ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sun, 04 Aug 2024 22:00:53 GMT2024-08-04T22:00:53Z501171- A 3D analysis of oxygen transfer in a low-cost micro-bioreactor for animal cell suspension culturehttps://scholarbank.nus.edu.sg/handle/10635/84768Title: A 3D analysis of oxygen transfer in a low-cost micro-bioreactor for animal cell suspension culture
Authors: Yu, P.; Lee, T.S.; Zeng, Y.; Low, H.T.
Abstract: A 3D numerical model was developed to study the flow field and oxygen transport in a micro-bioreactor with a rotating magnetic bar on the bottom to mix the culture medium. The Reynolds number (Re) was kept in the range of 100-716 to ensure laminar environment for animal cell culture. The volumetric oxygen transfer coefficient (kLa) was determined from the oxygen concentration distribution. It was found that the effect of the cell consumption on kLa could be negligible. A correlation was proposed to predict the liquid-phase oxygen transfer coefficient (kLm) as a function of Re. The overall oxygen transfer coefficient (kL) was obtained by the two-resistance model. Another correlation, within an error of 15%, was proposed to estimate the minimum oxygen concentration to avoid cell hypoxia. By combination of the correlations, the maximum cell density, which the present micro-bioreactor could support, was predicted to be in the order of 1012 cells m-3. The results are comparable with typical values reported for animal cell growth in mechanically stirred bioreactors. © 2006 Elsevier Ireland Ltd. All rights reserved.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/847682007-01-01T00:00:00Z
- Experimental investigation of pulsatile flows in tubeshttps://scholarbank.nus.edu.sg/handle/10635/58265Title: Experimental investigation of pulsatile flows in tubes
Authors: Shi, Z.D.; Winoto, S.H.; Lee, T.S.
Abstract: Based on cam-piston-valve arrangement, a mechanical pulsatile flow generator is designed to investigate sinusoidal flow and other types of pulsatile flow in straight rigid tube. Measurement reveals the relation between pressure gradient and flow rate. Numerical simulation using the k-ε turbulence model are carried out to compare the pulsatile flow produced by the generator with a sinusoidal flow and a physiological flow in a rigid tube. The results show that the pulsatile flow generated has similar dynamic properties to the physiological flow. Hence, the present setup can be used for in-vitro investigation of biofluid phenomena.
Thu, 01 May 1997 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582651997-05-01T00:00:00Z
- Effects of air entrainment on the ability of air vessels in the pressure surge suppressionshttps://scholarbank.nus.edu.sg/handle/10635/58213Title: Effects of air entrainment on the ability of air vessels in the pressure surge suppressions
Authors: Lee, T.S.
Abstract: This paper describes a new and efficient model for the study of air entrainment effects on the responses of a typical horizontal air vessel. The effects of air entrainment on the pressure surges for unsteady flow in a pipeline system were investigated. Studies showed that entrained, entrapped, or released gases in the transient fluid system tend to amplify the first pressure peak, increase surge damping, and produce asymmetric pressure surges with respect to the static head. The pressure surges showed longer periods of down-surge and shorter periods of upsurge. The upsurge was considerably amplified and down-surge was marginally reduced when compared with the gas-free case. With the horizontal air vessel installed, studies showed that the effects of air entrainment on the maximum transient pressure can be considerably reduced with an appropriately configured air vessel.
Fri, 01 Sep 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582132000-09-01T00:00:00Z
- Effects of incidence and afterbody shape on flow past bluff cylindershttps://scholarbank.nus.edu.sg/handle/10635/58226Title: Effects of incidence and afterbody shape on flow past bluff cylinders
Authors: Luo, S.C.; Yazdani, Md.G.; Chew, Y.T.; Lee, T.S.
Abstract: In the present paper the effects of the afterbody shape and the angle of incidence on the structure of the flow past a prismatic body are experimentally investigated, both quantitatively in the form of wind tunnel measurement and qualitatively in the form of water tunnel flow visualization. Four cross-sectional shapes with identical upstream facing side and streamwise dimension were chosen in the present investigation. They were a square, two trapeziums, and a triangle. By studying the structure of the flow associated with the above-mentioned shapes, a systematic investigation in which the effects of the afterbody are gradually reduced can be carried out. From the measured magnitude and frequency of the fluctuating aerodynamic forces, the main effect of differences in afterbody shape is the proximity of the two separated shear layers to each other and to their corresponding side faces. This proximity difference, in turn, results in differences in the normal force, the gradient of its variation with the angle of incidence, and, hence, the susceptibility of the shape (with respect) to flow-induced oscillation. Also, due to the difference in interaction between the separated shear layers and the sides of the prismatic structure, the vortex formation length, the base pressure and, hence, the axial force, the rate at which vortices are shed and, hence, the frequency of the aerodynamic loading on the structure and the longitudinal vortex spacing also vary. Changes in the angle of incidence can be viewed as similar to changes in the afterbody shape and, hence, its effects are similar to those caused by changing the afterbody shape. Experiments show that no cross-sectional shape is absolutely stable to galloping oscillation because a shape that is stable to galloping oscillation at a certain mean angle of incidence may become unstable at a different mean angle of incidence. By idealising the vortex wake as two parallel rows of vortices, various vortex street parameters, including the vortex spacing ratio and the strength of each vortex in the wake, are estimated. They show a dependence on the afterbody shape and the angle of incidence. © 1994.
Thu, 01 Dec 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582261994-12-01T00:00:00Z
- Effects of free end and turbulence intensities on triangular prismhttps://scholarbank.nus.edu.sg/handle/10635/58223Title: Effects of free end and turbulence intensities on triangular prism
Authors: Lee, T.S.
Abstract: For three-dimensional prisms of finite length in natural wind environments, the influx of fluid from the top end of the prism can cause considerable alterations to the vortex-shedding process near the free end, and hence variation of the loadings over the top end of the prism. It is thus desirable to know the extent of the wind load variations at the free end of a prism due to changes in the incident turbulence characteristics. This paper is concerned with the effects of turbulence intensities on the surface pressures of the prisms. Finite length triangular prisms are now being used as the main structural members of offshore oil rigs, telecommunication towers, etc.
Wed, 01 Feb 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582231995-02-01T00:00:00Z
- Effects of reynolds number on physiological-type pulsatile flows in a pipe with ring-type constrictionshttps://scholarbank.nus.edu.sg/handle/10635/58235Title: Effects of reynolds number on physiological-type pulsatile flows in a pipe with ring-type constrictions
Authors: Lee, T.S.; Shi, Z.D.
Abstract: The effects of Reynolds number on the physiological-type of laminar pulsatile flow fields within the vicinity of mechanical ring-type constriction in small pipes were studied numerically. The parameters considered are: the Reynolds number (Re) in the range of 50-1500; Strouhal number (St) in the range of 0.00156-3.98; Womersley number (Nw) from 0.0 to 50.0. The pulsatile flows considered were physiological-type of simulated flows. Within a pulsating cycle, detailed flow characteristics were studied through the pulsating contours of streamline (ψ), vorticity (Ω), shear stress (τ) and isobar. The relations between the instantaneous flow rate (Q) and instantaneous pressure gradients (dp/dz) are observed to be elliptic. The relations between the instantaneous flow rate (Q) and pressure loss (P(loss)) are quadratic. Linear relations were observed between the instantaneous flow rate (Q) and the maximum velocity, maximum vorticity and maximum shear stress. The Reynolds number of the flow in a pulsating cycle was found to have significant effects on the recirculation length and the pressure gradient within the pulsatile flow regime.
Fri, 30 Jul 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582351999-07-30T00:00:00Z
- Laminar fluid convection between concentric and eccentric heated horizontal rotating cylinders for low-Prandtl-number fluidshttps://scholarbank.nus.edu.sg/handle/10635/92733Title: Laminar fluid convection between concentric and eccentric heated horizontal rotating cylinders for low-Prandtl-number fluids
Authors: Lee, T.S.
