• Journal of the Korean Society for Nondestructive Testing
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• v.31 no.1
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• pp.47-52
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• 2011

An electromagnetic acoustic transducer(EMAT) is a non-contact transducer which can transmit the ultrasonic guided waves into specimens without couplant. And it can easily generate specific guided waves such as SH(shear horizontal) or Lamb waves by altering the design of coil and magnet. In this study, the SH wave, which is generated by EMAT, has been applied to estimate the thickness-reduction in a steel plate. Especially, the interesting feature of the dispersive behavior in selected wave modes is used to detect the thickness-reduction. Experimental results show that the reduction-level can be quantified by the measurement of the group velocity of the wave which passes though the thinning area.

• Tunnel and Underground Space
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• v.21 no.5
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• pp.349-358
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• 2011

This paper introduces a new P wave arrival time determination algorithm of acoustic emission (AE) suitable to identify P waves with low signal-to-noise ratio generated in rock masses around the high-level radioactive waste disposal repositories. The algorithms adopted for this paper were amplitude threshold picker, Akaike Information Criterion (AIC), two step AIC, and Hinkley criterion. The elastic waves were generated by Pencil Lead Break test on a granite sample, then mixed with white noise to make it difficult to distinguish P wave artificially. The results obtained from amplitude threshold picker, AIC, and Hinkley criterion produced relatively large error due to the low signal-to-noise ratio. On the other hand, two step AIC algorithm provided the correct results regardless of white noise so that the accuracy of source localization was more improved and could be satisfied with the error range.

• Geomechanics and Engineering
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• v.18 no.6
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• pp.605-614
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• 2019

The present paper seeks to investigate propagation and reflection of waves at free surfaces of homogeneous, anisotropic and rotating micropolar fibre-reinforced medium with voids. It has been observed that, in particular when P-wave is incident on the free surface, there exist four coupled reflected plane waves traveling in the medium; quasi-longitudinal displacement (qLD) wave, quasi-transverse displacement (qTD) wave, quasi-transverse microrotational wave and a wave due to voids. Normal mode Analysis usually called harmonic solution method is adopted in concomitant with Snell's laws and appropriate boundary conditions in determination of solution to the micropolar fibre reinforced modelled problem. Amplitude ratios which correspond to reflected waves in vertical and horizontal components are presented analytically. Also, the Reflection Coefficients are presented using numerical simulated results in graphical form for a particular chosen material by the help of Mathematica software. We observed that the micropolar fibre-reinforced, voids and rotational parameters have various degrees of effects to the modulation, propagation and reflection of waves in the medium. The study would have impact to micropolar fibre-reinforecd rotational-acoustic machination fields and future works about behavior of seismic waves.

• Proceedings of the KSR Conference
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• 2000.11a
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• pp.499-505
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• 2000

The shape of the KHST, and of the power car in particular, is largely determined by aerodynamic considerations. At high speeds, air resistance accounts for the major part of overall resistance to forward motion. Further points to be considered are environmentally undesirable acoustic phenomena and pressure waves. Minimizing power requirements and environmentally-unfriendly noise and pressure waves are thus major objectives in the development of the KHST. When deciding on the aerodynamic design of the power car, the entire train set has to be taken into consideration. This paper describes the design process and results about the front shape of the KHST.

• Proceedings of the KSR Conference
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• 1999.11a
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• pp.115-120
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• 1999

The shape of the KHST, and of the power car in particular. is largely determined by aerodynamic considerations. At high speeds, air resistance accounts for the major part of overall resistance to forward motion. Further points to be considered are environmentally undesirable acoustic phenomena and pressure waves. Minimizing power requirements and environmentally-unfriendly noise and pressure waves are thus major objectives in the development of the KHST. When deciding on the aerodynamic design of the power car, the entire train set has to be taken into consideration. This paper describes the design process and results about the front shape of the KHST.

• Transactions of the Korean Society of Automotive Engineers
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• v.8 no.4
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• pp.85-92
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• 2000

The pulsating pressure waves are composed of fundamental frequency and higher order harmonics in exhaust pipe of engines. The nonlinearity in exhaust pipe is caused by their interactions. The error which is between prediction and measurement is induced by the nonlinearity. We can not explain this phenomenon using linear acoustic theory which is existing theory. So power spectrum which was used in linear theory is not useful. Bispectrum and bicoherence functions which are a higher order spectrum are applicable to explain this phenomenon. This paper proposes a nonlinear effect of pulsating pressure waves. The phenomenon proposed here is identified by using of higher order spectrum density functions.

• Bulletin of the Korean Space Science Society
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• 2011.04a
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• pp.29.3-29.3
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• 2011

The turbulence in the nonlinear regime of the electromagnetic ion-cyclotron (EMIC) instability are investigated via a particle-in-cell (PIC) simulation. EMIC instability arises from anisotropic ion temperature and excites the Alfven ion-cyclotron (AIC) waves. The excited AIC waves undergo inverse-cascade via the nonlinear wave interaction of two AIC and one ion density modes. Induced ion density modes are the normal and second harmonic ion-acoustic (IA) waves. They have the same group velocity, but the second harmonic IA mode only generates the longitudinal electric field.

• 한국전산유체공학회:학술대회논문집
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• 2001.10a
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• pp.11-19
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• 2001

An adaptive nonlinear artificial dissipation model is presented for performing aeroacoustic computations by the high-order and high-resolution numerical schemes based on the central finite differences. An effective formalism of it is devised by combining a selective background smoothing term and a well-established nonlinear shock-capturing term which is for the temporal accuracy as well as the numerical stability. A conservative form of the selective background smoothing term is presented to keep accurate phase speeds of the propagating nonlinear waves. The nonlinear shock-capturing term that has been modeled by the second-order derivative term is combined with it to improve the resolution of discontinuities and stabilize the strong nonlinear waves. It is shown that the improved artificial dissipation model with an adaptive control constant which is independent of problem types reproduces the correct profiles and speeds of nonlinear waves, suppresses numerical oscillations near discontinuity and avoids unnecessary damping on the smooth linear acoustic waves. The feasibility and performance of the adaptive nonlinear artificial dissipation model are investigated by the applications to actual computational aeroacoustics problems.

• Proceedings of the Korea Electromagnetic Engineering Society Conference
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• 2001.11a
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• pp.332-336
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• 2001

The new RST is recently developed by the product expansion of the scattering functions in the field of acoustics. The new formulation suggests that the scattering coefficients consist of resonance, non-resonance, and their interactional components. In the scattering problems of acoustic waves, the moduli and phase of the resonance coefficient are obtained the appropriate results through the new RST. In our recent works the new RST was successfully applied to the scattering problem of electromagnetic waves for coated conducting cylinder and sphere. In this paper, the new RST is applied to the 2-dimensional scattering problem of electromagnetic waves for a homogeneous dielectric cylinder, and the numerical results are compared with the previous RST.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.243-246
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• 2000

This paper addresses a new technique of measuring the mean flow velocity not only over the cross sectional area but also along the pipe by exploiting the acoustic plane waves in the pipe. When fluid flows in the pipe and two plane waves propagate oppositely through the medium in it, the flow velocity causes a change of the wave number of the plane waves. The wave number of the positive going plane wave decreases but oppositely that of negative going one increases in comparison to no flow of the medium in the pipe. Theoretical backgrounds of this method are in details discussed and measurement results of the mean flow velocity are illustrated to reveal the feasibility and effectiveness of the suggested technique.