Computation of full-coverage film-cooled airfoil temperatures by two methods and comparison with high heat flux data

Two methods were used to calculate the heat flux to full-coverage film cooled airfoils and, subsequently, the airfoil wall temperatures. The calculated wall temperatures were compared to measured temperatures obtained in the Hot Section Facility operating at real engine conditions. Gas temperatures and pressures up to 1900 K and 18 atm with a Reynolds number up to 1.9 million were investigated. Heat flux was calculated by the convective heat transfer coefficient adiabatic wall method and by the superposition method which incorporates the film injection effects in the heat transfer coefficient. The results of the comparison indicate the first method can predict the experimental data reasonably well. However, superposition overpredicted the heat flux to the airfoil without a significant modification of the turbulent Prandtl number. The results suggest that additional research is required to model the physics of full-coverage film cooling where there is significant temperature/density differences between the gas and the coolant

Transverse Bragg-reflector injection lasers

A GaAs-GaAlAs injection laser has been tested that confines light in the lateral dimension (normal to junction plane) by a multilayer Bragg reflector. In the past, light has been confined as a result of the higher-index guiding region and resulting evanescent fields

Time-reversal in dynamically-tuned zero-gap periodic systems

We show that short pulses propagating in zero-gap periodic systems can be reversed with 100% efficiency by using weak non-adiabatic tuning of the wave velocity at time-scales that can be much slower than the period. Unlike previous schemes, we demonstrate reversal of {\em broadband} (few cycle) pulses with simple structures. Our scheme may thus open the way to time-reversal in a variety of systems for which it was not accessible before.Comment: Accepted for publication in Phys. Rev. Letter

Complete bandgaps in one-dimensional left-handed periodic structures

Artificially fabricated structures with periodically modulated parameters such as photonic crystals offer novel ways of controlling the flow of light due to the existence of a range of forbidden frequencies associated with a photonic bandgap. It is believed that modulation of the refractive index in all three spatial dimensions is required to open a complete bandgap and prevent the propagation of electromagnetic waves in all directions. Here we reveal that, in a sharp contrast to what was known before and contrary to the accepted physical intuition, a one-dimensional periodic structure containing the layers of transparent left-handed (or negative-index) metamaterial can trap light in three-dimensional space due to the existence of a complete bandgap.Comment: 4 pages, 5 figure

Spatial properties of entangled photon pairs generated in nonlinear layered structures

A spatial quantum model of spontaneous parametric down-conversion in nonlinear layered structures is developed expanding the interacting vectorial fields into monochromatic plane waves. A two-photon spectral amplitude depending on the signal- and idler-field frequencies and propagation directions is used to derive transverse profiles of the emitted fields as well as their spatial correlations. Intensity spatial profiles and their spatial correlations are mainly determined by the positions of transmission peaks formed in these structures with photonic bands. A method for geometry optimization of the structures with respect to efficiency of the nonlinear process is suggested. Several structures composed of GaN/AlN layers are analyzed as typical examples. They allow the generation of photon pairs correlated in several emission directions. Photon-pair generation rates increasing better than the second power of the number of layers can be reached. Also structures efficiently generated photon pairs showing anti-bunching and anti-coalescence can be obtained. Three reasons for splitting the correlated area in photonic-band-gap structures are revealed: zig-zag movement of photons inside the structure, spatial symmetry and polarization-dependent properties. Also spectral splitting can be observed in these structures.Comment: 13 pages, 17 figure

Effects of lattice distortion and Jahn–Teller coupling on the magnetoresistance of La0.7Ca0.3MnO3 and La0.5Ca0.5CoO3 epitaxial films

Studies of La0.7Ca0.3MnO3 epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (T → TC, TC being the Curie temperature), the magnetotransport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures (T ≪ TC/, the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associated with the presence of Jahn–Teller coupling in the manganites is further verified by comparing the manganites with epitaxial films of another ferromagnetic perovskite, La0.5Ca0.5CoO3. Regardless of the differences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attributed to the absence of Jahn–Teller coupling and the high electron mobility that prevents the formation of lattice polarons. We therefore suggest that lattice polaron conduction associated with the Jahn–Teller coupling is essential for the occurrence of CMR, and that lattice distortion further enhances the CMR effects in the manganites

Design of an intelligent spinal artificial disk prosthesis for the investigation of in-vivo loading on the spine

The knowledge of the in-vivo loading on the spinal disk is of paramount importance in the understanding of low back pain. In this study an artificial spinal disk is used as a base for making an in-body intelligent implantable load-cell which can measure the in-vivo loading on the spinal disk. A commercially available spinal disc was utilized and was loaded with eight strain gauges and two piezoresistive sensors placed at different locations on the disc in order to enable the complete local mapping on the disk. With the aid of a cadaveric animal spine the artificial disc with all sensor was loaded in a laboratory environment. The in-vitro loading produced reliable and repeatable results and therefore suggesting that such approach might aid in the development of an artificial intelligent disc which will aid in the better understanding of the in-vivo loading of the human spine

Chalcogen Height Dependence of Magnetism and Fermiology in FeTe_xSe_{1-x}

FeTexSe1-x (x=0, 0.25, 0.50, 0.75 and 1) system has been studied using density functional theory. Our results show that for FeSe, LDA seems better approximation in terms of magnitude of magnetic energy whereas GGA overestimates it largely. On the other hand for FeTe, GGA is better approximation that gives experimentally observed magnetic state. It has been shown that the height of chalcogen atoms above Fe layers has significant effect on band structure, electronic density of states (DOS) at Fermi level N(EF) and Fermi surfaces. For FeSe the value of N(EF) is small so as to satisfy Stoner criteria for ferromagnetism, (I\timesN(EF)\geq1) whereas for FeTe, since the value of N(EF) is large, the same is close to be satisfied. Force minimization done for FeTexSe1-x using supercell approach shows that in disordered system Se and Te do not share same site and have two distinct z coordinates. This has small effect on magnetic energy but no significant difference in band structure and DOS near EF when calculated using either relaxed or average value of z for chalcogen atoms. Thus substitution of Se at Te site decreases average value of chalcogen height above Fe layers which in turn affect the magnetism and Fermiology in the system. By using coherent-potential approximation for disordered system we found that height of chalcogen atoms above Fe layer rather than chalcogen species or disorder in the anion planes, affect magnetism and shape of Fermi surfaces (FS), thus significantly altering nesting conditions, which govern antiferromagnetic spin fluctuations in the system.Comment: 24 pages Text+Figs: comments/suggestions welcome ([email protected]

Scattering-free plasmonic optics with anisotropic metamaterials

We develop an approach to utilize anisotropic metamaterials to solve one of the fundamental problems of modern plasmonics -- parasitic scattering of surface waves into free-space modes, opening the road to truly two-dimensional plasmonic optics. We illustrate the developed formalism on examples of plasmonic refractor and plasmonic crystal, and discuss limitations of the developed technique and its possible applications for sensing and imaging structures, high-performance mode couplers, optical cloaking structures, and dynamically reconfigurable electro-plasmonic circuits