8,897 research outputs found
Using performance assessment in secondary school mathematics: an empirical study in a Singapore classroom
No full textThis article reports an exploratory study on using performance assessment in mathematics instruction in a high-performing secondary school in Singapore. An intact mathematics class participated in the study, and received chapter-based performance tasks as intervention during regular mathematics lessons for about one and a half school years. The performance tasks used included authentic and/or open-ended tasks. The students’ academic achievements and attitudes in mathematics were compared with a comparison class that did not receive the intervention. Both quantitative and qualitative data were collected, mainly through questionnaire surveys, performance task tests, conventional school exams, and interviews with students and teachers. The results suggest that the students receiving the intervention performed significantly better than their counterparts in solving conventional exam problems, and in general they also showed more positive changes in attitudes towards mathematics and mathematics learning. The students from the experimental class also expressed positive views about the benefits of using performance tasks in promoting their ability in higher order thinking, though no statistically significant difference was detected between the two classes of students in solving unconventional tasks before and after intervention. Overall, the results appear to support teachers’ using contextualised problems in real life situations and open-ended investigations in students’ learning of mathematic
Near electrical resonance signal enhancement (NERSE) in eddy-current crack detection
No full textAn investigation was performed into the effects of operating an absolute eddy-current testing (ECT) probe at frequencies close to its electrical resonance. A previously undocumented defect signal enhancement phenomenon, resulting from associated shifts in electrical resonant frequency, was observed and characterised. Experimental validation was performed on three notch defects on a typical aerospace superalloy, Titanium 6Al-4V. A conventional absolute ECT probe was operated by sweeping through a frequency range about the electrical resonance of the system (View the MathML source). The phenomenon results in signal-to-noise ratio (SNR) peak enhancements by a factor of up to 3.7, at frequencies approaching resonance, compared to those measured at 1 MHz. The defect signal enhancement peaks are shown to be a result of resonant frequency shifts of the system due to the presence of defects within the material. A simple, operational approach for raising the sensitivity of conventional industrial eddy-current testing is proposed, based on the principles of the observed near electrical resonance signal enhancement (NERSE) phenomenon. The simple procedural change of operating within the NERSE frequency band does not require complex probe design, data analysis or, necessarily, identical coils. Therefore, it is a valuable technique for improving sensitivity, which complements other ECT methods
Richardson, Barbauld, and the construction of an early modern fan club
No full textMPhilMuch has been written about the life and long works of the eighteenth century epistolary novelist, Samuel Richardson, but the prospect of his position as the first celebrity novelist – responsible for courting his own fame as well as initiating his own fan club – has largely been ignored. The body of manuscripts housed at the National Art Library in the Victoria and Albert Museum in London provides the modern scholar with evidence of the skeletal beginnings of an early fan club. This thesis aims to show how these manuscripts were turned into a saleable commodity by the publisher and entrepreneur Richard Phillips, while under the guiding hand of another, slightly later, literary celebrity, Anna Laetitia Barbauld. In order to restore Richardson’s reputation amongst a new nineteenth century audience, Barbauld was required to construct her own idea of him as an eighteenth century celebrity author, and in doing so the insecurities of a self-professed, apparently diffident man, are revealed. Barbauld’s capacious, but heavily edited selection of letters is analyzed in this thesis, providing ample evidence that Richardson’s correspondents were more than just eager letter writers. By using Barbauld’s biography of Richardson this thesis aims to show how she manipulates the genre of life writing in her construction of him.
This thesis offers an alternative reading of how the Richardson manuscripts are viewed, redefining them as not simply a collection of letters, but as a collective entity, deliberately selected and archived as evidence of an early modern fan club, and its celebrity managing director
Predicting fan noise propagation in aeroengine bypass ducts
No full textThis thesis explores the prediction of rearwards fan noise propagation within the bypass duct and its radiation into the far feld. Two recently developed models: B-induct and
GXMunt, are exploited in application to real engine bypass ducts and their performance is evaluated. These methods are an improvement on current industry standards, allowing
realistic duct geometry and flow conditions to be modelled with reasonable computation and time demands. The main focus is on the model b-induct.B-induct predictions for bypass attenuation are integrated into an industry standard whole engine model, and predictions of far-feld noise are obtained for a modern high bypass-ratio engine. These predictions compare more favourably with measured data
from full-scale static engine tests than similar predictions made using a standard uni-form rectangular duct model for the bypass attenuations, indicating that b-induct is an improvement over the current model.Initial studies on the effect of duct geometry on noise propagation suggest a noise benefit for a duct with higher curvature when compared to a typical Baseline design. This
suggestion is confirmed using measured data from zero-flow rig tests. Predictions for three-dimensional duct geometries are also performed to show the effect of scattering due to bifurcations within the duct.B-induct allows for the specific bypass geometry and liner positions to be taken into account when performing impedance optimisations. A new optimisation procedure is proposed in which b-induct predictions are used within an existing whole aircraft noise prediction model. This procedure is used to select liner impedances for a modern engine bypass duct.
