Lift and the leading-edge vortex

Flapping wings often feature a leading-edge vortex (LEV) that is thought to enhance the lift generated by the wing. Here the lift on a wing featuring a leading-edge vortex is considered by performing experiments on a translating flat-plate aerofoil that is accelerated from rest in a water towing tank at a fixed angle of attack of 15°. The unsteady flow is investigated with dye flow visualization, particle image velocimetry (PIV) and force measurements. Leading-and trailing-edge vortex circulation and position are calculated directly from the velocity vectors obtained using PIV. In order to determine the most appropriate value of bound circulation, a two-dimensional potential flow model is employed and flow fields are calculated for a range of values of bound circulation. In this way, the value of bound circulation is selected to give the best fit between the experimental velocity field and the potential flow field. Early in the trajectory, the value of bound circulation calculated using this potential flow method is in accordance with Kelvin's circulation theorem, but differs from the values predicted by Wagner's growth of bound circulation and the Kutta condition. Later the Kutta condition is established but the bound circulation remains small; most of the circulation is contained instead in the LEVs. The growth of wake circulation can be approximated by Wagner's circulation curve. Superimposing the non-circulatory lift, approximated from the potential flow model, and Wagner's lift curve gives a first-order approximation of the measured lift. Lift is generated by inertial effects and the slow buildup of circulation, which is contained in shed vortices rather than bound circulation. © 2013 Cambridge University Press.

Liquid crystalline chromophores for photonic band-edge laser devices

We present results on laser action from liquid crystal compounds whereby one sub-unit of the molecular structure consists of the cyano-substituted chromophore, {phenylene-bis (2-cyanopropene)}, similar to the basic unit of the semiconducting polymer structure poly(cyanoterephthalylidene). These compounds were found to exhibit nematic liquid crystal phases. In addition, by virtue of the liquid crystalline properties, the compounds were found to be highly miscible in wide temperature range commercial nematogen mixtures. When optically excited at λ = 355 nm, laser emission was observed in the blue/green region of the visible spectrum (480-530 nm) and at larger concentrations by weight than is achievable using conventional laser dyes. Upon increasing the concentration of dye from 2 to 5 wt.% the threshold was found to increase from Eth = 0.42 ± 0.02 μJ/pulse (≈20 mJ/cm2) to Eth = 0.66 ± 0.03 μJ/pulse (≈34 mJ/cm2). Laser emission was also observed at concentrations of 10 wt.% but was less stable than that observed for lower concentrations of the chromophore. © 2012 Elsevier B.V. All rights reserved.

Trailing edge noise theory for rotating blades in uniform flow

This paper presents a new formulation for trailing edge noise radiation from rotating blades based on an analytical solution of the convective wave equation. It accounts for distributed loading and the effect of mean flow and spanwise wavenumber. A commonly used theory due to Schlinker and Amiet (1981) predicts trailing edge noise radiation from rotating blades. However, different versions of the theory exist; it is not known which version is the correct one and what the range of validity of the theory is. This paper addresses both questions by deriving Schlinker and Amiet's theory in a simple way and by comparing it to the new formulation, using model blade elements representative of a wind turbine, a cooling fan and an aircraft propeller. The correct form of Schlinker and Amiet's theory (1981) is identified. It is valid at high enough frequency, i.e. for a Helmholtz number relative to chord greater than one and a rotational frequency much smaller than the angular frequency of the noise sources.