Abstract: Numerical experiments are performed to study rotational effects on the mixed convection of low-Prandtl-number fluids enclosed between the annuli of concentric and eccentric horizontal cylinders. The inner cylinder is assumed to be heated and rotating. The rotational Reynolds number considered is in the range where the effect of Taylor vortices is negligible. The Prandtl number of the fluid considered is in the range 0·01-1·0. The Rayleigh number considered is up to 106. A non-uniform mesh transformation technique coupled with the introduction of `false transient' parameters to the vorticity and streamfunction-vorticity expressions was used to solve the governing set of equations. Results show that when the inner cylinder is made to rotate, the multicellular flow patterns observed in stationary cylindrical annuli subside in a manner depending on the Prandtl number of the fluids. Eventually the flow tends toward a uniform flow similar to that of a solid body rotation. For a fixed Rayleigh number and with a Prandtl number of the order of 1·0, when the inner cylinder is made to rotate, the mean Nusselt number is observed to decrease throughout the flow. For lower Prandtl number of the order 0·1-0·01 the mean Nusselt number remained fairly constant when the inner cylinder was made to rotate. The mean Nusselt numbers obtained were also compared with available data from other investigators.
Wed, 01 Jan 1992 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927331992-01-01T00:00:00Z
- Laminar fluid and heat flow through an opened rectangular cooling chamberhttps://scholarbank.nus.edu.sg/handle/10635/92732Title: Laminar fluid and heat flow through an opened rectangular cooling chamber
Authors: Lee, T.S.
Abstract: A numerical model was developed to study the laminar flow and temperature fields in a flow-through rectangular cooling chamber. Hot fluid is introduced into one end of the chamber and withdrawn from the other end. The top of the chamber is exposed to the atmosphere for cooling and the remaining side-walls are all insulated. The Reynolds number Ro considered is in the range of 100 to 1000 and the densimetric Froude number Fo considered is in the range of 0.5 to 50.0. Numerical experiments that the flow fields and temperature profiles in the flow-through cooling chamber are strong functions of both the Fo and the R°. Comparisons were also made with available experimental and prototype data. © 1991.
Sun, 01 Sep 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927321991-09-01T00:00:00Z
- FLOW VISUALIZATION IN AN ESTUARY DISPERSION MODEL INVESTIGATION.https://scholarbank.nus.edu.sg/handle/10635/74178Title: FLOW VISUALIZATION IN AN ESTUARY DISPERSION MODEL INVESTIGATION.
Authors: Chew, Y.T.; Lee, T.S.; Cheong, H.F.; Ho, J.C.
Abstract: A hydraulic model investigation was conducted to determine the likely estuary dispersion pattern of the discharged effluent from a sewerage pumping station outfall extension work into the surrounding seas. The current motion, discharge rates and density effects were modelled. Isopropanol and rhodamine B dye were used for flow visualization and quantitative effluent dispersion concentration studies. Investigations show that isopropanol and rhodamine B dye simulated well the density stratification dispersion pattern around the estuary.
Tue, 01 Jan 1985 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/741781985-01-01T00:00:00Z
- Effects of conical lids on vortex breakdown in an enclosed cylindrical chamberhttps://scholarbank.nus.edu.sg/handle/10635/85074Title: Effects of conical lids on vortex breakdown in an enclosed cylindrical chamber
Authors: Yu, P.; Lee, T.S.; Zeng, Y.; Low, H.T.
Abstract: The effects of the conical lids on vortex breakdown have been investigated numerically. The boundaries for the onset of vortex breakdown are plotted in terms of the Reynolds number, the aspect ratio H1/R, where H1 is the height of the side wall, and the height ratio H2/H1, where H2 is the height of the axis. The concave-cone lid (H2/H1 >1) delays or even completely suppresses vortex breakdown, while the convex-cone lid (H2/H1
Wed, 01 Nov 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/850742006-11-01T00:00:00Z
- An efficient and robust numerical scheme for the SIMPLER algorithm on non-orthogonal curvilinear coordinates: CLEARERhttps://scholarbank.nus.edu.sg/handle/10635/73157Title: An efficient and robust numerical scheme for the SIMPLER algorithm on non-orthogonal curvilinear coordinates: CLEARER
Authors: Cheng, Y.P.; Lee, T.S.; Low, H.T.; Tao, W.Q.
Abstract: In this article, an Improved SIMPLER (CLEARER) algorithm is formulated to solve the incompressible fluid flow and heat transfer on the nonstaggered, nonorthogonal curvilinear grid system. By virtue of a modified momentum interpolation method in calculating the interface contravariant velocity in both the predictor step and the corrector step, the coupling between pressure and velocity is fully guaranteed, and the conservation law is also satisfied. A second relaxation factor is introduced in the corrector step, of which the convergent solution is independent. By setting the second relaxation factor less than the underrelaxation factor for the velocity to some extent, both the convergence rate and robustness can be greatly enhanced. Meanwhile, the CLEARER algorithm can also overcome the severe grid nonorthogonality. With the simplified pressure-correction equation, the convergent solution can still be obtained even when the intersection angle among grid lines is as low as 1, which may provide valuable guidance in studying the fluid flow in complex geometries.
Tue, 01 May 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/731572007-05-01T00:00:00Z
- Numerical experiments with fluid convection in tilted nonrectangular enclosureshttps://scholarbank.nus.edu.sg/handle/10635/92750Title: Numerical experiments with fluid convection in tilted nonrectangular enclosures
Authors: Lee, T.S.
Abstract: Numerical experiments were performed on an incompressible fluid contained in a tilted nonrectangular enclosure. Rayleigh numbers of 102-105 and Prandtl numbers of 0.001-100 are considered. The wall angles are 22.5°, 45°, and 77.5° with aspect ratios of 3 and 6. Results indicate that the heat transfer and fluid motion within the enclosure are strong functions of Rayleigh number, Prandtl number, and orientation angle of the enclosure. For Rayleigh numbers greater than 104 and Prandtl numbers greater than 0.1, a minimum and a maximum mean Nusselt number occurred as the angle of orientation was increased from 0° to 360°. A transition in the mode of circulation occurred at the angles corresponding to the minimum or maximum rate of heat transfer.
Sat, 01 Jun 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927501991-06-01T00:00:00Z
- Numerical computation of fluid convection with air enclosed between the annuli of eccentric heated horizontal rotating cylindershttps://scholarbank.nus.edu.sg/handle/10635/92749Title: Numerical computation of fluid convection with air enclosed between the annuli of eccentric heated horizontal rotating cylinders
Authors: Lee, T.S.
Abstract: In this paper, numerical experiments are performed to study the effects of the convective fluid motion of air enclosed between the annuli of eccentric horizontal cylinders. The inner cylinder is assumed heated and rotating. The rotational Reynolds number (Re) considered is in the range 0-1120; the Rayleigh number (Ra) considered is in the range 103-106. When the inner cylinder is made to rotate, numerical experiments show that the multicellular flow patterns observed in stationary cylindrical annuli subside in a manner dependent on the eccentricity and the rotational Re of the inner cylinder. At higher rotational Re, the flow tends toward a uniform flow. With a fixed Ra, when the inner cylinder is made to rotate, the mean Nusselt number decreases throughout the flow. © 1992.
Wed, 01 Jul 1992 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927491992-07-01T00:00:00Z
- Numerical studies of transitional turbulent pulsatile flow in pipes with ring-type constrictionshttps://scholarbank.nus.edu.sg/handle/10635/92755Title: Numerical studies of transitional turbulent pulsatile flow in pipes with ring-type constrictions
Authors: Lee, T.S.; Shi, Z.D.; Winoto, S.H.
