• Journal of the Computational Structural Engineering Institute of Korea
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• v.16 no.1
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• pp.1-8
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• 2003

Topology optimization has been evolved into a very efficient conceptual design tool and has been utilized into design engineering processes. Traditional topology optimization has been using homogenization method and optimality criteria method. homogenization method provides relationship equation between structure which includes many holes and stiffness matrix in FEM. Optimality criteria method is used to update design variables while maintaining that volume fraction is uniform. Traditional topology optimization has advantage of good convergence but has disadvantage of too much convergency time. In one way to solve this problem, element removal method using the criterion of an average stress is presented. As the result of examples, it is certified that convergency time is very reduced.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2010.05a
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• pp.705-708
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• 2010

Recently, HSRC (Half-Symbol-Rate-Carrier) OQPSK (Offset Quadrature Phase Shift Keying) signaling which reduces the bandwidth of transmitted signal for high-speed data communications has been introduced. Since the signal is based on QPSK modulation, it also has the characteristics of QPSK signal. This paper introduces architectures of the transceiver using differential coding to resolve the 4-phase ambiguity problem of the HSRC-QOPSK signaling for high-speed data communications. In addition, this paper proves the functionality of the transceiver with differential coding and shows the BER (bit-error-rate) performance of the transceiver by simulations.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.05a
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• pp.125-128
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• 2008

Atomization characteristics of small LRE-injector spray are investigated by using dual-mode phase Doppler anemometry (DPDA). Velocity, size, number density, and volume flux were measured at various injection pressures along the radial distance to make a close inquiry into spatial distribution characteristic of spray droplets. As the injection pressure increases, the velocity, turbulence intensity, number density, and volume flux of spray droplets become higher, whereas the droplet size ($D_{10}$ or $D_{32}$) gets smaller. Also, velocity and volume flux are proportional to Sauter mean diameter (SMD, $D_{32}$).

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.05a
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• pp.141-144
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• 2008

Atomization characteristics and spatial distribution of the spray emanating from an injector of liquid-propellant thruster are investigated by using dual-mode phase Doppler anemometry (DPDA). Spray characteristic parameters such as the mean velocity, Sauter mean diameter (SMD), and velocity fluctuation are measured at various locations along the spray axis as well as on the radial direction. Those data are quantified in radial profile and also used to scrutinize the correlation between diameter and turbulence intensity of spray droplets. For the better visual grasp, dynamic behavior of spray droplets along the spray stream is presented through the velocity vectors projected on the plane of geometric axis of nozzle orifice and radial coordinate.

• Journal of the Korean Society for Nondestructive Testing
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• v.19 no.2
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• pp.118-128
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• 1999

There are many ultrasonic measurement methods that are used in nondestructive testing applications. Some typical applications include material property determination, microstructural characterization. and flaw detection. Ultrasonic parameters such as velocity and attenuation are most commonly required in these applications. The accuracy and repeatability of testing results are dependent on both the hardware used to generate and receive the ultrasonic waves and on the analysis software for calculating these parameters. In this study, five analysis algorithms were implemented on a computer for measuring wave speed in a pulse echo. immersion testing configuration. In velocity measurements comparisons were made between the overlap. cross-correlation. Fourier transform. Hilbert transform, wavelet transform algorithms. Velocity measurement was applied to an isotropic steel sample using the five analysis algorithms. Frequency-dependent phase/group velocity and attenuation were also measured using the Fourier transform and wavelet transform algorithms on a composite laminate containing voids.

