25,479 research outputs found
On the relation between viscoelastic and magnetohydrodynamic flows and their instabilities
We demonstrate a close analogy between a viscoelastic medium and an
electrically conducting fluid containing a magnetic field. Specifically, the
dynamics of the Oldroyd-B fluid in the limit of large Deborah number
corresponds to that of a magnetohydrodynamic (MHD) fluid in the limit of large
magnetic Reynolds number. As a definite example of this analogy, we compare the
stability properties of differentially rotating viscoelastic and MHD flows. We
show that there is an instability of the Oldroyd-B fluid that is physically
distinct from both the inertial and elastic instabilities described previously
in the literature, but is directly equivalent to the magnetorotational
instability in MHD. It occurs even when the specific angular momentum increases
outwards, provided that the angular velocity decreases outwards; it derives
from the kinetic energy of the shear flow and does not depend on the curvature
of the streamlines. However, we argue that the elastic instability of
viscoelastic Couette flow has no direct equivalent in MHD.Comment: 21 pages, 3 figures, to be published in J. Fluid Mec
Laminated ferrite memory, phase II FINAL technical report, Jun. 1, 1965 - Jun. 30, 1966
Thermally stable ferrite materials tailored to laminated memory arrays operated at low current level
Magnetorotational-type instability in Couette-Taylor flow of a viscoelastic polymer liquid
We describe an instability of viscoelastic Couette-Taylor flow that is
directly analogous to the magnetorotational instability (MRI) in astrophysical
magnetohydrodynamics, with polymer molecules playing the role of magnetic field
lines. By determining the conditions required for the onset of instability and
the properties of the preferred modes, we distinguish it from the centrifugal
and elastic instabilities studied previously. Experimental demonstration and
investigation should be much easier for the viscoelastic instability than for
the MRI in a liquid metal. The analogy holds with the case of a predominantly
toroidal magnetic field such as is expected in an accretion disk and it may be
possible to access a turbulent regime in which many modes are unstable.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Secondary Electron Emission Yields
The secondary electron emission (SEE) characteristics for a variety of spacecraft materials were determined under UHV conditions using a commercial double pass CMA which permits sequential Auger electron electron spectroscopic analysis of the surface. The transparent conductive coating indium tin oxide (ITO) was examined on Kapton and borosilicate glass and indium oxide on FED Teflon. The total SEE coefficient ranges from 2.5 to 2.6 on as-received surfaces and from 1.5 to 1.6 on Ar(+) sputtered surfaces with 5 nm removed. A cylindrical sample carousel provides normal incidence of the primary beam as well as a multiple Faraday cup measurement of the approximately nA beam currents. Total and true secondary yields are obtained from target current measurements with biasing of the carousel. A primary beam pulsed mode to reduce electron beam dosage and minimize charging of insulating coatings was applied to Mg/F2 coated solar cell covers. Electron beam effects on ITO were found quite important at the current densities necessary to do Auger studies
Cauchy's residue theorem for a class of real valued functions
Let be an interval in and let be a real valued
function defined at the endpoints of and with a certain number of
discontinuities within . Having assumed to be differentiable on a
set to the derivative , where is a subset of at whose points can take values or not be defined at all,
we adopt the convention that and are equal to 0 at all points of
and show that %, where
denotes the total value of the \textit{% Kurzweil-Henstock} integral. The
paper ends with a few examples that illustrate the theory.Comment: 6 page
Enhanced Molecular Orientation Induced by Molecular Anti-Alignment
We explore the role of laser induced anti-alignment in enhancing molecular
orientation. A field-free enhanced orientation via anti-alignment scheme is
presented, which combines a linearly polarized femtosecond laser pulse with a
half-cycle pulse. The laser pulse induces transient anti-alignment in the plane
orthogonal to the field polarization, while the half-cycle pulse leads to the
orientation. We identify two qualitatively different enhancement mechanisms
depending on the pulse order, and optimize their effects using classical and
quantum models both at zero and non-zero temperature
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Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen
Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O-2. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at five temperatures in the range 297-600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-11.08 +/- 0.04) + (1.57 +/- 0.32 kJ mol(-1))/RT ln10 The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H2SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O + SiO. The decomposition of the intermediate cyclo-siladioxirane, via O-O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T -> S process in H2SiOO. This process has a small spin orbit coupling matrix element, consistent with an estimate of its rate constant of 1 x 10(9) s(-1) obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O-2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2 + O-2 and SiCl2 + O-2
Experiences with Lymphangiography in Rhodesia
A CAJM article on the adaptation of Lymphangiography in Zimbabwe (Rhodesia.)Lymphangiography has really only been attempted in Rhodesia since 1967, but our experience is rapidly growing. For a great many years improvised types of apparatus were used in an attempt to cannulate the vessels. With the introduction of lymphangiographic sets this is no longer necessary and the use of the proper tools makes lymphangiography an easier but still time- consuming procedure
Kondo Shuttling in Nanoelectromechanical Single-Electron Transistor
We investigate theoretically a mechanically assisted Kondo effect and
electric charge shuttling in nanoelectromechanical single-electron transistor
(NEM-SET). It is shown that the mechanical motion of the central island (a
small metallic particle) with the spin results in the time dependent tunneling
width which leads to effective increase of the Kondo temperature. The
time-dependent oscillating Kondo temperature T_K(t) changes the scaling
behavior of the differential conductance resulting in the suppression of
transport in a strong coupling- and its enhancement in a weak coupling regimes.
The conditions for fine-tuning of the Abrikosov-Suhl resonance and possible
experimental realization of the Kondo shuttling are discussed.Comment: 4 pages, 2 eps figure
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