• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2001년도 추계학술대회논문집 II
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• pp.1026-1031
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• 2001

In this paper. the dynamic characteristics of a super high-speed tilting-pad air bearing(TPGB) used in a turbo expander with high expansion ratio are analyzed. The dynamic behavior and stability of a rotary system supported by two journal air bearings are investigated numerically. The transient response of the shaft is obtained by simultaneously solving the equation of motion of the shaft and the dynamic Reynolds equation. The stiffness and damping coefficients of the bearing are calculated from the loading coefficients of the bearing are calculated from the loading capacity. shaft velocity and displacement by using a curve fitting method. The natural frequencies of the 1st and 2nd rigid modes can be calculated from these coefficients. The theoretical method of a rigid rotor system is verified by experimentsut.

• 대한기계학회논문집B
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• 제20권1호
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• pp.304-320
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• 1996

A multiple production .epsilon. equation model was developed in the low Reynolds number $\kappa$-$\varepsilon$ model with the aids of DNS data. We derived the model theoretically and avoided the use of empirical correlations as much as possible in order for the model to have generality in the prediction of complex turbulent flow. Unavoidable model constants were, however, optimized with the aids of DNS data. All the production and dissipation models in the $\varepsilon$ equation were modified with damping functions to satisfy the wall limiting behavior. A new $f_{\mu}$ function, turbulent diffusion and pressure diffusion model for the k and .epsilon. equations were also proposed to satisfy the wall limiting behavior. By, computational investigation on the plane channel flows, we found that the multiple production model for .epsilon. equation could improve the near wall turbulence behavior compared with the standard production model without the complicated empirical modification. Satisfication of the wall limiting conditions for each turbulence model term was found to be most important for the accurate prediction of near wall turbulence behaviors.

• 한국기계가공학회지
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• 제14권5호
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• pp.88-95
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• 2015

The thickness of adsorbed molecular layers is the most critical factor in studying thin-film lubrication, and it is the most essential parameter that distinguishes thin-film from thick-film lubrication analysis. The thin film between the shaft and bearing surface within a very narrow gap was considered. The general Reynolds equation has been derived for calculating thin-film lubrication parameters affecting the performance of the circular journal bearing. Investigation of the load-carrying capacity and pressure distribution for the journal bearing considering the adsorbed layer thickness has been carried out. A Reynolds equation appropriate for the journal bearing is used in this paper for the analysis, and it is discussed using the finite difference method of the central difference scheme. The parameters, such as eccentricity and attitude angle, are used for discussing the load-carrying capacity of the journal bearing. The results reported in this paper should be applied to analysis of the journal bearing with different lubrication factors. The steady-state analysis of the journal bearing is conducted using the Reynolds model under thin-film lubrication conditions. For a journal bearing, several parameters, such as a pressure, load capacity, and pressure components of the bearing can be obtained, and these results can be stored in a sequential data file for later analysis. Finally, their distribution can be displayed and analyzed easily by using the MATLAB GUI technique. The load-carrying capability of the journal bearing is observed for the specified operating conditions. This work could be helpful for the understanding and research of the mechanism of thin-film lubrication.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2000년도 추계 학술대회논문집
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• pp.115-122
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• 2000

본 연구에서는 2 차원 벽구동 캐비티 유동에 의하여 나타나는 이력효과에 의한 분기(Bifurcation)현상을 전산유체기법을 사용하여 연구하였다. 캐비티는 북쪽과 동쪽벽이 움직일 수 있고, 다른 두 벽은 고정되어있는 구조이다. 실험은 Reynolds 수 100 에서 1000까지 증가시켜가면서 북쪽벽과 동쪽벽을 동시에 가속 시켜 정상상태에 이르게 한 경우와 북쪽벽이 먼저 가속되어 정상해에 이른 후 동쪽벽을 나중에 가속하여 재차 정상상태에 이르게 한 경우를 비교하였다. 그 결과 Reynolds수가 약 200이상부터 벽에 작용하는 항력, 유량함수의 값, 재부착점등이 분기현상을 나타냄을 확인하였다.

• 대한기계학회논문집
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• 제17권8호
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• pp.1921-1930
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• 1993

In order to evaluate the inlet pressure correctly, the full Navier-Stokes equations are solved numerically for the computational domain which covers the cavity region between pads as well as the bearing film. A nonuiform grid system is adopted to reduce the number of grid points, and the numerical solutions are obtained for a wide range of Reynolds number in laminar regime with various values of the distance between pads. The numerical results show that the inlet pressure is significantly affected by Reynolds number and the distance between pads. An expression for the loss coefficient in terms of Reynolds number and non-dimensional distance between pads is obtained on the basis of the numerical results. It is found that the inlet pressure over the whole range of numerical solutions can be fairly accurately estimated by applying the formula for the loss coefficient to the extended Bernoulli equation.

