• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.23 no.7
• /
• pp.937-944
• /
• 1999

The near-wall flow structures of turbulent boundary layer over riblets having semi-circular grooves were investigated experimentally for the drag decreasing ($s^+=25.2$) and drag increasing ($s^+=40.6$) cases. The field of view used for tho velocity field measurement was $6.75{\times}6.75mm^2$ in physical dimension, containing two grooves. One thousand instantaneous velocity fields over the riblets were extracted for each case of drag increase and decrease. For comparison, five hundreds instantaneous velocity fields over a smooth flat plate were also obtained under the same flow conditions. To see the global flow structure qualitatively, the flow visualization was also performed using the synchronized smoke-wire technique. For the drag decreasing case ($s^+=25.2$), most of the streamwise vortices stay above the riblets, interacting with the riblet tips. The high-speed in-rush flow toward the riblet surface rarely influences the flow inside tho riblet valleys submerged in the viscous sublayer. The riblet tips seem to impede the spanwise movement of the longitudinal vortices and induce secondary vortices. The turbulent kinetic energy in the riblet valley is sufficiently small to compensate the increased wetted area of the riblets. In addition, in the logarithmic region, the turbulent kinetic energy are small or almost equal to that of a smooth flat plato. For the drag increasing case ($s^+=40.6$), however, the streamwise vortices move into the riblet valley freely, interacting directly with the riblet inner surface. The penetration of the high-speed in-rush flow on the riblets increases tho skin-friction. The turbulent kinetic energy is increased in the riblet valleys and even in the outer region compared to that over a flat plate.

• Nuclear Engineering and Technology
• /
• v.42 no.6
• /
• pp.670-679
• /
• 2010

In this paper, a new approach using a pixel-based correction method was developed to fix the non-uniform responses of flat-bed type scanners used for radiochromic film dosimetry. In order to validate the method's performance, two cases were tested: the first consisted of simple dose distributions delivered by a single port; the second was a complicated dose distribution composed of multiple beams. In the case of the simple individual dose condition, ten different doses, from 8.3 cGy to 307.1 cGy, were measured, horizontal profiles were analyzed using the pixel-based correcton method and compared with results measured by an ionization chamber and results corrected using the existing correction method. A complicated inverse pyramid dose distribution was made by piling up four different field shapes, which were measured with GAFCHROMIC$^{(R)}$EBT film and compared with the Monte Carlo calculation; as well as the dose distribution corrected using a conventional method. The results showed that a pixel-based correction method reduced dose difference from the reference measurement down to 1% in the flat dose distribution region or 2 mm in a steep dose gradient region compared to the reference data, which were ionization chamber measurement data for simple cases and the MC computed data for the complicated case, with an exception for very low doses of less than about 10 cGy in the simple case. Therefore, the pixel-based scanner correction method is expected to enhance the accuracy of GAFCHROMIC$^{(R)}$EBT film dosimetry, which is a widely used tool for two-dimensional dosimetry.

• Korean Journal of Remote Sensing
• /
• v.26 no.5
• /
• pp.497-510
• /
• 2010

In this study, comparisons of the backscattering coefficients and the coherence values which had been extracted from SAR (Synthetic Aperture Radar) images such as JERS-1, ENVISAT and ALOS satellites with surface roughness, surface geometric and soil moisture content were carried out. As the results of analysis using the backscattering coefficient and coherence values from SAR images, the coherence was shown high in the region containing more of mud fraction due to higher viscosity of fine grain-size. A lot of tidal channels were well developed in the Ganghwa tidal flat, affecting the drainage of seawater and subsequent soil moisture content by exposure time of tidal flat. The backscattering coefficient. consequently, appeared to be lower in sand flat and mix flat with decrease of soil moisture. In contrast, most mud flats were distributed at high elevation so that soil moisture was not much influenced by seawater. The backscattering coefficient in mud flat seemed to have a relationship with the density of tidal channel. In addition, lowering backscattering coefficients in the all Ganghwa tidal flat was observed when surface remnant water increased according to the amount of rainfall. The correlation between backscattering coefficient, coherence and sediment environment factors in the Ganghwa tidal flat was investigated. In the future, more quantitative spatial analysis will be helpful to well understand the sedimentary influence of various sediment environment factors.