Plane-strain discrete dislocation plasticity with climb-assisted glide motion of dislocations

A small-strain two-dimensional discrete dislocation plasticity (DDP) framework is developed wherein dislocation motion is caused by climb-assisted glide. The climb motion of the dislocations is assumed to be governed by a drag-type relation similar to the glide-only motion of dislocations: such a relation is valid when vacancy kinetics is either diffusion limited or sink limited. The DDP framework is employed to predict the effect of dislocation climb on the uniaxial tensile and pure bending response of single crystals. Under uniaxial tensile loading conditions, the ability of dislocations to bypass obstacles by climb results in a reduced dislocation density over a wide range of specimen sizes in the climb-assisted glide case compared to when dislocation motion is only by glide. A consequence is that, at least in a single slip situation, size effects due to dislocation starvation are reduced. By contrast, under pure bending loading conditions, the dislocation density is unaffected by dislocation climb as geometrically necessary dislocations (GNDs) dominate. However, climb enables the dislocations to arrange themselves into lower energy configurations which significantly reduces the predicted bending size effect as well as the amount of reverse plasticity observed during unloading. The results indicate that the intrinsic plasticity material length scale associated with GNDs is strongly affected by thermally activated processes and will be a function of temperature. © 2013 IOP Publishing Ltd.

Changes in the Mg profile and in dislocations induced by high temperature annealing of blue LEDs

The efficiency of the injection and recombination processes in InGaN/GaN LEDs is governed by the properties of the active region of the devices, which strongly depend on the conditions used for the growth of the epitaxial material. To improve device quality, it is very important to understand how the high temperatures used during the growth process can modify the quality of the epitaxial material. With this paper we present a study of the modifications in the properties of InGaN/GaN LED structures induced by high temperature annealing: thermal stress tests were carried out at 900 °C, in nitrogen atmosphere, on selected samples. The efficiency and the recombination dynamics were evaluated by photoluminescence measurements (both integrated and time-resolved), while the properties of the epitaxial material were studied by Secondary Ion Mass Spectroscopy (SIMS) and Rutherford Backscattering (RBS) channeling measurements. Results indicate that exposure to high temperatures may lead to: (i) a significant increase in the photoluminescence efficiency of the devices; (ii) a decrease in the parasitic emission bands located between 380 nm and 400 nm; (iii) an increase in carrier lifetime, as detected by time-resolved photoluminescence measurements. The increase in device efficiency is tentatively ascribed to an improvement in the crystallographic quality of the samples. © 2013 SPIE.

Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: An investigation into the role of the density of photon states near resonance

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data. © 2013 American Physical Society.

Removal of surface states and recovery of band-edge emission in InAs nanowires through surface passivation.

Surface states in semiconductor nanowires (NWs) are detrimental to the NW optical and electronic properties and to their light emission-based applications, due to the large surface-to-volume ratio of NWs and the congregation of defects states near surfaces. In this paper, we demonstrated an effective approach to eliminate surface states in InAs NWs of zinc-blende (ZB) and wurtzite (WZ) structures and a dramatic recovery of band edge emission through surface passivation with organic sulfide octadecylthiol (ODT). Microphotoluminescence (PL) measurements were carried out before and after passivation to study the dominant recombination mechanisms and surface state densities of the NWs. For WZ-NWs, we show that the passivation removed the surface states and recovered the band-edge emission, leading to a factor of ∼19 reduction of PL linewidth. For ZB-NWs, the deep surface states were removed and the PL peaks width became as narrow as ∼250 nm with some remaining emission of near band-edge surface states. The passivated NWs showed excellent stability in atmosphere, water, and heat environments. In particular, no observable changes occurred in the PL features from the passivated NWs exposed in air for more than five months.

Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: an investigation into the role of the density of photon states near resonance.

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data.

The effect of dislocations on the efficiency of InGaN/GaN solar cells

Two solar cells based on an InGaN/GaN p-i-n hetero-junction, but having different dislocation densities, were fabricated and characterized. The structures were grown on c-plane (0001) GaN-on-sapphire templates with different threading dislocation (TD) densities of 5×108 and 5×109 cm-2. Structural characterization revealed the presence of V-defects in the InGaN epilayer. Since each V-defect was associated with a TD, the structural as well as the optical properties worsened with a higher TD density in the GaN/sapphire template. It was also found that additional dislocations were generated in the p-GaN layer over the V-defects in the InGaN layer. Because of its superior structural quality, the peak external quantum efficiency (EQE) of the low TD density sample was three times higher than that of the high TD density sample. © 2013 Elsevier B.V.

