2,234 research outputs found
Mathematical models of some nematic and cholesteric liquid crystal devices
No full textThis thesis describes the mathematical modelling and analysis of liquid crystal systems when an electric field is applied. This analysis is performed for Nematic, Twisted nematic, and Super-twisted nematic cells and for Polymer Gels. The mathematical techniques employed are: linear and non-linear stability analysis; perturbation theory; and the method of matched asymptotic expansions. For conventional nematic systems analytic expressions are obtained which describe the distortion of the liquid crystal and the coupled electric field at low, intermediate, and high applied voltages. Aspects of the dynamics are considered for both strong and weak anchoring and also with the inclusion of a coupled flow. It is shown that certain weakly anchored nematic systems admit travelling wave solutions. This is particularly relevant to the relaxation of polymer gels. For these systems a model is proposed which treats the polymer matrix as comprising thin orientated fibrils which act as weak anchoring sites distributed throughout the liquid crystal. The model is shown to correctly predict many of the experimentally observed properties of polymer gels. Specifically it predicts the enhanced critical voltage for such systems and also indicates that they saturate at a voltage proportionately close to the critical voltage. The model predicts that the decay of a polymer gel from a highly distorted state occurs via a travelling wave. This in turn implies that the decay constant will depend linearly on the width of the cell containing the gel
The Annual Walter Rodney Symposium, 2022
No full textThe 19th Annual Walter Rodney Symposium titled "Walter Rodney: 50 Years of How Europe Underdeveloped Africa" took place on Saturday, March 26th, 2022 from 10:00am - 3:00pm EST. The virtual conference featured keynote speaker Dr. Joyce Ladner who highlights her relationship with Dr. Walter Rodney. The panel hosted by Kurt B. Young featured Dr. Horace G. Campbell, Professor Issa Shivji, and Walter Bgoya, and discusses the work of Walter Rodney and Julius Nyerere. The panel hosted by Zophia Edwards featured a lecture by Dr. Vijay Prashad and respondents Natasha Shivji, Tamnisha John, Kamau Franklin, and Cindy Peters about the text "How Europe Undeveloped Africa". There were Q & A segments and global remembrances. The 2022 symposium was co-hosted by The Walter Rodney Foundation and the AUC Woodruff Library
Aircraft classification for efficient modelling of environmental noise impact of aviation
No full textWith the environmental externalities of civil aviation under unprecedented scrutiny, and with the projected significant increase in air traffic demand over the next few decades, fleet-level studies are required to assess the potential benefit of novel aircraft technologies and operational procedures for minimising environmental impact of aviation. Using a statistical classification process, the UK commercial aircraft fleet is reduced to four representative-in-class aircraft onthe basis of aircraft physical characteristics, and aircraft noise and engine exhaust emissions. These four representative aircraft, that appropriately capture the noise and emissions characteristics for each category within the UK commercial fleet, are also selected to be used as baseline cases for the high-level assessment of the environmental benefit of novel aircraft technologies. For the particular case of aviation noise, the modelling tools are highly sensitiveto the number of aircraft types in the flight schedule. A reduction of about 80% in computational time with relatively minor decrease in accuracy (between -4% and +5%) is observed when the whole aircraft fleet is replaced with the four representative-in-class aircraft for computing noise contours. Therefore, the statistical classification and selection of representative-in-class aircraft presented in this paper is a valid approach for the rapid and accurate computation of a large number of exploratory cases to assess aviation noise reduction strategies
Evaluation of environmentally optimal descent and take-off slopes for existing and novel aircraft
No full textDescent and takeoff slopes of civil aircraft influence the associated noise and emissions impact around airports. Steep take-off and approach procedures are expected to reduce the noise footprint around airports whereas they could offer local air quality benefits as well. This paper appraises the optimal descent and take-off slopes in terms of noise and emissions for existing civil aircraft, as well as fora future blended wing-body (BWB) concept aircraft. The effect of the interdependencies between noise and emissions is demonstrated, whereas estimated Noise-Power-Distance (NPD) curves for the steep operations are presented. It is shown that a common optimum slope for both environmental concerns is unlikely to occur and that generally, noise benefits come to the expense of increased fuel consumption. However, it is also highlighted that new, more flexible ways of expressing the noise and emissions interdependencies may be required in order to determine optimum slopes more realistically
Investigation into the turbulence statistics of installed jets using hot-wire anemometry