B-induct is demonstrated to be a promising new tool within the engine design process, for both analysis of the impact of rear fan noise on whole engine noise, and assessment
of potential low noise bypass configurations
Development of a rotor model for the numerical simulation of helicopter exterior flow-fields
No full textIncludes bibliographical references (leaves 84-85).A numerical methodology is developed to model the effect of a rotor on the surrounding flow-field. The model calculates the time-averaged aerodynamic forces exerted on the air by the fan blades within the blade-swept region, and permits the user to specify blade properties such as cross-sectional profile and orientation at a particular radial and azimuthal location. The calculated forces are included as source terms within the Reynolds-averaged Navier-Stokes equations for an incompressible fluid, which are solved by the commercial CFD solver, FLUENT. The effects of turbulence are incorporated through the use of Launder and Spalding's k-g turbulence model. This method is selected as being the most efficient use of the resources available, giving the economic advantages of a steady simulation, while allowing radial and azimuthal variations of rotor characteristics. In order to validate the accuracy of the numerical model for both aligned and non-aligned inflow conditions, results are compared with experimental data reported for an axial flow fan. Agreement between experimental and numerical results is excellent to good. Fan static pressure rise is closely predicted by the numerical solution, while fan power consumption and fan static efficiency are under and over-predicted respectively. This error may be attributed to frictional losses not accounted for in the numerical model. These include physical rotational instabilities, leading to increased mechanical losses, and tip effects due to the clearance between the fan blade tips and the fan casing. Trends are nevertheless consistently predicted by the numerical model for inflow angles up to 45°, and for the range of blade pitch settings used. The adverse effect of off-axis inflow on the fan static pressure rise is numerically predicted, while fan power consumption is found to remain independent of inflow angle, as had been experimentally observed. The rotor model is finally integrated with the fuselage of the CIRSTEL (Combined Infra-Red Suppression and Tail rotor Elimination) prototype in an analysis of the helicopter exterior flow-field. No experimental data for this configuration was available for validation purposes. However, the model is used in the simulation of several common helicopter flight conditions. Results are presented graphically, and generally indicate good agreement with physically observed phenomena
Large eddy simulations for fan-OGV broadband noise prediction
No full textThe work in this paper forms part of a project on the use of large eddy simulation (LES) for broadband rotor-stator interaction noise prediction. Here we focus on LES of the flow field near a fan blade trailing edge. The first part of the paper aims to evaluate LES suitability for predicting the near-field velocity field for a blunt NACA-0012 airfoil at moderate Reynolds numbers (2× 105 and 4× 105). Preliminary computations of turbulent mean and root-mean-square velocities, as well as energy spectra at the trailing edge, are compared with those from a recent experiment.1 The second part of the paper describes preliminary progress on an LES calculation of the fan wakes on a fan rig.2 The CFD code uses a mixed element unstructured mesh with a median dual control volume. A wall-adapting local eddy-viscosity sub-grid scale model is employed. A very small amount of numerical dissipation is added in the numerical scheme to keep the compressible solver stable. Further results for the fan turbulentmean and RMS velocity, and especially the aeroacoustics field will be presented at a later stage. Copyright © 2008 by Qinling LI, Nigel Peake & Mark Savill
Large Eddy Simulation of the ACAT1 fan stage for broadband noise prediction
No full textInternational audienceA numerical study is carried out to assess the aerodynamic and noise radiation specific features of the ACAT1 fan stage, which was tested in the framework of the European project TurbonoiseBB. This study deals with the prediction of the broadband noise radiated by the fan stage at approach conditions, and particularly focuses on the rotor–stator interaction (RSI) noise, which results from the impact of the fan wakes onto the outlet guide vanes (OGV). To this end, two wall-modeled Large Eddy Simulations (LES) with different levels of mesh refinement have been performed on the full fan-OGV stage. The finer mesh significantly improved the aerodynamic predictions in terms of mean flow profiles, but also in terms of RMS profiles and velocity spectra, for which significant disparities are highlighted for the coarser LES. Only slight disparities regarding the turbulent content of the flow remain for the finer LES. The typical flow features at approach conditions are examined and the noise sources on both the stator and the rotor are analyzed, revealing the presence of additional broadband noise sources. Noise predictions are performed by using the free-field Ffowcs Williams and Hawkings (FW-H) analogy, and Goldstein’s in-duct acoustic analogy informed with the pressure fluctuations recorded on the stator and rotor surfaces. FW-H predictions of the stator noise, though well recovering the shape of the experimental spectra, overestimate the radiated noise especially upstream of the fan. Using the finer simulation data substantially reduces the gap with the measurements. Accounting for the duct effect on the sound propagation, further improves the predictions in terms of absolute levels, leading to a good agreement between the finer simulation and the noise measurements. The breakdown of the different stator noise source contributions, in conjunction with rotor broadband noise predictions made it possible to confirm the presence of additional significant broadband noise sources, questioning the common assumption considering the RSI sources as the only dominant fan noise mechanism at approach conditions
