Determination of dynamic properties of flax fibres reinforced laminate using vibration measurements

Experimental and numerical methods to identify the linear viscoelastic properties of flax fibre reinforced polymer (FFRP) composite are presented in this study. The method relies on the evolution of storage modulus and loss factor as observed through the frequency response. Free-free symmetrically guided beams were excited in the dynamic range of 10 Hz to 4 kHz with a swept sine excitation focused around their first modes. A fractional derivative Zener model has been identified to predict the complex moduli. A modified ply constitutive law has been then implemented in a classical laminates theory calculation (CLT) routine. Overall, the Zener model fitted the experimental results well. The storage modulus was not frequency dependant, while the loss factor increased with frequency and reached a maximum value for a fibre orientation of 70°. The damping of FFRP was, respectively, 5 and 2 times higher than for equivalent carbon and glass fibres reinforced epoxy composites.     

Frequency and development of slugs in a horizontal pipe at large liquid flows

The generation of slugs was studied for air–water flow in horizontal 0.0763 m and 0.095 m pipes. The emphasis was on high liquid rates (u_LS ? 0.5 m/s) for which slugs are formed close to the entry and the time intervals between slugs are stochastic. A “fully developed” slug flow is defined as consisting of slugs with different sizes interspersed in a stratified flow with a height slightly larger than the height, h₀, needed for a slug to be stable. Properties of this “fully developed” pattern are discussed. A correlation for the frequency of slugging is suggested, which describes our data as well as the data from other laboratories for a wide range of conditions. The possibility is explored that there is a further increase of slug length beyond the “fully developed” condition because slugs slowly overtake one another.     

Nanostructure development in nylon 6-Cloisite® 30B composites. Effects of the preparation conditions

PA6 composites with Cloisite® 30B (30B), prepared by different procedures, i.e., melt compounding, static annealing and solution blending, have been characterized by X-ray diffraction and microscopic analyses (TEM, SEM, POM) in order to shed more light on the mechanism of nanostructure development. It has been demonstrated that intercalation of the PA6 chains within the 30B galleries takes place very rapidly, in the absence of applied stresses, even when the size of the clay particles is relatively large (tens of microns) and the clay loading is very high (even 50 wt.%). It has also been shown that, if, conversely, the filler content is low (∼10 wt.% or less) and the particles are tiny (e.g., as for polymer/clay mixtures prepared by precipitation from a common solution), intercalation continues, under quiescent conditions, and leads in reasonable times to complete destruction of the silicate platelets stacking order. The composites with higher filler contents display a mixed exfoliated/intercalated morphology, with the intercalated silicate stacks characterized by an interlayer distance of about 3.7 nm. Contrary to statically annealed composites, the melt kneaded ones are characterized by a homogeneous dispersion of the filler particles and a local parallel orientation of the silicate platelets that induces, during polymer crystallization, an orientation of the polymer crystallites parallel to the faces of the compression molded specimens. Experiments carried out using 30B samples previously treated at 250 °C for 4 h under vacuum (30Bdegr) indicate that this treatment, probably due to the collapsed interlayer spaces, lowers the extent of PA6 chains intercalation. Thus, the relevant PA6/30Bdegr composites are characterized by the coexistence of unintercalated clay tactoids/agglomerates and individual silicate layers formed as result of intercalation on the edges of the filler particles.     

Analysis on acoustic characteristics of opposed multi-burner gasifier

Hilbert–Huang Transformation and Statistical Theory are used to analyze the impinging flame noise signals of a gasifier. The characteristic of the flame noise energy is studied by standard deviation and probability density, and frequency–time distribution is generated by the empirical mode decomposition (EMD) method and Hilbert spectrum. The results show that, in different conditions, the value of standard deviation of four burners impinging flame noise is greater than that of the two burners, but not for the probability density. The change of the standard deviation and probability density of four burners is smaller than that of two burners. It means that the flame of four burners combusts severely but stably. Under the condition of low feed and oxygen ratio, the two flame burner noise signals frequency and energy distribute mainly in 100 Hz regions; and four burners flame noise distribution is concentrated in the low-frequency bands below 45 Hz and the 45–100 Hz bands.     

