• Ocean and Polar Research
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• v.32 no.1
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• pp.53-61
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• 2010

The distribution of coral reefs can be an indicator of environmental or anthropogenic impacts. Here, we present a habitat map of coral reefs developed using high-spatial satellite images. The study area was located on the north-eastern part of Weno island, in the Chuuk lagoon of Federated States of Micronesia. Two fieldwork expeditions were carried out between 2007 and 2008 to acquire optical and environmental data from 121 stations. We used an IKONOS image obtained in December 2000, and a Kompsat-2 image obtained in September 2008 for the purpose of coral reef mapping. We employed an adapted version of the object-based classification method for efficient classification of the high-spatial satellite images. The habitat map generated using Kompsat-2 was 72.22% accurate in terms of comparative analysis with in-situ measurements. The result of change detection analysis between 2000 and 2008 showed that coral reef distribution had decreased by 6.27% while seagrass meadows had increased by 8.0%.

• International Journal of Naval Architecture and Ocean Engineering
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• v.7 no.3
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• pp.595-609
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• 2015

The wake characteristics of Contra-Rotating Propeller (CRP) were investigated using numerical simulation and flow measurement. The numerical simulation was carried out with a commercial CFD code based on a Reynolds Averaged Navier-Stokes (RANS) equations solver, and the flow measurement was performed with Stereoscopic Particle Image Velocimetry (SPIV) system. The simulation results were validated through the comparison with the experiment results measured around the leading edge of rudder to investigate the effect of propeller operation under the conditions without propeller, with forward propeller alone, and with both forward and aft propellers. The evolution of CRP wake was analyzed through velocity and vorticity contours on three transverse planes and one longitudinal plane based on CFD results. The trajectories of propeller tip vortex core in the cases with and without aft propeller were also compared, and larger wake contraction with CRP was confirmed.

• Journal of Ocean Engineering and Technology
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• v.33 no.1
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• pp.26-32
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• 2019

The vortex flow behind a bluff body has been a subject of interest for a very long time because of its engineering applicability such as to vortex induced vibration. In the near wake of a bluff body, vortices are periodically shed in two shear layers, which originate in the trailing edges. The far wake is made up of the classical Karman vortices, which are connected together by streamwise and spanwise vortices. These vortex formations have been studied in many experimental and numerical ways. However, most of the studies considered non-cavitating flow. In this study, we investigated cavitating flow in the wake of a two-dimensional wedge. Experiments were conducted in a cavitation tunnel of Chungnam National University. Using a particle image velocimetry (PIV), we measured the velocity fields under two different flow conditions: non-cavitating and cavitating regimes. We also investigated the vortex shedding frequencies using an absolute pressure transducer mounted on the top of the test window. Throughout the experiments, it was found that the shedding frequency of the vortex was strongly affected by cavitation, and the Strouhal number could exceed its value in the non-cavitating regime.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.58 no.1
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• pp.68-74
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• 2022

This study analyzed the wake characteristics of the rim-driven propeller (RDP) used in an underwater robot. For underwater robots to perform specific missions, not only propulsion characteristics but also wake characteristics must be considered. In this study, a blade was designed based on NAC 0012 with a symmetrical cross-section. The RDP was hubless with three or four blades. The influence of both the free water surface and the bottom was considered, and the wake was measured using a particle image velocimetry in the advance ratio of 0.2 to 1. Model 1 showed symmetrical wakes in the entire advance ratio section. Model 2 showed asymmetric wakes due to the influence of the free water surface and the bottom at low advance ratio.

• Journal of the Korean Association of Geographic Information Studies
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• v.16 no.1
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• pp.36-46
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• 2013

The Geostationary Ocean Color Imager (GOCI), the first geostationary ocean color observation instrument launched in 2010 on board the Communication, Ocean, and Meteorological Satellite (COMS), has been generating the operational level 1 data. This study describes a methodology for creating the GOCI true color image and data processing software, namely the GOCI RGB maker. The algorithm uses a generic atmospheric correction and reprojection technique to produce the color composite image. Especially, the program is designed for educational purpose in a way that the region of interest and image size can be determined by the user. By distributing software to public, it would maximize the understanding and utilizing the GOCI data. Moreover, images produced from the geostationary observations are expected to be an excellent tool for monitoring environmental changes.

