Applications of the dynamic mode decomposition

The decomposition of experimental data into dynamic modes using a data-based algorithm is applied to Schlieren snapshots of a helium jet and to time-resolved PIV-measurements of an unforced and harmonically forced jet. The algorithm relies on the reconstruction of a low-dimensional inter-snapshot map from the available flow field data. The spectral decomposition of this map results in an eigenvalue and eigenvector representation (referred to as dynamic modes) of the underlying fluid behavior contained in the processed flow fields. This dynamic mode decomposition allows the breakdown of a fluid process into dynamically revelant and coherent structures and thus aids in the characterization and quantification of physical mechanisms in fluid flow. © 2010 Springer-Verlag.

Measurement-Integrated simulations and Kalman filter applied to a co-flowing jet

This paper deals with the experimental evaluation of a flow analysis system based on the integration between an under-resolved Navier-Stokes simulation and experimental measurements with the mechanism of feedback (referred to as Measurement-Integrated simulation), applied to the case of a planar turbulent co-flowing jet. The experiments are performed with inner-to-outer-jet velocity ratio around 2 and the Reynolds number based on the inner-jet heights about 10000. The measurement system is a high-speed PIV, which provides time-resolved data of the flow-field, on a field of view which extends to 20 jet heights downstream the jet outlet. The experimental data can thus be used both for providing the feedback data for the simulations and for validation of the MI-simulations over a wide region. The effect of reduced data-rate and spatial extent of the feedback (i.e. measurements are not available at each simulation time-step or discretization point) was investigated. At first simulations were run with full information in order to obtain an upper limit of the MI-simulations performance. The results show the potential of this methodology of reproducing first and second order statistics of the turbulent flow with good accuracy. Then, to deal with the reduced data different feedback strategies were tested. It was found that for small data-rate reduction the results are basically equivalent to the case of full-information feedback but as the feedback data-rate is reduced further the error increases and tend to be localized in regions of high turbulent activity. Moreover, it is found that the spatial distribution of the error looks qualitatively different for different feedback strategies. Feedback gain distributions calculated by optimal control theory are presented and proposed as a mean to make it possible to perform MI-simulations based on localized measurements only. So far, we have not been able to low error between measurements and simulations by using these gain distributions.

Significant improvements in optical power budgets of real-time optical OFDM PON systems.

Based on a comprehensive theoretical optical orthogonal frequency division multiplexing (OOFDM) system model rigorously verified by comparing numerical results with end-to-end real-time experimental measurements at 11.25Gb/s, detailed explorations are undertaken, for the first time, of the impacts of various physical factors on the OOFDM system performance over directly modulated DFB laser (DML)-based, intensity modulation and direct detection (IMDD), single-mode fibre (SMF) systems without in-line optical amplification and chromatic dispersion compensation. It is shown that the low extinction ratio (ER) of the DML modulated OOFDM signal is the predominant factor limiting the maximum achievable optical power budget, and the subcarrier intermixing effect associated with square-law photon detection in the receiver reduces the optical power budget by at least 1dB. Results also indicate that, immediately after the DML in the transmitter, the insertion of a 0.02nm bandwidth optical Gaussian bandpass filter with a 0.01nm wavelength offset with respect to the optical carrier wavelength can enhance the OOFDM signal ER by approximately 1.24dB, thus resulting in a 7dB optical power budget improvement at a total channel BER of 1 × 10(-3).

Probing the electronic structure of multi-walled carbon nanotubes by transient optical transmittivity

High-resolution time resolved transmittivity measurements on horizontally aligned free-standing multi-walled carbon nanotubes reveal a different electronic transient behavior from that of graphite. This difference is ascribed to the presence of discrete energy states in the multishell carbon nanotube electronic structure. Probe polarization dependence suggests that the optical transitions involve definite selection rules. The origin of these states is discussed and a rate equation model is proposed to rationalize our findings. © 2013 Elsevier Ltd. All rights reserved.

