• 한국전산유체공학회:학술대회논문집
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• 2003.10a
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• pp.167-169
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• 2003

This paper presents a theoretical study concerning multibubble dynamics in a sound field and the numerical validation for it by employing our new CFD code MTS-DiCUP. In recent papers, the author has shown theoretically that an unknown characteristic frequency, named 'transition frequency,' exists in a multibubble system. For a N -bubble case, up to 2N -1 transition frequencies per bubble have been predicted, only N ones of them correspond to the natural frequencies of the system. The transition frequencies that do not correspond to the natural frequencies give rise to the phase reversal of bubbles' pulsation without resonant response. In this paper, it has been suggested theoretically that those transition frequencies may cause the sign reversal of the secondary Bjerknes force, which is an interaction force acting between acoustically coupled gas bubbles. This theoretical result has been validated by the direct numerical simulation, at least in a qualitative sense.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.04a
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• pp.347-351
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• 1997

A micromachined piezoelectic cantilever transducer,which works both as a microphone and as a microspeaker,has been fabricated and tested. The 2000*2000*3.mu.m .sap2. cantilever has oxide(ZnO)piezoelectric thin film on a supporting layer of low-pressure chemical-vapor-deposited(LPCVD)low-stress siliconnitride. A highlight of the fabrication process which may also be relevant for other micromachined stuctures is the technique for producing a flat,multilayer cantilever. The measured microphone sensitivity is fairly constant at 2 mV/.mu.bar in the low frequency range and rise to 20 mV/.mu.bar at the lowest resonant frequency of 890 Hz. The 2 mV/.mu.bar sensitivity is the highest report to data for a microphone with a micromachined diaphragm. When measured into a 2 cm/sap3 coupler with 4V (zero-park)drive,the microspeaker output sound pressure level(SPL) is 75 dB at 890 Hz. It increases to approximately 100dB SPL at 4.8kHz with 6V(zero-park)drive. The measured microphone frequency response agrees well with the results of an ABAQUS simulation.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.408-408
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• 2014

Graphene has received attention with its high electron mobility and visual transparency as a promising material for optoelectronic and photonic applications. Combination of graphene and conducting nanostructures i.e. plasmonic structures has recently been researched for enhancing light-matter interaction and overcoming diffraction limit of light. Here we show enhanced photodetection of incoherent visible light with graphene-mediated plasmonics. Gold nanoparticles fabricated by focused ion beam was used as an active element of photodetection and graphene was utilized as an interfacing material between nanostructures and electrodes. Hot electrons generated upon plasmon decay within nanoparticles pass over the potential barrier between nanostructure and graphene and give rise to a photocurrent with built-in electric field. We report 76.7% enhancement of photocurrent under resonant irradiation of fiber-coupled halogen lamp compared to the case without light illumination. We showed wavelength-dependent current response arisen from plasmonic nanostructure, providing a good agreement with theoretical calculation.

• Smart Structures and Systems
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• v.18 no.1
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• pp.183-196
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• 2016

In this paper a nonlinear, bistable, single degree of freedom system is considered. It consists of a Duffing oscillator externally excited by a non-resonant, harmonic force. A customized perturbation scheme is proposed to achieve an approximate expression for periodic solutions. It is based on the evaluation of the quasi-steady (slow) solution, and then on a variable change followed by two perturbation steps which aim to capture the fast, decaying contribution of the response. The reconstructed solution, given by the sum of the slow and fast contributions, is in a good agreement with the one obtained by numerical integration.

• Current Optics and Photonics
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• v.1 no.4
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• pp.278-283
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• 2017

We theoretically study the dynamics of water tetramer in intense femtosecond laser pulses with different frequencies. The simulations are carried out by incorporating the molecular dynamics method non-adiabatically into the time-dependent local-density approximation (TDLDA-MD). Three typical scenarios of water tetramer including the normal vibration with enlarged OH bonds, free OH bonds breaking and the pure Coulomb explosion are presented by investigating the electronic and ionic dynamics. The result indicates that the ionization is enhanced and the corresponding fragmentation effect as well as the damping of the dipole moment are found more notably when increasing the laser frequency especially when the frequency falls in the resonant region of the absorption spectra. The study of the level depletion reveals that the ratio of the emission amount from different levels can be controlled by changing the laser frequency referring to the Keldysh parameter.