Abstract: Pulsatile flows in the vicinity of mechanical ring-type constrictions in pipes were studied for transitional turbulent flow with a Reynolds number (Re) of the order of 104. The Womersley number (Nw) is in the range 30-50, with a corresponding Strouhal number (St) range of 0·0143-0·0398. The pulsatile flows considered are a pure sinusoidal flow, a physiological flow and an experimental pulsatile flow profile for mechanical aortic valve flow simulations. Transitional laminar and turbulent flow characteristics in an alternating manner within the pulsatile flow fields were studied numerically. It was observed that fluid accelerations tend to suppress the development of flow disturbances. All the instantaneous maximum values of turbulent kinetic energy, turbulent viscosity and turbulent shear stress are smaller during the acceleration phase than during the deceleration period. Various parametric equations have been formulated through numerical experimentation to better describe the relationships between the instantaneous flow rate (Q), the pressure loss (ΔP), the maximum velocity (Vmax), the maximum vorticity (ζmax), the maximum wall vorticity (ζw,max), the maximum shear stress (τmax) and the maximum wall shear stress (τw,max) for turbulent pulsatile flow in the vicinity of constrictions in the vascular tube. An elliptic relationship has been found to exist between the instantaneous flow rate and the instantaneous pressure gradient. Other linear and quadratic relations between various flow parameters were also obtained.
Sun, 30 Jun 1996 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927551996-06-30T00:00:00Z
- Numerical studies of mixed recirculatory flow in annuli of stationary and rotating horizontal cylinders with different radius ratioshttps://scholarbank.nus.edu.sg/handle/10635/92754Title: Numerical studies of mixed recirculatory flow in annuli of stationary and rotating horizontal cylinders with different radius ratios
Authors: Lee, T.S.
Abstract: Mixed recirculatory flow in the annuli of stationary and rotating horizontal cylinders were studied numerically. A set of distorted 'false transient' parameters were introduced to speed up the steady state solution of the unsteady vorticity, energy and stream function-vorticity equations. The inner cylinder of the annuli is assumed heated and rotating at Reynolds numbers that exclude the effects of centrifugal acceleration and three-dimensional Taylor vortices. The Prandtl number considered is in the range of 0.01 to 1.0 and Rayleigh number in the range of 102 to 106. Radius ratios of the cylinders considered are 1.25, 2.5 and 5.0. For a radius ratio of 2.5, inner cylinder rotation in the Reynolds number range of 0 to 1120 was considered. Vertical eccentricities in the range of ±2/3 were studied for cases of the rotating inner cylinder. Numerical experiments show that the mean Nusselt number increases with Rayleigh number for both cases of concentric and eccentric stationary inner cylinder. At a Prandtl number of order 1.0 with a fixed Rayleigh number, when the inner cylinder is made to rotate, the mean Nusselt number decreases throughout the flow. At lower Prandtl number of the order 0.1 to 0.01, the mean Nusselt number remained fairly constant with respect to the rotational Reynolds number.
Thu, 01 Dec 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927541994-12-01T00:00:00Z
- Numerical study of mixed heat and fluid flow in annuli of heated rotating cylindershttps://scholarbank.nus.edu.sg/handle/10635/92758Title: Numerical study of mixed heat and fluid flow in annuli of heated rotating cylinders
Authors: Lee, T.S.
Abstract: Convective flow in the annuli of rotating concentric cylinders were studied through a numerical model. The inner cylinder is heated and rotating in a range of Reynolds numbers where the effects of centrifugal acceleration and three dimensional Taylor vertices are considered negligible. The Prandtl number considered here varies from 0.01 to 1.0 and Rayleigh number varies from 103 to 105. Inner cylinder rotation in the Reynolds number range of 0 to 1120 was considered. Numerical experiments show that the mean Nusselt number increases generally with Rayleigh number. For Prandtl number of the order 0.01 to 0.1, the mean Nusselt number remains fairly constant when the inner cylinder is rotated. Above a critical Rayleigh number, for Prandtl number of order 1.0, when the inner cylinder is made to rotate, the mean Nusselt number decreases through out the flow. For both stationary and rotating cylinders, the flow patterns observed in the annular space are very different for the range of Prandtl number fluids considered here. Mono-thermal plume above the stationary inner was observed for higher Prandtl number fluids while bi-thermal plume above the stationary inner cylinder was observed for lower Prandtl number fluids. When the inner cylinder is made to rotate, the thermal plume for higher and lower Prandtl number fluids moved in different direction.
Thu, 01 Jan 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927581998-01-01T00:00:00Z
- Numerical Flow Visualization of Stall Suppression of a Symmetrical Aerofoil by Leading-edge Moving Surfacehttps://scholarbank.nus.edu.sg/handle/10635/92752Title: Numerical Flow Visualization of Stall Suppression of a Symmetrical Aerofoil by Leading-edge Moving Surface
Authors: Chew, Y.T.; Lee, T.S.; Li, W.P.; Pan, L.S.; Ng, Y.T.
Abstract: This paper applies the numerical simulation techniques based on the generalized conservation of circulation (GCC) method to investigate the effects of momentum injection by a leading-edge moving surface on flow past a two-dimensional aerofoil at a Reynolds number of 1000. The stream function and vorticity contours obtained together with the animated flow visualization show that the stall flow region is highly unsteady and consist mainly of large vortices being shed alternately. They are confined to a narrow region near the upper surface of aerofoil as Cu (the ratio of the speed of the moving surface to the free stream velocity) is raised. The proximity of vortices to the upper surface of aerofoil at high Cu is caused by the ability of free stream to negotiate around the leading edge since the leading-edge moving surface suppresses the growth of boundary layer by reducing the relative between the inviscid flow and the wall. As well-formed large scale vortices are associated with low pressure regions, their proximity to the aerofoil leads to increase in lift as speed ratio increases.
Sat, 01 Jan 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927522000-01-01T00:00:00Z
- Numerical simulation of the effect of a moving wall on separation of flow past a symmetrical aerofoilhttps://scholarbank.nus.edu.sg/handle/10635/92753Title: Numerical simulation of the effect of a moving wall on separation of flow past a symmetrical aerofoil
Authors: Chew, Y.T.; Pan, L.S.; Lee, T.S.
Abstract: This paper applies the numerical simulation technique based on the generalized conservation of circulation (GCC) method to investigate the effects of a leading-edge rotating circular cylinder on the suppression of stall flow past a symmetrical Joukowski aerofoil. The variables investigated were the angle of attack α and the ratio of the surface velocity of the cylinder to freestream velocity, C U. The Reynolds number based on chord length is 1.43 × 10 5. It was found that the separation point on the upper surface of the aerofoil shifts downstream with increasing C U and stall flow can be significantly suppressed even at α up to 30° when C U = 4. The lift coefficient C L increases and the drag coefficient C d decreases with increasing C U and the optimum C L/C d occurs at a = 8°. The maximum C L/C d obtained is about 60 at C U = 4. © IMechE 1998.
Thu, 01 Jan 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927531998-01-01T00:00:00Z
- Numerical experiments with laminar buoyant discharge into reservoirshttps://scholarbank.nus.edu.sg/handle/10635/92751Title: Numerical experiments with laminar buoyant discharge into reservoirs
Authors: Lee, T.S.
Abstract: Fluid flow and temperature fields in the two-dimensional vertical plane of a reservoir are numerically modeled for inflow Reynolds number R0 in the range 100-1000 and inflow densimetric Froude number F0 in the range 0.5-50.0. Both surface inflow and submerged inflow were studied for reservoir slopes of 45° and 90°. Numerical experiments show that the flow fields and temperature distributions in the reservoir are primarily a function of F0, while the dependence on R0 is implicitly coupled to F0. The strengths and extent of recirculation of the flow fields in the reservoirs were observed to be characterized by functions that are directly proportional to 1/F0 2 and R0.
Mon, 01 Jun 1992 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927511992-06-01T00:00:00Z
- Numerical study of effects of pulsatile amplitude for transitional turbulent pulsatile flow in pipes with ring-type constrictionshttps://scholarbank.nus.edu.sg/handle/10635/92756Title: Numerical study of effects of pulsatile amplitude for transitional turbulent pulsatile flow in pipes with ring-type constrictions
Authors: Lee, T.S.; Shi, Z.D.