• 동력기계공학회지
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• 제4권3호
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• pp.40-47
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• 2000

The turbulent flow with wake, reattachment and recirculation flow is very important from the viewpoint of engineering. But that is still difficult because of especially the unsteady problems which are related with the vehicle dynamics and the aerodynamics noise. This paper evaluate LES that can analyze about all fluid flow region including the laminar, transition and turbulent. So we compare the results of LES with those of PIV measurement and Reynolds averaging models. In conclusion, LES predicts flow behavior better than Reynolds averaging models.

• Water Engineering Research
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• 제3권4호
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• pp.247-258
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• 2002

This paper presents a non-isotropic turbulence modeling of flows over bedforms. The Reynolds stress model is used for the turbulence closure. In the model, Launder, Reece, and Rodi's model and Hanjalic and Launder's model are employed f3r the pressure strain correlation term and the diffusion term, respectively. The mean flow and turbulence structures are simulated and compared with profiles measured in the experiments. The numerical solutions from two-equation turbulence models are also provided for comparisons. The Reynolds stress model yields the separation length of eddy similar to the other numerical results. Using the developed model, the resistance coefficients are also estimated for the flows at different Froude numbers. Karim's (1999) relationship is used to determine the bedform geometry. It is found that the values of the form drag and the skin friction are very similar to those obtained by the other turbulence models. meaning higher values of the form drag and lower values of the skin friction compared with the empirical formulas.

• Tribology and Lubricants
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• 제29권3호
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• pp.149-159
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• 2013

In this research, the lubrication characteristics of spool valves with various cross-sectional groove shapes were studied. The validity of using the Reynolds equation for the analysis of spool valves with various groove shapes was also investigated. The cross-sectional shapes for the grooves included a triangle, square, and U shape. The characteristics of the flow in the groove were investigated using streamlines. When the number of grooves was increased, the difference between the results obtained from the Reynolds equation and those obtained from the Navier-Stokes equation increased according to the groove shape. Thus, it was found that the Navier-Stokes equation should be used to investigate the lubrication characteristics of the spool valves in those cases. Moreover, in the case where the cross section of the groove was U-shaped, the groove prevented the small eddy current from occurring in the groove. Therefore, the lateral force and friction force of the spool valve with the U-shaped groove were lower than those of the spool valves with other groove shapes.

• Tribology and Lubricants
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• 제27권6호
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• pp.326-331
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• 2011

The purpose of this paper is to analyze and discuss the effects of surface roughness by comparing the elastohydrodynamic lubrication(EHL) analysis of smooth surface and rough surface as the ball bearing. In order to do this, The average flow model is adapted for the interaction of the flow rheology of lubricant and surface roughness. The average Reynolds equation and the related flow factor which describes the coupled effects of surface roughness and flow rheology, the viscosity-pressure and density-pressure relations equations, the elastic deformation equation, and the force balance equation are solved simultaneously. The results show that effects of surface roughness on the film thickness and pressre distribution should be considered especially in EHL contact problems.

• Tribology and Lubricants
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• 제37권1호
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• pp.14-24
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• 2021

This study shows the effect of the thermal boundary condition around the tilting pad journal bearing on the static and dynamic characteristics of the bearing through a high-precision numerical model. In many cases, it is very difficult to predict or measure the exact thermal boundary conditions around bearings at the operating site of a turbomachine, not even in a laboratory. The purpose of this study is not to predict the thermal boundary conditions around the bearing, but to find out how the performance of the bearing changes under different thermal boundary conditions. Lubricating oil, bearing pads and shafts were modeled in three dimensions using the finite element method, and the heat transfer between these three elements and the resulting thermal deformation were considered. The Generalized Reynolds equation and three-dimensional energy equation that can take into account the viscosity change in the direction of the film thickness are connected and analyzed by the relationship between viscosity and temperature. The numerical model was written in in-house code using MATLAB, and a parallel processing algorithm was used to improve the analysis speed. Constant temperature and convection temperature conditions are used as the thermal boundary conditions. Notably, the conditions around the bearing pad, rather than the temperature boundary conditions around the shaft, have a greater influence on the performance changes of the bearing.