• Journal of Electrical Engineering and information Science
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• v.3 no.2
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• pp.245-250
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• 1998

Novel fabrication process of vertical spring for micro mirror array is proposed. The proposed fabrication process adopts a shadow evaporation process using shielding screen structure on top of the sacrificial layer. The 50${\times}$50 micro mirror arrays are fabricated using the proposed process and ceramic packaged. The static and dynamic characteristics of mirror are measured. The mirror plate touches substrate at 16V and the response time of about 16.8 ${\mu}\textrm{s}$. The resonant frequency of mirror is 16kHz. The spring thickness is calculated from static characteristic to be 1075${\AA}$.

• Journal of Mechanical Science and Technology
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• v.18 no.7
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• pp.1246-1257
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• 2004

The objective of this paper is to characterize dynamic pressure traces measured at self-excited combustion instabilities occurring in two combustion systems of different hardware. One system is a model lean premixed gas turbine combustor and the other a fullscale bipropellant liquid rocket thrust chamber. It is commonly observed in both systems that low frequency waves at around 300㎐ are first excited at the onset of combustion instabilities and after a short duration, the instability mode becomes coupled to the resonant acoustic modes of the combustion chamber, the first longitudinal mode for the lean premixed combustor and the first tangential mode for the rocket thrust chamber. Low frequency waves seem to get excited at first since flame shows the higher heat release response on the lower frequency perturbations with the smaller phase differences between heat release and pressure fluctuations. Nonlinear time series analysis of pressure traces reveals that even stable combustion might have chaotic behavior with the positive maximum Lyapunov exponent. Also, pressure fluctuations under combustion instabilities reach a limit cycle or quasi-periodic oscillations at the very similar run conditions, which manifest that a self-excited high frequency instability has strong nonlinear characteristics.

• International Journal of Railway
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• v.6 no.4
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• pp.155-159
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• 2013

The purpose of this study was to develop a theoretical model to analyze the vibration of finite railways forced by distributed moving loads. The vibration characteristics of compliantly supported beam utilizing compressional damping model were investigated through the Rayleigh-Ritz method. The distributed moving load was analyzed as the cross correlation function on railways. This allowed the use of statistical characteristics for simulation of the moving train wheels on the rail. The results showed there is a critical velocity inducing resonant vibration of the rail. The mass spring resonance from the rail fastening systems exhibited significant influence on the resulting vibration response. In particular, the effect of the viscoelastic core damping was investigated as an efficient method for minimizing rail vibration. The decrease of the averaged vibration and rolling noise generation by the damping core was maximized at the mass-stiffness-mass resonance frequency.

• Proceedings of the KSR Conference
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• 2002.10a
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• pp.626-632
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• 2002

Geogrid is widely used as the reinforcement materials in railway earth structures in order to achieve efficient land utilization as well as securing safety in railway service lines in other countries. In this study, the real scale test was carried out to investigate the application of geogrid reinforced soil segmental retaining walls(SRWs) in railway. For this goal, the vibration transfer characteristics of reinforced soil segmental retaining walls was evaluated. The resonant frequencies of SRWs, vertical ground vibration in backfill and vertical/horizontal vibration at segmental units were acquired. This experimental data and analysis result can contribute to understand the vibration response behavior of SRWs.

• Journal of Mechanical Science and Technology
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• v.20 no.10
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• pp.1702-1715
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• 2006

The increasing use of nano-positioners in a wide variety of laboratory and industrial applications has created a need for nano-mechatronics education in all engineering disciplines. The subject of nano-mechatronics is broad and interdisciplinary. This article focuses on the way nano-mechatronics is taught in department of mechanical engineering at Korea Advanced Institute of Science and Technology (KAIST). As one model of nano-positioning systems, design and experimental methodology is presented in this article. For design phase, the stiffness and resonant frequencies are found analytically and verified by using a commercial finite element analysis program. Next, for experimental phase, various tests are performed to access the performances of the designed nano-positioner, for example, sine-tracking, multi-step response and travel-range check etc. Finally, the definition of 'separation frequency' is described and some comments are discussed.