• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.2
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• pp.269-275
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• 1990

Nonaxisymmetric bifurcation behaviours of composite tubes two different materials subjected to uniform radial shrinkage at the external surface have been investigated and compared with those of single tube. The effect of material parameters normalized with respect to those of outer tube upon the bifurcation point and corresponding mode has been clarified. The parameters substantially affect the bifurcation mode with long-wavelength so that the composite tube with low hardening exponent or with high yield stress of inner tube destabilizes the overall deformation of the tube. However surface type bifurcation, short-wavelength mode, shown on the traction-free inner surface is hardly affected by the material parameters. The surface type bifurcation completely depends on the material characteristics of inner tube and the bifurcation point of composite tube almost coincides with the of single tube.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.05a
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• pp.495-500
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• 2003

The vibration of a flexible cantilever tube with nonlinear constraints when it is subjected to flow internally with fluids is examined by experiment and theoretical analysis. These kind of studies have often been performed that finds the existence of chaotic motion. In this paper, the important parameters of the system leading to such a chaotic motion such as Young's modulus and coefficient of viscoelasticity in tube material are discussed. The parameters are investigated by means of a system identification so that comparisons are made between numerical analysis using the parameters of a handbook and the experimental results. The chaotic region led by several period-doubling bifurcations beyond the Hopf bifurcation is also re-established with phase portraits and bifurcation diagram so that one can define optimal parameters for system design.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.11a
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• pp.270-275
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• 2003

The nonlinear dynamic characteristic of a straight tube conveying fluid with constraints and an attached mass on the tube is examined in this study. An experimental apparatus composed of an elastomer tube conveying water which has an attached mass and constraints is made and comparisons are done between the theoretical results from non-linear equation of motion of piping system and experimental results. And the results show that the tube is destabilized as the mass of the attached mass increases, and stabilized as the position of the attached mass close to the fixed end. In case of a small end-mass, the system shows rich and different types of periodic solutions. For a constant end-mass, the system undergoes a series of bifurcations after the first Hopf bifurcation, as the flow velocity increases, which causes chaotic motion of the tube eventually.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.14 no.7
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• pp.561-568
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• 2004

The nonlinear dynamic characteristic of a straight tube conveying fluid with constraints and an attached mass on the tube is examined in this study An experimental apparatus with an elastomer tube conveying water which has an attached mass and constraints is made and comparisons are made between the theoretical results from the non-linear equation of motion of piping system and the experimental results. The comparisons show that the tube is destabilized as the magnitude of the attached mass increases, and stabilized as the position of the attached mass closes to the fixed end. In case of a small end-mass, the system shows complicated and different types of solutions. For a constant end-mass. the system undergoes a series of bifurcations after the first Hopf bifurcation, as the flow velocity increases. which causes chaotic motions of the tube eventually.

• Journal of Mechanical Science and Technology
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• v.17 no.12
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• pp.1994-2003
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• 2003

The vibration of a flexible cantilever tube with nonlinear constraints when it is subjected to flow internally with fluids is examined by experimental and theoretical analysis. These kinds of studies have been performed to find the existence of chaotic motion. In this paper, the important parameters of the system leading to such a chaotic motion such as Young's modulus and the coefficient of viscoelastic damping are discussed. The parameters are investigated by means of system identification so that comparisons are made between numerical analysis using the design parameters and the experimental results. The chaotic region led by several period-doubling bifurcations beyond the Hopf bifurcation is also re-established with phase portraits, bifurcation diagram and Lyapunov exponent so that one can define optimal parameters for system design.

• Transactions of the Korean Society of Automotive Engineers
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• v.7 no.6
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• pp.270-278
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• 1999

This paper describes the collapse characteristics of the rectangular tube under eccentric compressive load. Overall buckling stress and bifurcation criterion (slenderness ration)are investigated. modified secant formula(MSF) is proposed to decide overall buckling stress. The bifurcation criterion which can distinguish between the local and overall buckling mode shapes is suggest by equating the local and overall buckling stresses. Additionally the effect of initial imperfection on bifurcation criterion is investigated.