No full textThis work presents a detailed study of theturbulence flow statistics of a jet mounted with its axisparallel to a rigid flat plate. Hot-wire constant tem-perature anemometry has been used to measure thesingle-point and two-point statistics of the axial velocitycomponent at several locations within the jet flow field.Results show that the jet mean flow near the plate sur-face is subjected to a local acceleration and redirectiondue to a Coand ̆a-type effect. The propagation of theseeffects downstream of the plate trailing edge are stronglydependent on the plate position. Regarding the velocityfluctuations, the mean turbulence intensity levels areseen to decrease as the radial distance between the jetand surface decreases. Analysis of the single-point powerspectral density data on the shear layer close to the plateshows that the reduction in magnitude of the low fre-quency content of the energy spectrum is responsiblefor the decrease in turbulence intensity. Additionally,the characteristic time and length-scales computed fromtwo-point measurements reduce as the plate is mountedcloser to the jet centre-line. The axial eddy convectionvelocity is seen to increase in the region of high turbulentkinetic energy in the shear layer adjacent to the surface.Empirical models for turbulence characteristic scalesand eddy convection velocity are presented. These find-ings suggest that both the amplitude and distributionof the jet mixing noise sources are affected when closelyinstalled next to a surface. This paper is a continuationof a recent investigation on the turbulence statistics ofisolated jets presented in Proen ̧ca et al. (2019
Data for: Aircraft classification for efficient modelling of environmental noise impact of aviation
No full textAircraft database used in this research, with the associated noise and engine exhaust emission data.</span
Effects of a hovering unmanned aerial vehicle on urban soundscapes perception
No full textSeveral industry leaders and governmental agencies are currently investigating the use of Unmanned Aerial Vehicles (UAVs), or ‘drones’ as commonly known, for an ever-growing number of applications from blue light services to parcel delivery. For the specific case of the delivery sector, drones can alleviate road space usage and also lead to reductions in CO
2 and air pollution emissions, compared to traditional diesel-powered vehicles. However, due to their unconventional acoustic characteristics and operational manoeuvres, it is uncertain how communities will respond to drone operations. Noise has been suggested as a major barrier to public acceptance of drone operations in urban areas. In this paper, a series of audio-visual scenarios were created to investigate the effects of drone noise on the reported loudness, annoyance and pleasantness of seven different types of urban soundscapes. In soundscapes highly impacted by road traffic noise, the presence of drone noise lead to small changes in the perceived loudness, annoyance and pleasantness. In soundscapes with reduced road traffic noise, the participants reported a significantly higher perceived loudness and annoyance and a lower pleasantness with the presence of the same drone noise. For instance, the reported annoyance increased from 2.3 ± 0.8 (without drone noise) to 6.8 ± 0.3 (with drone noise), in an 11-point scale (0-not at all, 10-extremely). Based on these results, the concentration of drone operations along flight paths through busy roads might aid in the mitigation of the overall community noise impact caused by drones.
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Noise assessment of aircraft with distributed electric propulsion using a new noise estimation framework
No full textFuture aircraft are envisioned to have significantly smaller noise footprint and emissions impact in order to satisfy the ambitious long-term aircraft noise and emissions goals set by several organisations worldwide, for example ACARE and NASA. Distributed electric propulsion (DEP) is anticipated as one of the most suitable and efficient options for powering these aircraft. DEP is the dispersion of thrust among multiple propulsors that are driven electrically rather than mechanically. This paper presents preliminary noise estimations for a civil aircraft that uses various DEP architectures (e.g. different number of electric propulsors, powered by either batteries or gas turbine engines), obtained through a new noise estimation framework that estimates noise variations arising from technological and/or operational changes with respect to a baseline scenario, where the noise levels are known. The aim of the paper is therefore twofold; investigate the possible noise benefits of DEP aircraft, whilst on the other hand demonstrate the core methodology and capabilities of our framework for estimating the noise impact of future aircraft concepts. This preliminary study indicates the framework’s potential in correctly capturing trends
Modelling Jet-Surface Interaction Noise from Cranked Wings
No full textA strip implementation of Amiet’s theory has been used to model the jet-surface interaction noise produced by the cranked wing geometry seen on commercial jet airliners. Because it is the geometry that has changed between strips, not the incident pressure, the noise produced by each strip has been summed coherently. This coherent addition has been verified as converging on the infinite-span solution as the number of strips is increased, and to equal that of a single strip of the same overall span. Jet-surface interaction noise predictions from both strip and infinite-span implementations of Amiet’s theory have been validated against installed jet noise measurements with a low aspect ratio plate. The finite span implementation shows an improvement in spectra at low frequencies, however, it under-predicts at azimuthal observer angles near the plane of the plate. Finally, the OASPL directivity of a cranked wing has been calculated both using strip theory and assuming infinite span (both swept and unswept). Of the three, the strip theory model agrees best with the measured data
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