• Ocean Systems Engineering
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• v.8 no.3
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• pp.281-312
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• 2018

Characteristics of a turbulence wind model control the magnitude and frequency distribution of wind loading on floating offshore wind turbines (FOWTs), and an in-depth understanding of how wind spectral characteristics affect the responses, and ultimately the design cost of system components, is in shortage in the offshore wind industry. Wind spectrum models as well as turbulence intensity curves recommended by the International Electrotechnical Commission (IEC) have characteristics derived from land-based sites, and have been widely adopted in offshore wind projects (in the absence of site-specific offshore data) without sufficient assessment of design implications. In this paper, effects of wind spectra and turbulence intensities on the strength or extreme responses of a 5 MW floating offshore wind turbine are investigated. The impact of different wind spectral parameters on the extreme blade loads, nacelle accelerations, towertop motions, towerbase loads, platform motions and accelerations, and mooring line tensions are presented and discussed. Results highlight the need to consider the appropriateness of a wind spectral model implemented in the strength design of FOWT structures.

• The Journal of the Acoustical Society of Korea
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• v.28 no.4
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• pp.328-335
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• 2009

Phase variance of the acoustic signals has to be investigated with the research of the medium, because the phase of the acoustic signals carries the information of the medium. The phase compensation of the received signals is required for the signal processing of SAS (Synthetic Aperture Sonar) and underwater communication. In this paper, the phase variance of the acoustic signals was studied depending on the micro-scale-turbulence of ocean. The turbulence strength of the locally isotropic and homogeneous turbulence was calculated, and the phase variance affected by the turbulence strength was computed along the ray paths. The CTD and ADCP data were acquired from a buoy system near the Mukho port in the East Sea of Korea and the ray paths were calculated by the Bellhop algorithm. As a result, the turbulence strength was mainly determined by the variation of temperature and flow speed, changing the phase variance of the received signals. Hence, we thought the phase variance should be considered in the sonar operating system.

• Journal of the Korean Society of Visualization
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• v.16 no.1
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• pp.21-29
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• 2018

Homogeneous and isotropic turbulence (HIT) with no mean flow is a very useful type of flow for basic turbulence research. However, it is difficult to generate HIT in the lab. In this study, we implemented HIT in a confined box through synthetic jet actuators using sub-woofer speakers. Characteristics of HIT are varied depending on the strength of the jets. We used 2D PIV to measure the velocity field. Turbulence statistics such as homogeneity, isotropy ratio, turbulence kinetic energy, dissipation rate, Taylor microscale, Kolmogorov scale, and velocity correlation coefficient were calculated. Most of the turbulence statistics increased exponentially according to the strength of the jets, and the Taylor Reynolds number reached up to 185.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.10a
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• pp.1132-1135
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• 2008

In this study, a circular and indoor soil tank foundation was manufactured to study the improvement according to the degree of turbulence arising from PBD penetration, using the existing plate-type shoe and improved V-type shoe to change the degree of turbulence. Furthermore, to study the foundation improvement effect, the strength, settlement speed in the turbulence area were compared according to the shoe penetration. The results of the study showed that the V-type shoe reduced the strength coefficient decrease effect, and the foundation improvement effect according to the degree of turbulence was identified.

• Journal of Environmental Science International
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• v.11 no.3
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• pp.155-160
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• 2002

Using measured data at Daegu by tethersonde for the period of 1984∼1987, we have investigated the lower atmospheric boundary layer structure including relationships between inversion layer and meteorological factors(wind and temperature), and the inversion strength and inversion height. The inversion layer was defined from the vertical temperature profile and its strength was analyzed with the wind shear as well as the vertical temperature gradient. From October to January, measured inversion layer isn't destroyed, however, in June, after sun rise, it is destroyed by surface heating and mixed layer is developed from surface. According to Pasquill stability classes, the moderately stable cases dominated. It's the larger vertical temperature gradient the lower SBL height. We have introduced B(bulk turbulence scale) which indicated SBL height. It's larger B, the higher SBL height and vice versa. It was noted that the bulk turbulence scale (B) is appropriate to determine the stable boundary layer height.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.86.1-86.1
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• 2011

Most astrophysical systems are turbulent and magnetized. Magnetic field plays an important role in the dynamics of ISM and influence all of properties of astrophysical system. Information of magnetic field is very important to understand properties of astrophysical systems. For example, one way to obtain information of magnetic field is to use Rotation Measure. Mean strength of the magnetic field along the line of sight can be estimated from RM/DM. (where RM is rotation measure, DM is dispersion measure) For the estimation of magnetic field strength using RM/DM, the correlation between density and magnetic field is very important. When there is no correlation between density and magnetic field the relation gives exact mean magnetic field strength. But, if the correlation is positive, it overestimates the magnetic field strength, while if the correlation is negative, it underestimate the strength. We calculate correlation between density and magnetic field in compressible MHD turbulence.

