Comparison of acoustic velocity perturbation measurements using PIV vs. two-microphone technique

Understanding combustion instabilities requires accurate measurements of the acoustic velocity perturbation into injectors. This is often accomplished via the use of the two microphone technique, as this only requires two pressure transducers. However, measurements of the actual velocities emerging from the injectors are not often taken, leaving questions regarding the assumptions about the acoustic velocity. A comparison of velocity measured at downstream of the injector with that of two-microphone technique can show the accuracy and limitations of two-microphone technique. In this paper, velocity measurements are taken using both particle image velocimetry (PIV) and the two-microphone technique in a high pressure facility designed for aeroengine injector measurements. The flow is excited using an area modulation device installed on the choked end of the combustion chamber, with PIV measurements enabled by optical access downstream of the injector through a quartz tube and windows. Acoustic velocity perturbations at the injector are determined by considering the Fourier transformed pressure fluctuations for two microphones installed at a known distance upstream of the injector. PIV measurements are realized by seeding the air flow with micrometric water particles under 2.5 bar pressure at ambient temperature. Phase locked velocity fields are realized by synchronizing the acquisition of PIV images with the revolution of the acoustic modulator using the pressure signal measured at the face of injector. The mean velocity fluctuation is calculated as the difference between maximum and minimum velocities, normalized by the mean velocity of the unforced case. Those values are compared with the peak-to-peak velocity fluctuation amplitude calculated by the two-microphone technique. Although the ranges of velocity fluctuations for both techniques are similar, the variation of fluctuation with forcing frequencies diverges significantly with frequency. The differences can be attributed to several limitations associated with of both techniques, such as the quality of the signal, the signal/noise ratio, the accuracy of PIV measurements and the assumption of isentropic flow of the particle velocity from the plenum through the injector. We conclude that two-microphone methods can be used as a reference value for the velocity fluctuation in low order applications such as flame transfer functions, but not for drawing conclusions regarding the absolute velocity fluctuations in the injector. Copyright © 2013 by ASME.

Response of clamped sandwich beams subjected to high-velocity impact by sand slugs

The dynamic response of end-clamped sandwich and monolithic beams of equal areal mass subjected to loading via high-velocity slugs of dry and water-saturated sand is measured using a novel laboratory-based method. The sandwich beams comprise aluminium face sheets and an aluminium honeycomb core: the effect of sandwich core strength and beam thickness on the dynamic beam deflection is investigated by varying the orientation and height of the anisotropic aluminium honeycomb core material. High-speed imaging is used to measure the transient transverse deflection of the beams and to record the dynamic modes of deformation. The measurements show that sandwich beams with thick, strong cores are optimal and that these beams significantly outperform monolithic beams of equal mass. The water-saturated sand slugs cause significantly higher deflections compared to the dry sand slugs having the same mean slug velocity and we demonstrate that this enhanced deflection is due to the larger mass of the water-saturated slugs. Finally, we show that the impact of sand slugs is equivalent to the impact of a crushable foam projectile. The experiments using foam projectiles are significantly simpler to perform and thus represent a more convenient laboratory technique. © 2014 Elsevier Ltd. All rights reserved.

Sparse recovery of complex phase-encoded velocity images using iterative thresholding

In this paper we propose a new algorithm for reconstructing phase-encoded velocity images of catalytic reactors from undersampled NMR acquisitions. Previous work on this application has employed total variation and nonlinear conjugate gradients which, although promising, yields unsatisfactory, unphysical visual results. Our approach leverages prior knowledge about the piecewise-smoothness of the phase map and physical constraints imposed by the system under study. We show how iteratively regularizing the real and imaginary parts of the acquired complex image separately in a shift-invariant wavelet domain works to produce a piecewise-smooth velocity map, in general. Using appropriately defined metrics we demonstrate higher fidelity to the ground truth and physical system constraints than previous methods for this specific application. © 2013 IEEE.

Rapid hologram generation utilizing layer-based approach and graphic rendering for realistic three-dimensional image reconstruction by angular tiling

An approach of rapid hologram generation for the realistic three-dimensional (3-D) image reconstruction based on the angular tiling concept is proposed, using a new graphic rendering approach integrated with a previously developed layer-based method for hologram calculation. A 3-D object is simplified as layered cross-sectional images perpendicular to a chosen viewing direction, and our graphics rendering approach allows the incorporation of clear depth cues, occlusion, and shading in the generated holograms for angular tiling. The combination of these techniques together with parallel computing reduces the computation time of a single-view hologram for a 3-D image of extended graphics array resolution to 176 ms using a single consumer graphics processing unit card. © 2014 SPIE and IS and T.

