JOURNAL OF VIBRATION AND SOUND
Year:2020 | Volume:9 | Issue:18|Papers Count:10

In the present study, the effect of circumferential mode number on natural frequencies of cylindrical shells was investigated numerically and experimentally. Three cylindrical shells with different diameter to length ratios were examined. Different contact type with water were compared using both numerical simulation and experimental tests. The effects of immersion depth, diameter to length ratio, variations of natural frequency with different circumferential mode number were investigated. The reduction of the natural frequencies of the cylindrical shell for the minimum frequency of different circumferential mode number is low, but it increases for higher and lower frequencies, and at the beginning of immersion and at full immersion, the frequency decreases suddenly. Also, as the ratio of diameter to shell length decreases, the frequencies of the low circumferential mode number decrease and the lower value of the natural frequency tends to lower circumferential mode number. At low circumferential mode number, the natural frequencies differ considerably, and with increase of n number, the difference decreases and converges.

The dynamic properties and characteristics of engineering structures may change throughout their lives for a variety of reasons, often over time. In such engineering structures, for which the term "time-varying systems" can be used, it is important to identify time-varying dynamic parameters, including instantaneous modal frequency, as it can enable engineers to monitor the performance of structures. It also helps to diagnose structural damage and assess the condition of the structure in a cost-effective manner. In fact, tools that can correctly obtain characteristics similar to the instantaneous frequency in the analysis of such systems have particular importance. Tools for extracting such characteristics from dynamic responses are known as time-frequency domain analysis methods. These instantaneous characteristics can be used to identify time-varying parameters in these structures, such as time-varying stiffness and damping. In the current paper, two of the most powerful methods of time-frequency domain analysis, namely wavelet analysis, and analytical mode decomposition-Hilbert, are used to extract instantaneous characteristics. These two methods in extracting the characteristics are compared with each other, their superiority over each other in noise, and no noise conditions are examined. Then, by combining each of these two methods with existing algorithms in the parametric identification of multi-degree of freedom time-varying systems, new combinations are introduced that report performance improvements in the identification of time-variable parameters.

Ultrasonication in liquids is one of the most important approaches to exfoliate 2D materials. Here we described some experiments based on Liquid Phase Exfoliation of Molybdenum Disulfide (MoS2) powder in the water-ethanol solution without any additives. It is reported that MoS2 as a prototype 2D Transition Metal Dichalcogenide, has a bandgap that increases with decreasing layer thickness. There are some factors that affect the average band gap energy value (and the thickness of the exfoliated flakes). In this study, 2D nanoflakes of Molybdenum Disulfide (MoS2) were produced using an ultrasonic probe in various solutions. Because of the key role of surface tension in the exfoliation process, and also to reduce the negative effects of chemical residues on the environment, we used a binary solution consisting of water and ethanol with the ratio determining based on the Connors-Wright equation. Our findings from UV-Visible spectroscopy show that the best efficiency, i. e. the minimum thickness of MoS2 nanoflakes, is obtained for the solution with the mentioned solution. The FESEM and TEM images, and the XRD pattern confirmed that we obtained the few-layers of MoS2.

In the maintenance system in industries, there are different strategies that in each industry, a specific strategy is tried to be implemented as a superior strategy. What is certain is that trying to implement a specific strategy in the product maintenance system is a closed view and it is necessary to select and implement the desired strategies according to the situation of each industry. In order to properly implement the situation monitoring techniques, it is necessary to implement each technique in a principled manner and compare the results obtained from each technique with the output results of other techniques. In some cases, a particular technique may be effective in diagnostics, but in most cases it can be said that the combination of different techniques helps to make the right decision. In the present study, while explaining how to use different situation monitoring techniques, the achievements of the simultaneous implementation of these techniques in one of the country's oil fields have been discussed.

This paper presents the use of Lyapunov direct method to boundary control of the upper end of the riser so as to reduce the top angle and transverse vibration of the riser during time-varying disturbances. Two cases are taken into account for analysis. In the first case, the surface current of sea is assumed to be linearly reduced from the sea surface to the sea bed; while, in the second case, the declination is exponential. In both cases, it is assumed that the current decreases to almost zero. To address the problem, a distributed parameter system is modeled with partial differential equation constrained to boundary conditions. As all control signals can be measured using sensors and/or can be calculated using the backward difference algorithm, the boundary control has the potential to be practically implemented. The Lyapunov direct method has also been utilized for stability analysis of the closed-loop system. Eventually, the efficiency and robustness of the proposed controller is verified, and a comparison is made between the related results of linear and exponential profiles.

In order to deal with the problems of noise pollution, designing systems and structures that can have targeted control over the propagation of sound waves is of great importance. One of the most common and widely used methods in controlling noise pollution is the use of sandwich panels as acoustic insulation. The distinctive feature of this type of panels is the high variability of their core, so that by changing the material and arrangement of the core, the acoustic and mechanical properties of the panel change. In recent years, nesting structures have been used as the core of sandwich panels. Nesting cores are a good option for designing sound insulation due to their light weight and high stiffness. The various features and characteristics of the nest structure, including cell geometry, the type of structure used, and the type of arrangement and orientation of the cells next to each other, indicate that an optimal structure should be designed to achieve better acoustic behavior. In this research, genetic algorithm has been used to improve the acoustic performance of sandwich panel structure with honeycomb core. This optimization is done for two common types of nesting cell orientations, taking into account that the panel is under in-plane loading. Optimization has been performed on the geometric variables of the nesting cell for a number of materials with different physical and mechanical properties, and the aim was to reduce the sound transmission through this panel in the low frequency range.

In the present study, an efficient numerical algorithm based on theoretical methods is developed for the prediction of the noise of aerodynamic flows around two-dimensional geometries. The theoretical formulation for calculating different types of noise is proofed and a solver with a negligible computational cost is developed accordingly. By using the present noise solver, the aeroacoustic characteristics around two test cases are predicted and the obtained results are presented in comparison with the available numerical and experimental data. The present results for noise around NACA0012 and S822 are in excellent agreement with those reported in the literature which shows the accuracy and performance of the present solver based on the theoretical formulation. Employing this quick solver is very crucial to save costs in the preliminary design process of low noise geometries in aerospace engineering. The extension of the present two-dimensional noise solver for the prediction of noise properties around three-dimensional geometries is undergoing.

The multipurpose hall studied here is the part of design project of gorgan music hall designed in the centeral region of gorgan and submitted in the final project of first author’ s master thesis. The aim of establishment of multipurpose hall is creation of a space that be suitable for speech and music performance simultaneously. According to the evidences the reverberation time is most important parameter for performance design. This value was considered 1. 0(s) for speech and 2. 0(s) for music in average. Having volume and effective absorber surfaces values and using Sabine equation for RT, absorption coefficient was obtained for different parts of the hall. Due to assumed data, the values of C50, C80, ALcons, CD, D/R ratio, both for the music and speech were presented. Some of the results given to standards were suitable and some another were rather acceptable. Finally, several solutions were presented in the analyzing part for the best result.