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Bai L.,Peking University | Huang X.,Peking University | Huang X.,State Key Laboratory of Turbulence and Complex Systems
16th AIAA/CEAS Aeroacoustics Conference (31st AIAA Aeroacoustics Conference) | Year: 2010

Acoustic arrays have become an important testing tool in noise identification for industry applications, where the typical beamforming algorithm has been adopted as a classical processing technique. In most practical cases the beamforming computations have to be conducted off-line due to the excessive computational requirements. An alternative algorithm with a real-time capability is proposed. The algorithm is similar to a classic observer while array processing is performed in the frequency domain. The performance of this observer-based algorithm is studied here through comparing with the typical beamforming method, particulary for a case of coherent noise sources. In this paper it is shown that the observer-based algorithm could resolve the coherence restriction between the background noise and the signal of interest. The proposed algorithm is also beneficial for its capability of operating over sampling blocks recursively. The convergence rate of this recursive algorithm is fast enough to satisfy the requirements for practical cases. The experimental efforts could be saved extensively as any testing defects could be revealed instantaneously and corrected on site. In addition, this innovative approach provides an alternative perspective, from which many techniques already developed in control and filtering could be extended to this new application area of array processin. © 2010 by Xun Huang, Long Bai.

Chen J.,Ningbo University | Chen J.,State Key Laboratory of Turbulence and Complex Systems | Song H.,Ningbo University | Zhang M.,Ningbo University
Acta Mechanica Solida Sinica | Year: 2013

Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated. Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techniques. The formation of delayed ettringite crystal is analyzed using scanning electron microscope and energy disperse spectrum methods. Deformation of micro-voids and the distribution of stress at the surface of a micro-void are calculated. It is found that the nucleation of micro-cracks is caused by the tensile stress at the voids' surface, and such damage nucleation will be speeded up by the remote uniaxial compressive load. © 2013 The Chinese Society of Theoretical and Applied Mechanics.

Liu X.,Peking University | Huang X.,State Key Laboratory of Turbulence and Complex Systems
21st AIAA/CEAS Aeroacoustics Conference | Year: 2016

sound propagation in a lined duct with uniform mean flow has important practical interest for noise emission by aero-engines. In this paper we present the compensated impedance boundary conditions for stable time domain simulations of that. Under the unusual perspective of control, impedance boundary conditions act as closed-loop feedbacks to an overall duct acoustic system. It turns out that those numerical instabilities of time domain simulations are caused by deficient phase margins of the corresponding control-oriented model. A particular instability of very low frequencies in the presence of steady uniform background mean flow, in addition to the well known high frequency numerical instabilities at the grid size, can be identified using this analysis approach. By introducing appropriate phase-lead compensators to achieve desired phase margins, we could formulate stable time domain impedance boundary conditions. The compensated impedance boundary conditions can be simply designed with no empirical parameter, straightforwardly integrated with ordinary linear acoustic models, and efficiently calculated with no need of resolving sheared boundary layers. In order to validate the proposed boundary conditions, comparison is conducted between the corresponding results and asymptotic solutions of spinning modal sound propagation in a duct with a hard-soft interface. Reasonable agreement is achieved. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Chen J.-K.,Ningbo University | Chen J.-K.,State Key Laboratory of Turbulence and Complex Systems | Lei J.-T.,Ningbo University | Zhang M.-H.,Ningbo University | Bai S.-L.,State Key Laboratory of Turbulence and Complex Systems
Mechanics of Materials | Year: 2015

The piezoresistive effect is an important property of conductive polymer composites (CPCs), but the conductive behavior of the composite under shock wave of high pressure and high strain rate, including incident wave and reflected wave, needs to be further studied. To characterize such a system, we study the variation in electrical conductivity of a conductive polymer composite under shock waves, including loading wave and unloading wave, in this paper. The polymer matrix is polyamide-6 with carbon nanotubes and stainless steel fibers are the fillers. The data indicate that the resistivity decreases under the loading shock wave, but sharply increases due to the unloading shock wave. Scanning electron microscopy (SEM) reveals that this resistivity increase is due to damage by the unloading wave and subsequent failure of many micro-conductive channels in the material. © 2015 Elsevier Ltd.

Lei J.-T.,Ningbo University | Chen J.-K.,Ningbo University | Chen J.-K.,State Key Laboratory of Turbulence and Complex Systems | Zhang M.-H.,Ningbo University
Polymers and Polymer Composites | Year: 2012

In order to detect the variation of electro-conductive property of short steel fiber (SSF) filled polyamide-6 (PA6) under impact loading, a modified split Hopkinson pressure bar (M-SHPB) is suggested. Such M-SHPB is constructed by designing a new test electrocircuit, and connecting it to the specimen and oscillograph. On the other hand, a copper foil cover is designed and placed on the whole SHPB equipment for avoiding interference of electromagnetic wave existing in the testing environment. By means of M-SHPB, the relation between the resistance and dynamic stress or dynamic strain is effectively detected. The experimental results indicate that the resistance of the material decreases with the increase of strain or stress. The sensitive variation of the resistance appears at the beginning of the deformation of the material. © Smithers Rapra Technology, 2012.

Sun C.,Ningbo University | Chen J.,Ningbo University | Chen J.,State Key Laboratory of Turbulence and Complex Systems | Zhu J.,Ningbo University | And 2 more authors.
Construction and Building Materials | Year: 2013

Sulfate attack is one of the major durability problems in concrete structures. Sulfate ions present in concrete can react with cement hydration products to form gypsum and ettringite, and this may affect the pore structure and cause structural damage. In this paper, an experimental study was performed on the diffusion of sulfate ions in concrete. Based on the experimentally obtained results, a new diffusion model for sulfate ions was developed, taking into account the influence of the evolution of sulfate-induced damage on the diffusion of ions. The evolution of damage with immersion time and concentration of sulfate ions can be determined using ultrasonic tests. A numerical method was utilized to solve the nonlinear parabolic partial differential equation describing diffusion. A comparison between numerical results and experimental data is provided to validate the proposed models. © 2012 Elsevier Ltd. All rights reserved.

Chu H.-Y.,Ningbo University | Chen J.-K.,Ningbo University | Chen J.-K.,State Key Laboratory of Turbulence and Complex Systems
Construction and Building Materials | Year: 2013

This paper presents a theoretical and experimental study on the attenuation coefficient of the ultrasonic expansion wave in concrete specimens made of different mixes and suffering from sulfate attack. The relationships between the attenuation coefficient, relaxation time, and the wave velocity of the elastic expansion wave were derived. By using simple ultrasonic techniques, the variation of the attenuation coefficient with time during the process of sulfate attack was determined experimentally. The results showed that the magnitude of the attenuation coefficient of concrete increased with time, reflecting damage evolution in the material caused by sulfate attack. This suggested that the damage caused by sulfate attack in concrete can be determined by using simple ultrasonic testing methods. Also, it was found that the concrete specimens with added fly ash had a better performance (reduced sulfate attack) than those with added fine ground slag. © 2012 Elsevier Ltd. All rights reserved.

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