• Tribology and Lubricants
• /
• 제29권4호
• /
• pp.218-222
• /
• 2013

Water-lubricated ball bearings consist of rolling elements, an inner raceway, an outer raceway, a retainer, and an operating lubricant. In the water environment, ball bearings are required to sustain high loads at high speeds under poorly lubricated conditions. For the analysis of bearing behavior, friction torque is considered as the main factor at various flow rates, rotating speeds, and roughnesses between the rolling element and raceways. When the bearing operates at high rotating speeds, the friction torque between the raceway and rolling elements increases considerably. This frictional torque is an important factor affecting bearing reliability and life cycle duration. For understanding the flow conditions in the inner part of the bearing, this study focuses on the flow around the rotating and revolving rolling elements. A simple model of fluid flow inside the ball bearing is designed using the commercial CFD program ANSYS.

• 한국전산유체공학회:학술대회논문집
• /
• 한국전산유체공학회 2005년도 춘계 학술대회논문집
• /
• pp.219-222
• /
• 2005

A compressible two-fluid two-phase flow computation model using the stiffened-gas equation of state is formulated. Since the conservation equation system is of mixed type, it gives complex eigenvalues. The sonic speeds obtained from the individual single phase have been simply used in the literature for the fastest wave speeds necessary in the HLL scheme. This method has worked fine but proved to be quite diffusive according to our test. To improve the accuracy, we here propose to utilize the analytic eigenvalues evaluated from an approximate Jacobian matrix lot the fastest wave speeds. The interfacial transfer terms were dropped in constituting the Jacobian matrix for this purpose. The present scheme proved efficient, robust and accurate in comparison with other existing methods. We solved the cavitating flow problem using the present scheme. The result shows more detailed wave structure in the cavitating process caused by the strong expansion waves.

• Tribology and Lubricants
• /
• 제35권3호
• /
• pp.190-198
• /
• 2019

Gas foil bearings (GFBs) enable small- to medium-sized turbomachinery to operate at ultra-high speeds in a compact design by using ambient air or process gas as a lubricant. When using air or process gas, which have lower viscosity than lubricant oil, the turbomachinery has the advantage of reduced power loss from bearing friction drag. However, GFBs may have high Reynolds number, which causes turbulent flows due to process gas with low viscosity and high density. This paper analyzes gas foil journal bearings (GFJBs) with high Reynolds numbers and studies the effects of turbulent flows on the static and dynamic performance of bearings. For comparison purposes, air and R-134a gas lubricants are applied to the GFJBs. For the air lubricant, turbulence is dominant only at rotor speeds higher than 200 krpm. At those speeds, the journal eccentricity decreases, but the film thickness, power loss, and direct stiffness and damping coefficients increase. On the other hand, the R-134a gas lubricant, which that has much higher density than air, causes dominant turbulence at rotor speeds greater than 10 krpm. The turbulent flow model predicts decreased journal eccentricity but increased film thickness and power loss when compared with the lamina flow model predictions. The vertical direct stiffness and damping coefficients are lower at speeds below 100 krpm, but higher beyond that speeds for the turbulent model. The present results indicate that turbulent flow effects should be considered for accurate performance predictions of GFJBs with high Reynolds number.

• 대한기계학회논문집B
• /
• 제20권7호
• /
• pp.2289-2297
• /
• 1996

Flows in the combustion chamber near the spark plug are measured using LDv.A single cylinder DOHC S.I. engine of compression ratio 9.5:1 with a transparent quartz window piston is used. Combustion chamber shape is semi-wedge type. Measured data are analyzed using the ensemble averaged analysis and the cycle resolved analysis which uses FFT Filtering. Turbulent intensity and mean velocity are studied in the main flow direction and the normal to main flow direction as a function of engine speeds. The results shows that the turbulent intensity obtained by the ensemble averaged analysis is greater than that calculated by the cycle resolved analysis. Especially, the ensemble averaged analysis shows increase in turbulence at the end of compression stroke although the cycle resolved analysis shows increase only in the cycle-by-cycle variation with no noticeable increase in turbulence. The mean velocity in the main flow direction increase as engine speed increase. But the mean velocity normal to the main flow does not show such increase. Turbulent intensity in both direction increase in proportion to engine speeds. The magnitude of turbulent intensity is about 0.3 ~ 0.4 times the mean piston speeds at the end of the compression stroke.

• Management Science and Financial Engineering
• /
• 제4권2호
• /
• pp.1-13
• /
• 1998

The Shortest Path Problem in Time-dependent Networks, where the travel time of each link depends on the time interval, is not realistic since the model and its solution violate the Non-passing Property (NPP:often referred to as FIFO) of real phenomena. Furthermore, solving the problem needs much more computational and memory complexity than the general shortest path problem. A new model for Time-dependent Networks where the flow speeds of each link depend on time interval, is suggested. The model is more realistic since its solution maintains the NPP. Solving the problem needs just a little more computational complexity, and the same memory complexity, as the general shortest path problem. A solution algorithm modified from Dijkstra's label setting algorithm is presented. We extend this model to the problem of Minimum Expected Time Path in Time-dependent Stochastic Networks where flow speeds of each link change statistically on each time interval. A solution method using the Kth-shortest Path algorithm is presented.