Comparative studies of RANS versus large eddy simulation for fan-intake interaction
No full textThe present research applied a mixed-fidelity approach to examine the fan-intake interaction. Flow separation induced by a distortion generator (DG) is either resolved using large eddy simulation (LES) or modeled using the standard k-x model, Spalart- Allmaras (SA) model, etc. The immersed boundary method with smeared geometry (immersed boundary method with smeared geometry (IBMSG)) is employed to represent the effect of the fan and a wide range of test cases is studied by varying the (a) height of the DG and (b) proximity of the fan to the DG. Comparisons are drawn between the LES and the Reynolds-averaged Navier-Stokes (RANS) approaches with/without the fan effect. It is found that in the "absence of fan," the discrepancies between RANS and LES are significant within the separation and reattachment region due to the well-known limitations of the standard RANS models. "With the fan installed," the deviation between RANS and LES decreases substantially. It becomes minimal when the fan is closest to the DG. It implies that with an installed fan, the inaccuracies of the turbulence model are mitigated by the strong flow acceleration at the casing due to the fan. More precisely, the mass flow redistribution due to the fan has a dominant primary effect on the final predictions and the effect of turbulence model becomes secondary, thereby suggesting that high fidelity eddy resolving simulations provide marginal improvements to the accuracy for the installed cases, particularly for the short intake-fan strategies with fan getting closer to intake lip
Large-Eddy Simulation of a Tunnel Ventilation Fan
No full textIn this paper we discuss a computational method focused on the prediction of unsteady aerodynamics, adequate for industrial turbomachinery. Here we focus on a single rotor device selected from a new family of large tunnel ventilation axial flow fans. The flow field in the fan was simulated using the open source code OPENFOAM, with a large-eddy simulation (LES) approach. The sub-grid scale (SGS) closure relied on a one-equation model, that requires us to solve a differential transport equation for the modeled SGS turbulent kinetic energy. The use of such closure was here considered as a remedial strategy in LES of high-Reynolds industrial flows, being able to tackle the otherwise insufficient resolution of turbulence spectrum. The results show that LES of the fan allows to predict the pressure rise capability of the fan and to reproduce the most relevant flow features, such as three-dimensional separation and secondary flows
Large-Eddy Simulation of Time Evolution and Instability of Highly Underexpanded Sonic Jets
No full textHigh-pressure jet injection into quiescent air is a challenging fluid dynamics problem in the field of aerospace engineering. Although plenty of experimental, theoretical, and numerical studies have been conducted to explore this flow, there is a dearth of literature detailing the flow evolution and instability characteristics, which is vital to the mixing enhancement design and jet noise reduction. In this paper, a density-based solver for compressible supersonic flow, astroFoam, is developed based on the OpenFOAM library. Large-eddy simulations of highly underexpanded jets with nozzle pressure ratios from 5.60 to 11.21 at a Reynolds number around 10(5) are carried out with a highresolution grid. A grid-convergence study has been conducted to confirm the fidelity of the large-eddy simulation results. The large-eddy simulation results have also been validated against available literature data in terms of the time-averaged near-field properties of underexpanded jets. The turbulent transition processes are revealed based on the instantaneous flow features and are quantitatively resolved according to the jet penetration and maximum width. The vorticity analysis is conducted to understand the turbulent transition mechanism, and it is found that the vortex stretching term plays a leading role on the distortion of the vortex rings in the near field of the jets. The dominant instability modes of jets, visualized by helicity, are quantitatively revealed based on the spectrum and relative phase of pressure fluctuation. The single helical modes corresponding to a phase angle close to +/- 180 deg with the 1 + 1 helices are dominant for nozzle pressure ratios of 5.60 and 7.47, whereas the complex and multiple helices for the other two higher nozzle pressure ratios are due to the superposition of the single and double helical modes. In addition, the performance of the coarse mesh and different subgrid-scale models on capturing the dominant instability characteristics in large-eddy simulation of underexpanded jets is investigated
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