Browsing by Author "Tsui, YY"

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A pressure-correction method for incompressible flows using unstructured meshes
(2006-01-01)
Tsui, YY
;
Pan, YF
;
機械工程學系
;
Department of Mechanical Engineering
A pressure-correction method is presented to solve incompressible viscous flows. The development of this method is aimed at dealing with unstructured grids, which are made of control volumes with arbitrary topology. To enhance the robustness of the method, all variables are collocated on the cell centers. The divergence theorem of Gauss is employed for discretization, and vector forms are used throughout the formulation. In this way the method is equally applicable to two- and three-dimensional problems. An overrelaxed approach is adopted for the approximation of the cross-diffusion flux to deal with "skew" grids. It can he seen that this approach is equivalent to some other approximations available in the literature. However, the present approach is more suitable for three-dimensional calculations without causing complication. This overrelaxed approach is also employed in the pressure-correction equation derived from the continuity constraint. Most prevailing "methods simply ignore the cross-derivative term of the pressure-correction equation, which not only causes instability but also slows down convergence rate if the grid is skew. This cross-derivative term is taken into account in the present calculations by using a successive correction procedure. The application of the methodology to flows in lid-driven cavities and diffusers shows that no more than two pressure-correction steps are enough to obtain fast and stable convergence. The method is also applied to a three-dimensional flow in an impeller-stirred tank.
A study of the recirculating flow in planar, symmetrical branching channels
(2006-01-20)
Tsui, YY
;
Lu, CY
;
機械工程學系
;
Department of Mechanical Engineering
A numerical method is employed to examine the now in symmetrical, two-dimensional branches of Y shape and Tee shape. The methodology is based oil a pressure-correction procedure within the frame of unstructured grids. Specified pressures are imposed at the outlets of the two branches. The area ratio of the branch is allowed to vary in the range of 2-3. Separation of the flow in the bifurcating region is inevitable. Willi equal Millet pressures, symmetrical flow patterns prevail except for the Y type branch Under the conditions of high Reynolds numbers and large area ratios. This implies that the Y-branch flow is more sensitive to small disturbances. It is shown that with a slightly higher pressure imposed oil one of the two branches the structure Of the recirculating flow for the Y type is greatly affected and the flow rate is reduced dramatically in the high-pressure branch channel. In contrast, the influence on the Tee type branch is much lower since the flow behaves like a jet impinging oil a confined duct. Copyright (c) 2005 John Wiley & Sons, Ltd.
Analysis of the flow in the grooves of a molecular pump
(2001-07-01)
Tsui, YY
;
Kung, CP
;
Cheng, HP
;
機械工程學系
;
Department of Mechanical Engineering
A computational procedure used to solve conservation equations governing the slip flow in a molecular pump of the Holweck type is developed, The finite volume method is used for discretization and the gr id is arranged in the curvilinear, nonstaggered form The slip boundary condition is imposed on the solid walls. The clearance gap between the rotor and the casing is ignored It is evident fi um the calculation that the pressure difference between the two side walls of the channel, iel med pressure side and suction sick, varies in accordance with the pressure difference between the inlet and the outlet of the pump, Thus, the side-wall pressure difference can play. the role of an indicator of pumping effectiveness, Tests on a variety. of configurations show that to achieve the best performance both the spiral angle and the channel height need to be optimized, Besides, it is better to have fewer flow channels and to place these channels on the rotor instead of on the casing.
Calculation of laminar separated flow in symmetric two dimensional diffusers
(1995-12-01)
Tsui, YY
;
Wang, CK
;
交大名義發表
;
機械工程學系
;
National Chiao Tung University
This study is concerned with numerical analysis of laminar separated flow in symmetric, two-dimensional, straight-walled diffusers. With Reynolds numbers Re = 56 and 114 and expansion ratios ER = 3 and 4, totally, there are four cases considered. At the low Reynolds number and the low expansion reaction the flow in the diffuser is nearly symmetric to the center line, irrespective of the diffusion angle. As Reynolds number or expansion ratio increases, a large recirculation region forms at one side wall and a small one at the other side. For the case with Re = 114 and ER = 4 the small recirculating flow disappears at small diffusion angles and a third recirculating flow appear in the same side of the small main recirculation region for large diffusion angles. The pressure recovery reaches its peak value somewhere downstream of the reattachment point of the large recirculating flow. The effectiveness of the diffuser deteriorates as the diffusion angle increases, apart from that at Re = 56 the effectiveness increases from theta = 15 to 30 deg. Symmetric flow solutions can be obtained by incorporating a symmetric relaxation method. The pressure recovery is higher for the symmetric flow than that for the asymmetric flow owing to the weaker recirculating strength in the former.
Effects of buoyancy and orientation on the flow in a duct preceded with a double-step expansion
(1998-09-01)
Tsui, YY
;
Shu, SJ
;
機械工程學系
;
Department of Mechanical Engineering
A numerical method, employing nonstaggered grid arrangement, is incorporated to deal with mixed convection flow in a duct with a double-step expansion at inlet. The walls downstream of the expansion are maintained at a uniform temperature. In the first series of tests, the duct is placed vertically (gamma = 0 degrees), and the buoyancy effect is examined via gradually increasing Grashof number until no converged solution is obtained. With the aid of buoyancy force the recirculating flows behind the sudden expansion attach to the two step walls when the Grashof number is large enough. In the meanwhile, reversed flow is detected in the centerline region. At Re = 56 the flow remains symmetric throughout the Grashof numbers tested whereas for Re = 114 the flow held may be symmetric or asymmetric, depending on the Grashof number. Also examined is the effect of inclination angle gamma for Ga = 1000 and 3000. In the horizontal position (gamma = 90 degrees), two recirculations exist behind the two step walls. When gamma > 90 degrees, a number of recirculations are formed on the two side walls and the main stream exhibits a wavy structure. The wavy how helps enhance heat transfer. The above results for different duct orientations are equally applicable to both heating and cooling hows. (C) 1998 Elsevier Science Ltd. All rights reserved.
