114 research outputs found
Accurate aeroacoustic measurements in closed-section hard-walled wind tunnels
No full textNoise emissions from aircraft are of major concern to aircraft manufacturers. There are various analytical, empirical and numerical tools to help in the design of quieter aircraft, however aeroacoustic measurements in wind tunnels are still required. There is a growing interest in simultaneous aerodynamic and aeroacoustic measurements in hard-walled closed-section wind tunnels. The research hypothesis of this work is whether accurate aeroacoustic measurements are possible in this type of wind tunnel. Two issues are of particular concern: the reverberation sound field and high background noise levels. De-reverberation, based on an Image Source Model (ISM), is proposed to tackle the first issue by incorporating the reflections in the focused beamformer. This technique is computationally fast and easy to implement. Source Power Integration and deconvolution techniques are shown to be still valid in de-reverberation. Measurements in a closed section wind tunnel have shown that an ISM gives a better estimate of the Green's functions, when compared to free-space Green's functions. Furthermore de-reverberation yielded more accurate source strength estimates from the beamformer. Qualitatively, de-convolved results were no different than when using free-space Green's functions. Simulations have shown that the ISM can become unstable at high frequencies if position errors are present. It is therefore recommended to limit the application of the ISM to frequencies below 10 kHz. At low frequencies the accuracy of beamforming levels is highly dependent on the level of noise contamination of the input data. Removing the diagonal of the cross spectral matrix might not be sufficient to eliminate this noise
Frequency response analysis of microphone preamplifiers in the audible and ultrasonic regime
No full textThis report presents an investigation on the frequency response of two types of electret microphone preamplifiers currently in use with SotonArray, Southampton University's wind tunnel microphone array system. The investigation is presented as a comparison between the response of an electret microphone coupled to the preamplifiers and a reference instrumentation-grade microphone and preamplifier assembly. The results show that electret microphones can potentially be used above their rated frequency range of 20kHz; in fact no degradation in signal was noticed up to 48kHz. The results also show that the newly-built SES preamps have a much better high-frequency performance than the original ISVR preamps
SotonArray: Southampton University wind tunnel microphone array system guide
No full textThis report shall serve as an introductory background to SotonArray, the microphone array system at the University of Southampton. It covers the practical aspects only; namely the instrumentation setup and the beamforming software. It aims to give a basic level of understanding of the system to the researcher who wants to use it, and especially to highlight both the capabilities and limitations of the system. Since the microphone array system is a new capability for the Aerodynamics and Flight Mechanics research group (AFM), the hardware and software is constantly being upgraded, and the details given here are correct only at the time of going to print
In search of a personal and national identity : the English poems of Victor Fenech
No full textThis article examines the English poems that the Maltese contemporary writer Victor Fenech (1935-) published during the 1970s. Fenech considers the Maltese language to be quite limited since it keeps the Maltese people "locked out of the mainstream of international literature". English thus becomes the international medium that has the power to liberate the author from the restrictions imposed upon him by the national Maltese language. This article highlights the fact that Fenech feels a deep sense of sorrow at the way human beings act. The writer therefore decides to distance himself in solitude from a valueless society. Solitude, however, does not necessarily coincide with contemplative life. So much so, that the poet even indulges in 'political'poetry. The key elements that surface in Fenech's poetry seem to be : lack of faith in human beings; lack of faith in politics; even of faith in God. However, the author does find some consolation in his ideal female figure and in the way of life in little centres : happiness, if this is ever found, lies far away from the hustle and bustle of the big metropolis. The author of this article reaches the conclusion that Fenech's poetry should not be seen exclusively as a transcription of the poet:S feelings: it is also, and more especially so, the faithful chronicle of the epic story of a people who are always "licking wounds sustained for others" and who continuously search for a proper fixed identity.peer-reviewe
Towards more accurate beamforming levels in closed-section wind tunnels via de-reverberation
No full textThe ability to perform aeroacoustic measurements in closed-section wind tunnels is important in vehicle design, but presents significant problems compared with anechoic open-jet tunnel testing. The use of microphone arrays and beamforming processing can yield useful source localisation information, but there are concerns as to the consistency of quantitative results from such methods. In this paper some of the deficiencies of current beamforming practice are highlighted; in particular, inaccuracies in levels arising from the
reverberant sound field in hard-walled wind tunnels. Notably, the use of free-space Green's functions when beamforming in such conditions is shown to be a significant source of errror, and an image source method is proposed to improve accuracy by de-reverberating the
measuring environment. Comparisons are made between the conventional and proposed approaches using microphone array measurements of a compact source in a closed section wind tunnel. Results show that the new de-reverberation method can give improvements in accuracy even if the wind tunnel geometry is not modelled accurately, provided that the number of image sources is chosen correctly. The results also highlight the significant challenges associated with Green's functions measurements in highly-reverberant environments
Possibility of Stabilizing the Greenland Ice Sheet
No full textRecent acceleration in the retreat of the Greenland ice sheet under a warming climate has caused unprecedented challenges and threats to coastal communities due to the rising sea level and increasing storm surges. This raises a critical question from a climate mitigation perspective: Would there still be a chance to stabilize the Greenland ice sheet if the carbon reduction goals of the Paris Agreement could be met? Here, we show that there is indeed a possibility for stabilizing the Greenland ice sheet with the low-emission scenario of RCP2.6. In particular, RCP2.6 would potentially limit the warming in Greenland below 1°C within next 30years and constrain its loss of ice sheet coverage below 10%. After 2050, the annual mean temperature in Greenland is likely to be stabilized and no further loss is expected to its ice sheet. However, the effective window for this chance will be closing after 2020. If no effective carbon reduction policies are being taken now, we are very likely to enter a continuous warming pathway and lose the chance of stabilizing the Greenland ice sheet.Natural Sciences and Engineering Research Council of CanadaAtlantic Canada Opportunities AgencyNanjing Agricultural UniversityACEnet (Canada
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