• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.3
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• pp.346-359
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• 1998

The aerodynamic forces and wake structure of the non-rotating downstream circular cylinder, of which the uniform freestream flow is interfered with another spinning upstream cylinder having the same diameter that is located upstream in a line have been investigated experimentally. When the spin rate of the downstream cylinder defined as the ratio of tangential surface velocity of the spinning cylinder to the freestream velocity increases gradually from zero to 1.4, the change of surface pressure distribution, aerodynamic forces of the non-rotating downstream cylinder were measured in case of several distance ratios of 1.5, 3.0, and 4.5 defined as the ratio of distance between the centers of two cylinders to the diameter. The wake flow patterns behind the cylinder were also investigated in each case. From the present experiments, it has been found that the spin rate significantly influences the aerodynamic forces and near-wake flow phenomena of the downstream cylinder in such a way that the drag increases as the spin rate and distance ratio increase and the wake width increases as the distance ratio increases.

• Journal of Korean Society on Water Environment
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• v.28 no.3
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• pp.418-425
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• 2012

Dam construction in a river can change its hydrological pattern and trap sediments, which results in ecological changes in the downstream. It is a common phenomenon in the downstream of dams to have decreased sediment flow and increased periphyton. Artificial floods and sediment application are suggested as mitigation practices in order to simulate natural process of flood; transporting sediment and sloughing periphyton off. In this study the effects of artificial floods on periphyton were examined by applying sand artificially and discharging water from a dam (Yangyang Dam, Korea). The study area has been suffering from turbidity problems caused by shore erosion of the dam. The accumulation of inorganic sediments and increase of periphyton on the river bottom are the major factors of habitat deterioration in the downstream reaches. Artificial flood and artificial addition of sand was performed in summer and the effects were measured. Piles of applied sands were washed off easily by discharge and it enhanced the periphyton sloughing effect. The removal efficiency of periphyton was 50 ~ 80% within the 2 km reach from the dam. In conclusion artificial floods and sand application can be a good mitigation measure for the habitat rehabilitation after a dam construction in streams.

• Korean Journal of Ecology and Environment
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• v.42 no.4
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• pp.465-475
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• 2009

The purpose of the study was to evaluate spatial and temporal effects of wastewater treatment plants (WWTPs) on the water quality of downstreams (Tan Stream, TS; Daemyeong Stream, DS; Gwangju Stream, GS, and Kap Stream, KS) located in four major watersheds along with impact analysis of nutrient enrichments on the WWTPs during 2004~2008. In the four streams, seasonal means of BOD, COD, TN, and TP were significantly (p<0.01) greater in the downstreams ($D_s$) than the upstreams ($U_s$). The removal effect of nutrients (nitrogen, and phosphorus) from the WWTPs was much less than the BOD, indicating a greater nutrient impact on the downstreams. Seasonal dilution of organic matter, based on BOD, during the summer monsoon of July~September was most pronounced in the downstreams of all four watersheds. However, mean TN in the downstreams during the monsoon varied little in all four streams. Regression analysis of TN in the downstreams against TN from the WWTPs showed that in the TS, and DS regression slopes in the upstreams were similar to the slopes of downstream but there was a significant difference in the GS (p<0.001) and KS (p<0.01). Tan-Stream WWTP showed low removal efficiency of BOD and COD concentrations, compared to the nutrients, whereas, two WWTPs of Gwangju and Kap Stream had low removal effects in TN and TP. Regression analysis of TN and BOD in the downstreams showed that they was closely related (p<0.01) with stream water volume only in the GS. Our data analysis suggests that greater treatment efficiencies of phosphorus and nitrogen from the WWTPs may improve the downstream water quality.

