• Journal of Mechanical Science and Technology
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• 제15권5호
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• pp.646-655
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• 2001

This paper reports the effects of nozzle exit boundary layer swirl on the instability modes of underexpanded supersonic jets emerging from plane rectangular nozzles. The effects of boundary layer swirl at the nozzle exit on thrust and mixing of supersonic rectangular jets are also considered. The previous study was performed with a 30°boundary layer swirl (S=0.41) in a plane rectangular nozzle exit. At this study, a 45°boundary layer swirl (S=1.0) is applied in a plane rectangular nozzle exit. A three-dimensional unsteady compressible Reynolds-Averaged Navier-Stokes code with Baldwin-Lomax and Chiens $\kappa$-$\xi$ two-equation turbulence models was used for numerical simulation. A shock adaptive grid system was applied to enhance shock resolution. The nozzle aspect ratio used in this study was 5.0, and the fully-expanded jet Mach number was 1.526. The \"flapping\" and \"pumping\" oscillations were observed in the jets small dimension at frequencies of about 3,900Hz and 7,800Hz, respectively. In the jets large dimension, \"spanwise\" oscillations at the same frequency as the small dimensions \"flapping\" oscillations were captured. As reported before with a 30°nozzle exit boundary layer swirl, the induction of 45°swirl to the nozzle exit boundary layer also strongly enhances jet mixing with the reduction of thrust by 10%.

• 대한기계학회논문집
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• 제11권1호
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• pp.160-169
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• 1987

본 논문에서는 위의 연구에서 제안되는 즉, 유동장 내에서 형식이 변화되는 새롭고 보다 정확한 형태의 와도방정식을 사용하여 평면 4:1수축을 지나는 유동을 수 치모사하고자 한다. 2장에서는 UCM유체의 평면 Poiseuille 유동 및 평면 4:1수축을 지나는 유동을 개략적으로 고찰하고, 유동장이 와도에 관하여 타원형 혹은 쌍곡선형으 로 구별될 수 있음을 밝힌다. 3장에서는 Murman과 Cole의 형식에 따른 차분법을 원 용하여 Crochet, Davies와 Walters의 유한차분법을 개선함으로써, 와도방정식을 수치 근사하는데 있어서, 타원형 구역에서는 중심차분 근사, 쌍곡선형 구역에서는 후진차분 근사하는 방법을 도입 한다. 4장에서는 본 연구의 계산 결과를 R=0에 대한 Davies, Lee와 Webster의 계산결과와 비교하고, 레이저를 이용한 Walters와 Rawlinson의 유동 가시화 실험과도 정성적인 비교를 한다.즉 유동함수의 분포를 실험 및 종래의 계산 결과와 비교하며, 와도, 응력의 분포를 와도의 형식변화 관점에서 검토하였다.

• 대한기계학회논문집A
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• 제29권5호
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• pp.762-769
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• 2005

This paper presents a method to investigate the characteristics of a hydrodynamic bearing of a HDD spindle motor due to elevated temperature considering the variation of the clearance as well as the lubricant viscosity. Iterative finite element analysis of the heat conduction and the thermal deformation is performed to determine the viscosity and clearance of a hydrodynamic bearing due to elevated temperature until the temperature of the bearing area converges. Proposed method is verified by comparing the calculated temperature with the measured one in elevated surrounding temperature as well as in room temperature. This research shows that elevated temperature changes the clearance as well as the lubricant viscosity of the hydrodynamic bearing of a HDD spindle motor. Once the viscosity and the clearance of a hydrodynamic bearing of a HDD spindle motor are determined, finite element analysis of the Reynolds equation is performed to investigate the static and dynamic characteristics of a hydrodynamic bearing of a HDB spindle motor due to elevated temperature. It also shows that the variation of clearance due to elevated temperature is another important design consideration to affect the static and dynamic characteristics of a hydrodynamic bearing of a HDD spindle motor

• 한국항공우주학회지
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• 제32권8호
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• pp.1-11
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• 2004

본 연구에서는 비정렬 격자계에서 가장 많이 쓰이는 근사 해법 중에 하나인 LU 기법의 Navier-Stokse 방정식에 대한 수렴성 및 안정성에 관한 연구를 수행하였다. 적절한 스칼라 모델 방정식을 사용하여 LU 기법이 갖는 고유한 특성에 관한 해석적 논의를 수행하였으며, 이를 Navier-Stokes 방정식으로 확장하여 해석하였다. 그 결과 LU 기법의 강성도는 격자 종횡비가 높아짐에 띠라, 그리고 격자 레이놀즈 수 감소함에 따라 증가하게 된다. 또한 내부반복계산을 통해서 이러한 강성도가 부분적으로 극복될 수 있음을 보였으며, 평판 난류 유동 해석을 통해서 해석 결과를 검증하였다.