Effects of lobe geometry on the mixing flow in multilobe mixers
(2001-01-01)
Tsui, YY
;
Wu, PW
;
機械工程學系
;
Department of Mechanical Engineering
This study is concerned with a numerical investigation of the mixing flow in multilobe mixers. Predictions are obtained using a finite-volume method which incorporates a second-order difference scheme. The irregular boundaries are fitted by curvilinear nonorthogonal grids and the grid points are arranged in the collocated manner. A method is described to generate suitable three-dimensional grids to cover both the lobe region and the mixing duct. Computations ave undertaken to examine the effects of lobe geometry on the mixing performance. Results indicated that the convex lobe contour can lead to stronger vortex flow and, thus, better mixing because of its large slope. As for the geometry of the lobe trailing edge, a sinusoidal shape with wide peak region gives rise to roell-organized vortices and the mixing performance is enhanced. Also revealed is the mechanism which, resulting in efficient mixing in the mixer, is clearly identified.
Heat transfer enhancement by a multilobe vortex generator in internally finned tubes
(1999-04-01)
Tsui, YY
;
Leu, SW
;
機械工程學系
;
Department of Mechanical Engineering
A three-dimensional computational method is employed to study the flow and heat transfer in internally finned tubes with a multilobe vortex generator inserted. Governing equations are discretized using the finite volume method The irregular lobe geometry is treated using curvilinear nonstaggered grids. The linear interpolation method is adopted to calculate face velocities. The results show that secondary flows induced by the lobes are transformed to become axial vortices downstream of the vortex generator. As a consequence of the transport by the vortex flow, the core flow is moved to the fins and the tube wall, while the wall flow moves to the core. In this way, both heat transfer and flow mixing are enhanced. When the fin height is increased the axial vortex is more restricted in the centerline region, and the strength of the vortex flow, represented by circulation, is decreased. In turn, the total pressure loss is also decreased However, the heat transfer increases with fin height. Consequently, efficiency is greatly promoted.
Heat transfer enhancement by multilobe vortex generators: Effects of lore parameters
(2000-05-12)
Tsui, YY
;
Leu, SW
;
Lin, CC
;
Wu, PW
;
機械工程學系
A three-dimensional computational method is developed to study the flow and heat transfer in a circular tube with a multilobe vortex generator inserted. Governing equations are discretized by the finite volume technique. The irregular lobe geometry is treated using curvilinear nonstaggered grids. Examination of the flow field reveals that the radial and counterradial flows induced inside and outside the lobe are forced to form axial vortices downstream of the vortex generator Because of the transport of the high-speed, low-temperature core flow toward the tube wad by the vortex, heat transfer is enhanced. Also because of the vortex flow, the high-temperature wall flow is carried away from the wall to mix with the low-temperature core flow. Consequently, very effective augmentation in the heat transfer and mixing of flow temperature results. It is shown also that since the slope of the lobe is increased by enlarging the lobe penetration, reducing the lobe length, and making a concave contour geometry, the circulation, representing the strength of the axial vortex induced by the lobe, is promoted, leading to higher heat transfer and flow mixing By increasing the lobe number both the circulation and Nusselt number (Nu) decrease because of the narrowed lobe passage.
Investigation of the mixing flow structure in multilobe mixers
(1996-07-01)
Tsui, YY
;
Wu, PW
;
交大名義發表
;
機械工程學系
;
National Chiao Tung University
A computational procedure is described to study the mixing flow in a multilobe turbofan mixer. The predictions have been obtained using a finite volume method to solve the density-weighted time-averaged Navier-Stokes equations, Turbulence is characterized by the k-epsilon eddy viscosity model, To fit the irregular boundaries of the flowfield, curvilinear nonorthogonal coordinates are employed, The robustness of the computational procedure is enhanced by making use of nonstaggered grids. In the calculation the computational domain contains not only the mixing duct but also the lobe itself, Three kinds of configuration are under consideration: a confluent mixer, a convoluted mixer, and a forced miser, Results show that the streamwise vortex generated at the trailing edge is the most prominent flow structure for the forced mixer, responsible for the effective mixing, The strength of the streamwise vortex can be characterized by the circulation. A parameter termed mixedness is defined to describe the effects of the mixing process. In addition to the streamwise vortex, normal vortices are generated by the velocity difference in the shear layer between the core and fan streams. To illustrate the mechanism of the mixing procedure, velocity vectors and contours of streamwise and normal vorticities and turbulence contours on transverse planes at selected axial locations are presented, The roles played by the streamwise vortex and the normal vortex are clearly identified.
Tumbling/squish interaction in loop-scavenged two-stroke engines
(1997-12-01)
Tsui, YY
;
Cheng, HP
;
機械工程學系
;
Department of Mechanical Engineering
This article describes a numerical investigation of the flow in a loop-scavenged two-stroke engine equipped with a head-bowl combustion chamber. The bowl is either placed in the center of the cylinder head or displaced away from the center. Attention is focused on the period of compression and early expansion. Results show that the effects of compression of the tumbling vortex by the piston become prominent after the midcompression stage, resulting in significant cascading of energy from mean flow to turbulence, followed by a fast decay of turbulence. The subsequent emergence of squish and reverse squish helps retard die decay. As a consequence, the cases with head bowls possess higher levels of mean flow and turbulence than those without bowls at top dead center and the later stage. It is also shown in the results that the offset of the head bowl has significant effects on the mean flow and turbulence characteristics, highly dependent on the offset direction.