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941.
942.
Xavier Amandolese Pascal Hemon Françoise Santi Jan Wojciechowski 《Comptes Rendus Mecanique》2002,330(2):101-106
We present experimental results obtained with a deep cavity, such as a Helmholtz resonator, excited by an airflow. The resonance under the action of the vortices generated in the shear layer is well described and quantified. The mounting of actuators, based on a few piezo-electric elements, allows us to generate a series of two-dimensional vortices forced at a frequency which is different than the natural resonance frequency. The sound level in the cavity is strongly decreased and only the broadband noise of the turbulence remains. To cite this article: X. Amandolese et al., C. R. Mecanique 330 (2002) 101–106. 相似文献
943.
Experimental results concerning the pressure oscillations induced by a grazing flow over a deep cavity like a Helmholtz resonator are presented. The study deals with the forcing of the neck shear layer instability in an opened-loop control scheme by means of pulsed micro-jets. The effects of the frequency and amplitude are investigated. It is found that efficient attenuation of the pressure oscillations can be reached when the forcing frequency is larger than the cavity resonance frequency. Then the shear layer is locked with the forcing and resonance with the cavity is lost, inducing a significant decrease of the acoustic pressure level in the cavity. Effects of the jet amplitude are weak, a very small amplitude being capable of forcing the shear layer. By contrast, when the forcing frequency is lower than the cavity resonance frequency (the forcing wave length is greater than twice the neck length) the forcing is ineffective. 相似文献
944.
Piotr Kozlowski Paulina Rosicka Jie Liu Andrew C. Yung Wolfram Tetzlaff 《Magnetic resonance imaging》2014
Longitudinal Myelin Water Imaging was carried out in vivo to characterize white matter damage following dorsal column transection (DC Tx) injury at the lumbar level L1 of rat spinal cords. A transmit–receive implantable coil system was used to acquire multiple spin-echo (MSE) quantitative T2 data from the lumbar spinal cords of 16 rats at one week pre-injury as well as 3 and 8 weeks post-injury (117 microns in-plane resolution and 1.5 mm slice thickness). In addition, ex vivo MSE and DTI data were acquired from cords fixed and excised at 3 or 8 weeks post injury using a solenoid coil. The MSE data were used to generate Myelin Water Fractions (MWFs) as a surrogate measure of myelin content, while DTI data were acquired to study damage to the axons. Myelin damage was assessed histologically with Eriochrome cyanine (EC) and Myelin Basic Protein in degenerated myelin (dgen-MBP) staining, and axonal damage was assessed by neurofilament-H in combination with neuron specific beta-III-tubulin (NF/Tub) staining. These MRI and histological measures of injury were studied in the dorsal column at 5 mm cranial and 5 mm caudal to injury epicenter. MWF increased significantly at 3 weeks post-injury at both the cranial and caudal sites, relative to baseline. The values on the cranial side of injury returned to baseline at 8 weeks post-injury but remained elevated on the caudal side. This trend was found in both in vivo and ex vivo data. This MWF increase was likely due to the presence of myelin debris, which were cleared by 8 weeks on the cranial, but not the caudal, side. Both EC and dgen-MBP stains displayed similar trends. MWF showed significant correlation with EC staining (R = 0.63, p = 0.005 in vivo and R = 0.74, p = 0.0001 ex vivo). MWF also correlated strongly with the dgen-MBP stain, but only on the cranial side (R = 0.64, p = 0.05 in vivo; R = 0.63, p = 0.038 ex vivo). This study demonstrates that longitudinal MWI in vivo can accurately characterize white matter damage in DC Tx model of injury in the rat spinal cord. 相似文献
945.
Evgeniy Skoptsov Vladimir Agabekov Mohammed A. Binhussain Dmitriy Egorov Piotr Ropot 《Applied Physics A: Materials Science & Processing》2014,117(2):713-718
The techniques of formation of thin metallic coatings by deposition of silver nanoparticles (NPs) from solution onto the surface of glass and silicon substrates modified by polyelectrolytes were developed. The possibilities of the adsorption of individual silver particles, as well as creation of continuous films on their basis, were shown. The transmission, absorption and reflection spectra of obtained coatings were investigated. The most promising structure for metamaterials creation from the list of formed one was determined—solid thin film of Ag NPs on a glass substrate modified by polyethylenimine. 相似文献
946.
Bożena Burtan Maciej Sitarz Radosław Lisiecki Witold Ryba-Romanowski Piotr Jeleń Jan Cisowski Manuela Reben 《Central European Journal of Physics》2014,12(1):57-62
The goal of this work was to investigate the spectroscopic properties of Pr3+ ions, embedded in two different tellurite glass matrices, TeO2-WO3-PbO-La2O3 and TeO2-WO3-PbO-Lu2O3. The absorption and fluorescence spectra have been recorded and analyzed in terms of the Judd-Ofelt theory along with the luminescence decay of the 3P0 and 1D2 levels of the Pr3+ ion. The spectroscopic studies were completed with ellipsometric measurements providing the dispersion relation of the refractive index of the investigated glasses. 相似文献
947.
Piotr Budzyński Zenon Jan Jabłoński Il Bong Jung Jan Stochel 《Annali di Matematica Pura ed Applicata》2014,193(3):663-688
Fundamental properties of unbounded composition operators in \(L^2\) -spaces are studied. Characterizations of normal and quasinormal composition operators are provided. Formally normal composition operators are shown to be normal. Composition operators generating Stieltjes moment sequences are completely characterized. The unbounded counterparts of the celebrated Lambert’s characterizations of subnormality of bounded composition operators are shown to be false. Various illustrative examples are supplied. 相似文献
948.
Piotr Zakrzewski 《Acta Mathematica Hungarica》2014,143(2):367-377
We study structural properties of the collection of all σ-ideals in the σ-algebra of Borel subsets of the Cantor group \(2^{\mathbb{N}}\) , especially those which satisfy the countable chain condition (ccc) and are translation invariant. We prove that the latter collection contains an uncountable family of pairwise orthogonal members and, as a consequence, a strictly decreasing sequence of length ω 1. We also make some observations related to the σ-ideal I ccc on \(2^{\mathbb{N}}\) , consisting of all Borel sets which belong to every translation invariant ccc σ-ideal on \(2^{\mathbb{N}}\) . In particular, improving earlier results of Rec?aw, Kraszewski and Komjáth, we show that:
- every subset of \(2^{\mathbb{N}}\) of cardinality less than can be covered by a set from I ccc,
- there exists a set C∈I ccc such that every countable subset Y of \(2^{\mathbb{N}}\) is contained in a translate of C.
949.
950.
We demonstrate a new approach for the asymmetric synthesis of bissetone. The key reaction is the highly enantioselective hetero-Diels–Alder cycloaddition of triene 3 with ethyl glyoxylate catalyzed by readily available BINOL–Ti complexes. The HDA cycloadduct 4 was then transformed in five steps into O-protected bissetone (8) and its C5-epimer in good yield. 相似文献