• 한국해양학회지
• /
• v.24 no.4
• /
• pp.184-193
• /
• 1989

An investigation on the soft-bottom macrobenthic community in the inner Kyeonggi Bay was conducted in July, October of 1987 and February, May of 1988. The benthic fauna comprised a total of 87 species and showed mean density of 550 indiv./$m^2$. Major faunal groups are polychaetes, molluks, and crustaceans, corresponding to 64.7%, 17.4%, and 15.2% in total abundance respectively. The most abundant species was Tharyx sp. (10.9%), and followed by Mactra veneriformis (10.5%), Mediomastus sp. (8.6%), and Glycinde sp. (8.0%) etc.. The important species in the intertidal zone were Mactra veneriformis, Hinia festiva, and Nephtys californiensis, whereas those in subtidal channel region were Tharyx sp., mediomastus sp., Sternaspis scutata, and Nephtys polybranchia. The study area could be divided into 4 station groups based on the species composition; two groups within interidal flat and two in channel regions. Each station group sustained its specific benthic faunal assemblage; Mactra-Hinia, Aricidea-Haploscoloplos assemblages in the intertidal flat and Harpiniopsis-Urothoe, Mediomastus-Sternaspis assemblages in the subtidal channel region. The Mediomastus-Sternaspis assemblage showed the highest species number and population density, and also did the highest diversity and evenness indices. The Harpiniopsis-Urothoe assemblage showed the poor species composition and abundance, and so recorded the highest dominance index. Two assemblages in the intertidal flat had the intermediate values in all the biological parameters. The northern channel region of zone S-2 showed the high abundance in species composition, which seems to be caused by the heterogeneous sediment and the organic enrichment from the Han River and the Incheon industrial complex.

• Journal of the Korean Geographical Society
• /
• v.45 no.2
• /
• pp.275-292
• /
• 2010

This study investigated the land-cover changes in the tidal flat of the Taean peninsula due to man-made environmental changes between 1972 and 2008, through time-series analysis based on a modified post-classification comparison method and multi-temporal satellite images. The analysis revealed that the land-cover of the tidal flat has changed from tidal flat to wetland and from wetland to paddy field between 1972 and 2008. Also, the pattern of detailed land-cover changes is as follows: tidal flat to wetland; lake and saltpan to bare land and paddy field. The accurate classification of each image is needed for the application of the post-classification comparison method. The overall accuracy of the classified images was found to be 95.33% on average, and the Kappa value was 0.941 on average.

• Journal of Environmental Impact Assessment
• /
• v.22 no.5
• /
• pp.409-426
• /
• 2013

The purpose of this study is to present the raw data for management and conservation of tidal flat by objective surveying and analysing the halophytes and vegetation distributed in Seocheon tidal flat wetland conservation area, Korea. The results are as follows. The numbers of halophytes in this site were summarized as 27 taxa including 13 families, 21 genera, 26 species and 1 variety. In the results of ecologically important species, rare plant was 1 taxa, 10 taxa of the specific plants by floristic region, 1 taxa of naturalized plant and 1 taxa of the plant with approval for delivering oversea. The life form spectrum consisted of therophytes(44.4%), hemicryptophytes(25.9%), geophytes(14.8%), nanophanerophytes(7.5%), chamaephytes and hydatophytes(each 3.7%). The types of vegetation of Seocheon tidal flat wetland conservation area were classified with 17 communities including Vitex rotundifolia community, Suaeda maritima community, Calystegia soldanella community and so forth. In the halophytes composition, section C and E-1 had the largest character species and companion species. In the results of vegetation amount, section C, D, E-1 and E-2 were the highest score, on the other hand, section A and B were the lowest. The final rating was calculated by adding up values of two factors, and section C and E-1 had the highest rating of II. In future, we will survey the whole flora in Seocheon tidal flat, we will offer the help to establishing the conservation plan of coastal plant ecosystem in West Sea.