Abstract: The effects of pulsatile amplitude on sinusoidal transitional turbulent flows through a rigid pipe in the vicinity of a sharp-edged mechanical ring-type constriction have been studied numerically. Pulsatile flows were studied for transitional turbulent flow with Reynolds number (Re) of the order of 104. Womersley number (Nw) of the order of 50 with a corresponding Strouhal number (St) of the order of 0.04. The pulsatile flow considered is a sinusoidal flow with dimensionless amplitudes varying from 0.0 to 1.0. Transitional laminar and turbulent flow characteristics in an alternative manner within the pulsatile flow fields were observed and studied numerically. The flow characteristics were studied through the pulsatile contours of streamlines, vorticity, shear stress and isobars. It was observed that fluid accelerations tend to suppress the development of flow disturbances. All the instantaneous maximum values of turbulent kinetic energy, turbulent viscosity, turbulent shear stress are smaller during the acceleration phase when compared with those during deceleration period. Various parametric equations within a pulsatile cycle have also been formulated through numerical experimentations with different pulsatile amplitudes. In the vicinity of constrictions, the empirical relationships were obtained for the instantaneous flow rate (Q), the pressure gradient (dp/dz), the pressure loss (P(loss)), the maximum velocity (V(max)), the maximum vorticity (ζ(max)), the maximum wall vorticity (ζ(w,max)), the maximum shear stress (τ(max)) and the maximum wall shear stress (τ(w,max)). Elliptic relation was observed between flow rate and pressure gradient. Quadratic relations were observed between flow rate and the pressure loss, the maximum values of shear stress, wall shear stress, turbulent kinematic energy and the turbulent viscosity. Linear relationships exist between the instantaneous flow rate and the maximum values of vorticity, wall vorticity and velocity. The time-average axial pressure gradient and the time average pressure loss across the constriction were observed to increase linearly with the pulsatile amplitude.
Fri, 01 Jan 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927561999-01-01T00:00:00Z
- Numerical study of effects of pulsatile amplitude on unsteady laminar flows in rigid pipe with ring-type constrictionshttps://scholarbank.nus.edu.sg/handle/10635/92757Title: Numerical study of effects of pulsatile amplitude on unsteady laminar flows in rigid pipe with ring-type constrictions
Authors: Lee, T.S.; Ng, T.W.; Shi, Z.D.
Abstract: The effects of pulsatile amplitude on sinusoidal laminar flows through a rigid pipe with sharp-edged ring-type constrictions have been studied numerically. The parameters considered are: mean Reynolds number (Re) of the order of 100; Strouhal number (St) in the range 0.0-3.98; Womersley number (Nw) in the range 0.0-50.0. The pulsatile amplitude (A) varies in the range 0.0-2.0. The flow characteristics were studied through the pulsatile contours of streamline, vorticity, shear stress and isobars. Within a pulsatile cycle the relations between instantaneous flow rate (Q) and instantaneous pressure gradient (dp/dz) are observed to be elliptic. The relations between instantaneous flow rate (Q) and pressure loss (Ploss) are quadratic. Linear relations exist between instantaneous flow rate (Q) and maximum velocity, maximum vorticity and maximum shear stress.
Sat, 15 Feb 1997 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927571997-02-15T00:00:00Z
- Separation and reattachment of fluid flow through series vascular constrictionshttps://scholarbank.nus.edu.sg/handle/10635/92782Title: Separation and reattachment of fluid flow through series vascular constrictions
Authors: Lee, T.S.; Low, H.T.
Abstract: The flow fields in the neighbourhoods of vascular constrictions in series are studied numerically for a Reynolds number range of 5 to 200, spacing ratios of 1,2,3 and ∞ for diameter constriction ratios of 0.2 to 0.8. The effect on streamlines, vorticity, flow separation and reattachment, velocity distribution and wall vorticity as the fluid flow passes through two adjacent vascular stenoses are numerically investigated. With the Reynolds number increased, the separation point of the eddy was noted to move upstream from the constriction surface; and the reattachment point of the recirculating flow moved downstream. If the upstream constriction was more severe than the downstream constriction (c1>c2), a recirculation zone usually filled the valley region between the two constrictions and the pattern of flow tends to amplify the wall vorticity values of the downstream constrictions. For a less severe upstream constriction (c1
Thu, 01 Dec 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927821994-12-01T00:00:00Z
- Wave equation model for ship waves in bounded shallow waterhttps://scholarbank.nus.edu.sg/handle/10635/92816Title: Wave equation model for ship waves in bounded shallow water
Authors: Lee, T.S.; Wu, J.K.; Xiong, C.G.; Shu, C.
Abstract: Ships were modeled as moving pressure disturbances on the free surface of a shallow water basin. The moving-pressure generating waves were subjected to the reflection of land boundaries and the radiation of open boundaries. This paper proposed and examined a wave equation model (WEM) to solve the shallow water equations with moving surface pressures simulating ship waves in a bounded shallow water region. The Galerkin finite element method was used to solve a second order wave equation for the free surface elevations and the hydrodynamic pressure of the ship bottom simultaneously. Horizontal velocities were obtained from the momentum equations. Numerical solutions of Series 60 CB=0.6 ships moving with the depth Froude number of Fh=0.6, 1.0, 1.3 in a rectangular shallow water harbor were investigated. Three dimensional surface elevation profiles and the depth-averaged horizontal velocities were analysed. The numerical results characterized very well the ship waves in shallow water. Strong boundary reflection waves were found in the case of high depth Froude number (Fh=1.3) Waves generated by the interactions of two ships moving in the same directions and in the opposite directions were also numerically investigated.
Fri, 01 Dec 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928162000-12-01T00:00:00Z
- Steady laminar fluid flow through variable constrictions in vascular tubehttps://scholarbank.nus.edu.sg/handle/10635/92786Title: Steady laminar fluid flow through variable constrictions in vascular tube
Authors: Lee, T.S.
Abstract: Steady laminar flow fields in the neighborhoods of two consecutive constrictions in a vascular tube were studied for approaching Reynolds number Re in the range of 5 to 200. The upstream stenosis was set at a dimensionless diameter constriction c1 of 0.5 while the downstream stenoses were allowed to vary from c2=0.2 to 0.6. The proximity of the constrictions was determined by the spacing of S/D=1, 2, 3, and ∞. When c2>c1, a recirculation zone filled the valley between the two constrictions with little changes to the separation and reattachment points as Re was further increased. For c2
Tue, 01 Mar 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927861994-03-01T00:00:00Z
- Galloping of prismatic bodies with different afterbody shapeshttps://scholarbank.nus.edu.sg/handle/10635/92827Title: Galloping of prismatic bodies with different afterbody shapes
Authors: Lee, T.S.; Luo, S.C.; Chew, Y.T.; Yazdani, M.G.
Abstract: Many offshore and polar structures consists of slender prismatic bodies with different cross-sectional shapes. In an ISOPE-93 article by Yazdani et. al (1993) it was shown that at a mean angle of attack [ α ] of zero degrees square cylinder become unstable to galloping oscillation. However, at this same condition, trapezoidal and triangular cylinders exhibit a stable and neutrally stable condition to galloping oscillation respectively. In the present paper, the conditions of galloping stability of cylinders with the same three shapes are examined experimentally at both zero and non zero α. All the experiments were carried out in an open loop wind tunnel. At the experimental wind speed the level of turbulence in the tunnel was no more than 0.5%, which is considered equivalent to a calm sea-state. Dynamic tests, where the cylinder was forced to oscillate in a direction that is transverse to the free stream, were also carried out at amplitude [ a], oscillation frequency [fN] and free stream velocity [UINF]. While a/d (d = cylinder cross-stream dimension) was kept constant at a value of 1 in the experiment, fN and UINF were varied over the range of 1 Hz to 3 Hz and 5 m/s to 20 m/s respectively. The reduced velocity [Ur=UINF/(fNd)] and Reynolds number therefore varied in the range of 33.34 to 400 and 1.5 × 104 to 6.0 × 104 respectively. Dynamic tests were carried out at α of 5°, 10°, 14°, 20° and 30° for the trapezoidal and at α = 5°, 10°, 15°, 22° and 35° for the triangular cylinder. By analyzing the normal force [CN] versus α curves and in particular the sign and magnitude of ∂CN/∂α at different α, it was observed that there exists several ranges of α within which galloping instability for these three shapes becomes possible.
