• 대한조선학회논문집
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• 제51권4호
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• pp.311-320
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• 2014

Test uncertainty of a towed underwater Stereoscopic Particle Image Velocimetry (SPIV) system was assessed in a towing tank. To estimate the systematic error and random error of mean velocity and turbulence properties measurement, velocity field of uniform flow was measured. Total uncertainty of the axial component of mean velocity was 1.45% of the uniform flow speed and total uncertainty of turbulence properties was 3.03%. Besides, variation of particle displacement was applied to identify the change of error distribution. In results for variation of particle displacement, the error rapidly increases with particle movement under one pixel. In addition, a nominal wake of a model ship was measured and compared with existing experimental data by five-hole Pitot tubes, Pitot-static tube, and hot wire anemometer. For mean velocity, small local vortex was identified with high spatial resolution of SPIV, but has serious disagreement in local maxima of turbulence properties due to limited sampling rate.

• Wind and Structures
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• 제30권6호
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• pp.617-631
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• 2020

Wake-induced aerodynamics of yawed circular cylinders with smooth and grooved surfaces in a tandem arrangement was studied. This pair of cylinders represent sections of stay-cables with smooth surfaces and high-voltage power conductors with grooved surfaces that are vulnerable to flow-induced structural failure. The study provides some insight for a better understanding of wake-induced loads and galloping problem of bundled cables. All experiments in this study were conducted using a pair of stationary section models of circular cylinders in a wind tunnel subjected to uniform and smooth flow. The aerodynamic force coefficients and vortex-shedding frequency of the downstream model were extracted from the surface pressure distribution. For measurement, polished aluminum tubes were used as smooth cables; and hollow tubes with a helically grooved surface were used as power conductors. The aerodynamic properties of the downstream model were captured at wind speeds of about 6-23 m/s (Reynolds number of 5×10⁴ to 2.67×10⁵ for smooth cable and 2×10⁴ to 1.01×10⁵ for grooved cable) and yaw angles ranging from 0° to 45° while the upstream model was fixed at the various spacing between the two model cylinders. The results showed that the Strouhal number of yawed cable is less than the non-yawed case at a given Reynolds number, and its value is smaller than the Strouhal number of a single cable. Additionally, compared to the single smooth cable, it was observed that there was a reduction of drag coefficient of the downstream model, but no change in a drag coefficient of the downstream grooved case in the range of Reynolds number in this study.

• 대한기계학회논문집B
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• 제35권4호
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• pp.385-390
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• 2011

본 연구에서는 미세채널과 나노채널의 교차부에 불균형 동전기성을 이용한 미세혼합기를 개발하였다. 채널 내 용액의 혼합은 인가된 전압에 의한 전기삼투유동과 미세채널과 나노채널 교차 부에서의 불균형 동전기성에 의한 와류현상에 의해 이루어진다. 미세채널은 PDMS 을 이용하여 소프트리소그래피 공정으로 제작하였고, 나노채널은 미세채널의 특정위치에 전기적 충격에 의한 PDMS 의 파괴로 매우 간단하게 제작하였다. 혼합성능을 평가하기 위하여 형광물질인 Rhodamine B 용액을 이용하여 혼합 전과 후의 형광 분포를 분석하였으며, 약 90%의 혼합을 얻을 수 있었다. 본 연구의 미세혼합기는 복잡한 공정을 요구하지 않고 매우 간단하게 제작되었으며, 생화학시료 분석을 위한 미세시스템에 활용될 수 있다.

• 한국유체기계학회 논문집
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• 제15권1호
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• pp.58-63
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• 2012

Three dimensional unsteady numerical calculations were performed to investigate unsteadiness of the tip leakage flow in an axial compressor. The first stage of the four-stage low-speed research axial compressor was examined. Since this compressor has a relatively large tip clearance, the unsteadiness of the tip leakage flow is induced. Through the results from the unsteady calculations, the process of the induced unsteady tip leakage flow was investigated. It was shown that the leakage flow that occurred at a rotor blade tip clearance affected the pressure distribution on the pressure side near the tip of the adjacent blade, thus caused the fluctuation of the pressure difference between the pressure side and suction side. Consequently, the unsteady tip leakage flow was induced at the adjacent rotor blade. The unsteady feature of the tip leakage flow was changed as the operating point was moved. The interface between the tip leakage flow and the main flow only affected the trailing edge region at the design point whereas the interface influenced up to the leading edge at the low flow rate point. As the flow rate decreased, additionally, it was seen that the vortex size of the tip leakage flow increased and the relatively large length scale disturbance occurred. On the other hand, using frequency analysis, it was shown that the unsteadiness was not associated with the rotor speed and was about 40% of the blade passing frequency. This feature was explained in the rotor relative frame of reference, and the frequency decreased as the flow rate decreased.