• Wind and Structures
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• v.29 no.6
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• pp.417-430
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• 2019

To study the wake influence of an upstream bridge on the wind-resistance performance of a downstream bridge, two adjacent long-span cable-stayed bridges are taken as examples. Based on wind tunnel tests, the static aerodynamic coefficients and the dynamic response of the downstream bridge are measured in the wake of the upstream one. Considering different horizontal and vertical distances, the flutter derivatives of the downstream bridge at different angles of attack are extracted by Computational Fluid Dynamics (CFD) simulations and discussed, and the change in critical flutter state is further studied. The results show that a train passing through the downstream bridge could significantly increase the lift coefficient of the bridge which has the same direction with the gravity of the train, leading to possible vertical deformation and vibration. In the wake of the upstream bridge, the change in lift coefficient of the downstream bridge is reduced, but the dynamic response seems to be strong. The effect of aerodynamic interference on flutter stability is related to the horizontal and vertical distances between the two adjacent bridges as well as the attack angle of incoming flow. At large angles of attack, the aerodynamic condition around the downstream girder which may drive the bridge to torsional flutter instability is weakened by the wake of the upstream bridge, and the critical flutter wind speed increases at this situation.

• Journal of Environmental Science International
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• v.7 no.3
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• pp.349-360
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• 1998

Effects of uniform flow on a two-dimensional mesoscale horizontal convection were investigated by using the vorticity and thermodynamic equations. For thins purpose, We simulated properties of a thermal convection m a stably stratified Boussinesq flued caused by partial heating at the center of a lower boundary If we don't consider effects of the uniform flow, the convection takes the form of aidsymmetrlc with respect to the z-alds. But when uniform flow Is strong, velocity field and temperature field consist of a sin91e cell structure which spreads upstream side of the partial heating area. The flow pattern for strong uniform flows takes the form of positive temperatue near the ground and negative temperature perturbation soft over the partial heating area, and downward motion directly over the upwind portion of the partial heating area and upward motion on the downstream side. The downstream edge of the upstream cell Is shifted in the downstream direction with the Increase of uniform flow almost linearly.

• 한국전산유체공학회:학술대회논문집
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• 1999.11a
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• pp.181-186
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• 1999

The effects of the frequency of upstream gust on the unsteady boundary characteristics on a downstream blade was simulated by using a Navier-Stokes code. The Navier-Stokes code is based on an unstructured finite volume method and uses a low Reynolds k-e turbulence model to close the momentum equations. The MIT flapping foil experiment set-up is used to simulate the interaction between the upstream wake and a blade. The frequency of the upstream wake is simulated by varying rate of pitching motion of the flapping airfoils. Three reduced frequencies. 3.62. 7.24. and 10.86. are simulated. As the frequency increases, the unsteady fluctuation on the surfaces of the downstream hydrofoil is shown to decrease while the upstream flapper wake has larger first harmonics of y-velocity component. The unsteady vortices are shown to interact with each other and. as a result. the upstream wake becomes undiscernible inside the inner layer. The turbulence kinetic energy shows a similar behavior. Limiting streamlines around the trailing edge of the flapper are shown to conform with the unsteady Kutta condition for a round trailing edge. while limiting streamlines around the trailing edge of the hydrofoil conforms with the unsteady Kutta condition for a sharp edge.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.113-118
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• 2000

Experimental results describing the effects of blowing ratio on film cooling from two rows of holes with opposite orientation angles are presented. The inclination angle was fixed at $35^{\circ}$ and the orientation angles were set to be $45^{\circ}$ for downstream row. and $-45^{\circ}$ for upsream row. The studied blowing ratios were 0.5, 1.0 and 2.0. The boundary layer temperature distributions were measured using thermocouple at two downstream loundary layer temperature distributions were measured using thermocouple at two downstream locations. Detailed adiabatic film cooling effectiveness and heat transfer coefficient distributions were measured with TLC(Thermochromic Liquid Crystal). The adiabatic film cooling effectiveness and heat transfer coefficient distributions are discussed in connection with the injectant behaviors inferred from the boundary layer temperature distributions. Film cooling performance, represented by heat flux was calculated with the adiabatic film cooling effectiveness and heat transfer coefficient data.

