The low Reynolds number aerodynamics of leading edge flaps

Within the low Reynolds number regime at which birds and small air vehicles operate (Re=15,000-500,000), flow is beset with laminar separation bubbles and bubble burst which can lead to loss of lift and early onset of stall. Recent video footage of an eagle's wings in flight reveals an inconspicuous wing feature: the sudden deployment of a row of feathers from the lower surface of the wing to create a leading edge flap. An understanding of the aerodynamic function of this flap has been developed through a series of low speed wind tunnel tests performed on an Eppler E423 aerofoil. Experiments took place at Reynolds numbers ranging from 40000 to 140000 and angles of attack up to 30°. In the lower range of tested Reynolds numbers, application of the flap was found to substantially enhance aerofoil performance by augmenting the lift and limiting the drag at certain incidences. The leading edge flap was determined to act as a transition device at low Reynolds numbers, preventing the formation of separation bubbles and consequently decreasing the speed at which stall occurs during landing and manoeuvring.

Machine Vision-Based Infrastructure As-Built Documentation Using Edge Points

The existing machine vision-based 3D reconstruction software programs provide a promising low-cost and in some cases automatic solution for infrastructure as-built documentation. However in several steps of the reconstruction process, they only rely on detecting and matching corner-like features in multiple views of a scene. Therefore, in infrastructure scenes which include uniform materials and poorly textured surfaces, these programs fail with high probabilities due to lack of feature points. Moreover, except few programs that generate dense 3D models through significantly time-consuming algorithms, most of them only provide a sparse reconstruction which does not necessarily include required points such as corners or edges; hence these points have to be manually matched across different views that could make the process considerably laborious. To address these limitations, this paper presents a video-based as-built documentation method that automatically builds detailed 3D maps of a scene by aligning edge points between video frames. Compared to corner-like features, edge points are far more plentiful even in untextured scenes and often carry important semantic associations. The method has been tested for poorly textured infrastructure scenes and the results indicate that a combination of edge and corner-like features would allow dealing with a broader range of scenes.

THE SENSITIVITY OF 2D COMPRESSOR INCIDENCE RANGE TO IN-SERVICE GEOMETRIC VARIATION

The loss mechanisms which control 2D incidence range are discussed with an emphasis on determining which real in-service geometric variations will have the largest impact. For the majority of engine compressor blades (Minlet>0.55) both the negative and positive incidence limits are controlled by supersonic patches. It is shown that these patches are highly sensitive to the geometric variations close to, and around the leading edge. The variations used in this study were measured from newly manufactured as well as ex-service blades. Over most the high pressure compressor considered, it was shown that manufacture variations dominated. The first part of the paper shows that, despite large geometric variations (~10% of leading edge thickness), the incidence range responded in a linear way. The result of this is that the geometric variations have little effect on the mean incidence range of a row of blades. In the second part of the paper a region of the design space is identified where non-linear behavior can result in a 10% reduction in positive incidence range. The mechanism for this is reported and design guidelines for its avoidance offered. In the final part of the paper, the linear behavior at negative incidence and the transonic nature of the flow is exploited to design a robust asymmetric leading edge with a 5% increase in incidence range.

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.

Oxygen vacancy defects in Ta2O5 showing long-range atomic re-arrangements

The structure, formation energy, and energy levels of the various oxygen vacancies in Ta2O5 have been calculated using the λ phase model. The intra-layer vacancies give rise to unusual, long-range bonding rearrangements, which are different for each defect charge state. The 2-fold coordinated intra-layer vacancy is the lowest cost vacancy and forms a deep level 1.5 eV below the conduction band edge. The 3-fold intra-layer vacancy and the 2-fold inter-layer vacancy are higher cost defects, and form shallower levels. The unusual bonding rearrangements lead to low oxygen migration barriers, which are useful for resistive random access memory applications. © 2014 AIP Publishing LLC.

Effects of surface roughness on airframe noise

The generation of sound by turbulent boundary layer flow at low Mach number over a rough wall is investigated by applying the theoretical model which describes the scattering of the turbulence near field into sound by roughness elements. Attention is focused on the numerical method to approximately quantify the absolute level of the roughness noise radiated to far field. Empirical models for the source statistics are obtained by scaling smooth-wall data through increased skin friction velocity and boundary layer thickness for the rough surface. Numerical integration is performed to determine the roughness noise, and it reproduces the spectral characteristics of the available empirical formula and experimental data. Experiments are conducted to measure the radiated sound from two rough plates in an open jet by four 1/2'' free-field condenser microphones. The measured noise spectra of the rough plates are above that of a smooth plate in 1-2.5 kHz frequency and exhibits encouraging agreement with the predicted spectra. Also, a phased microphone array is utilized to localize the sound source, and it confirms that the rough plates generate higher source strengthes in this frequency range. A parametric study illustrates that the roughness height and roughness density significantly affect the far-field radiated roughness noise with the roughness height having the dominant effect. The estimates of the roughness noise for a Boeing 757 sized aircraft wing show that in high frequency region the sound radiated from surface roughness may exceed that from the trailing edge, and higher overall sound pressure levels for the roughness noise are also observed.

High tuning range AlSi RF MEMS capacitors fabricated with sacrificial amorphous silicon surface micromachining

This paper reports the fabrication and electrical characterization of high tuning range AlSi RF MEMS capacitors. We present experimental results obtained by a surface micromachining process that uses dry etching of sacrificial amorphous silicon to release Al-1%Si membranes and has a low thermal budget (<450 °C) being compatible with CMOS post-processing. The proposed silicon sacrificial layer dry etching (SSLDE) process is able to provide very high Si etch rates (3-15 μm/min, depending on process parameters) with high Si: SiO2 selectivity (>10,000:1). Single- and double-air-gap MEMS capacitors, as well as some dedicated test structures needed to calibrate the electro-mechanical parameters and explore the reliability of the proposed technology, have been fabricated with the new process. S-parameter measurements from 100 MHz up to 2 GHz have shown a capacitance tuning range higher than 100% with the double-air-gap architecture. The tuning range can be enlarged with a proper DC electrical bias of the capacitor electrodes. Finally, the reported results make the proposed MEMS tuneable capacitor a good candidate for above-IC integration in communications applications. © 2004 Elsevier B.V. All rights reserved.