• 대한기계학회논문집B
• /
• 제22권2호
• /
• pp.153-161
• /
• 1998

This study describes the results of the analysis of measured rotating stall signal in a centrifugal compressor with vaneless diffuser. Unsteady flow and rotating stall in vaneless diffuser were investigated by measuring of unsteady velocity fluctuation using hot-wire anemometer. Experiments were carried out in several impeller rotating speeds, at different radius ratios. Single hot -wire was used to study the characteristics of rotating stall. As a result, the abrupt rotating stall was detected at all measured impeller rotating speeds and the several flow coefficients which are less than 0.16. The number of the stall cell was one at all measured rotational speeds, and the rotating direction was the same as that of the impeller. As the flow rate decreased, the profile of the phase averaged radial velocity component with time changed from a sawtooth to a sine wave.

• Wind and Structures
• /
• 제13권2호
• /
• pp.145-167
• /
• 2010

By using the 'failure' model approach, the effects of wind direction on the flight of sheathing panels from the roof of a model house in extreme winds was investigated. A complex relationship between the initial conditions, failure velocities, flight trajectories and speeds was observed. It was found that the local flow field above the roof and in the wake of the house have important effects on the flight of the panels. For example, when the initial panel location is oblique to the wind direction and in the region of separated flow near the roof edge, the panels do not fly from the roof since the resultant aerodynamic forces are small, even though the pressure coefficients at failure are high. For panels that do fly, wake effects from the building are a source of significant variation of flight trajectories and speeds. It was observed that the horizontal velocities of the panels span a range of about 20% - 95% of the roof height gust speed at failure. Numerical calculations assuming uniform, smooth flow appear to be useful for determining panel speeds; in particular, using the mean roof height, 3 sec gust speed provides a useful upper bound for determining panel speeds for the configuration examined. However, there are significant challenges for estimating trajectories using this method.

• 한국도로학회논문집
• /
• 제14권6호
• /
• pp.131-138
• /
• 2012

PURPOSES : Two-lane highways have one lane in each direction, and lane changing and passing maneuvers take place in the opposing lane depending on the availability of passing sight distance. 2001 Korea Highway Capacity Manual (KHCM) is classified into two classes of two-lane highways (Type I, II), and average travel speed and time-delayed rate are used as measures of effectiveness (MOEs). However, since existing two-lane highways have both uninterrupted and interrupted traffic flow-system elements, a variety of free-flow speeds exhibits in two-lane highways. In addition, it is necessary to check if the linear-relationship between volumes and time-delayed rate is appropriate. Then, this study is to reestablish the relationship between average travel speed, time-delayed rate, and flow. METHODS : TWOPAS model was selected to conduct this study, and the free-flow speeds of passenger cars and the percentage of following vehicles observed in two-lane highways were applied to the model as the input. The revised relationships were developed from the computer simulation. RESULTS : In the revised average travel speed vs. flow relationship, the free-flow speed of 90km/h and 70km/h were added. It shows that the relationship between time delayed-rate and flow appeared to be appropriate with the log-function form and that there was no difference in time-delayed rate between the free flow speeds. In addition to revise the relationships, the speed prediction model and the time-delayed rate prediction model were also developed. CONCLUSIONS : The revised relationships between average travel speed, time-delayed rate, and flow would be useful in estimating the Level of Service(LOS) of a two-lane highway.

• 한국지능시스템학회논문지
• /
• 제4권2호
• /
• pp.13-23
• /
• 1994

This study proposed a method of accessment for traffic flow on roads based on the driver's decision making. In order to, an attempt is carried out to express driving speeds through driver's congnitive language theoretically and experimentally. Membership function is derived to express driver's congnitive language about driving speed through a fuzzy set theory and examines the applicability for speed evaluation. As a resul, the membership function of the recognized as medium by drivers almost agrees with the frequency distribution of speeds on roads constrtained at 50km/h.

• International Journal of Fluid Machinery and Systems
• /
• 제7권3호
• /
• pp.110-124
• /
• 2014

Surge phenomena in the zone of reduced speeds in a system of a nine-stage axial flow compressor coupled with ducts were studied analytically by use of a surge transient simulation code. Main results are as follows. (1) Expansion of apparently stable, non-surge working area of the pressure vs. flow field beyond the initial stage-stall line was predicted by the code in the lower speed region. The area proved analytically to be caused by significantly mismatched stage-working conditions, particularly with the front stages deep in the rotating stall branch of the characteristics, as was already known in situ and in steady-state calculations also. (2) Surge frequencies were found to increase for decreasing compressor speeds as far as the particular compressor system was concerned. (3) The tendency was found to be explained by a newly introduced volume-modified reduced surge frequency. It suggests that the surge frequency is related intimately with the process of emptying and filling of air into the delivery volume. (4) The upstream range of movement of the fluid mass having once passed through the compressor in surge was found to reduce toward the lower speeds, which could have caused additionally the increase in surge frequency. (5) The concept of the volume-modified reduced surge frequency was able to explain, though qualitatively at present, the behaviors of the area-pressure ratio parameter for the stall stagnation boundary proposed earlier by the author.