Sat, 01 Jan 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928271994-01-01T00:00:00Z
- Numerical studies on effect of check valve performance on pressure surges during pump trip in pumping systems with air entrainmenthttps://scholarbank.nus.edu.sg/handle/10635/68007Title: Numerical studies on effect of check valve performance on pressure surges during pump trip in pumping systems with air entrainment
Authors: Lee, T.S.
Abstract: Recent numerical investigations on pressure surges during pump trip in pumping installations showed that by including an air entrainment variable wave speed model, reasonable predictions of transient pressure surges with proper phasing and attenuation of pressure peaks can be obtained. These calculated results are consistent with similar field measurements made with the pumps operating at low pump cut-out levels, when air entrainment due to an attached surface vortex was observed. However, in the numerical calculation procedures it is assumed that the inertia of the moving elements of the check valve is small and that the check valve closes at zero reverse flow velocity. In practice, check valves seldom close precisely at zero reverse flow velocity. With the check valves not closing at zero reverse velocity, the present numerical computations show that the air content in a fluid system can adversely affect the check valve performance. (from Author)
Sun, 01 Jan 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/680071995-01-01T00:00:00Z
- Computer visualization of fluid circulation in annuli of heated rotating cylinders of low prandtl number fluidshttps://scholarbank.nus.edu.sg/handle/10635/84932Title: Computer visualization of fluid circulation in annuli of heated rotating cylinders of low prandtl number fluids
Authors: Lee, T.S.; Xu, Z.; Huo, Y.
Abstract: The present work considered fluid circulation in an annular region between two cylinders with the inner cylinder heated and rotating. The Prandtl number (Pr) considered here varies from 0.01 to 1.0 and Rayleigh number (Ra) is of the order of 10 5. Reynolds number Re in the range of 0 to 1120 was considered. Mono-thermal plume above the stationary inner was observed for higher Prandtl number fluids (Pr≫0.1) while bi-thermal plume above the stationary inner cylinder was observed for lower Prandtl number fluids (Pr≪0.1). However, when the inner cylinder is made to rotate, the thermal plume for higher and lower Prandtl number fluids were observed to move in different directions. The mechanism of the mono- and bi-thermal plumes movements were investigated through numerical flow simulations.
Tue, 01 Jun 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/849322004-06-01T00:00:00Z
- Flow in a pipe with a ring-type obstaclehttps://scholarbank.nus.edu.sg/handle/10635/92707Title: Flow in a pipe with a ring-type obstacle
Authors: Shi, Z.D.; Lee, T.S.; Winoto, S.H.
Abstract: Turbulent calculations have been carried out to investigate flows in a circular pipe with a ring-type obstacle attached to the wall for Reynolds numbers from 102 to 105. The numerical procedure is based on the solution of the primitive variable formulation of the Reynolds-averaged Navier-Stokes governing equations and the k - ε turbulence model in axisymmetric co-ordinate system and with a non-staggered grid. The obstacle opening ratio (d/D) is 0.5, and the obstacle thickness ratio (h/D) is 0.13. The numerical results reveal the effect of the Reynolds number on the flow fields. With the Reynolds number varying from 102 to 105, the reattachment length (Zr/D) increases to a maximum of 3.1 at Re = 5 × 102 and then decreases gradually to a value of 2.1. The velocity profiles of the fully developed flow change from parabolic to power-law curves. The non-dimensional maximum turbulent shear stress (τmax) varies in a range from 0.2 to 0.4 and the pressure loss (fioss) across the obstacle varies between 8.0 and 11.0, while the maximum vorticity (Ωmax) remains unchanged at a value of 16.2. At Re = 5 × 102 and 103, laminar cores exist downstream of the obstacle. For the cases of Re>3 × 104, the τmax and Ploss remain unchanged at 0.32 and 10.5, respectively. The flow field structures are similar. The distributions of the centreline turbulent kinetic energy, velocity and pressure along axial distance remain unchanged.
Mon, 01 Jan 1996 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927071996-01-01T00:00:00Z
- Visualization of fluid motions in parallelogrammic enclosures with varying gravitational fieldshttps://scholarbank.nus.edu.sg/handle/10635/92836Title: Visualization of fluid motions in parallelogrammic enclosures with varying gravitational fields
Authors: Lee, T.S.
Abstract: Numerical flow visualization was made on air contained in parallelogrammic thermal enclosures under various angles of gravitational fields for enclosure aspect ratio of 3.0. Rayleigh numbers considered are in the range of 10 3 to 106. The gravitational fields act on the enclosures in angles varying from 0°to 360°in steps of 15°. For air with a Prandtl number of 0.7, a transition in the mode of fluid circulation was observed when the gravitational field is varied and the mean Nusselt changed from maximum to minimum. Flow visualization experiments were made for air with Pra0.7 at Raa 2x105 with aspect ratios of the order ≥ 1.
Sat, 01 Jan 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928361994-01-01T00:00:00Z
- Numerical simulation on mass transport in a microchannel bioreactor for co-culture applicationshttps://scholarbank.nus.edu.sg/handle/10635/60955Title: Numerical simulation on mass transport in a microchannel bioreactor for co-culture applications
Authors: Zeng, Y.; Lee, T.-S.; Yu, P.; Low, H.-T.
Abstract: Microchannel bioreactors have applications for manipulating and investigating the fluid microenvironment on cell growth and functions in either single culture or co-culture. This study considers two different types of cells distributed randomly as a co-culture at the base of a microchannel bioreactor: absorption cells, which only consume species based on the Michaelis-Menten process, and release cells, which secrete species, assuming zeroth order reaction, to support the absorption cells. The species concentrations at the co-culture cell base are computed from a three-dimensional numerical flow-model incorporating mass transport. Combined dimensionless parameters are proposed for the co-culture system, developed from a simplified analysis under the condition of decreasing axial-concentration. The numerical results of species concentration at the co-culture cell-base are approximately correlated by the combined parameters under the condition of positive flux-parameter. Based on the correlated results, the critical value of the inlet concentration is determined, which depends on the effective microchannel length. For the flow to develop to the critical inlet concentration, an upstream length consisting only of release cells is needed; this upstream length is determined from an analytical solution. The generalized results may find applications in analyzing the mass transport requirements in a co-culture microchannel bioreactor. Copyright © 2007 by ASME.
Fri, 01 Jun 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609552007-06-01T00:00:00Z
- Numerical study of physiological turbulent flows through stenosed arterieshttps://scholarbank.nus.edu.sg/handle/10635/60968Title: Numerical study of physiological turbulent flows through stenosed arteries
Authors: Liao, W.; Lee, T.S.; Low, H.T.
Abstract: A detailed analysis on the characteristics of transitional turbulent flow over a bell-shape stenosis for a physiological pulsatile flow is presented. The comparison of the numerical solutions to three types of pulsatile flows, including a physiological flow, an equivalent pulsatile flow and a simple pulsatile flow, are made in this work. Then the effects of the Reynolds number, Womersley number and constriction ratio of stenosis on the pulsatile turbulent flow fields for the physiological flow are considered. The comparison of the three pulsatile flows shows that the flow characteristics cannot be properly estimated if an equivalent or simple pulsatile inflow is used instead of actual physiological one in the study of the pulsatile flows through arterial stenosis. The equivalent or simple pulsatile inflow can lead to higher disturbance intensity in the vicinity of the stenosis than the physiological inflow. For a physiological flow, the recirculation zones with high disturbance intensity occur mainly in the distal of the stenosis. The larger Reynolds number and severer constriction ratio may result in more complex flow field and cause some important flow variables to increase dramatically near stenosis. The higher Womersley number leads to a larger phase lag between the imposed flow rate changes and the final converged flow field in one cycle. The turbulence intensity decreases with the increase of Womersley number for the same Reynolds number.
