2,361 research outputs found
The Kraichnan-Kazantsev dynamo
The problem of the dynamo effect for a Kraichnan incompressible helicity-free
velocity field is considered. Exploiting a quantum formalism first introduced
by Kazantsev (A.P. Kazantsev, Sov. Phys. JETP 26, 1031-1034 (1968)), we show
that a critical magnetic Reynolds number exists for the presence of dynamo. The
value of the Prandtl number influences the spatial distribution of the magnetic
field and its growth in time. The magnetic field correlation length is always
the largest between the diffusive scale and the viscous scale of the flow. In
the same way the field growth is characterized by a time scale that corresponds
to the largest between the diffusive and the viscous characteristic time.Comment: 16 pages, 6 figure
Nonlinear elastic polymers in random flow
Polymer stretching in random smooth flows is investigated within the
framework of the FENE dumbbell model. The advecting flow is Gaussian and
short-correlated in time. The stationary probability density function of
polymer extension is derived exactly. The characteristic time needed for the
system to attain the stationary regime is computed as a function of the
Weissenberg number and the maximum length of polymers. The transient relaxation
to the stationary regime is predicted to be exceptionally slow in the proximity
of the coil-stretch transition.Comment: 10 pages, to be published in J. Fluid Mec
Shear effects on passive scalar spectra
The effects of a large-scale shear on the energy spectrum of a passively
advected scalar field are investigated. The shear is superimposed on a
turbulent isotropic flow, yielding an Obukhov-Corrsin scalar
spectrum at small scales. Shear effects appear at large scales, where a
different, anisotropic behavior is observed. The scalar spectrum is shown to
behave as for a shear fixed in intensity and direction. For other
types of shear characteristics, the slope is generally intermediate between the
-5/3 Obukhov-Corrsin's and the -1 Batchelor's values. The physical mechanisms
at the origin of this behaviour are illustrated in terms of the motion of
Lagrangian particles. They provide an explanation to the scalar spectra shallow
and dependent on the experimental conditions observed in shear flows at
moderate Reynolds numbers.Comment: 10 LaTeX pages,3 eps Figure
A model for alignment between microscopic rods and vorticity
Numerical simulations show that microscopic rod-like bodies suspended in a
turbulent flow tend to align with the vorticity vector, rather than with the
dominant eignevector of the strain-rate tensor. This paper investigates an
analytically solvable limit of a model for alignment in a random velocity field
with isotropic statistics. The vorticity varies very slowly and the isotropic
random flow is equivalent to a pure strain with statistics which are
axisymmetric about the direction of the vorticity. We analyse the alignment in
a weakly fluctuating uniaxial strain field, as a function of the product of the
strain relaxation time and the angular velocity about
the vorticity axis. We find that when , the rods are
predominantly either perpendicular or parallel to the vorticity
Interference fit effect on holed single plates loaded with tension-tension stresses
This paper deals with the influence of interference fit coupling on the fatigue strength of holed plates. The effect was investigated both experimentally and numerically. Axial fatigue tests have been carried out on holed specimens made of high performance steel (1075MPa of Ultimate strength and 990MPa of Yield strength) with or without a pin, made of the same material, press fitted into their central hole. Three different conditions have been investigated: free hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress-life (S–N) curves pointed out an increased fatigue life of the interference fit specimens compared with the free hole ones. The numericalinvestigation was performed in order to analyse the stress fields by applying an elastic plastic 2D simulation witha commercial Finite Element software. The stress history and distribution along the contact interference of the fitted samples indicates a significant reduction of the local stress range due to the externally applied loading (remote stress) since a residual and compressive stress field is generated by the pin insertion
UAS Regulation in Foreign Countries: Alternatives to UAS Operations in the United States
Wide range interest in Unmanned Aerial Systems (UAS) across the globe Rapid growth has been stymied in the U.S. due to slow progression of integration and privacy concerns U.S. may lose competitive advantage Other countries provide opportunities for potential research and collaboratio
Interference fit effect on holed single plates loaded with tension-tension stresses
This paper deals with the influence of interference fit coupling on the fatigue strength of holed plates. The effect was investigated both experimentally and numerically. Axial fatigue tests have been carried out on holed specimens made of high performance steel (1075MPa of Ultimate strength and 990MPa of Yield strength) with or without a pin, made of the same material, press fitted into their central hole. Three different conditions have been investigated: free hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress-life (S–N) curves pointed out an increased fatigue life of the interference fit specimens compared with the free hole ones. The numerical investigation was performed in order to analyse the stress fields by applying an elastic plastic 2D simulation with a commercial Finite Element software. The stress history and distribution along the contact interference of the fitted samples indicates a significant reduction of the local stress range due to the externally applied loading (remote stress) since a residual and compressive stress field is generated by the pin insertion
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