrupolar
symmetry can be generated by taking into account non-axisymmetric components of
the flow. Moreover, we show that by breaking a symmetry of the flow, the
magnetic field becomes oscillatory. This leads to reversals of the axial dipole
polarity, involving a competition with the quadrupolar component.Comment: 6 pages, 5 figure
Identifying Deficiencies of Standard Accretion Disk Theory: Lessons from a Mean-Field Approach
Turbulent viscosity is frequently used in accretion disk theory to replace
the microphysical viscosity in order to accomodate the observational need for
in- stabilities in disks that lead to enhanced transport. However, simply
replacing the microphysical transport coefficient by a single turbulent
transport coeffi- cient hides the fact that the procedure should formally arise
as part of a closure in which the hydrodynamic or magnetohydrodynamic equations
are averaged, and correlations of turbulent fluctuations are replaced by
transport coefficients. Here we show how a mean field approach leads quite
naturally two transport coefficients, not one, that govern mass and angular
momentum transport. In particular, we highlight that the conventional approach
suffers from a seemingly inconsistent neglect of turbulent diffusion in the
surface density equation. We constrain these new transport coefficients for
specific cases of inward, outward, and zero net mass transport. In addition, we
find that one of the new transport terms can lead to oscillations in the mean
surface density which then requires a constant or small inverse Rossby number
for disks to maintain a monotonic power-law surface density.Comment: 11 page
Residual stresses in aerospace structures reinforced with bonded crack retarders
Bonded crack retarder technology is an innovative concept to improve the fatigue performance of aircraft structures. Stiffening âstrapsâ are adhesively bonded to areas where potential fatigue cracking may occur. The straps retard the growth of fatigue cracks, by a combination of the local stiffening effect that reduces the crack driving force, and bridging in the wake of the crack. However, bonded crack retarder results in thermal residual stresses that may adversely affect the performance of the reinforced structure due to extensive fatigue loads in service. This is the first study where we have looked at the application of GLARE6/5 fibre-metal laminate as a bonded crack retarder onto a structural butt joint and simulated manhole mock-up assemblies containing cold-worked holes. Neutron diffraction was used for residual stress measurements. Results indicate that the strap-bonding process has no discernible effect on the magnitude of the compressive cold-working stresses. The use of bonded crack retarders should not, therefore, impair the benefits of cold working of fastener joints in aircraft structures
Chaotic magnetic field reversals in turbulent dynamos
We present direct numerical simulations of reversals of the magnetic field
generated by swirling flows in a spherical domain. In agreement with a recent
model, we observe that coupling dipolar and quadrupolar magnetic modes by an
asymmetric forcing of the flow generates field reversals. In addition, we show
that this mechanism strongly depends on the value of the magnetic Prandtl
number.Comment: 4 pages, 5 figure
Fatigue and fracture of a 316 stainless steel metal matrix composite reinforced with 25% titanium diboride
Fatigue and fracture mechanisms have been studied in a steel-based metal matrix composite (MMC), comprising a 316L austenitic matrix reinforced with 25 wt% particulate titanium diboride (TiB2). The fracture toughness was determined in the as-HIPped condition as being slightly below 30 MPaâm. Fatigue crack growth rates have been determined, and corrected for the effects of crack closure. The fracture surfaces have been studied to determine the mechanisms of damage during crack advance, which are determined as matrix fatigue, reinforcement particle fracture, and ductile rupture of the matrix. We show that the occurrence of damage mechanisms during fatigue of the material is linked to Kmax, rather than to âK. This is rationalised in terms of a semi-cohesive process zone within the monotonic plastic zone ahead of the crack tip
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