• 한국항공우주학회지
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• 제30권5호
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• pp.24-31
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• 2002

축 대칭 원형노즐 출구에 삼각형 형태의 작은 탭이나 얇은 테이프가 부착되었을 때 나타나는 초음속 제트유동장의 변화에 관한 연구가 쉴리렌 영상과 피토압력 측정을 통하여 수행되었다. 노즐 출구면적의 1 %에 해당하는 작은 탭에서 나타나는 유동방향 와류에 의한 제트유동 변화는 유동이 과소팽창 될 때 완전팽창과 과대팽창의 경우에 비하여 더 크게 나타나며 이로 인한 유동유입이 더 크게 나타남을 알 수 있었다. 탭의 설치각도 변화에 의한 영향도 평가되었으며, 유동이 과대팽창 될 때 노즐 안으로 구부러진 탭의 설치가 충격파의 간섭강도를 약화시키고 이로 인한 마하디스크의 소멸현상을 관찰할 수 있었다. 아울러 노즐내부 낮은 높이의 돌출부가 후방 유동장의 원주방향으로 압력변화를 나타내고, 이러한 압력변화는 유동의 팽창 정도에 영향을 받음을 알 수 있었다.

• 한국항공우주학회지
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• 제44권6호
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• pp.520-529
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• 2016

본 연구의 목적은 중고도-장기체공 무인기의 성능을 역설계를 통해 분석하는 것이며, 역설계를 위한 모델로 RQ-1 Predator를 선정하였다. CATIA 프로그램을 사용하여 RQ-1 Predator의 외형에 대한 형상과 중량 분포를 생성하였다. RQ-1 Predator의 공력특성은 주로 와류격자법과 경험식을 조합하여 수행하였으며, 추진 성능은 Howe의 경험식을 통해 수행되었다. 상승률, 실용상승한도, 항속 거리, 그리고 체공 시간에 대한 역설계를 수행하였으며, 역설계 결과가 참고문헌과 잘 일치함을 확인하였다.

• 한국자동차공학회논문집
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• 제7권8호
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• pp.91-98
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• 1999

This paper describes the air flow characteristics inside the throttle valve. Tow-dimensional steady incompressible Navier-Strokes equation are solved numerically with embedding the conceopt of the artificial compressibility and adopting the Baldwin-Lomax turbulence model. With varying the valve opening angles(the Reynolds number )such as 15$^{\circ}$(5000) , 45$^{\circ}$(3000) , 75$^{\circ}$(7000) and 90$^{\circ}$(10000), respectively. tow cases, with a valve shaft and without one, are analysed. The pressure loss between the entrance and exit is severe at 15$^{\circ}$, 100 times as larger as that of 90$^{\circ}$ case, which also depends much on the existece of the valve shaft. The counter rotating vortices are formed over the valve plate with the shaft at only 75$^{\circ}$. They are smally and very large scale in front and back of the valve shaft , respectively. The velocity profiles of 15$^{\circ}$ and 90$^{\circ}$ at the exit are almost symmetric to the horizontal center line, however, the symmetricity is no longer maintained at 45$^{\circ}$ and 75$^{\circ}$ , and in addition, the flow at 75$^{\circ}$ is enforced a lot below center line. The pressure distribution on the walls is largely changed near the valve shaft, and its magnitude becomes great as the valve angle decreases.

