• Geophysics and Geophysical Exploration
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• v.16 no.4
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• pp.293-299
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• 2013

The recent Multiple Lapse Time Window (MLTW) analysis of Korean Peninsula event showed that the focal depth was far greater influence factor than the velocity structure of the model, applying the analysis of the direct simulation Monte Carlo (DSMC) method. Thus, using the events with focal depth of about 10 km, this study considered 330 paths connecting 41 events and 71 stations, and re-examined uniform and depth-dependent velocity models previously studied. As a result, the residual of misfit function greatly decrease from analytic model to DSMC model, reflecting variation of the focal depth from 0 to 10 km. On the other hand, the difference of residuals for each velocity model were relatively small.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.11a
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• pp.254-259
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• 1996

Phase Doppler particle analyze. (PDPA) is a instrument which can be used to obtain simultaneous size and velocity measurements in a multiphase flow. In this study, the size of the water droplets entrained from a bubbling surface of a stagnant liquid column is measured by PDPA with a specially designed transmitter of long focal length and large beam diameter. The test section tube is made of acryle with 18 mm I.D. and 900 mm length. The experimental data are obtained for the air superficial velocity between 0.7 m/s to 3.4 m/s at atmospheric pressure. The experimental results show that there exists large difference in the entrainment mechanism between the churn-turbulent flow and annular flow. Through the present study, the phase Doppler analyzer system is shown to be successfully applied to measure particle sizes larger than $2,000\mu\textrm{m}$ if a transmitter of long focal length is utilized.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.6
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• pp.531-537
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• 2008

One of the fundamental features of time reversal acoustic (TRA) techniques is the ability to focus the propagating ultrasonic beam to a specific point within the test material. Therefore, it is important to understand the focusing properties of a TR device in many applications including nondestructive testing. In this paper, we employ an analytical scheme for the analysis of TR beam focusing in a homogeneous medium. More specifically, a nonparaxial multi-Gaussian beam (NMGB) model is used to simulate the focusing behavior of array transducers composed of multiple rectangular elements. The NMGB model is found to generate accurate beam fields beyond the nonparaxial region. Two different simulation cases are considered here for the focal points specified on and off from the central axis of the array transducer. The simulation results show that the focal spot size increases with increasing focal length and focal angle. Furthermore, the maximum velocity amplitude does not always coincide with the specified focal point. Simulation results for the off-axis focusing cases do demonstrate the accurate steering capability of the TR focusing.

• Journal of the Korean Society of Visualization
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• v.8 no.3
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• pp.22-28
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• 2010

In the present study, we performed the velocity field measurement in a microchannel using a focal length variable micro liquid lens. The liquid lens is used as a beam expander in a micro-PIV system to acquire the scatter image of the seeded particle. A thin film-type micro liquid lens was made of PDMS material and it was attached on top of the 700-micron-wide working fluid supply channel trench. As a result, the focal length and contact angle of the liquid lens changed with variations in applied pressure.

• Journal of the Earthquake Engineering Society of Korea
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• v.11 no.6
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• pp.33-39
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• 2007

The seismic source parameters of the Uljin earthquake on 29 May 2004, including focal depth, focal mechanism, magnitude, and moment tensor elements for source characteristics, are analysed using moment tensor seismic source inversion. The Green‘s function for 3 crust models representing the southern Korean Peninsula are used. Also 3 kinds of epicenters are used to find optimum solution for seismic source parameters. Results show that seismic source parameters have a little dependency of azimuthal distribution and epicentral distances of seismic stations. Final results show that the event, considering 6 moment tensor elements, is caused by the typical reverse fault with nearly NS strike. The focal mechanism implies that the tectonic force around epicenter area currently has compressive environment, with nearly EW principal axis. The focal depth is estimated to be about 12km. The resultant focal mechanism show fairly good agreement to those of other studies. However, focal depth is much different from that of other studies.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.21 no.4
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• pp.70-76
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• 2022

Calcium-phosphate-based bioceramics are promising biomaterials for scaffolds because they can assist in bone regeneration. In this study, a laser sintering deposition system was developed, and 3D hydroxyapatite (HA) scaffolds were fabricated. The main process conditions of the HA scaffolds were laser power, table velocity, and laser focal distance. As the laser power increased, the line width, line height, and layer thickness also increased. Further, the line width, line height, and layer thickness decreased as the table velocity increased. As the laser focal distance increased, the line width increased, but the line height and layer thickness decreased. The fabricated green scaffolds were sintered at 1050 ℃ and 1150 ℃. The sintered scaffolds had a uniform and continuous interconnected shape, with pore sizes ranging from 850 to 950 ㎛ having 53% porosity. The compressive strength of the scaffolds decreased from 0.72 MPa (1050 ℃) to 0.53 MPa (1150 ℃). The biocompatibility of the scaffolds was investigated by analyzing the adhesion of osteoblast-like MG-63 cells cultured on the surfaces of the scaffolds. The results indicate that the scaffold sintered at 1050 ℃ had good mechanical and biological properties compared to that at 1150 ℃.

