Spectral properties of blue upconversion luminescences in Tm 3ϩ doped tellurite ͑PWT, PbF 2 -WO 3 -TeO 2 ͒ glasses and germanate ͑PWG, PbF 2 -WO 3 -GeO 2 ͒ glasses pumped by a tunable dye laser were studied at room temperature. Two emission bands centered at 453 and 477 nm, corresponding to the 1 D 2 → 3 H 4 and 1 G 4 → 3 H 6 transitions of Tm 3ϩ ions respectively, were observed. The two-photon absorption mechanism responsible for the 477 nm luminescence was confirmed by a quadratic dependence of luminescent intensities on the excitation power. Tellurite glasses showed a weaker upconversion luminescence than germanate glasses. This observation was inconsistent with the prediction from the phonon sideband measurement. In this article, Raman spectroscopy and transmittance measurement were employed to investigate the origin of the difference in upconversion luminescences in the two glasses. Compared with phonon sideband spectroscopy, Raman spectroscopy extracts more information, including both phonon energy and phonon density, and therefore, is a more effective analytical tool for understanding upconversion luminescence. Our results showed that the phonon energy as well as phonon density of the host glass is important in determining the upconversion efficiency.
A preliminary experimental study was carried out to investigate the aerodynamics and aeroacoustic characteristics of small propellers. A baseline propeller with 120mm radius was designed at first, and four different saw-tooth serrated trailing edges were added to the baseline propeller. The heights (2h) of the saw-tooth serration are 6mm, and the width to height ratio (λ/h) are 0.3, 0.6, 0.9, and 1.2, respectively. The aerodynamic forces, sound pressure level, and detailed flow field structures are achieved via a high sensitive load cell, sound measurement system, and PIV system at hover motion (thrust=3N). The objective of this study was to reduce the noise and maintain the thrust of the small propellers at low Reynold number of 60,000. By adding the saw-tooth serration feature to the baseline propeller, the broadband noise reduction was substantially suppressed at high frequency. Meanwhile, the aerodynamics performance was maintained. The detailed flow measurements of different propellers are also studied to illustrate the mechanism of noise reduction.
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