Sun, 01 Jun 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609682003-06-01T00:00:00Z
- Numerical study of physiological turbulent flows through series arterial stenoseshttps://scholarbank.nus.edu.sg/handle/10635/60967Title: Numerical study of physiological turbulent flows through series arterial stenoses
Authors: Lee, T.S.; Liao, W.; Low, H.T.
Abstract: A numerical investigation on the characteristics of transitional turbulent flow over series bell-shape stenoses for a physiological pulsatile flow is presented in the present study. The flow behaviours for the physiological pulsatile flow are studied by considering the effects of the Reynolds number, Womersley number, constriction ratio and spacing ratio of the stenoses on the pulsatile turbulent flow fields. Especially, the mutual influences between the double stenoses under different flow conditions are considered. The numerical results show that the variation of these flow parameters puts significant impacts on the flow developments in the arteries with series stenoses. The double stenoses lead to the higher peak turbulence disturbance and the greater area with comparatively high turbulence intensity distal to the stenoses in comparison with the single stenosis. The analysis shows that for the physiological pulsatile flow, the downstream stenosis usually does not have perceptible influences on the upstream flow fields. © 2004 John Wiley and Sons, Ltd.
Thu, 30 Sep 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609672004-09-30T00:00:00Z
- Operator-splitting method for the analysis of cavitation in liquid-lubricated herringbone grooved journal bearingshttps://scholarbank.nus.edu.sg/handle/10635/61007Title: Operator-splitting method for the analysis of cavitation in liquid-lubricated herringbone grooved journal bearings
Authors: Wu, J.; Li, A.; Lee, T.S.; Shu, C.; Wan, J.
Abstract: This paper presents an operator-splitting method (OSM) for the solution of the universal Reynolds equation. Jakobsson-Floberg-Olsson (JFO) pressure conditions are used to study cavitation in liquid-lubricated journal bearings. The shear flow component of the oil film is first solved by a modified upwind finite difference method. The solution of the pressure gradient flow component is computed by the Galerkin finite element method. Present OSM solutions for slider bearings are in good agreement with available analytical and experimental results. OSM is then applied to herringbone grooved journal bearings. The film pressure, cavitation areas, load capacity and attitude angle are obtained with JFO pressure conditions. The calculated load capacities are in agreement with available experimental data. However, a detailed comparison of the present results with those predicted using Reynolds pressure conditions shows some differences. The numerical results showed that the load capacity and the critical mass of the journal (linear stability indicator) are higher and the attitude angle is lower than those predicted by Reynolds pressure conditions for cases of high eccentricities. © 2004 John Wiley and Sons, Ltd.
Wed, 10 Mar 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/610072004-03-10T00:00:00Z
- Numerical studies of physiological pulsatile flow through constricted tubehttps://scholarbank.nus.edu.sg/handle/10635/60957Title: Numerical studies of physiological pulsatile flow through constricted tube
Authors: Liao, W.; Lee, T.S.; Low, H.T.
Abstract: A detailed analysis on the characteristics of laminar flow over a bell-shaped stenosis for a physiological pulsatile flow is presented in this study. In order to have a good understanding of the physiological pulsatile flow, a comparison of the numerical solutions to three types of pulsatile flows, including a physiological flow, an equivalent pulsatile flow and a pure sinusoidal flow, are made in this work. The comparison shows that the flow behavior cannot be properly estimated if the equivalent or simple pulsatile inlet flow is used in the study of flow fields through stenosed arteries instead of actual physiological one. Then the physiological pulsatile flow is further studied by considering the effect of constriction ratio of stenosis, Womersley number and Reynolds number on the flow behavior through stenosed arteries. The analysis shows that the variation of these flow parameters puts significant impacts on the pulsatile flow field for the physiological flow.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609572004-01-01T00:00:00Z
- Numerical study of fluid flow through double bell-shaped constrictions in a tubehttps://scholarbank.nus.edu.sg/handle/10635/60962Title: Numerical study of fluid flow through double bell-shaped constrictions in a tube
Authors: Lee, T.S.
Abstract: The effects of steady fluid flow through double bell-shaped constrictions in tubes were investigated numerically for the Reynolds number range of 5 to 400. The double constrictions studied were for similar first and second constrictions of 1/3, 1/2 and 2/3. A dimensionless constriction spacing of 1.0 was considered. Study showed that the major part of the mean dimensionless pressure drop in the constricted tube occurs predominantly across the first constriction when flow moves towards the valley region formed by the two constrictions. Minimum pressures along the constricted tubes occurs downstream of each constrictions. When the constriction magnitudes increased, the pressure drop across the same length of the tube increases exponentially. The effect of increasing the Reynolds number for all the constriction values considered here is to increase the spreading of the recirculation region between the valley region of the constrictions. The recirculation region formed between the two constrictions has a deminishing effect on the generation of wall vorticity near the second constriction. The effects are more pronounce when the recirculatory flow from the first constriction has spread over the second constriction. In general, a peak wall vorticity is found slightly upstream of each of the constrictions. When the Reynolds number is increased, the peak wall vorticity increases and its location moved upstream. It is noted for the cases considered here that the peak wall vorticity generated by the first constriction is always greater than the peak wall vorticity generated by the second constriction.
Tue, 01 Jan 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609622002-01-01T00:00:00Z
- Numerical study of fluid transient in pipes with air entrainmenthttps://scholarbank.nus.edu.sg/handle/10635/60963Title: Numerical study of fluid transient in pipes with air entrainment
Authors: Lee, T.S.; Low, H.T.; Huang, W.D.
Abstract: The four-equation two-fluid model of two-component flow has been developed to study the effects of air entrainment and air release on the pressure transient due to the rapid valve closure. A nearly-implicit hydrodynamic numerical scheme is applied. Numerical experiments show that entrained, entrapped and released gases decrease the pressure wave frequency, produce asymmetric pressure surges with respect to the static pressure and result in the difference between up-surge duration and down-surge duration. These results are consistent with the experimental and field data observed by other investigators. A decrease in pressure peaks is also discovered in the given conditions.
Thu, 01 Jul 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609632004-07-01T00:00:00Z
- Numerical simulation on steady flow around and through a porous spherehttps://scholarbank.nus.edu.sg/handle/10635/60956Title: Numerical simulation on steady flow around and through a porous sphere
Authors: Yu, P.; Zeng, Y.; Lee, T.S.; Chen, X.B.; Low, H.T.
Abstract: The flow around and through a porous sphere is investigated numerically. It is found that the recirculating wake existing downstream of the porous sphere is either completely detached from or penetrates it. Specially, in a certain range of Darcy number, the wake may initially increase in size with an increase in Reynolds number but then decrease in size and eventually disappear when the Reynolds number is sufficiently large. For a small Darcy number, the critical Reynolds number for the onset of recirculating wake may be significantly smaller than that of a solid sphere. The present study suggests that the surface curvature has an important effect on the initial position of the onset of recirculating wake. © 2012 Elsevier Inc.
Wed, 01 Aug 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609562012-08-01T00:00:00Z
- Numerical simulation of Richtmyer-Meshkov instability driven by imploding shockshttps://scholarbank.nus.edu.sg/handle/10635/60953Title: Numerical simulation of Richtmyer-Meshkov instability driven by imploding shocks
Authors: Zheng, J.G.; Lee, T.S.; Winoto, S.H.
Abstract: In this paper, the classical piecewise parabolic method (PPM) is generalized to compressible two-fluid flows, and is applied to simulate Richtmyer-Meshkov instability (RMI) induced by imploding shocks. We use the compressible Euler equations together with an advection equation for volume fraction of one fluid component as model system, which is valid for both pure fluid and two-component mixture. The Lagrangian-remapping version of PPM is employed to solve the governing equations with dimensional-splitting technique incorporated for multi-dimensional implementation, and the scheme proves to be non-oscillatory near material interfaces. We simulate RMI driven by imploding shocks, examining cases of single-mode and random-mode perturbations on the interfaces and comparing results of this instability in planar and cylindrical geometries. Effects of perturbation amplitude and shock strength are also studied. © 2008 IMACS.
