Current Issue : October - December Volume : 2018 Issue Number : 4 Articles : 6 Articles
To investigate relation between fracture scale and acoustic emission time-frequency parameters in rocks, experiments of acoustic\nemission monitoring of granite uniaxial compression were carried out. The AE signal energy and dominant-frequency of granite\nfracture processwere extracted bymeans of AE time-frequency analysis.Therelation between fracture scale andAE time-frequency\nparameters (energy and frequency) in granite fracture process was analyzed. The evaluation model of rock fracture scale based on\nAE energy and dominant-frequency was established by using the intrinsic relation between the scale of rock fracture and the timefrequency\nparameters of rock mass. The evolution of crack scale in the process of uniaxial compression was analyzed based on the\nevaluation model of rock fracture scale. Results show that the AE energy and the dominant-frequency can reflect the crack scale\ninside the rock. The scale of rock fracture is proportional to the AE energy, which is inversely proportional to the AE dominantfrequency.\nSignals with low frequency and high energy usually represent large-scale cracks. On the contrary, if the high frequency\nhas low energy value, it indicates a small-scale crack. The theory andmethod of evaluation of rock rupture scale based on AE timefrequency\ninformation (energy, frequency) can describe the failure process of rock crack scale variation characteristics. It provides\na way and method for investigating the characterization of fracture size evolution process of rock fracture....
Five well-known azimuth angle estimation methods using a single acoustic vector sensor (AVS) are investigated in open-lake\nexperiments. A single AVS can measure both the acoustic pressure and acoustic particle velocity at a signal point in space and\noutput multichannel signals. The azimuth angle of one source can be estimated by using a single AVS in a passive sonar system.\nOpen-lake experiments are carried out to evaluate how these different techniques perform in estimating azimuth angle of a\nsource. The AVS that was applied in these open-lake experiments is a two-dimensional accelerometer structure sensor. It\nconsists of two identical uniaxial velocity sensors in orthogonal orientations, plus a pressure sensorââ?¬â?all in spatial collocation.\nThese experimental results indicate that all these methods can effectively realize the azimuth angle estimation using only one\nAVS. The results presented in this paper reveal that AVS can be applied in a wider range of application in distributed\nunderwater acoustic systems for passive detection, localization, classification, and so on....
Mechanical parameters of the rock are important for the design of geotechnical, mining engineering, and petroleum reservoir\nprojects. Many researches have suggested that the mechanical variables of rock specimens, such as compressive strength and\nelastic modulus, do not have a single fixed value. Uncertainty in the basic mechanical variables of the rock material can significantly\naffect the structural performance and safety. In this study, a series of compression experiments with acoustic emission\nhave been performed on rock specimens. The damage evolution characteristics of the rock in the process of loading were studied,\nand the macromechanical behaviors were obtained at the same time. Distribution characteristics of the strength and elastic\nmodulus as random variables are illustrated, and the statistical damage model is presented by the authors to formulate analytical\nconstitutive relations for deformation behavior. The comparisons between predicted results and experimental data show that the\nstatistical damage constitutive model could well reproduce the deformation process of rock materials....
The noise produced by a contra-rotating ventilator can cause injury to humans.Therefore, it is important to reduce noise caused\nby ventilators. In this study, the FfowcsWilliams and Hawkings (FW-H) model was used to simulate the acoustics of four different\naxial impeller spacing points based on the unsteady flow field through a FBD No. 8.0 contra-rotating ventilator. Experiments were\nconducted to verify the correctness of the numericalmodel.Meanwhile, the Variable FrequencyDrive (VFD) drives the two motors\nof 55kW to give the impellers different speeds to distinguish different conditions. The results showed that the main noise source\nof the ventilator was the two rotating impellers and the area between them. For the same axial space, the noise decreased with the\nincrease of flow rate and then decreased. And the amplitude of the discrete pulse increased gradually. It can be concluded that the\nvortex acoustics decreased gradually with the increase of flow rate and the rotating acoustics were the major contributor.With the\naxial distance increasing, the noise caused by the two impellers was weak, and the frequencies of sound pressure level moved toward\nmedium- and low-frequency bands gradually. The suitable axial space could reduce noise and improve the working environment....
There has been an explosion of interest in acoustic metamaterial in the last ten years. The tunable negative acoustic metamaterial\nis an important issue for designing metamaterials. The acoustic metamaterial is restricted by the narrow bandgap of sound waves\nfor the local resonance of metamaterial unit cells. By shifting the sizes of the unit cell, the acoustic metamaterial could potentially\novercome the limit of the narrow resonance frequency. In this research, we focus on the resonant behavior of split hollow sphere\n(SHS) in the waveguide. Firstly, we analyze the resonance characteristics of SHS and get an analytical formula for the effect of\ngeometrical parameters on the resonance frequency. Furthermore, the resonance frequency of SHS is verified with finite-element\nmethod analysis based on COMSOLMultiphysics simulation. The results are in good agreement with theory model. It is observed\nthat there is a blue shift of the resonance frequency with the gradual increase of the neck radius of SHS, a V-type response curve\nwith the increase of inner radius of SHS, and a red shift with the increase of outer radius of SHS. Using the method of estimate\nof resonance, we could get a precisely controllable unit structure with negative effective modulus and offer a way to optimize the\nrealization of double negative acoustic metamaterial....
The unmanned surface vehicle (USV) integrated with acoustic modems has some advantages such as easy integration, rapid\nplacement, and low cost, which becomes a kind of selective novel node in the underwater acoustic (UWA) communication network\nand a kind of underwater or overwater communication relay as well. However, it is difficult to ensure the communication quality\namong the nodes on the network due to the randomunderwater acoustic channel, the severemarine environment, and the complex\nmobile node system. Aiming to model the communication characteristics of the USV, the multipath effect and Doppler effect are\nmain concerns for the UWA communication in this paper, so that the ray beam method is utilized, the channel transmission\nfunction and the channel gain are obtained, and the mobile communication quality evaluation model is built. The simulation and\nlake experiments verify that the built mobile UWA communication quality evaluation model on USV can provide preference and\ntechnique support for USV applications....
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