Modelling of turbulent lifted jet flames using flamelets: A priori assessment and a posteriori validation

This study focuses on the modelling of turbulent lifted jet flames using flamelets and a presumed Probability Density Function (PDF) approach with interest in both flame lift-off height and flame brush structure. First, flamelet models used to capture contributions from premixed and non-premixed modes of the partially premixed combustion in the lifted jet flame are assessed using a Direct Numerical Simulation (DNS) data for a turbulent lifted hydrogen jet flame. The joint PDFs of mixture fraction Z and progress variable c, including their statistical correlation, are obtained using a copula method, which is also validated using the DNS data. The statistically independent PDFs are found to be generally inadequate to represent the joint PDFs from the DNS data. The effects of Z-c correlation and the contribution from the non-premixed combustion mode on the flame lift-off height are studied systematically by including one effect at a time in the simulations used for a posteriori validation. A simple model including the effects of chemical kinetics and scalar dissipation rate is suggested and used for non-premixed combustion contributions. The results clearly show that both Z-c correlation and non-premixed combustion effects are required in the premixed flamelets approach to get good agreement with the measured flame lift-off heights as a function of jet velocity. The flame brush structure reported in earlier experimental studies is also captured reasonably well for various axial positions. It seems that flame stabilisation is influenced by both premixed and non-premixed combustion modes, and their mutual influences. © 2014 Taylor & Francis.

Bipedal walking and running with compliant legs

Passive dynamics plays an important role in legged locomotion of the biological systems. The use of passive dynamics provides a number of advantages in legged locomotion such as energy efficiency, self-stabilization against disturbances, and generating gait patterns and behavioral diversity. Inspired from the theoretical and experimental studies in biomechanics, this paper presents a novel bipedal locomotion model for walking and running behavior which uses compliant legs. This model consists of three-segment legs, two servomotors, and four passive joints that are constrained by eight tension springs. The self-organization of two gait patterns (walking and running) is demonstrated in simulation and in a real-world robot. The analysis of joint kinematics and ground reaction force explains how a minimalistic control architecture can exploit the particular leg design for generating different gait patterns. Moreover, it is shown how the proposed model can be extended for controlling locomotion velocity and gait patterns with the simplest control architecture. © 2007 IEEE.

One-legged locomotion with a compliant passive joint

There is an increasing attention of exploiting compliant materials for the purpose of legged locomotion, because they provide significant advantages in locomotion performance with respect to energy efficiency and stability. Toward establishing a fundamental basis for this line of research, a minimalistic locomotion model of a single legged system is explored in this paper. By analyzing the dynamic behavior of the system in simulation and a physical robotic platform, it is shown that a stable locomotion process can be achieved without the necessity of sensory feedback. In addition, further analysis characterizes the relation between motor control and the natural body dynamics determined by morphological properties such as body mass and spring constant. © 2006 The authors.

Scaling of velocity fluctuations in off-wall boundary conditions for turbulent flows

A model for off-wall boundary conditions for turbulent flow is investigated. The objective of such a model is to circumvent the need to resolve the buffer layer near the wall, by providing conditions in the logarithmic layer for the overlying flow. The model is based on the self-similarity of the flow at different heights in the logarithmic layer. It was first proposed by Mizuno and Jiménez (2013), imposing at the boundary plane a velocity field obtained on-the-fly from an overlying region. The key feature of the model was that the lengthscales of the field were rescaled to account for the self-similarity law. The model was successful at sustaining a turbulent logarithmic layer, but resulted in some disagreements in the flow statistics, compared to fully-resolved flows. These disagreements needed to be addressed for the model to be of practical application. In the present paper, a more refined, wavelength-dependent rescaling law is proposed, based on the wavelength-dependent dynamics in fully-resolved flows. Results for channel flow show that the new model eliminates the large artificial pressure fluctuations found in the previous one, and a better agreement is obtained in the bulk properties, the flow fluctuations, and their spectral distribution across the whole domain. © Published under licence by IOP Publishing Ltd.

