• International Journal of Fluid Machinery and Systems
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• 제9권1호
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• pp.66-74
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• 2016

Concerning the numerical simulation of high-speed water jet with intensive cavitation this paper presents a practical compressible mixture flow method by coupling a simplified estimation of bubble cavitation and a compressible mixture flow computation. The mean flow of two-phase mixture is calculated by URANS for compressible fluid. The intensity of cavitation in a local field is evaluated by the volume fraction of gas phase varying with the mean flow, and the effect of cavitation on the flow turbulence is considered by applying a density correction to the evaluation of eddy viscosity. High-speed submerged water jets issuing from a sheathed sharp-edge orifice nozzle are treated when the cavitation number, ${\sigma}=0.1$, and the computation result is compared with experimental data The result reveals that cavitation occurs initially at the entrance of orifice and bubble cloud develops gradually while flowing downstream along the shear layer. Developed bubble cloud breaks up and then sheds downstream periodically near the sheath exit. The pattern of cavitation cloud shedding evaluated by simulation agrees experimental one, and the possibility to capture the unsteadily shedding of cavitation clouds is demonstrated. The decay of core velocity in cavitating jet is delayed greatly compared to that in no-activation jet, and the effect of the nozzle sheath is demonstrated.

• 한국추진공학회지
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• 제20권4호
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• pp.50-58
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• 2016

한국형 발사체(KSLV-II) 추진제 공급시스템은 각 단에 위치한 추진제 탱크에 산화제와 케로신을 공급해주는 시스템이다. 발사체의 추진제 탱크를 안정적으로 충전하기 위해, 충전시나리오와 충전유량을 결정하였다. 다음으로 1D 유동 해석프로그램을 이용하여 추진제 공급시스템을 모델링 하였다. 1D steady state 해석을 통하여 각 시스템에서의 충전모드에 따른 밸브용량과 orifice 사이즈를 계산하였다. Steady state 결과를 이용하여 1D transient 해석을 수행하였다. 해석 결과 추진제가 각 추진제 탱크로 충전됨에 따라 탱크의 수두가 상승하여 충전유량이 감소하는 것으로 나타났다. 최종적으로 해석을 통해 제안된 충전시스템 모델이 요구되는 충전설계조건을 만족하는 것을 확인하였다.

• 한국산학기술학회논문지
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• 제19권10호
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• pp.437-443
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• 2018

본 연구에서는 라인믹서 내의 압력손실과 혼화성능에 대한 혼합요소의 영향을 해석하였다. 기체와 액체가 혼합된 고농도 오존수 장치는 단체급식시설의 살균 및 소독, 양식장의 수질관리 등 다양한 분야에서 사용되고 있다. 그러나 기존에 사용되는 장치는 가압방식을 사용하여 부피가 매우 크다. 최근에는 기후 변화로 강수량이 감소하여 지하수 및 지표수의 오염으로 음용수의 살균은 매우 중요하다. 따라서 기존의 장치와 대비하여 작은 공간을 사용하면서도 정수기능을 갖는 라인믹서형 기체-액체 혼합장치의 개발이 필요하다. 본 연구에서는 라인믹서 장치 내에 삽입한 혼합요소인 static mixer의 개수, 오리피스의 형태와 개수, 오리피스에 부착된 베인의 모양에 따른 유입유체의 혼화상태와 유동특성을 전산유체역학 모사를 통해 해석하고 혼화상태에 미치는 영향을 평가하였다. 곡면의 베인을 부착한 오리피스를 라인믹서에 사용한 경우 static mixer를 사용한 경우와 비교하여 비슷한 혼화정도를 나타내면서 압력손실양은 50% 이상 감소하는 것을 확인하였다.

• 한국자동차공학회논문집
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• 제9권5호
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• pp.179-187
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• 2001

In this paper, a primary pressure regulating type ratio control system is developed for a metal belt CVT, and the CVT ratio changing characteristics are investigated by simulation and experiment. The hydraulic part of the ratio control system has a simple structure with one 3-way spool valve as a main ratio control valve and one bleed type variable force solenoid as a pilot valve. The mathematical modelling of the CVT hydraulic system is derived by considering the CVT shift dynamics. Simulation results of CVT speed ratio and the primary pressure agree with the experimental results demonstrating the validity of the dynamic models. It is found from the simulation and experimental results that the response time of speed ratio and primary pressure can be shortened by increasing the ratio control valve port area, and the size of feedback orifice of ratio control valve gives a damping effect on the primary pressure oscillation.

• 상하수도학회지
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• 제24권4호
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• pp.425-432
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• 2010

From the results of tracer test for the existing clearwell in Y water treatment plant, $T_{10}$ and T10/T were calculated as 150 min and 0.24, respectively. Therefore it required the modification schemes for improving hydraulic efficiency, surrogated by $T_{10}$ and $T_{10}$/T, and disinfection performance. In this study, using transient CFD(Computational Fluid Dynamics) simulation technique, tracer tests on dynamic condition for the suggested schemes were simulated. From the results of simulation, it was revealed that 8~6 baffles are necessary to guarantee the disinfection ability in the existing clearwell. Also, installing orifice baffle in the vicinity of inlet could increase plug flow fraction within clearwell.