• 유체기계공업학회:학술대회논문집
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• 2005.12a
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• pp.311-317
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• 2005

Measurements of the leakage flow in the shrouded cavity were performed in axial compressor cascades at $Re=2.6{\times}10^5$. This paper describes the effects of the leakage flow tangential velocity on kinematics of the leakage flow in the shrouded cavity and consequent overall loss and exit flow turning at stator blade row downstream. Flow data and flow visualization images consistently indicate that leakage flow circumferentially migrates 2, 4 and 5 blade passages in the direction of rotation for ${\upsilon}_y/c=0.09$, 0.35 and 0.45, respectively where ${\upsilon}_y$ is the leakage tangential velocity and c is the mainstream velocity. The leakage flow contracts to a jet across the seal-tooth resulting in an increase in the leakage axial velocity-doubling the leakage axial velocity in upstream cavity compared to that in the downstream cavity. Consequently, two flow regions are distinguished before and after the seal-tooth. As increasing the leakage tangential velocity, the overall loss downstream of stator blade row decreases and the exit flow turning in the range of span. from the hub endwall to 15% increases while the decreases in the flow turning from 15% to 30% span is observed.

• Korean Journal of Ecology and Environment
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• v.43 no.1
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• pp.24-34
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• 2010

This study was carried out to analyze some influences on ecological health conditions, threaten by various stressors such as physical, chemical and biological parameters. We collected samples in 2008 from three zones of upstream, midstream and downstream, Gap Stream. We applied multi-metric fish assessment index (MFAI), based on biotic integrity model to the three zones along with habitat evaluations based on Qualitative Habitat Evaluation Index (QHEI). We also examined fish fauna and compositions, and analyzed relations with MFAI values, QHEI values, and various guild types. Chemical parameters such as oragnic matter (BOD, COD), nutrients (TP, $NH_3$-N), coli-form number (as MPN), and suspended solids (SS) were analyzed to identify the relationship among multiple stressor effects. Using the sentinel species of Zacco platypus, the population structures and condition factors were analyzed along with DNA damages related with genotoxicant effects by comet assay. This study using all these parameters showed that stream condition was degraded along the longitudinal gradient from upstream to downstream, and the downstream, especially, was impacted by nutrient enrichment and toxicant exposure from the point source, wastewater treatment plant. Overall results indicated that our approaches applying various parameters may be used as a cause-effect technique in the stream health assessments and also used as a pre-warning tool for diagnosis of ecological degradation.

• Journal of The Geomorphological Association of Korea
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• v.26 no.1
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• pp.15-26
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• 2019

In this study, we investigated the grain size, lithological type, and shape of coarse bed materials in the upper and middle reaches of Gongneungcheon River. For this purpose, 11 sampling points were selected along the river. For 100 samples of the coarse bed materials at each point, three axes (long, intermediate, and short) of samples were measured, and their lithological types were also identified. By measuring grain size, the sphericity and flatness of samples were calculated. Finally, every particle was classified into four shape categories: sphere, disc, blade and rod. We found that the grain size in incised meandering reach is the largest. This is mainly due to the supply of coarse materials from steep valley sides along the meandering channel. According to the lithological analysis, all samples were identified as granite, gneiss and schist, and quartz. The proportion of granite decreased, whereas the proportion of gneiss and schist increased downstream. These patterns indicate that the bedrock distribution within the study area accounts for the downstream lithological variation of coarse bed materials. With regard to the grain shape, sphericity gradually decreased while flatness gradually increased downstream. In the case of the shape classification, unlike the general downstream pattern of grain shape, the proportion of the sphere type decreased and the proportion of the blade type increased downstream. Such a reversal change in the downstream direction turns out to be determined by the lithology (such as foliation, bedding and the pattern of weathering) of coarse bed materials.