• Journal of The Korean Astronomical Society
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• v.34 no.4
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• pp.321-323
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• 2001

Compressible, magnetohydrodynamic (MHD) turbulence in two dimension is studied through high-resolution, numerical simulations with the isothermal equation of state. First, hydrodynamic turbulence with Mach number $(M)_{rms}\;\~$1 is generated by enforcing a random force. Next, initial, uniform magnetic field of various strengths with Alfvenic Mach number Ma $\gg$ 1 is added. Then, the simulations are followed until MHD turbulence is fully developed. Such turbulence is expected to exist in a variety of astrophysical environments including clusters of galaxies. Although no dissipation is included explicitly in our simulations, truncation errors produce dissipation which induces numerical resistivity. It mimics a hyper-resistivity in our second-order accurate code. After saturation, the resulting flows are categorized as SF (strong field), WF (weak field), and VWF (very weak field) classes respectively, depending on the average magnetic field strength described with Alfvenic Mach number, $(Ma)_{rms}{\ge}1$, $(Ma)_{rms}{\~}1$, and $(Ma)_{rms}{\gg}1$. The characteristics of each class are discussed.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.49.2-49.2
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• 2019

Clusters of galaxies shape up through a series of hierarchical mergers. It is believed that major mergers lead to cluster-wide shock waves, which are manifested as radio relics. The 1RXS J0603.0+4213 and CIZA J2242.8+5301 clusters, for instance, contain Mpc-size giant radio relics in the outskirts. Synchrotron emission from these radio relics reveals the presence of relativistic electrons and the magnetic fields of a few ${\mu}G$ strength. The presence of such magnetic fields in the ICM has been explained by the so-called small-scale turbulent dynamo. To get quantitative measures for magnetic fields in clusters of galaxies, we investigate the development of turbulence and the follow-up amplification of magnetic fields through three-dimensional numerical magnetohydrodynamical (MHD) simulations. The turbulence is induced in highly stratified cluster media, and driven sporadically by major mergers. We here present the results, aiming to answer whether the turbulence dynamo scenario can explain the observed strength and scale of magnetic fields in clusters. Also, we discuss whether the observed properties of giant radio relics can be reproduced in our simulations.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.75.2-75.2
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• 2010

We have studied the statistical properties of turbulence in molecular clouds identified in the Boston University - Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS). Toward this end, the probability density function (PDF) and velocity distribution were measured for about 50 molecular clouds. We found there exists a good correlation between the PDF width and the velocity dispersion for these molecular clouds. In order to investigate how general properties of astrophysical turbulence depends on the plasma parameters such as magnetic field strength and sonic Mach number, we performed three-dimensional MHD simulations. We then examined if the observed characteristics of interstellar turbulence are consistent with theoretical results from MHD simulations.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.35.2-35.2
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• 2018

Magnetic fields in clusters of galaxies play a critical role in shaping up the intracluster medium. Their existence has been established through observations of synchrotron emission, especially from radio relics and halos, as well as observations of rotation measure. In the so-called Sausage relic, which is one of Mpc-size giant radio relics detected in the outskirts of merging clusters, for instance, the magnetic fields are believed to have a few ${\mu}G$ strength and a Mpc scale. The observed magnetic fields are conjectured to be produced by the process of small-scale turbulence dynamo. To investigate the dynamo origin, we simulate the development of turbulence and the follow-up amplification of magnetic fields in galaxy clusters using a three-dimensional magnetohydrodynamical(MHD) code. Turbulence is induced in highly stratified backgrounds expected in clusters, and driven sporadically mimicking major mergers. We here present preliminary results, aiming to answer whether the turbulence dynamo scenario can explain observed magnetic fields in clusters of galaxies.