• Korean Journal of Remote Sensing
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• v.26 no.3
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• pp.297-304
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• 2010

Since the change in Doppler centroid according to moving targets brings alteration to the phase in azimuth differential signals of synthetic aperture radar (SAR) data, one can measure the velocity of the moving targets using this effect. In this study, we will investigate theoretically measuring the velocity of an object from azimuth differential signals by using range compressed data which is the interim outcome of treatment from the simulated SAR raw data of moving targets on the background of sea clutter. Also, it will provide evaluation for the elements that affect the estimation error of velocity from a single SAR sensor. By making RADARSAT-1 simulated image as a specific case, the research includes comparisons for the means of velocity measurement classified by the directions of movement in the four following cases. 1. A case of a single target without currents, 2. A case of a single target with tidal currents of 0.5 m/s, 1 m/s, and 3 m/s, 3. A case of two targets on a same azimuth line moving in a same direction and velocity, 4. A case of a single target contiguous to land where radar backscatter is strong. As a result, when two moving targets exist in SAR image outside the range of approximately 256 pixels, the velocity of the object can be measured with high accuracy. However, when other moving targets exist in the range of approximately 128 pixels or when the target was contiguous to the land of strong backscatter coefficient (NRCS: normalized radar cross section), the estimated velocity was in error by 10% at the maximum. This is because in the process of assuming the target's location, an error occurs due to the differential signals affected by other scatterers.

• Journal of Ocean Engineering and Technology
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• v.32 no.1
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• pp.28-35
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• 2018

An analytical method for predicting the velocity potential around a pulsating bubble close to a free or rigid wall was established using an image method. Because the velocity potential should satisfy two boundary conditions at the bubble surface and rigid wall, we investigated the velocity in the normal direction at the two boundaries by adding the image bubbles. The potential was analyzed by decomposing the bubble motion as two independent motions, pulsation and translation, and we found that when the number of image bubbles was greater than ten, the two boundary conditions were satisfied for the translation term. By adding many image bubbles after the approximation of the pulsation term, we also confirmed that the boundary condition at the wall was satisfied.

• Journal of the Korean earth science society
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• v.24 no.5
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• pp.467-476
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• 2003

For the seafloor acoustic image mapping of side scan sonar, the beginning step of the procedure is to fix the absolute sonar (tow-fish) position since the sonar is not hull mounted but towed astern. The technical algorithm used to calculate the actual sonar position without any other additional sub-system, i.e., the underwater acoustic position tracking system or the sonar attitude measuring device, was proposed. In the seafloor image mosaic mapping results using the sonar track (not ship track) developed in this study, any ambiguity or inconsistency of seafloor features was not found. The incidental effect from the sonar position determination procedure orients the towing direction of sonar to be smooth, consequently the swath pattern on the across-track direction becomes stable and the blanking phenomenon of the insonification area is reduced conspicuously. This technical method is considered to be an useful tool when applied toother underwater towing vehicle surveys.

• Journal of the Optical Society of Korea
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• v.5 no.1
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• pp.9-15
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• 2001

The ocean Scanning Multi-spectral Imager (OSMI) is a payload on the KOrea Multi-Purpose SATellite (KOMPSAT) to perform worldwide ocean color monitoring f the study of biological oceanography. OSMI performs solar and dark calibrations for on-orbit instrument calibration. The purpose of the solar calibration is to monitor the degradation of imaging performance for each pixel of 6 spectral bands and to correct the degradation effect on OSMI image during the ground station date processing. The design, the operation concept, and the radiometric characteristics of the solar calibration are investigated. A linear model of image response and a solar calibration radiance model are proposed to study the instrument characteristics using the solar calibration data. The performance of spectral responsivity and spatial response uniformity. The first solar calibration data and the analysis results are important references for further study on the on-orbit stability of OSMI response during its lifetime.

• International Journal of Naval Architecture and Ocean Engineering
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• v.5 no.3
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• pp.404-413
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• 2013

In the present study, a towed underwater particle image velocimetry (PIV) system was validated in uniform flow and used to investigate the free-surface effects on the turbulent wake of a simple surface-piercing body. The selected test model was a cylindrical geometry formed by extruding the Wigley hull's waterplane shape in the vertical direction. Due to the constraints of the two-dimensional (2D) PIV system used for the present study, the velocity field measurements were done separately for the vertical and horizontal planes. Using the measured data at several different locations, it was possible to identify the free-surface effects on the turbulent wake in terms of the mean velocity components and turbulence quantities. In order to provide an accuracy level of the data, uncertainty assessment was done following the International Towing Tank Conference standard procedure.