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.

Nearly intrinsic exciton lifetimes in single twin-free GaAsAlGaAs core-shell nanowire heterostructures

CW and time-resolved photoluminescence measurements are used to investigate exciton recombination dynamics in GaAsAlGaAs heterostructure nanowires grown with a recently developed technique which minimizes twinning. A thin capping layer is deposited to eliminate the possibility of oxidation of the AlGaAs shell as a source of oxygen defects in the GaAs core. We observe exciton lifetimes of ∼1 ns, comparable to high quality two-dimensional double heterostructures. These GaAs nanowires allow one to observe state filling and many-body effects resulting from the increased carrier densities accessible with pulsed laser excitation. © 2008 American Institute of Physics.

REGROWTH RATES OF AMORPHOUS LAYERS IN SILICON-ON-SAPPHIRE FILMS.

The rate and direction of regrowth of amorphous layers, created by self-implantation, in silicon-on-sapphire (SOS) have been studied using time resolved reflectivity (TRR) experiments performed simultaneously at two wavelengths. Regrowth of an amorphous layer towards the surface was observed in specimens implanted with 3 multiplied by (times) 10**1**5Si** plus /cm**2 at 50keV and regrowth of a buried amorphous layer, from a surface seed towards the sapphire, was observed in specimens implanted with 1 multiplied by (times) 10**1**5Si** plus /cm**2 at 175keV. Rapid isothermal heating to regrow the layers was performed in an electron beam annealing system. The combination of 514. 5nm and 632. 8nm wavelengths was found to be particularly useful for TRR studies since the high absorption in amorphous silicon, at the shorter wavelength, means that the TRR trace is not complicated by reflection from the silicon-sapphire interface until regrowth is nearly complete.

Wake, shock and potential field interactions in a 1.5 stage turbine: Part I: Vane-rotor and rotor-vane interaction

The composition of the time-resolved surface pressure field around a high-pressure rotor blade caused by the presence of neighboring blade rows was studied, with the individual effects of wake, shock and potential field interaction being determined. Two test geometries were considered: first, a high-pressure turbine stage coupled with a swan-necked diffuser exit duct; secondly, the same high-pressure stage but with a vane located in the downstream duct. Both tests were carried out at engine-representative Mach and Reynolds numbers. By comparing the results to time-resolved computational predictions of the flowfield, the accuracy with which the computation predicts blade interaction was determined. It was found that in addition to upstream vane-rotor and rotor-downstream vane interactions, a new interaction mechanism was found resulting from the interaction between the downstream vane's potential field and the upstream vane's trailing edge potential field and shock.

Wake, shock and potential field interactions in a 1.5 stage turbine: Part II: Vane-vane interaction and discussion of results

The composition of the time-resolved surface pressure field around a high-pressure rotor blade caused by the presence of neighboring blade rows was studied, with the individual effects of wake, shock and potential field interaction being determined. Two test geometries were considered: first, a high-pressure turbine stage coupled with a swan-necked diffuser exit duct; secondly, the same high-pressure stage but with a vane located in the downstream duct. Both tests were carried out at engine-representative Mach and Reynolds numbers. By comparing the results to time-resolved computational predictions of the flowfield, the accuracy with which the computation predicts blade interaction was determined. Evidence was obtained that for a large downstream vane, the flow conditions in the rotor passage, at any instant in time, are close to being steady state.