Aerodynamic noise from a poroelastic trailing edge with implications for the silent flight of owls

The interaction of a turbulent eddy with a semi-infinite, poroelastic edge is examined with respect to the effects of both elasticity and porosity on the efficiency of scattered aerodynamic noise. The scattering problem is solved using the Wiener-Hopf technique for constant plate properties to identify their scaling dependence on the resulting aerodynamic noise, including the dependence on flight velocity, where special attention is paid to the limiting cases of rigid, porous and elastic, impermeable plate conditions. Results from these analyses attempt to address how trailing edge noise may be mitigated by porosity and seek to deepen the understanding of how owls hunt in acoustic stealth. © 2012 by Justin W. Jaworski and Nigel Peake. Published by the American Institute of Aeronautics and Astronautics, Inc.

Acoustic analogy in swirling mean flow applied to predict rotor trailing-edge noise

The present study aims at accounting for swirling mean flow effects on rotor trailing-edge noise. Indeed, the mean flow in between the rotor and the stator of the fan or of a compressor stage is highly swirling. The extension of Ffowcs-Williams & Hawkings' acoustic analogy in a medium at rest with moving surfaces and of Goldstein's acoustic analogy in a circular duct with uniform mean flow to a swirling mean flow in an annular duct is introduced. It is first applied to tonal noise. In most cases, the swirl modifies the pressure distribution downstream of the fan. In several configurations, when the swirl is rather close to a solid body swirl, it is often sufficient to apply a simple Doppler effect correction when predicting the duct modes in uniform mean flow in order to predict accurately the noise radiated with swirl. However, in other realistic configurations, the swirling mean-flow effect cannot be addressed using this simple Doppler effect correction. Second, a rotor trailing-edge noise model accounting for both the effects of the annular duct and the swirling mean flow is developed and applied to a realistic fan rotor with different swirling and sheared mean flows (and as a result different associated blade stagger angles). The benchmark cases are built from the Boeing 18-inch Fan Rig Broadband Noise Test. In all cases the swirling mean flow has an effect. In some cases the a simple Doppler effect may address it, but, in other realistic configurations our acoustic analogy with swirl is needed. © 2012 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.

The dynamic response of edge clamped plates loaded by spherically expanding sand shells

The dynamic deformation of both edge clamped stainless steel sandwich panels with a pyramidal truss core and equal mass monolithic plates loaded by spherically expanding shells of dry and water saturated sand has been investigated, both experimentally and via a particle based simulation methodology. The spherically expanding sand shell is generated by detonating a sphere of explosive surrounded by a shell of either dry or water saturated synthetic sand. The measurements show that the sandwich panel and plate deflections decrease with increasing stand-off between the center of the charge and the front of the test structures. Moreover, for the same charge and sand mass, the deflections of the plates are significantly higher in the water saturated sand case compared to that of dry sand. For a given stand-off, the mid-span deflection of the sandwich panel rear faces was substantially less than that of the corresponding monolithic plate for both the dry and water saturated sand cases. The experiments were simulated via a coupled discrete-particle/ finite element scheme wherein the high velocity impacting sand is modeled by interacting particles while the plate is modeled within a Lagrangian finite element setting. The simulations are in good agreement with the measurements for the dry sand impact of both the monolithic and sandwich structures. However, the simulations underestimate the effect of stand-off in the case of the water saturated sand explosion, i.e. the deflections decrease more sharply with increasing stand-off in the experiments compared to the simulations. The simulations reveal that the momentum transmitted into the sandwich and monolithic plate structures by the sand shell is approximately the same, consistent with a small fluid-structure interaction effect. The smaller deflection of the sandwich panels is therefore primarily due to the higher bending strength of sandwich structures. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.