361 research outputs found
Aerothermodynamic radiation studies
We have built and made operational a 6 in. electric arc driven shock tube which alloys us to study the non-equilibrium radiation and kinetics of low pressure (0.1 to 1 torr) gases processed by 6 to 12 km/s shock waves. The diagnostic system allows simultaneous monitoring of shock radiation temporal histories by a bank of up to six radiometers, and spectral histories with two optical multi-channel analyzers. A data set of eight shots was assembled, comprising shocks in N2 and air at pressures between 0.1 and 1 torr and velocities of 6 to 12 km/s. Spectrally resolved data was taken in both the non-equilibrium and equilibrium shock regions on all shots. The present data appear to be the first spectrally resolved shock radiation measurements in N2 performed at 12 km/s. The data base was partially analyzed with salient features identified
Intrinsic mechanism of phase locking in two-dimensional Josephson junction networks in presence of an external magnetic field
We present numerical simulations of the dynamics of two-dimensional Josephson
junction arrays to study the mechanism of mutual phase locking. We show that in
the presence of an external magnetic field two mechanisms are playing a role in
phase locking: feedback through the external load and internal coupling between
rows due to microwave currents induced by the field. We have found the
parameter values (junction capacitance, cell loop inductance, impedance of the
external load) for which the interplay of both these mechanisms leads to the
in-phase solution. The case of unshunted arrays is discussed as well.Comment: 13 pages, incl. 6 ps figures, Subm. to Europhysics Letter
Theory of phase-locking in generalized hybrid Josephson junction arrays
A recently proposed scheme for the analytical treatment of the dynamics of
two-dimensional hybrid Josephson junction arrays is extended to a class of
generalized hybrid arrays with ''horizontal'' shunts involving a capacitive as
well as an inductive component. This class of arrays is of special interest,
because the internal cell coupling has been shown numerically to favor in-phase
synchronization for certain parameter values. As a result, we derive limits on
the circuit design parameters for realizing this state. In addition, we obtain
formulas for the flux-dependent frequency including flux-induced switching
processes between the in-phase and anti-phase oscillation regime. The treatment
covers unloaded arrays as well as arrays shunted via an external load.Comment: 24 pages, REVTeX, 5 Postscript figures, Subm. to Phys. Rev.
Crossover from Attractive to Repulsive Casimir Forces and Vice Versa
Systems described by an O(n) symmetrical Hamiltonian are considered
in a -dimensional film geometry at their bulk critical points. The critical
Casimir forces between the film's boundary planes , are
investigated as functions of film thickness for generic symmetry-preserving
boundary conditions . The
-dependent part of the reduced excess free energy per cross-sectional area
takes the scaling form when , where are scaling
fields associated with the variables , and is a surface
crossover exponent. Explicit two-loop renormalization group results for the
function at dimensions are
presented. These show that (i) the Casimir force can have either sign,
depending on and , and (ii) for appropriate
choices of the enhancements , crossovers from attraction to
repulsion and vice versa occur as increases.Comment: 4 RevTeX pages, 2 eps figures; minor misprints corrected and 3
references adde
Fluctuations of the Casimir-like force between two membrane inclusions
Although Casimir forces are inseparable from their fluctuations, little is
known about these fluctuations in soft matter systems. We use the membrane
stress tensor to study the fluctuations of the membrane-mediated Casimir-like
force. This method enables us to recover the Casimir force between two
inclusions and to calculate its variance. We show that the Casimir force is
dominated by its fluctuations. Furthermore, when the distance d between the
inclusions is decreased from infinity, the variance of the Casimir force
decreases as -1/d^2. This distance dependence shares a common physical origin
with the Casimir force itself.Comment: 5 pages, 3 figure
Influence of Capillary Condensation on the Near-Critical Solvation Force
We argue that in a fluid, or magnet, confined by adsorbing walls which favour
liquid, or (+) phase, the solvation (Casimir) force in the vicinity of the
critical point is strongly influenced by capillary condensation which occurs
below the bulk critical temperature T_c. At T slightly below and above T_c, a
small bulk field h<0, which favours gas, or (-) phase, leads to residual
condensation and a solvation force which is much more attractive (at the same
large wall separation) than that found exactly at the critical point. Our
predictions are supported by results obtained from density-matrix
renormalization-group calculations in a two-dimensional Ising strip subject to
identical surface fields.Comment: 4 Pages, RevTeX, and 3 figures include
Forces Induced by Non-Equilibrium Fluctuations: The Soret-Casimir Effect
The notion of fluctuation-induced forces is generalized to the cases where
the fluctuations have nonequilibrium origin. It is shown that a net force is
exerted on a single flat plate that restricts scale-free fluctuations of a
scalar field in a temperature gradient. This force tends to push the object to
the colder regions, which is a manifestation of thermophoresis or the Soret
effect. In the classic two-plate geometry, it is shown that the Casimir forces
exerted on the two plates differ from each other, and thus the Newton's third
law is violated.Comment: 8 pages, 5 postscript figures, uses (old) RevTe
Casimir Forces between Spherical Particles in a Critical Fluid and Conformal Invariance
Mesoscopic particles immersed in a critical fluid experience long-range
Casimir forces due to critical fluctuations. Using field theoretical methods,
we investigate the Casimir interaction between two spherical particles and
between a single particle and a planar boundary of the fluid. We exploit the
conformal symmetry at the critical point to map both cases onto a highly
symmetric geometry where the fluid is bounded by two concentric spheres with
radii R_- and R_+. In this geometry the singular part of the free energy F only
depends upon the ratio R_-/R_+, and the stress tensor, which we use to
calculate F, has a particularly simple form. Different boundary conditions
(surface universality classes) are considered, which either break or preserve
the order-parameter symmetry. We also consider profiles of thermodynamic
densities in the presence of two spheres. Explicit results are presented for an
ordinary critical point to leading order in epsilon=4-d and, in the case of
preserved symmetry, for the Gaussian model in arbitrary spatial dimension d.
Fundamental short-distance properties, such as profile behavior near a surface
or the behavior if a sphere has a `small' radius, are discussed and verified.
The relevance for colloidal solutions is pointed out.Comment: 37 pages, 2 postscript figures, REVTEX 3.0, published in Phys. Rev. B
51, 13717 (1995
Critical Casimir effect and wetting by helium mixtures
We have measured the contact angle of the interface of phase-separated
He-He mixtures against a sapphire window. We have found that this
angle is finite and does not tend to zero when the temperature approaches
, the temperature of the tri-critical point. On the contrary, it increases
with temperature. This behavior is a remarkable exception to what is generally
observed near critical points, i.e. "critical point wetting''. We propose that
it is a consequence of the "critical Casimir effect'' which leads to an
effective attraction of the He-He interface by the sapphire near
.Comment: submitted july 13 (2002), published march 20 (2003
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