Numerical analysis of active chordwise flexibility on the performance of non-symmetrical flapping airfoils

This paper investigates the effect of active chordwise flexing on the lift, thrust and propulsive efficiency of three types of airfoils. The factors studied are the flexing center location, standard two-sided flexing as well as a type of single-sided flexing. The airfoils are simulated to flap with four configurations, and the effects of flexing under these configurations are investigated. Results show that flexing is not necessarily beneficial for the performance of the airfoils. However, with the correct parameters, efficiency is as high as 0.76 by placing the flexing centre at the trailing edge. The average thrust coefficient is more than twice as high, from 1.63 to 3.57 with flapping and flexing under the right conditions. Moreover, the single-sided flexing also gives an average lift coefficient as high as 4.61 for the S1020 airfoil. The shape of the airfoil does alter the effect of flexing too. Deviating the flexing phase angle away from 90° does not give a significant improvement to the airfoil’s performance. These results greatly enhance the design of a better performing ornithopter wing.     

Encouraging problem-solving disposition in a Singapore classroom

In this article, we share our learning experience as a Lesson Study team. The Research Lesson was on Figural Patterns taught in Year 7. In addition to helping students learn the skills of the topic, we wanted them to develop a problem-solving disposition. The management of these two objectives was a challenge to us. From the lesson observation and the students’ classwork, it turned out better than we expected.     

Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs

In this study, we report on the enhancement in the light extraction efficiency of GaN blue LEDs topped with ZnO nanorods. The ZnO nanorods were grown by a two-step hydrothermal synthesis with pre-coated ZnO nanoparticles under optimized condition to give the appropriate size and quality, giving an increase in the light output efficiency of 66%. This improvement is attributed to the optimal rod size and spacing with improved thermal dissipation as compared to light extraction from plain GaN surface. During the ZnO growth on the LEDs, 0.55 M of NH₃ was added and the ZnO sample was later annealed at 475 °C in N₂ ambient, to drive out interstitial oxygen atoms from the tetrahedral unstable site. As a result, a high ratio of UV to orange defect band emission was achieved. The two-step growth of ZnO nanorods on GaN LEDs was effective in generating array of ZnO nanorods which serve as reflector to enhance light extraction from LEDs.     

Femtosecond laser drilling of alumina ceramic substrates

In the paper, the result on femtosecond laser drilling of alumina ceramic substrate was reported. The effects of various laser parameters such as different focus position, traverse speed, drilling pattern, pausing time, etc. on the drilled hole quality in terms of surface finish, heat affected zone (HAZ), hole circularity, debris, microcracks were studied. The quality of laser-drilled holes on alumina ceramic substrates was evaluated with optical microscope, SEM/EDX, and X-ray μ-CT analysis. The optimum drilling conditions were identified. High-quality laser-drilled holes on alumina ceramic substrates were demonstrated. The developed process has potential application in manufacturing of alumina substrate based electronic devices.     

Photorefractive polymer device with video-rate response time operating at low voltages

The high-voltage bias required for video-rate compatible, efficient operation of a photorefractive polymer composite is reduced from 6-8 to 1.3 kV. At this low voltage, the device can hold erasable Bragg holograms with 80% efficiency in addition to having a video-rate response time. The transition of the hologram's state from thick to thin is analyzed in detail.     

Triosmium-antimony clusters containing the μ,η-phenylene ligand: Unusually long osmium-osmium bonds

The reaction of the osmium-antimony cluster Os₃(μ-H)(μ-SbPh₂)(μ₃,η²-C₆H₄)(CO)₉ with AsPh₃ at room temperature afforded the o-phenylene cluster Os₃(μ-H)(SbPh₂)(μ₂,η²-C₆H₄)(CO)₉(AsPh₃) by nucleophilic addition via a metal-metal bond cleavage, and the substitution product Os₃(μ-H)(SbPh₂)(μ₃,η²-C₆H₄)(CO)₈(AsPh₃). It reacted with ^tBuNC to afford the adduct Os₃(μ-H)(SbPh₂)(μ₂,η²-C₆H₄)(CO)₉(CN^tBu) quantitatively. This adduct isomerised slowly on standing via migration of the isonitrile, while photolysis led to decarbonylation to Os₃(μ-H)(SbPh₂)(μ₂,η²-C₆H₄)(CO)₈(CN^tBu). All the products have been characterised completely, including by X-ray crystallography, and their structures exhibit very long Os-Os bonds.     

Effect of rounded edged dimple arrays on the boundary layer development

The effect of a turbulent boundary layer subjected to a series of rounded edged shallow dimple arrays with dimple depth ratios, d/D of 4%, 8% and 12% were experimentally studied. Measurements show the existence of a higher flow speed region at the center of each dimple. The spanwise distribution of the mean wall shear stress immediately downstream of the centers of the last row of dimples does not vary with dimple depth, and is about 45% over that without the dimple array. Turbulence measurements and surface flow visualization shows that the flow over the shallowest dimple differs from the deeper dimples. Flow separation observed with the deeper rounded edged dimples produce similar flow structures as those from sharp edged dimples reported in the literature. However flow separation is not observed when d/D=4% but instead two other higher speed regions either side accompany the central flow. The effects of the dimples are rapidly suppressed by the flat surfaces between of the dimples, and the flow rapidly reverts back to an unmanipulated flat boundary layer flow in these areas.     

ContributionsOn optimal orientations of cartesian products of graphs (I)

For a graph G, let D(G) be the family of strong orientations of G, and define [ovbar|d] (G) = min[d(D) vb D ] D(G), where d(D) denotes the diameter of the digraph D. Let G × H denote the cartesian product of the graphs G and H. In this paper, we determine completely the values of and , except , where K_n, P_n and C_n denote the complete graph, path and cycle of order n, respectively.     

Analysis of phase distortion in phase-shifted fringe projection

This paper describes the analysis of phase distortion in phase-shifted fringe projection method. A phase distortion occurs when the phase shifting technique is applied to extract the phase values from projected fringe patterns in surface contouring. The phase distortion will induce measurement errors especially in the measurement of micro-components. The cause of such phase distortion is investigated and the influence of phase distortion on the measurement of micro-components is discussed. To eliminate the phase distortion, a continuous wavelet transform (CWT) is employed to extract phase values from object surface modulated fringe patterns. Principle of the proposed CWT phase extraction method is described and experiments are conducted to verify the proposed method. It is shown that by the use of CWT phase extraction method phase distortion induced in conventional phase-shifting technique can be completely eliminated.