• KSTLE International Journal
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• 제3권2호
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• pp.90-94
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• 2002

A press shoe is an element of a machine for squeezing water from wood pulp in the field of manufacturing paper. This is used to compress the pulp enveloped by felt sheet with a large roller. The squeezing farce is made by hydraulic pressure. The press shoe has a mechanism similar to a partial hydrostatic bearing. The pressure profile between press shoe and roller affects their squeezing ability, and partial peak pressure can tear the wet pulp. The curvature of the surface of press shoe varies to reduce the peak pressure and increase the mean pressure simultaneously, Therefore, the prediction of pressure distribution considering partially changed curvature of hydrostatic bearing is very important far designing the press shoe. In this study, the difference formulation of Reynolds equation far partial hydrostatic bearing is derived by direct numerical method and a computer program to calculate the pressure distribution is developed. We investigate the effect of partially changed curvature of bearing surface on the pressure distribution. Other design parameter far hydrostatic bearing such as depth of pocket and relative velocity are also studied.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 2002년도 제35회 춘계학술대회
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• pp.137-142
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• 2002

The fluid mechanics and operating conditions of gas-lubricated proceeding bearings in micro rotating machinery such as micro polarization modulator and micro gas turbine are different from their larger size ones. Due to non-continuum effects, there is a slip of gas at the walls. Thus in this paper, the slip flow effect is considered to estimate the pressure distribution and load-carrying capacity of micro gas-lubricated proceeding bearings as the local Knudsen number at the minimum film thickness is greater than 0.01. Based on the compressible Reynolds equation with slip flow, the static characteristics of micro gas-lubricated proceeding bearings are obtained. Numerical predictions compare the pressure distribution and load capacity considering slip flow with the performance of micro proceeding bearings without slip f]ow for a range of bearing numbers and eccentricities. The results clearly show that the slip flow effect on the static characteristics is considerable and becomes more significant as temperature increases.

• 물과 미래
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• 제21권3호
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• pp.291-291
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• 1988

2차원 흐름이 원주주위를 지날 때 발생하는 흐름의 변화가 기본방정식인 연속방정식과 운동량방정식에 의하여 수치적으로 해석된다. 수치해석 과정은 기본방정식에 유함수, 와도 및 흐름의 특성을 나타내는 무차원 매개변수를 도입하여 무차원 유함수-와소수송식을 유도한 후, successive over relaxation scheme과 alternating direct implicit scheme으로 수행된다. 수치실험은 레이놀즈수 125-275를 기존의 수치해석에서는 주로 수치실험 결과와 비교한다. 원주표면의 압력을 구하는 방법에 있어서 기존의 수치해석에서는 주로 방사 운동량방정식만을 사용하였으나, 본 논문에서는 기존의 방법외에 방사 운동량방정식 및 접선 운동량방정식에 의해 압력을 계한하고, 두 값을 비교하여 레이놀즈수에 따른 압력을 구하는 방법을 제시한다. 또한 와도의 분포를 도시하여 원주에 의한 후류의 영향을 받지 않는 외부경계의 한계를 새로이 설정한다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2005년도 추계 학술대회논문집
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• pp.21-26
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• 2005

Gas Chromatography (GC) is a wisely technique used for the separation and analysis of liquid and gas sample. Separation of the sample vapors is achieved via their differential migration through a capillary column with an insert carrier gas. The identity and quantity of each vapor in the mixer can be determined from its retention time in the column and a particular property of the gas, such as thermal conductivity, which can be related to the concentration of sample vapor in the carrier gas. Therefore, the flow characteristics in the spiral gas chromatographic column are numerically investigated in this study. Especially, different pressure drop between the front and the rear of GC column with various flow rates is estimated the governing equations are derived from making using of three-dimensional Naver-Stokes equation with incompressible and laminar model due to the nature of low Reynolds number flow. Using a commercial code, FLUENT, the pressure and flow fields in GC column are calculated with various flow rates. The characteristics of thermal cycling which is one of the most important factors affecting the column efficiency and analysis time is also estimated. Furthermore, numerical analyses are also carried out by using commercial code, ANSYS, with various values of power, which is applied to the heating element located at lower GC column.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2005년도 추계 학술대회논문집
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• pp.287-292
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• 2005

The flowfields generated by gaseous slot injection into a supersonic flow at a Mach number of 3.75 and a Reynolds number of $2.07{\times}10^7$ are simulated numerically. Fine-scale turbulence effects are represented by a two-equation(k-w SST model) closure model which includes $y^+$ effects on the turbulence model. Grid convergence index(GCI) is also considered to provide a measure of uncertainty of the grid convergence. Comparison is made with experimental data and other turbulence model in term of surface static pressure distributions, the length of the upstream separation region, and the penetration height. Results indicate that the k-w SST model correctly predicts mean surface pressure distribution and upstream separation length. However, it is also observed that the numerical simulation over predicts the pressure spike and penetration height compared with experimental data. All these results are taken within $1\%$ error band of grid convergence.

• Journal of Mechanical Science and Technology
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• 제14권3호
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• pp.350-357
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• 2000

Experiments were performed by using PF-5060 and water to investigate the influence of an aspect ratio of a horizontal rectangular channel on the cooling characteristics from an in-line $6{\times}1$ array of discrete heat sources which were flush mounted on the top wall of the channel. The experimental parameters were aspect ratio of rectangular channel, heat flux of simulated VLSI chip, and channel Reynolds number. The chip surface temperatures decreased with the aspect ratio at the first and sixth rows, and decreased more rapidly at a high heat flux than at a low heat flux. The measured friction factors at each aspect ratio for both water and PF-5060 gave a good agreement with the values predicted by the modified Blasius equation within ${\pm}7%$. The Nusselt number increased as the aspect ratio decreased, but the increasing rate of Nusselt number reduced as the aspect ratio decreased. A 5:1 rectangular channel yields the most efficient cooling performance when the heat transfer and pressure drop in the test section were considered simultaneously.