Simultaneous measurements of multiple flow velocity components using frequency modulated lasers and a single molecular absorption cell

Since unsteady, complex flow phenomena play an important role, optical measurements techniques are required for flow investigations, which provide simultaneous measurements of multiple velocity components. Doppler global velocimetry has this potential. It is a flow measurement technique, where the Doppler shift of scattered light is measured by a molecular absorption cell for frequency-to-intensity conversion. However, novel Doppler global velocimeters with laser frequency modulation were only used for single component measurements yet. In order to enhance such a system for the simultaneous measurement of multiple components, a concept based on frequency division multiplexing is introduced for the first time. Besides multiple lasers, only a single molecular absorption cell and a single detector unit is required. Two-component measurements of velocity profiles from nozzle flows are presented as a proof of principle. The designed measurement system provides high measurement rates of up to 20 kHz, which is three orders of magnitude higher than for typical Doppler global velocimetry setups.     

AgGaS2- and Al-doped GaSe Crystals for IR Applications

We report a systematic study of AgGaS₂- and Al-doped GaSe crystals in comparison with pure GaSe and S-doped GaSe crystals. AgGaS₂-doped GaSe (GaSe:AgGaS₂) crystal was grown by Bridgman technique from the melt of GaSe:AgGaS₂ (10.6 wt.%). Its real composition was identified as GaSe:S (2 wt.%). Al-doped GaSe (GaSe:Al) crystals were grown from the melt of GaSe and 0.01, 0.05, 0.1, 0.5, 1, 2 mass % of aluminium. Al content in the grown crystals is too small to be measured. The hardness of GaSe:S (2 wt.%) crystal grown from the melt of GaSe:AgGaS₂ is 25% higher than that of GaSe:S (2 wt.%) crystal grown by a conventional S-doping technique and 1.5- to 1.9-times higher than that of pure GaSe. GaSe:Al crystals are characterized by 2.5- to 3-times higher hardness than that of pure GaSe and by extremely low conductivity of ≤ 10^− 7 Om^− 1 cm^− 1. A comparative experiment on SHG in AgGaS₂-, Al-, S-doped GaSe and pure GaSe is carried out under the pumps of 2.12-2.9 μm fs OPA and 9.2−10.8 μm ns CO₂ laser. It was found that GaSe:S crystals possess the best physical properties for mid-IR applications among these doped GaSe crystals. GaSe:Al crystals have relatively low conductivity which have strong potential for THz application.     

Spin-spin coupling edition in chiral liquid crystal NMR solvent

The application of the G-SERFph pulse sequence is presented on enantiomeric mixtures dissolved in a chiral liquid crystal. It aims at editing, within one single 2D spectrum, every proton coupling which is experienced by a given proton site in the molecule, and leads to real phased T-edited spectroscopy (T=J+2D). This NMR experiment is based on the combination of homonuclear semi-selective refocusing techniques with a spatial frequency encoding of the sample. This approach, which consists in handling selectively each coupling in separate cross sections of the sample, is applied to the visualization of enantiomers dissolved in a chiral liquid crystalline phase. Advantages and limits of this methodology are widely discussed.     

舰船辐射噪声模拟发射系统的设计研究

本文依据典型舰船噪声的频谱特性规律,设计研制了一套舰船辐射噪声发射系统实验样机,包括信号结构设计、换能器选择配置、系统构成设计等,本噪声模拟发射系统具有100Hz～16kHz频带噪声的模拟发射功能,包括线谱、连续谱成分,频谱分配和谱线形式可调可控,信号总声源级动态范围达60dB。     

Calculation of the frequency response in step-index plastic optical fibers using the time-dependent power flow equation

The time-dependent power flow equation, which is reduced to its time-independent counterpart is employed to calculate frequency response and bandwidth in addition to mode coupling and mode-dependent attenuation in a step-index plastic optical fiber. The frequency response is specified as a function of distance from the input fiber end. This is compared to reported measurements. Mode-dependent attenuation and mode dispersion and coupling are known to be strong in plastic optical fibers, leading to major implications for their frequency response in data transmission systems.     

Complete synchronization of Kuramoto oscillators with finite inertia

We present an approach based on Gronwall’s inequalities for the asymptotic complete phase-frequency synchronization of Kuramoto oscillators with finite inertia. For given finite inertia and coupling strength, we present admissible classes of initial configurations and natural frequency distributions, which lead to the complete phase-frequency synchronization asymptotically. For this, we explicitly identify invariant regions for the Kuramoto flow, and derive second-order Gronwall’s inequalities for the evolution of phase and frequency diameters. Our detailed time-decay estimates for phase and frequency diameters are independent of the number of oscillators. We also compare our analytical results with numerical simulations.