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Flexible rotors must pass through at least one critical speed before reaching its nominal or working speed. During passage through this resonance, transient vibrations are introduced in the rotor. For a better performance of the machinery, these vibrations must be kept below a certain threshold to keep a small clearance between the rotor and the stator. Modal coupling by periodic control is a modern approach which is capable of transferring kinetic energy between two selected modes. By introducing a time-periodic variation of the bearing characteristics, a modal energy transfer is induced in a flexible rotor during run-up and run-down. This concept reduces the main resonance peak during passage through resonance compared to the nominal run-up. By the application of this technique, we were able to reduce the maximum amplitude response during the run-up to 25.4 percent and during the run-down to 27.1 percent.
Semiconducting metal oxides are widely used for solar cells, photo-catalysis, bio-active materials and gas sensors. Besides the material properties of the semiconductor being used, the specific surface topology of the sensors determines device performance. This study presents different approaches for increasing the sensing area of semiconducting metal oxide gas sensors. Micro- and nanopatterned laser-induced periodic surface structures (LIPSSs) are generated on silicon, Si/SiO2 and glass substrates. The surface morphologies of the fabricated samples are examined by FE SEM. We selected the nanostructuring and characterization of nanostructured source Ni/Au and Ti/Au films prepared on glass using laser ablation as the most suitable of the investigated approaches. Surface structures produced on glass by backside ablation provide 100 nm features with a high surface area; they are also transparent and have high resistivity. The value of the hydrogen sensitivity in the range concentrations from 100 to 500 ppm was recorded using transmittance measurements to be twice as great for the nanostructured target TiO2/Au as compared to the NiO/Au. It was found that such transparent materials present additional possibilities for producing optical gas sensors.
Introduction: Acoustic black holes (ABH) are capable to mitigate structural vibrations efficiently above a certain cut-on frequency. The most commonly used geometry for a flexible beam is a simple wedge following a power-law curve. A simple wedge demands large dimensions for achieving mitigation in the low-frequency range below 1000 Hz. It was shown recently by experiments and numerical simulation that a multi-wedge configuration is beneficial for realizing a compact design and still showing good performance at low frequencies.
Materials and Methods: The WKB approximation is extended for a single-wedge design. Expressions for the reflection coefficient and cut-on frequency are discussed for an arbitrary number of wedges—the suggested multi-wedge ABH.
Results: The main benefit of the stacked multi-wedge ABH is a great improvement in performance in the low-frequency range. A numerical example highlights the successful vibration mitigation. It is shown how a multi-wedge ABH is tuned towards low-frequency in terms of cut-on frequency and reflections’ coefficient. The improved performance of a multi-wedge ABH is benchmarked against the well-established simple ABH.
Noise from machine vibrations and oscillations is a growing problem in today’s society. The use of acoustic black holes (ABH) in the area of passive vibration damping as an absorbing metamaterial is an active research field. Previous work has been successful mainly in the higher frequency range above 1500 Hz. This work aims at vibration damping in the lower frequency range below 1500 Hz. Here, additively manufactured multi-wedge ABH with two, three, four and ten wedges were welded to a beam structure and measured to estimate which number of wedges produces the best damping for a specific frequency range. The manufactured wedges largely absorbed a vast amount of the vibration energy induced into the structure and showed promising results. It was found that the more wedges were welded to the beam, the more natural frequencies occurred in the low frequency range. In the case of the ABH with ten wedges, ten eigenmodes were detected in this range, all of which absorbed the induced vibration energy effectively in the low frequency range.
Machines and other driving components like compressors or fans usually generate vibrations which frequently lead to acoustic noise. Flexible structures equipped with acoustic black holes minimise acoustic radiation by confining structural vibrations locally. One main restriction of its usage in the broad engineering field is its limited effectiveness at low frequencies. Recent investigations shifted the frequency range of attenuation successfully down to 1500 Hz. Moving the existing designs towards an even lower frequency demands a large structure. However, in general, sufficient space is often not available in machines and facilities. We propose a new design that enables a geometrically compact and simultaneously broadband vibration attenuation in the low-frequency below to 100 Hz: stacked wedges. The proposed design is calculated and optimised numerically by combining CAD and finite element calculations. The influence of geometrical parameters on the effectiveness of vibration attenuation is analysed with the help of transfer functions and dispersion curves. Successful designs of multi-stacked wedges at different lengths confirm their effectiveness at low frequency.
This article aims to compare different types of optical amplifiers implemented in metropolitan xWDM-PON networks. The first part is an analysis of the issue with the focus on the used optical access network based on xWDM-PON. In the next part the deployment of optical amplifiers (SOA, EDFA and Raman) into the network and their analysis is presented. The following part is dedicated to the simulation of WDM-PON networks in the OptiSystem tool from Optiwave, with applied appropriate optical amplifier for chosen channels. In the last part, the simulated results (based on optical power, BER, Q-factor, eye-diagram and spectral analysis) are evaluated and discussed.