Mon, 01 Dec 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609532008-12-01T00:00:00Z
- Numerical study of mass transfer coefficient in a 3D flat-plate rectangular microchannel bioreactorhttps://scholarbank.nus.edu.sg/handle/10635/60965Title: Numerical study of mass transfer coefficient in a 3D flat-plate rectangular microchannel bioreactor
Authors: Zeng, Y.; Lee, T.-S.; Yu, P.; Low, H.-T.
Abstract: Microchannel bioreactors have been used in many studies to investigate the fluid microenvironment around cells. In the present study, a three-dimensional numerical model was developed to simulate the fluid flow and mass transfer in a 3D flat-plate rectangular microchannel bioreactor with a monolayer of cells adherent to the bottom. The numerical results of mass transfer were well correlated in terms of two dimensionless parameters: Sherwood number at base (Sh) and effective distance (κ). The generalized results show the mass transfer characteristic in the flat-plate microchannel bioreactors, which may find applications in culturing cells in these bioreactors. © 2006 Elsevier B.V. All rights reserved.
Thu, 01 Feb 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609652007-02-01T00:00:00Z
- Numerical simulation of mass transport in a microchannel bioreactor with cell micropatterninghttps://scholarbank.nus.edu.sg/handle/10635/60950Title: Numerical simulation of mass transport in a microchannel bioreactor with cell micropatterning
Authors: Zeng, Y.; Lee, T.-S.; Yu, P.; Low, H.-T.
Abstract: Micropatterning of two different cell types based on surface modification allows spatial control over two distinct cell subpopulations. This study considers a micropatterned coculture system, which has release and absorption parts alternately arranged at the base, and each part has a single cell type. A micropattern unit was defined and within each unit, there are one release part and one absorption part. The cells in the absorption parts consume species, which are secreted by the cells in the release parts. The species concentrations at the micropatterned cell base were computed from a three-dimensional numerical flow model incorporating mass transport. Different combined parameters were developed for the release and absorption parts to make the data collapse in each part. Combination of the corpse data in the release and absorption parts can be used to predict the concentration distribution through the whole channel. The correlated results were applied to predict the critical length ratio of the release and absorption parts for an actual micropatterned system (Bhatia et al., 1999, "Effect of Cell-Cell Interactions in Preservation of Cellular Phenotype: Co-Cultivation of Hepatocytes and Nonparenchymal Cell," FASEB J. 13, pp. 1883-1900) to avoid species insufficiency based on basic fibroblast growth factor (bFGF). The mass transfer effectiveness was found to be higher with more numbers of micropattern units. The optimal condition for micropatterned coculture bioreactors is achieved by having the product of the length ratio and the reaction ratio equal to 1. This condition was used to optimize the mass transfer in the micropatterned system (Bhatia et al., 1999, "Effect of Cell-Cell Interactions in Preservation of Cellular henotype: Co-Cultivation of Hepatocytes and Nonparenchymal Cell," FASEB J. 13, pp. 1883-1900) based on bFGF. Copyright © 2008 by ASME.
Sun, 01 Jun 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609502008-06-01T00:00:00Z
- Numerical prediction of periodically developed fluid flow and heat transfer characteristics in the sinusoid wavy fin-and-tube heat exchangerhttps://scholarbank.nus.edu.sg/handle/10635/60936Title: Numerical prediction of periodically developed fluid flow and heat transfer characteristics in the sinusoid wavy fin-and-tube heat exchanger
Authors: Cheng, Y.P.; Lee, T.S.; Low, H.T.
Abstract: Purpose: In this paper three-dimensional numerical simulations were conducted for the periodically developed laminar flow in the sinusoid wavy fin-and-tube heat exchanger. Design/methodology/approach: A novel CLEARER algorithm is adopted to guarantee the fully coupling between the pressure and velocity, and it can not only speed up the convergence rate, but also overcome the severe grid non-orthogonality in the wavy fin-and-tube heat exchanger. The influence of wave amplitude, fin pitch, tube diameter and wave density on fluid flow and heat transfer characteristics is analyzed under different Reynolds numbers. Findings: The numerical results show that with the increase of wave amplitude, tube diameter or wave density, both the friction factor and Nusselt number will increase, and the increase rate of friction factor is higher than that of Nusselt number. It is interesting to note that, at low Reynolds numbers the Nusselt number increases with the decrease of fin pitch, while at high Reynolds numbers, the Nusselt number increases with the increase of fin pitch. Originality/value: The numerical results presented in this paper may provide some useful guidance in the design of the wavy fin-and-tube heat exchanger with large number of rows of tubes.© Emerald Group Publishing Limited.
Thu, 01 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609362009-01-01T00:00:00Z
- Numerical simulation of conjugate heat transfer in electronic cooling and analysis based on field synergy principlehttps://scholarbank.nus.edu.sg/handle/10635/60941Title: Numerical simulation of conjugate heat transfer in electronic cooling and analysis based on field synergy principle
Authors: Cheng, Y.P.; Lee, T.S.; Low, H.T.
Abstract: In this paper, the conjugate heat transfer in electronic cooling is numerically simulated with the newly proposed algorithm CLEARER on collocated grid. Because the solid heat source and substrate are isolated from the boundary, special attention is given to deal with the velocity and temperature in the solid region in the full field computation. The influence of openings on the substrate, heat source height and their distribution along the substrate on the maximum temperature and overall Nusselt number is investigated. The numerical results show that the openings on the substrate can enhance the heat transfer as well as increasing the heat source height, meanwhile, by arranging the heat sources coarsely in the front part and densely in the rear part of the substrate, the thermal performance can also be increased. Then the results are analyzed from the viewpoint of field synergy principle, and it is shown that the heat transfer improvement can all be attributed to the better synergy between the velocity field and temperature field, which may offer some guidance in the design of electronic devices. © 2007 Elsevier Ltd. All rights reserved.
Wed, 01 Oct 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609412008-10-01T00:00:00Z
- Numerical study on effect of check valve closure flow conditions on pressure surges in pumping station with air entrainmenthttps://scholarbank.nus.edu.sg/handle/10635/60971Title: Numerical study on effect of check valve closure flow conditions on pressure surges in pumping station with air entrainment
Authors: Lee, T.S.; Leow, L.C.
Abstract: This paper proposes a numerical procedure to better compute the characteristics of pressure surges when check valves close under different flow conditions in a pumping station. Studies have shown that the effects of check valve closure on the pressure transients are predominantly dependent on the magnitude and gradient of the flow velocities immediately downstream of the check valve at the instant of valve closure. Through the present study, it was noted that the transient flow velocities near the check valve of a fluid system are also dependent on the characteristics of the air entrained into the fluid system. An improved numerical computational procedure for the fluid system with air entrainment under different transient conditions downstream of the check valve is also proposed in this paper. With a fluid system operating within the critical range of air entrainment values, the present analysis showed that there is a possibility of 'high pressure surges' when the check valves were closed at flow rates other than the positive flow conditions. This phenomenon was confirmed through field observations. This study thus concludes that a detailed numerical transient analysis of the fluid system, with various assumed amounts of entrained air, is necessary whenever there is the possibility of air entrainment into the fluid system, and that the flow conditions at the instant of check valve closure need to be modelled.
Mon, 01 Jan 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609712001-01-01T00:00:00Z
- Numerical study on sinusoidal fluctuated pulsatile laminar flow through various constrictionshttps://scholarbank.nus.edu.sg/handle/10635/60972Title: Numerical study on sinusoidal fluctuated pulsatile laminar flow through various constrictions
Authors: Lee, T.S.; Liu, X.; Li, G.C.; Low, H.T.
