• Korea-Australia Rheology Journal
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• v.21 no.1
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• pp.27-37
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• 2009

In this work, we numerically investigate the effect of viscoelasticity on 2D laminar vortex dynamics in flows past a single rotating cylinder for rotational rates $0{\leq}{\alpha}{\leq}5$ (the rotational rate ex is defined by the ratio of the circumferential rotating velocity to free stream velocity) at Re=100, in which the vortex shedding has been predicted to occur in literature for Newtonian fluids. The objective of the present research is to develop a promising technique to fully suppress the vortex shedding past a bluff body by rotating a cylinder and controlling fluid elasticity. The predicted vortex dynamics with the present method is consistent with the previous works for Newtonian flows past a rotating cylinder. We also verified our method by comparing our data with the literature in the case of viscoelastic flow past a non-rotating cylinder. For $0{\leq}{\alpha}{\leq}1.8$, the frequency of vortex shedding slightly decreases but the fluctuation of drag and lift coefficient significantly decreases with increasing fluid elasticity. We observe that the vortex shedding of viscoelastic flow disappears at lower ${\alpha}$ than the Newtonian case. At ${\alpha}$=5, the relationship between the frequency of vortex shedding and Weissenberg number (Wi) is predicted to be non-monotonic and have a minimum around Wi=0.25. The vortex shedding finally disappears over critical Wi number. The present results suggest that the vortex shedding in the flow around a rotating cylinder can be more effectively suppressed for viscoelastic fluids than Newtonian fluids.

• International Journal of Fluid Machinery and Systems
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• v.2 no.4
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• pp.295-302
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• 2009

The flow in the draft tube cone of Francis turbines operated at partial discharge is a complex hydrodynamic phenomenon where an incoming steady axisymmetric swirling flow evolves into a three-dimensional unsteady flow field with precessing helical vortex (also called vortex rope) and associated pressure fluctuations. The paper addresses the following fundamental question: is it possible to compute the circumferentially averaged flow field induced by the precessing vortex rope by using an axisymmetric turbulent swirling flow model? In other words, instead of averaging the measured or computed 3D velocity and pressure fields we would like to solve directly the circumferentially averaged governing equations. As a result, one could use a 2D axi-symmetric model instead of the full 3D flow simulation, with huge savings in both computing time and resources. In order to answer this question we first compute the axisymmetric turbulent swirling flow using available solvers by introducing a stagnant region model (SRM), essentially enforcing a unidirectional circumferentially averaged meridian flow as suggested by the experimental data. Numerical results obtained with both models are compared against measured axial and circumferential velocity profiles, as well as for the vortex rope location. Although the circumferentially averaged flow field cannot capture the unsteadiness of the 3D flow, it can be reliably used for further stability analysis, as well as for assessing and optimizing various techniques to stabilize the swirling flow. In particular, the methodology presented and validated in this paper is particularly useful in optimizing the blade design in order to reduce the stagnant region extent, thus mitigating the vortex rope and expending the operating range for Francis turbines.

• Transactions of Materials Processing
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• v.22 no.5
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• pp.250-257
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• 2013

A series of finite element analyses of the rolling process were performed and a neural network algorithm was employed to calculate the amount of ski-effect for an arbitrary thickness of incoming material in the roll gap. Pilot hot plate rolling tests were also conducted to verify the usefulness of the finite element analyzes conducted in this study. In these experiments, plates with thicknesses varying from 25 to 65 mm were tested. In addition, a number of rolling reductions of up to 31% were examined. Finally, a number of circumferential upper and lower rolls were investigated. Experimental validations demonstrated that the neural network algorithm predicted the proper amount of ski when rolling conditions(material thickness, reduction ratio, roll velocity differential) changed arbitrarily.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.4
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• pp.537-549
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• 1996

A numerical study of heat and mass transfer has been carried out for a frost formation process on a circular cylinder in a cross flow including the effect of buoyancy. Studies include cases of low and high Reynolds number flows. The effect of normal velocity at the surface which is produced due to mass transfer was included in the analysis as well as heat transfer contribution generated due to mass transfer. Variations of heat transfer and frost growth both in time and in the circumferential direction have been obtained for various buoyancy parameters. The effect of flow directions(identical or opposite directions to the gravity) has been studied to yield different frost growth. Our results have been compared with existing experimental data and are in good agreement. Buoyancy analyses for a high Reynolds number flow agree with full numerical solutions for the case of having the same flow direction as gravity. However, for the opposite direction case, the boundary layer analyses would not be applicable to predict frost growth except the region near the stagnation point.