• Journal of Oral Medicine and Pain
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• 제18권1호
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• pp.73-82
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• 1993

To investigate the changes in aerobic and facultative anaerobic oral microflora during remission-induction chemotherapy in patients with acute myeloid leukemia, 10 consecutive patients were studied during a period of 28 days. One day before, during and after the induction therapy, patients were given 10% Betadine solution for mouthrinses after breakfast and kept from eating and drinking. After 3 hours, paraffin-stimulated whole saliva was obtained for 2 minutes and transported to the laboratory. The samples were dispersed and homogenized by use of vortex mixer for 20 seconds. From these samples 10-fold serial dilutions (from 10-1 through 10-3) were prepared. Each dilution of 0.1 ml was plated on duplicate set of one nonselective medium (Blood agar) and four selective media (Sabourauds dextrose agar, Mannitol salt agar, Mac-Conkey agar, SF medium ) using applicator woods. All agar plate were incubated at 37$^{\circ}C$ for 48 hours. The total number of microorganisms was calculated and the percentage distribution of the various microorganisms from each specimen was drawn. 1. The salivary flow rate decreased by 66%, going from 5.38 ml/2min to 1.81 ml/2min over two days during the chemotherapy. 2. The total number of microorganisms in saliva increased by 22%, going from 4.88$\times$105/ml to 6.00$\times$105/ml over two days during the chemotherapy. 3. The salivary flow rate and the total number of microorganisms in saliva were recovered within 28 days after the chemotherapy. 4. The quantitative alteration in oral Enterobacteria, Enterococci, Staphylococci, Cndida during the chemotherapy had no statistical significance. 5. In saliva of the patients with acute myeloid leukemia who ahd intraoral ulcer, Enterobacteria was quantitatively predominent. Our study suggests that chemotherapy-induced transient xerostomia may induce acute oral infection. Consequently, the use of saliva substitute, the removal of intraoral infection source and the consistent oral hygiene care seem to be required to avoid the transmission of potential pathogenes in this group of patients.

• International Journal of Fluid Machinery and Systems
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• 제3권4호
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• pp.342-351
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• 2010

During the operation of a hydro turbine the fluid mechanical pressure loading on the turbine blades provides the driving torque on the turbine shaft. This fluid loading results in a structural load on the component which in turn causes the turbine blade to deflect. Classically, these mechanical stresses and deflections are calculated by means of finite element analysis (FEA) which applies the pressure distribution on the blade surface calculated by computational fluid dynamics (CFD) as a major boundary condition. Such an approach can be seen as a one-way coupled simulation of the fluid structure interaction (FSI) problem. In this analysis the reverse influence of the deformation on the fluid is generally neglected. Especially in axial machines the blade deformation can result in a significant impact on the turbine performance. The present paper analyzes this influence by means of fully two-way coupled FSI simulations of a propeller turbine utilizing two different approaches. The configuration has been simulated by coupling the two commercial solvers ANSYS CFX for the fluid mechanical simulation with ANSYS Classic for the structure mechanical simulation. A detailed comparison of the results for various blade stiffness by means of changing Young's Modulus are presented. The influence of the blade deformation on the runner discharge and performance will be discussed and shows for the configuration investigated no significant influence under normal structural conditions. This study also highlights that a two-way coupled fluid structure interaction simulation of a real engineering configuration is still a challenging task for today's commercially available simulation tools.

• 상하수도학회지
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• 제19권2호
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• pp.117-124
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• 2005

Appropriate removal of pollutants from combined sewer overflows(CSOs) and stormwater runoff is of primary concern to watershed managers trying to meet water quality standards even under a wet weather condition. Harmful substances associated with particles besides TSS and BOD are subjected to removal prior to discharge into the natural waters. Effectiveness of five major hydrodynamic separation technologies, Vortechs, Downstream Defender including Storm King for CSOs control, CDS, Stormceptor, and IHS, were evaluated in this study. There is not sufficient information for accurate evaluation of the removal efficiency for the pollutants from the stormwater runoff and CSOs. Based upon limited engineering data, however, all technologies were found to be effective in separation of heavy particles and floating solids. Technologies utilizing screens seem to have advantage in the treatment capacity than the other technologies relied fully on hydrodynamic behavior. The IHS system seems to have a strong potential in application for control of CSOs because of unique hydrodynamic behavior as well as a flexibility in opening size of the screens. Size of the particulate matter in the CSOs and stormwater runoff is found to be the most important parameter in selection of the type of the hydrodynamic separators. There exists an upper limit in the solids removal efficiency of a hydrodynamic separator, which is strongly dependent upon the particle size distribution of the CSOs and stormwater runoff.