4,580 research outputs found
Analysis of dynamic stall using unsteady boundary-layer theory
The unsteady turbulent boundary layer and potential flow about a pitching airfoil are analyzed using numerical methods to determine the effect of pitch rate on the delay in forward movement of the rear flow reversal point. An explicit finite difference scheme is used to integrate the unsteady boundary layer equations, which are coupled at each instant of time to a fully unsteady and nonlinear potential flow analysis. A substantial delay in forward movement of the reversal point is demonstrated with increasing pitch rate, and it is shown that the delay results partly from the alleviation of the gradients in the potential flow, and partly from the effects of unsteadiness in the boundary layer itself. The predicted delay in flow-reversal onset, and its variation with pitch rate, are shown to be in reasonable agreement with experimental data relating to the delay in dynamic stall. From the comparisons it can be concluded (a) that the effects of time-dependence are sufficient to explain the failure of the boundary layer to separate during the dynamic overshoot, and (b) that there may be some link between forward movement of the reversal point and dynamic stall
A statistical model for the intrinsically broad superconducting to normal transition in quasi-two-dimensional crystalline organic metals
Although quasi-two-dimensional organic superconductors such as
-(BEDT-TTF)Cu(NCS) seem to be very clean systems, with apparent
quasiparticle mean-free paths of several thousand \AA, the superconducting
transition is intrinsically broad (e.g K wide for K).
We propose that this is due to the extreme anisotropy of these materials, which
greatly exacerbates the statistical effects of spatial variations in the
potential experienced by the quasiparticles. Using a statistical model, we are
able to account for the experimental observations. A parameter , which
characterises the spatial potential variations, may be derived from
Shubnikov-de Haas oscillation experiments. Using this value, we are able to
predict a transition width which is in good agreement with that observed in MHz
penetration-depth measurements on the same sample.Comment: 8 pages, 2 figures, submitted to J. Phys. Condens. Matte
Cyclotron resonance of the quasi-two-dimensional electron gas at Hg1-xCdxTe grain boundaries
The magnetotransmission of a p-type Hg0.766Cd0.234Te bicrystal containing a single grain boundary with an inversion layer has been investigated in the submillimetre wavelength range. For the first time the cyclotron resonance lines belonging to the various electric subbands of a quasi-two-dimensional carrier system at a grain boundary could be detected. The measured cyclotron masses and the subband densities determined from Shubnikov-de Haas experiments are compared with theoretical predictions and it is found that the data can be explained very well within the framework of a triangular well approximation model which allows for non-parabolic effects
Weak anisotropy of the superconducting upper critical field in Fe1.11Te0.6Se0.4 single crystals
We have determined the resistive upper critical field Hc2 for single crystals
of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to
60T. A rather high zero-temperature upper critical field of mu0Hc2(0) approx
47T is obtained, in spite of the relatively low superconducting transition
temperature (Tc approx 14K). Moreover, Hc2 follows an unusual temperature
dependence, becoming almost independent of the magnetic field orientation as
the temperature T=0. We suggest that the isotropic superconductivity in
Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface
topology. An analogous result was obtained for (Ba,K)Fe2As2, indicating that
all layered iron-based superconductors exhibit generic behavior that is
significantly different from that of the high-Tc cuprates.Comment: 4 pages, 4 figures, submit to PR
Fermion Production in the Background of Minkowski Space Classical Solutions in Spontaneously Broken Gauge Theory
We investigate fermion production in the background of Minkowski space
solutions to the equations of motion of gauge theory spontaneously
broken via the Higgs mechanism. First, we attempt to evaluate the topological
charge of the solutions. We find that for solutions is not well-defined
as an integral over all space-time. Solutions can profitably be characterized
by the (integer-valued) change in Higgs winding number . We show
that solutions which dissipate at early and late times and which have nonzero
must have at least the sphaleron energy. We show that if we couple
a quantized massive chiral fermion to a classical background given by a
solution, the number of fermions produced is , and is not related
to .Comment: Version to be published. Argument showing that the topological charge
of solutions is undefined has been strengthened and clarified. Conclusions
unchange
The magnetoresistance and Hall effect in CeFeAsO: a high magnetic field study
The longitudinal electrical resistivity and the transverse Hall resistivity
of CeFeAsO are simultaneously measured up to a magnetic field of 45T using the
facilities of pulsed magnetic field at Los Alamos. Distinct behaviour is
observed in both the magnetoresistance Rxx({\mu}0H) and the Hall resistance
Rxy({\mu}0H) while crossing the structural phase transition at Ts \approx 150K.
At temperatures above Ts, little magnetoresistance is observed and the Hall
resistivity follows linear field dependence. Upon cooling down the system below
Ts, large magnetoresistance develops and the Hall resistivity deviates from the
linear field dependence. Furthermore, we found that the transition at Ts is
extremely robust against the external magnetic field. We argue that the
magnetic state in CeFeAsO is unlikely a conventional type of spin-density-wave
(SDW).Comment: 4 pages, 3 figures SCES2010, To appear in J. Phys.: Conf. Ser. for
SCES201
Vibratory Loads Data from a Wind-Tunnel Test of Structurally Tailored Model Helicopter Rotors
An experimental study was conducted in the Langley Transonic Dynamics Tunnel to investigate the use of a Bell Helicopter Textron (BHT) rotor structural tailoring concept, known as rotor nodalization, in conjunction with advanced blade aerodynamics as well as to evaluate rotor blade aerodynamic design methodologies. A 1/5-size, four-bladed bearingless hub, three sets of Mach-scaled model rotor blades were tested in forward flight from transition up to an advance ratio of 0.35. The data presented pertain only to the evaluation of the structural tailoring concept and consist of fixed-system and rotating system vibratory loads. These data will be useful for evaluating the effects of tailoring blade structural properties on fixed-system vibratory loads, as well as validating analyses used in the design of advanced rotor systems
A photonic bandgap resonator to facilitate GHz frequency conductivity experiments in pulsed magnetic fields
We describe instrumentation designed to perform millimeter-wave conductivity
measurements in pulsed high magnetic fields at low temperatures. The main
component of this system is an entirely non-metallic microwave resonator. The
resonator utilizes periodic dielectric arrays (photonic bandgap structures) to
confine the radiation, such that the resonant modes have a high Q-factor, and
the system possesses sufficient sensitivity to measure small samples within the
duration of a magnet pulse. As well as measuring the sample conductivity to
probe orbital physics in metallic systems, this technique can detect the sample
permittivity and permeability allowing measurement of spin physics in
insulating systems. We demonstrate the system performance in pulsed magnetic
fields with both electron paramagnetic resonance experiments and conductivity
measurements of correlated electron systems.Comment: Submitted to the Review of Scientific instrument
Complex Lagrangians and phantom cosmology
Motivated by the generalization of quantum theory for the case of
non-Hermitian Hamiltonians with PT symmetry, we show how a classical
cosmological model describes a smooth transition from ordinary dark energy to
the phantom one. The model is based on a classical complex Lagrangian of a
scalar field. Specific symmetry properties analogous to PT in non-Hermitian
quantum mechanics lead to purely real equation of motion.Comment: 11 pages, to be published in J.Phys.A, refs. adde
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