The development of turbine exit flow in a swan-necked inter-stage diffuser

This paper describes both the migration and dissipation of flow phenomena downstream of a transonic high-pressure turbine stage. The geometry of the HP stage exit duct considered is a swan-necked diffuser similar to those likely to be used in future engine designs. The paper contains results both from an experimental programme in a turbine test facility and from numerical predictions. Experimental data was acquired using three fast-response aerodynamic probes capable of measuring Mach number, whirl angle, pitch angle, total pressure and static pressure. The probes were used to make time-resolved area traverses at two axial locations downstream of the rotor trailing edge. A 3D time-unsteady viscous Navier-Stokes solver was used for the numerical predictions. The unsteady exit flow from a turbine stage is formed from rotordependent phenomena (such as the rotor wake, the rotor trailing edge recompression shock, the tip-leakage flow and the hub secondary flow) and vane-rotor interaction dependant phenomena. This paper describes the time-resolved behaviour and three-dimensional migration paths of both of these phenomena as they convect downstream. It is shown that the inlet flow to a downstream vane is dominated by two corotating vortices, the first caused by the rotor tip-leakage flow and the second by the rotor hub secondary flow. At the inlet plane of the downstream vane the wake is extremely weak and the radial pressure gradient is shown to have caused the majority of the high loss wake fluid to be located between the mid-height of the passage and the casing wall. The structure of the flow indicates that between a high pressure stage and a downstream vane simple two-dimensional blade row interaction does not occur. The results presented in this paper indicate that the presence of an upstream stage is likely to significantly alter the structure of the secondary flow within a downstream vane. The paper also shows that vane-rotor interaction within the upstream stage causes a 10° circumferential variation in the inlet flow angle of the 2nd stage vane.

74-fs nanotube-mode-locked fiber laser

We report an erbium-doped, nanotube mode-locked fiber oscillator generating 74 fs pulses with 63 nm spectral width. This all-fiber-based laser is a simple, low-cost source for time-resolved optical spectroscopy, as well as for many applications where high resolution driven by short pulse durations is required. © 2012 American Institute of Physics.

Particulate matter emission during start-up and transient operation of a spark-ignition engine

In order to understand why emissions of Particulate Matter (PM) from Spark-Ignition (SI) automobiles peak during periods of transient operation such as rapid accelerations, a study of controlled, repeatable transients was performed. Time-resolved engine-out PM emissions from a modern four-cylinder engine during transient load and air/fuel ratio operation were examined, and the results could be fit in most cases to a first order time response. The time constants for the transient response are similar to those measured for changes in intake valve temperature, reflecting the strong dependence of PM emissions on the amount of liquid fuel in the combustion chamber. In only one unrepeatable case did the time response differ from a first order function: showing an overshoot in PM emissions during transition from the initial to the final steady state PM emission level. PM emissions during controlled, motored start-up experiments show a peak at start-up followed by a period during which emissions are either relatively constant or drift somewhat. When the fuel injection and ignition are shut off, PM emissions also peak briefly, but rapidly decay to low levels. Qualitative implications on the study and modeling of PM emissions during transient engine operation are discussed. Copyright © 1999 Society of Automotive Engineers, Inc.

Strong carrier lifetime enhancement in GaAs nanowires coated with semiconducting polymer.

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.

III-V semiconductor nanowires for optoelectronic device applications

Semiconductor nanowires have recently emerged as a new class of materials with significant potential to reveal new fundamental physics and to propel new applications in quantum electronic and optoelectronic devices. Semiconductor nanowires show exceptional promise as nanostructured materials for exploring physics in reduced dimensions and in complex geometries, as well as in one-dimensional nanowire devices. They are compatible with existing semiconductor technologies and can be tailored into unique axial and radial heterostructures. In this contribution we review the recent efforts of our international collaboration which have resulted in significant advances in the growth of exceptionally high quality IIIV nanowires and nanowire heterostructures, and major developments in understanding the electronic energy landscapes of these nanowires and the dynamics of carriers in these nanowires using photoluminescence, time-resolved photoluminescence and terahertz conductivity spectroscopy. © 2011 Elsevier Ltd. All rights reserved.

Characterisation of nanostructures via terahertz spectroscopy

We have used terahertz spectroscopy to measure the conductivity and time-resolved photoconductivity of a range of semiconducting nanostructures. This article focuses on our recent terahertz conductivity studies on semiconductor nanowires and single walled carbon nanotubes. © 2010 IEEE.