• International Journal of Fluid Machinery and Systems
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• v.2 no.1
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• pp.72-79
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• 2009

The effect of symmetric and asymmetric micro regenerative pump impellers on their pressure performance was studied. The shut off head of the pump with the symmetric impeller was about 2.5 times as that with the asymmetric impeller. The computation of the internal flow was performed to clarify the cause of the increase of the head. It was found that the contribution of the angular momentum supply was larger than that of shear stress for the head development in both cases. The larger head and momentum supply in the case of the symmetric impeller were caused by larger recirculated flow rate and larger angular momentum difference between the inlet and outlet to the impeller. The larger recirculated flow rate was caused by smaller pressure gradient in the direction of recirculated flow. The decrease of the circumferential velocity in the casing was attributed to the smaller local flow rate in the casing.

• Transactions of the Korean Society of Automotive Engineers
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• v.22 no.5
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• pp.73-83
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• 2014

CFD(computational fluid dynamics) model is developed to simulate direct injection of ammonia gas phase from ammonia transporting materials into the SCR catalyst in the exhaust pipe of the engine with solid SCR. Configurations of one-hole and four-hole nozzle, circumferential type, porous tube type, and the effect of mixer configurations which commonly used in liquid injection of AdBlue are considered for complex geometries. Mal-distribution index related to concentration of ammonia gas, flow uniformity index related to velocity distribution, and pressure drop related to flow resistance are compared for different configurations of complex geometries at the front section of SCR catalyst. These results are used to design the injection system of ammonia gas phase for solid SCR of target vehicle.

• Journal of Mechanical Science and Technology
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• v.18 no.6
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• pp.972-978
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• 2004

Forward slip is an important parameter often used in rolling-speed control models for tandem hot strip rolling mills. In a hot strip mill, on-line measurement of strip speed is inherently very difficult. Therefore, for the set-up of the finishing mill, a forward slip model is used to calculate the strip speed from roll circumferential velocity at each mill stand. Due to its complexity, most previous researches have used semi-empirical methods in determining values for the forward slip. Although these investigations may be useful in process design and control, they do not have a theoretical basis. In the present study, a better forward slip model has been developed, which provides for a better set-up and more precise control of the mill. Factors such as neutral point, friction coefficient, width spread, shape of deformation zone in the roll bite are incorporated into the model. Implementation of the new forward slip model for the control of a 7-stand hot strip tandem rolling mill shows significant improvement in roll speed set-up accuracy.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.11b
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• pp.1022-1027
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• 2002

This paper describes the fluttering characteristics of the upper pad in a tilting pad journal bearing(6-pad, LOP type) using a steam turbine. In order to investigate the phenomena of the upper pad fluttering experimentally, the absolute vibration of the upper pads the relative vibration between bearing and shaft and the circumferential distribution of the film thickness are measured under the different values of supply oil flow rate, shaft speed and bearing load. It can be known that the fluttering mechanism of the upper pads has a tendency of the self-excited vibration from the study of fluttering frequencies and amplitudes with the change of shaft speed. furthermore, it is observed that the incipient upper pad fluttering velocity is increased by the increase of oil supply flow rate and fluttering amplitude of the upper pads is increased by the decrease of the oil flow rate and by the increase of the bearing load.

• International Journal of Fluid Machinery and Systems
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• v.4 no.2
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• pp.217-222
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• 2011

The rotordynamic fluid forces acting on a closed type impeller in whirling motion were measured and the influence of the clearance geometry on the stability of the impeller was examined. At small positive whirling speed, the rotordynamic forces acted as destabilizing forces for all casings. A small clearance between the shroud of the impeller and the casing caused large fluid force, but did not change the destabilizing region. Radial grooves in the clearance were effective for reducing the fluid forces and destabilizing region due to the reduction of the circumferential velocity without the deterioration of the pump performance. A rotating phenomenon like a rotating stall of the impeller occurred at low flow rate and the resonance between it and the whirling motion led to a sudden increase in force at the whirling speed ratio of 0.7.

• International Journal of Fluid Machinery and Systems
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• v.2 no.3
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• pp.189-196
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• 2009

The effect of inlet and outlet blade angles on a micro regenerative pump head was examined in experiments. The pump head was little increased by changing the blade angles compared with the original pump with the inlet and outlet blade angles of 0 degree. The effect of the axial clearance between the impeller and the casing on the pump head was also examined. The head was increased largely by decreasing the axial clearance. The computation of the internal flow was performed to clarify the cause of the increase of the pump head due to the decrease of the clearance. The local flow rate in the casing decreased as the leakage flow rate through the axial clearance decreased due to the decrease of the clearance. It was found that the larger head in the smaller clearance was just caused by the smaller local flow rate in the casing. In the case of the smaller clearance, the smaller local flow rate caused the smaller circumferential velocity near the front and rear sides of the impeller. This caused the increase of the angular momentum in the casing and the head.