Effective recombination velocity of textured surfaces

Surface texturization is an effective way to enhance the absorption of light for optoelectronic devices but it also aggravates the surface recombination by enlarging the surface area. In order to evaluate the influence of texture structures on the surface recombination, an effective surface recombination velocity is defined which is assumed to have an equivalent recombination effect on a flat surface. Based on numerical and analytical calculation, the dependences of effective surface recombination on the pattern geometry, the surface recombination velocity, and the diffusion length are analyzed.

High efficiency operation of butt joint line-defect-waveguide microlaser in two-dimensional photonic crystal slab

Butt joint line-defect-waveguide microlasers are demonstrated on photonic crystal slabs with airholes in a triangular lattice. Such microlaser is designed to increase the output power from the waveguide edge directly. The output power is remarkably enhanced to 214 times higher by introducing chirped structure in the output waveguide. The lasing mode operates in the linear dispersion region of the output waveguide so that the absorption loss due to the band-edge effect is reduced. The laser resonance is illustrated theoretically using the finite difference time domain method. A practical high power efficiency of 20% is obtained in this microlaser. (C) 2008 American Institute of Physics.

A novel butt-joint scheme for the preparation of electro-absorptive lasers

A 1.55 mu m InGaAsP/InGaAsP multiple-quantum-well electro-absorption modulator (EAM) monolithically integrated with a distributed feedback laser (DFB) diode has been realized based on a novel butt-joint scheme by ultra-low metal-organic vapour phase epitaxy for the first time. The threshold current of 25 mA and an extinction ratio of more than 30 dB are obtained by using the novel structure. The beam divergence angles at the horizontal and vertical directions are as small as 19.3 degrees x 13 degrees, respectively, without a spot-size converter by undercutting the InGaAsP active region. The capacitance of the ridge waveguide device with a deep mesa buried by polyimide was reduced down to 0.30 pF.

Angular dependent characteristics of a 1.3-mu m GaInNAs/GaAs quantum-well resonant cavity enhanced photodetect

Characteristics of a 1.3-mum GaInNAs RCE PD with respect to the incident light angle were analyzed both in theoretical simulation and experiments. The results show the influence can be neglected when the light incidence angle is less than 3degrees. This is a requirement for the PD to be applied in WDM networks. (C) 2002 Wiley Periodicals, Inc.

Surface acoustic wave velocity and electromechanical coupling coefficient of GaN grown on (0001) sapphire by metal-organic vapour phase epitaxy

High-quality and high-resistivity GaN films were grown on (0001) sapphire face by metal-organic vapour phase epitaxy. To measure the surface acoustic wave properties accurately, we deposited metallized interdigital transducers on the GaN surface. The acoustic surface wave velocity and electromechanical coupling coefficient were measured, respectively, to be 5667 m/s and 1.9% by the pulse method.

Experimental investigation of slow light phenomenon in photonic crystal waveguide line defect laser

The guide mode whose frequency locates in the band edge in photonic crystal single line defect waveguide has very low group velocity. So the confinement and gain of electromagnetic field in the band edge are strongly enhanced. Photonic crystal waveguide laser is fabricated and the slow light phenomenon is investigated. The laser is pumped by pulsed pumping light at 980nm whose duty ratio is 0.05%. The active layer in photonic crystal slab is InGaAsP multiple quantum well. Light is transimited by a photonic crystal chirp waveguide in one facet of the laser. Then the output light is coupled to a fiber and the character of laser is analysis by an optical spectrometer. It is found that single mode and multimode happens with different power of pumping light. Meanwhile the plane wave expansion and finite-difference time-domain methods are used to simulate the phenomenon of slow light. And the result of the experiment is compared with the theory which proves the slow light results in lasing oscillation.

Methane hydrate growth velocity in seepage system of Qiongdongnan Basin, South China Sea

The occurrences of diapirs, gas-filled zones and gas plumes in seawater in Qiongdongnan Basin of South China Sea indicate that there may exist seepage system gas-hydrate reservoirs. Assuming there has a methane venting zone of 1500 m in diameter, and the methane flux is 1000 kmol/a, and the temperature of methane hydrate-bearing sediments ranges from 3 degrees C to 20 degrees C, then according to the hydrate film growth theory, by numerical simulation, this paper computes the temperatures and velocities in 0 mbsf, 100 mbsf, 200 mbsf, 425 mbsf over discrete length, and gives the change charts. The results show that the cementation velocity in sediments matrix of methane hydrate is about 0.2 nm/s, and the seepage system will evolve into diffusion system over probably 35000 years. Meanwhile, the methane hydrate growth velocity in leakage system is 20 similar to 40 times faster than in diffusion system.