• 한국유체기계학회 논문집
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• 제9권5호
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• pp.22-27
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• 2006

Sedimentation phenomenon of suspended solids occurs by the gravitational force. Pollution particles are separated from slowly flowing waste water in clarifier. Recently, the sludge suction collector is Preferred rather than the scraper type sludge collector due to the enhancement of the clarifier efficiency. The sludge suction collector is usually operated by the user's experiences without any scientific and/or technical consideration. To evaluate the performance of sludge suction collector, the three dimensional numerical simulation was conducted by the finite volume method. To analyze the performance, the velocity vectors and the suction flow rates of the orifices were investigated. The result showed that each suction flow rate through out the collector was equivalent in the sludge suction collector and the efficiency of suction collector was good to remove high concentrated sludge in clarifier.

• 한국정밀공학회지
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• 제13권8호
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• pp.120-127
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• 1996

Anti-lock Brake System has been developed to reduce tendency for wheel lock and improve vehicle control during sudden braking on slippery road surfaces. This is achieved by controlling the braking pressure, avoiding wheel lock, while retaining handling and brake performance. This paper is concerned about pressurecharacteristics of hydraulic modulator. Experimental sets which is consists of hydraulic modulator, duty controller, pressure regulator, pressure senset is consuructed. System modelling and computer simulation are performed for comparison with experimental results. Brake wheel pressure are measured under various driving pulse. The result of experiment show fairly agreement with the simulation. As a result, it is known that wheel pressure is affected by duty ratio, orifice diameter through computer simulation.

• Structural Engineering and Mechanics
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• 제28권1호
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• pp.89-105
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• 2008

The paper presents a study on the effects of soil-structure-interaction (SSI) on the performance of the compliant liquid column damper (CLCD) for the seismic vibration control of short period structures. The frequency-domain formulation for the input-output relation of a flexible-base structure with CLCD has been derived. The superstructure has been modeled as a linear, single degreeof-freedom (SDOF) system. The foundation has been considered to be attached to the underlying soil medium through linear springs and viscous dashpots, the properties of which have been represented by complex valued impedance functions. By using a standard equivalent linearization technique, the nonlinear orifice damping of the CLCD has been replaced by equivalent linear viscous damping. A numerical stochastic study has been carried out to study the functioning of the CLCD for varying degrees of SSI. Comparison of the damper performance when it is tuned to the fixed-base structural frequency and when tuned to the flexible-base structural frequency has been made. The effects of SSI on the optimal value of the orifice damping coefficient of the damper has also been studied. A more convenient approach for designing the damper while considering SSI, by using an established model of a replacement oscillator for the structure-soil system has also been presented. Finally, a simulation study, using a recorded accelerogram, has been carried out on the CLCD performance for the flexible-base structure.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2002년도 유체기계 연구개발 발표회 논문집
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• pp.493-498
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• 2002

The letdown system of pressurized water reactor (PWR) nuclear fewer plants had experienced instabilities in letdown system due to unacceptable flow characteristics of control valves. The Korean Standard Nuclear Power Plants (KSNPs) have three flow paths in parallel for letdown new control. Each flow path consists of two offices and one isolation valve. This study evaluates the effect of orifice arrangement and valve stroke time of letdown isolation valve on the system transients because sudden flow changes due to valve actuation can generate high pressure peaks in letdown line. A pressure transient analysis has been preformed to evaluate the impact of dynamic transients. This analysis uses MMS which is a simulation code developed by EPRI based on the method of characteristics. The result shows that the pressure peak is reduced in the continuous arrangement but negligible. Additionally, it shows that the stroke time of linear type flog valve greater than 15 seconds can give more stable performance.

• 대한기계학회논문집B
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• 제29권10호
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• pp.1101-1110
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• 2005

In this paper we present the numerical results of the behavior of the horizontal vortex generated by ejecting a liquid vertically upward from an orifice into the bulk fluid above the orifice. The numerical calculation has been performed for the axi-symmetric Navier-Stokes equation. A simple flow-visualization experiment was also conducted to qualitatively verify the numerical solutions. Three cases of the flow configurations studied in this paper are; firstly, the vortex was generated without any background rotation, secondly, the vortex was made under a full background rotation, and thirdly, the vortex was made during the spin-up process such that only the region adjacent to the side wall was set into motion viewed in the inertial frame of reference. It was shown that the swirl flow at the inlet boundary affects considerably the formation and development of the vortex for the second case. In the third case, it was remarkable to see that the vortex cannot penetrate into the region near to the side wall of the tank, because of the strong swirl flow and corresponding high pressure gradient in the region.