Abstract: Numerical simulations have been carried out for laminar sinusoidal pulsating flow in a tube with smooth single constriction. A second-order finite volume method has been developed to solve the fluid flow governing equations on a non-staggered non-orthogonal grid. The effects of the Reynolds number, the Womersley number, the pulsatile amplitude, the constriction ratio and the constriction length on fluid flow in constricted tube will be investigated. It will be demonstrated that the dynamic nature of the pulsating flow greatly depends on the frequency of the flow changes. It is observed that the peak wall vorticity seems to increase with the increase of Reynolds number, the pulsating amplitude and the constriction ratio. The peak values of instantaneous wall vorticity are not greatly affected by the variation of Womersly number. The constriction length does not put a significant impact on the flow instantaneous streamline behaviors compared with other parameters. However, the peak wall vorticity increases monotonically with the decrease of the constriction length. © 2007 Global-Science Press.
Thu, 01 Feb 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609722007-02-01T00:00:00Z
- Numerical analysis of mixed convection in three-dimensional rectangular channel with flush-mounted heat sources based on field synergy principlehttps://scholarbank.nus.edu.sg/handle/10635/60913Title: Numerical analysis of mixed convection in three-dimensional rectangular channel with flush-mounted heat sources based on field synergy principle
Authors: Cheng, Y.P.; Lee, T.S.; Low, H.T.
Abstract: In this paper the fluid flow and heat transfer characteristics of mixed convection in three-dimensional rectangular channel with four heat sources are investigated numerically. The SIMPLEC algorithm is applied to deal with the coupling between pressure and velocity, and a new high-order stability-guaranteed second-order difference (SGSD) scheme is adopted to discretize the convection term. The influence of four parameters is studied: Richardson number, heat source distribution, channel height and inclination angle. The numerical results are analysed from the viewpoint of the field synergy principle, which says that the enhanced convective heat transfer is related not only to the velocity field and temperature field, but also to the synergy between them. It is found that the effects of the four parameters on the thermal performance can all be explained with the field synergy principle. To obtain better electronic cooling, the synergy between the velocity and temperature gradient should be increased when other conditions are unchanged. Copyright © 2006 John Wiley & Sons, Ltd.
Thu, 30 Nov 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609132006-11-30T00:00:00Z
- Hybrid lattice Boltzmann finite-difference simulation of axisymmetric swirling and rotating flowshttps://scholarbank.nus.edu.sg/handle/10635/60465Title: Hybrid lattice Boltzmann finite-difference simulation of axisymmetric swirling and rotating flows
Authors: Huang, H.; Lee, T.S.; Shu, C.
Abstract: The axisymmetric flows with swirl or rotation were solved by a hybrid scheme with lattice Boltzmann method for the axial and radial velocities and finite-difference method for the azimuthal (or swirl) velocity and the temperature. An incompressible axisymmetric lattice Boltzmann D2Q9 model was proposed to solve the axial and radial velocities through inserting source terms into the two-dimensional lattice Boltzmann equation. Present hybrid scheme was firstly validated by simulations of Taylor-Couette flows between two concentric cylinders. Then the benchmark problems of melt flow in Czochralski crystal growth were studied and accurate results were obtained. Numerical experiment demonstrated that present axisymmetric D2Q9 model is more stable than the previous axisymmetric D2Q9 model (J. Comp. Phys. 2003; 186(1):295-307). Hence, compared with the previous model, present numerical method provides a significant advantage in simulation melt flow cases with high Reynolds number and high Grashof number. Copyright © 2006 John Wiley & Sons, Ltd.
Fri, 20 Apr 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/604652007-04-20T00:00:00Z
- Improvement of simpler algorithm for incompressible flow on staggered grid systemhttps://scholarbank.nus.edu.sg/handle/10635/60497Title: Improvement of simpler algorithm for incompressible flow on staggered grid system
Authors: Lee, T.S.; Cheng, Y.P.; Low, H.T.
Abstract: In this paper, the updating of the intermediate velocity in the correction stage in SIMPLE-like algorithms on staggered grid is discussed in detail, based on the discussion CLEARER algorithm that is extended from the non-staggered grid to the staggered grid. The performance of SIMPLER and CLEARER algorithms are compared using two numerical examples with reliable solutions. The results show that CLEARER can predict the numerical results as accurately as SIMPLER, and it can also enhance the convergence rate greatly under low under-relaxation factors. In some cases CLEARER algorithm can only need 27% of iteration number required by SIMPLER to reach the same convergence criterion. © World Scientific Publishing Company.
Sun, 01 Jul 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/604972007-07-01T00:00:00Z
- Natural convection from an eccentric square cylinder using a novel flexible forcing IB-LBM methodhttps://scholarbank.nus.edu.sg/handle/10635/60875Title: Natural convection from an eccentric square cylinder using a novel flexible forcing IB-LBM method
Authors: Dash, S.M.; Lee, T.S.; Huang, H.
Abstract: The natural convection process in an annulus of a cold square enclosure with a hot eccentric square cylinder is simulated using a novel flexible forcing immerse boundary (IB)-lattice Boltzmann method (LBM). In the conventional IB-LBM schemes, the explicit force density calculation may not ensure the exact no-slip velocity/no-jump temperature boundary condition, which may produce unphysical streamline/isotherm penetration into the cylinder. Here, we proposed a single Lagrangian velocity/temperature correction that satisfies the boundary conditions accurately. The developed scheme is applied to analyze the flow regime and heat transfer, as functions of Rayleigh number and eccentricity of the cylinder. © 2014 Copyright Taylor and Francis Group, LLC.
Sat, 15 Mar 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/608752014-03-15T00:00:00Z
- Lattice-BGK simulation of steady flow through vascular tubes with double constrictionshttps://scholarbank.nus.edu.sg/handle/10635/60641Title: Lattice-BGK simulation of steady flow through vascular tubes with double constrictions
Authors: Huang, H.; Lee, T.S.; Shu, C.
Abstract: Purpose - The aim of the present study is to evaluate the accuracy and efficiency of Lattice-BGK (LBGK) method application in simulation of the 3D flow through complex geometry. On the other hand, the steady flows through vascular tube with Reynolds number 10-150 and different constriction spacing ratios are simulated. Design/methodology/approach - The numerical method is based on the LGBK method with an incompressible D3Q19 model. To treat the curved boundary, the "bounce back" scheme combined with spatial interpolation of second order is applied. Findings - The highly axisymmetric property in the direct 3D tube flow simulation is observed. Solutions obtained from LBGK method are quite consistent with that of finite volume method (FVM). The overall order of accuracy of these LBGK solutions is about 1.89. The LBGK incompressible D3Q19 model with the curved boundary treatment can handle the problems of 3D steady flow through complex geometry. Research limitations/implications - Investigating the flow in constricted vascular tubes with different stenose shape and higher Reynolds number is left for future work. Practical implications - Lattice BGK method is the very useful tool to investigate the steady vascular flow. Originality/value - Applying LBGK method with incompressible D3Q19 model to simulate the steady flow through complex geometry. The accuracy and efficiency of the present LBGK solver are examined. © Emerald Group Publishing Limited.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/606412006-01-01T00:00:00Z
- Investigation of fluid transients in pipelines with air entrainmenthttps://scholarbank.nus.edu.sg/handle/10635/60603Title: Investigation of fluid transients in pipelines with air entrainment
Authors: Lee, T.S.; Nguyen, D.T.; Low, H.T.; Koh, J.Y.
Abstract: This paper introduces a variable wave speed model for the calculation of fluid transients in pipelines with air entrainment in which wave speed at every point along the pipeline depends on the local pressure and local air void fraction at that point. A set of experiments was carried out in a single pump pipeline system to test the fluid transient behaviour for the case of pump trip due to power failure and to validate the numerical model. In comparison with experimental results, numerical results show that the variable wave speed model can provide a closely transient pressure prediction for the first pressure peak. However, there are the discrepancies between the computational predicted results and the experimental results for the next pressure surges especially in the rate of pressure damping. This limitation points to the need for further refinements of the variable wave speed model to improve the accurate of the pressure transient prediction. © 2008 Pushpa Publishing House.
Wed, 01 Oct 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/606032008-10-01T00:00:00Z