• Journal of Korean Society on Water Environment
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• v.21 no.1
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• pp.79-83
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• 2005

Foraging behaviour of false dace, Pseudorasbora parva, was investigated in water flowing at various velocities with the existence of a cavity for rest. The pursuit comprised three succeeding processes such as, approaching, chasing and attacking. Angles between the fish body and the water flow direction and swimming speeds increased in the latter stages of approaching, chasing and attacking. All pursuit angles, swimming speeds and distances increased with flow velocity and peaked at the flow velocity of 7 cm/sec. At higher velocities, however, the fish avoided the use of much energy against the large drag force. The probability of capture and the feeding rate steadily decreased with increasing flow velocity. Under the fast flow, the fish adjusted their swimming speed to get the optimum velocity relative to the flowing water for the energetic budget. Fish spent more time in the cavity as flow velocity increased to avoid the energy expenditure necessitated by the high velocity.

• Journal of the Earthquake Engineering Society of Korea
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• v.18 no.5
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• pp.213-219
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• 2014

The source parameters of four earthquakes with magnitude larger than 4.0 nearby the offshore Sinan are analyzed. The waveform inversion method is used for analyzing the source parameters of two events (20 August 2012 and 21 April 2013) with C&B and AK135-c crustal velocity structure models. The source parameters of the other two events (26 July 1994 and 23 March 2003) are obtained from references. Focal mechanisms of the events are strike slip faulting or strike slip faulting with a thrust component. The directions of P-axes of the events are ENE-WSW or NE-SW which are similar to previous studies on P-axes in and around the Korean peninsula. With regard to the events nearby the offshore Sinan, the regional stress, rather than being local stress, is seen to be the result of the combination of tectonic forces from the compression of plates colliding to the Eurasian Plate.

• Journal of the Korean Society for Nondestructive Testing
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• v.9 no.2
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• pp.18-24
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• 1989

Computer-aided high frequency ultrasonic is applied to aluminum oxide(85w%, 94w%, 96w%, and 99w%) MOR(modulus of rupture) samples to evaluate mechanical properties such as density variation, pore content, elastic modulus, shear modulus, and poisson's ratio. Ultrasonic wave velocity and attenuation measurement techniques were used as an evaluator of such properties. Pulse-echo C-Scan images with different fate setting method using 50MHz center frequency 1 inch focal length transducer allows evaluation of density variation and pore content. Elastic modulus calculated with the relation of density and ultrasonic velocity. It shows good reliability as compared with resonance method. Sintered density variation of $0.025g/cm^{3}$, that is 0.6% of theoretical density in $Al_{2}O_{3}$ samples can be observed by ultrasonic velocity measurement. Attenuation measurement method qualitatively agree with 4-point fracture testing result concerning of porosity content.

• Journal of the Earthquake Engineering Society of Korea
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• v.24 no.5
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• pp.219-232
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• 2020

The slip-weakening model developed by Ohnaka and Yamashita is extended over the breakdown zone by equating the scaling relationships for the breakdown zone and the whole rupture area. For the extension, the study uses the relationship between rupture velocity and radiation efficiency, which was derived in the theory of linear elastic fracture mechanics, and the definition of f_max given in the specific barrier model proposed by Papageorgiou and Aki. The results clearly show that the extended scaling relationship is governed by the ratio of rupture velocity to S wave velocity, and the velocity ratio can be determined by the ratio of characteristic frequencies of a Fourier amplitude spectrum, which are corner frequency, f_c, and source-controlled cut-off frequency, f_max, or vice versa. The derived relationship is tested by using the characteristic frequencies extracted from previous studies of more than 130 shallow crustal events (focal depth less than 25 km, M_W 3.0~7.5) that occurred in Japan. Under the assumption of a dynamic similarity, the rupture velocity estimated from f_max/f_c and the modified integral timescale give quite similar scale-dependence of the rupture area to that given by Kanamori and Anderson. Also, the results for large earthquakes show good agreement to the values from a kinematic inversion in previous studies. The test results also indicate the unavailability of the spectral self-similarity proposed by Aki because of the scale-dependent rupture velocity and the rupture velocity-dependent f_max/f_c; however, the results do support the local similarity asserted by Ohnaka. It is also remarkable that the relationship between the rupture velocity and f_max/f_c is quite similar to Kolmogorov's hypothesis on a similarity in the theory of isotropic turbulence.