• Wind and Structures
• /
• v.35 no.5
• /
• pp.309-322
• /
• 2022

Due to the complex terrain around high-speed railways, the windbreaks were established along different landforms, resulting in irregular windbreak transition regions between different subgrade infrastructures (flat ground, cutting, embankment, etc). In this paper, the effect of a windbreak transition on the wind flow around railways subjected to crosswinds was studied. Wind tunnel testing was conducted to study the wind speed change around a windbreak transition on flat ground with a uniform wind speed inflow, and the collected data were used to validate a numerical simulation based on a detached eddy simulation method. The validated numerical method was then used to investigate the effect of the windbreak transition from the flat ground to cutting (the "cutting" is a railway subgrade type formed by digging down from the original ground) for three different wind incidence angles of 90°, 75°, and 105°. The deterioration mechanism of the flow fields and the reasons behind the occurrence of the peak wind velocities were explained in detail. The results showed that for the windbreak transition on flat ground, the impact was small. For the transition from the flat ground to the cutting, the influence was relatively large. The significant increase in the wind speeds was due to the right-angle structure of the windbreak transition, which resulted in sudden changes of the wind velocity as well as the direction. In addition, the height mismatch in the transition region worsened the protective effect of a typical windbreak.

• 한국정보디스플레이학회:학술대회논문집
• /
• 2005.07b
• /
• pp.1284-1287
• /
• 2005

Peripheral pentacene region gives a significant influence on C-V characteristics of metal-oxide-pentacene capacitor structure. When the gate voltage goes toward negative, the effect of peripheral pentacene region becomes larger. Remaining gate DC bias constant and changing small signal frequency, the capacitance of peripheral pentacene changes along with frequency so that the total capacitance value also changes. The influence of peripheral pentacene region should be removed to measure accurate C-V characteristics, because it is hard to take into account the effect of the region quantitatively. After removing the influence of peripheral pentacene region, acceptor concentration, flat band voltage and depletion width of pentacene thin film are extracted from an accurate C-V curve as $1.58{\times}10^{17}cm^{-3}$, -1.54 V and 39.4 nm, respectively.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.29 no.2 s.233
• /
• pp.279-286
• /
• 2005

This study presents experimental results on the heat transfer coefficients in the film boiling region of spray cooling for actual metallurgical process. In this study, the heat flux distributions of a two dimensional dilute spray impinging on a hot plate were experimentally investigated. A stainless steel block was cooled down from intial temperature of about $800^{\circ}C$ by twin fluid (air-water) flat spray. It was found from the experimental results that the heat transfer area was classified into the stagnation region and wall-flow region. In the stagnation region, the experimental data of local heat transfer coefficient was closely correlated with the local droplet-flow-rate supplied from the spray nozzle directly. Thus, the local heat transfer coefficients are in good agreement with the predicted values from the correlations proposed by our previous study. In wall-flow region, however, remarkable differences are observed between experimental data and predicted values because the number of rebound droplets increase with increasing the distance from the stagnation point.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.32 no.10
• /
• pp.1-11
• /
• 2004

An experimental study was carried out to investigate the influence of flow conditions of a boundary layer on the near-wake of a flat plate. Various attaching positions of tripping wires were selected to change flow conditions on a boundary layer. Laminar, transitional, and turbulent boundary layer conditions at 0.98C from the leading edge are imposed to investigate the evolution of symmetric and asymmetric wake. An x-type hot-wire probe(55P61) is employed to measure at 8 stations of the near-wake region. Measured mean velocity distributions are presented in terms of similarity parameter. It is found that the symmetric wake collapses well to the universal profile in the central part of the wake. However, the universal profile is not suitable in describing an asymmetric wake.