• Korean Journal of Optics and Photonics
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• v.32 no.4
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• pp.180-186
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• 2021

Fluorinated polyimide has excellent light transmittance and a high optical refractive index, and is useful for producing optical-waveguide phase modulators with low optical loss and low electric power consumption. In this work, an optical-waveguide phase modulator is designed and fabricated based on a high-refractive-index fluorinated polyimide, and its characteristics are measured. An efficient protocol for characterizing the loss components of the optical waveguide is proposed, and the propagation loss of the fabricated polyimide optical waveguide is confirmed to be 0.9 dB/cm. The phase modulator requires 9.1 mW of power for phase change of π, and the response time is 290 μs, which is improved by tenfold compared to a previous demonstration of polymeric phase modulators.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.26 no.6
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• pp.567-579
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• 1993

The response of sea levels to a typhoon in the north Japanese coast in the Japan Sea is investigated by using hourly ses level data($1966{\sim}1986$) and a numerical shallow water model with high resolution($5'{\times}5'$). The observed sea level analysis shows (1) progressive waves exist between Simonoseki(SS) and Maizuru(MZ) with the mean phase speed of about 4 m/s during the passage of the typhoon, (2) the phase speed between Sasebo(SB) and HK(Hakata) is slower(about 1.7 m/s), and (3) the maximum sea level at HK is achieved about 0.5 day later than that of SS. In many aspects, the numerical model results correspond well to the above observed features. In the model the progressive waves are identified as a topographic wave with the phase speed of about 4 m/s. Before the typhoon passes through the Korea Strait/ the Tsushima Strait, the wave propagations along the Japanese coast are significantly influenced by the southwestward coastal jet induced by the wind stress parallel to the coast. The waves start to propagate northeastward along the coast when the coastal jet is weakened.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.2
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• pp.211-216
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• 2008

In this study, we have developed a program which can calculate phase and group velocities, attenuation and wave structures of each mode in multi-layered plates. The wave structures of each mode are obtained, varying material properties and number of layers. The key in the success of guided wave NDE is how to optimize the mode selection scheme by minimizing energy loss when a structure is in contact with liquid. In this study, the normalized out-of-plane displacements at the surface of a free plate are used to predict the variation of modal attenuation and verily the correlation between attenuation and wave structure. It turns out that the guided wave attenuation can be efficiently obtain from the out-of-plane displacement variation of a free wave guide alleviating such mathematical difficulties in extracting complex roots for the eigenvalue problem of a liquid loaded wave guide. Through this study, the concert to optimize guided wave mode selection is accomplished to enhance sensitivity and efficiency in nondestructive evaluation for multi-layered structures.

• Journal of the Korean Society for Nondestructive Testing
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• v.13 no.4
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• pp.9-17
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• 1994

It is well recognized recently that ultrasonic technique is one of the most widely used methods of nondestructive evaluation to characterize material properties of nonconventional engineering materials. Therefore it is very important to understand physical phenomenon on propagation behavior of elastic wave in these materials, which is directly associated with ultrasonic signals in the test. In this study, the theoretical analysis on multi-scattering of harmonic elastic wave due to the particulate with interface between matrix and fiber in metal matrix composites(MMCs) was done on the basis of Lax's quasi-crystalline approximation and extinction theorem. SiC particulate (SiCp) reinforced A16061-T6 composite material was chosen for this analysis. From this analysis, frequency dependences of phase velocity and amplitude attenuation of effective plane wave due to the change of volume fraction of SiC particulate were clearly found. It was also shown that the interface condition between matrix and fiber in MMCs gives a direct effect on the variation of phase velocity of plane wave in MMCs.

• Journal of the Korean Society for Nondestructive Testing
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• v.25 no.1
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• pp.20-26
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• 2005

Ultrasonic guided waves have been widely employed for long range inspection of structures such as plates, rods and pipes. There are numerous modes with different wave velocities, and the appropriate mode selection is one of key techniques in the application of guided waves. In the present work, phase tuning by an array transducer was applied to generate ultrasonic guided waves. For this purpose, 8-channel ultrasonic pulser/receiver and their controller which enables sequential activation of each channels with given time delay were developed. Eight transducers were fabricated in order to generate guided waves by using an array transducer. The selective tuning of wave mode can be achieved by changing the interval between elements of an array transducer.