• Nuclear Engineering and Technology
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• v.51 no.1
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• pp.310-316
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• 2019

Fluidelastic instability is a key issue in steam generator tube bundles subjected in cross-flow. With a low flow velocity, a large amplitude vibration of the tube observed by many researchers. However, the mechanism of this vibration is seldom analyzed. In this paper, the mechanism of cross-flow effects on fluidelastic instability of tube bundles was investigated. Analysis reveals that when the system reaches the critical state, there would be two forms, with two critical velocities, and thus two expressions for the critical velocities were obtained. Fluidelastic instability experiment and numerical analysis were conducted to obtain the critical velocity. And, if system damping is small, with increases of the flow velocity, the stability behavior of tube array changes. At a certain flow velocity, the stability of tube array reaches the first critical state, a dynamic bifurcation occurs. The tube array returns to a stable state with continues to increase the flow velocity. At another certain flow velocity, the stability of tube array reaches the second critical state, another dynamic bifurcation occurs. However, if system damping is big, there is only one critical state with increases the flow velocity. Compared the results of experiments to numerical analysis, it shows a good agreement.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.385-388
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• 2008

The objective of this paper is simulating blood flow through the branched and stenotic tube numerically. SC-Tetra, which is one of the commercial code using FVM method, was utilized for this analysis. The flow is assumed as an incompressible laminar flow with the additional condition of non-Newtonian fluid. As the constitutive equation for the fluid viscosity, the following models were solved with governing equations ; Cross Model, Modified Cross Model, Carreau Model and Carreau-Yasuda Model. Final goal was achieved to get analytic data about shear stress, at specific points, changing the geometry with various factors like the bifurcation angle, diameter of the branches, the ratio of stenosis, and etc. The material property of blood was referred from the related papers. Furthermore, to verify results they were compared with those of the published papers. There were some discrepancies based on the different solver and the different data post-processing method. However, many parameters like the location of low shear stress, which arised from bifurcation or stenosis, and the tendency of various factors were found to be very similar.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.4
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• pp.895-904
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• 1998

This paper presents the results of experimental investigation of the buckling behavior of thin-walled box-section column. The experiments for finding the buckling stress and bifurcation slenderness ratio are performed by the method from AISC. The sets of boundary conditions are both end simply supported, one end simply supported and the other end clamped, and both ends clamped. The types of specimens are clssified by thickness to width ratio. The experiments for the thin-walled rectangular tubes are closely concurrent with the theoretical values of overall buckling load and bifurcation slenderness ratio that are suggested by the part (I) of this paper.

• 한국전산유체공학회:학술대회논문집
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• 2008.03a
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• pp.177-180
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• 2008

The objective of this study is to get simulation data about pulsatile flow of a non-Newtonian fluid through a bifurcated tube. All the process was based on CFD method, with a commercial FVM code, SC/Tetra ver. 6.0 for solving, and with CATIA R16 for generating geometries. To define a non-Newtonian fluid, the following viscous models are used; the Powell-Eyring model, the modified Powell-Eyring model, the Cross model, the modified Cross model, the Carreau model, the Carreau-Yasuda model and the modified Power Law model. The flow calculation data using each model were compared with the other data of a existing paper. Finally, the Carreau model was recognized to give the best result with the SC/Tetra code, and the succeeding simulations are made with the model. For the pulsating flow condition, the sine wave type velocity profile is given as the inlet boundary condition. To investigate the effect of geometries and mesh, the pre-test is carried out with various curvature conditions of the bifurcated corner, and then with various mesh conditions. The final process is to calculate flow variables such as the wall shear stress (WSS) and the wall shear stress gradient (WSSG). To validate all the result, the simulation is compared with the existing data of the other papers. Generally speaking, there is a noticeable difference in the maximum and minimum value of WSS. It is not sure that the values in each data are on the exactly same location. However, the overall trend is similar. The next study needs to investigate the same situation by experimental method. Furthermore, if the flow is simulated with more pulsatile conditions, more data of flow field through a bifurcated tube could be achieved.