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  • 1
    Electronic Resource
    Electronic Resource
    Multigrid computations with the immersed boundary technique for multiphase flows (2004)
    Bradford : Emerald
    International journal of numerical methods for heat & fluid flow 14 (2004), S. 98-115 
    Details
    ISSN: 0961-5539
    Source: Emerald Fulltext Archive Database 1994-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Multiphase flow computations involve coupled momentum, mass and energy transfer between moving and irregularly shaped boundaries, large property jumps between materials, and computational stiffness. In this study, we focus on the immersed boundary technique, which is a combined Eulerian-Lagrangian method, to investigate the performance improvement using the multigrid technique in the context of the projection method. The main emphasis is on the interplay between the multigrid computation and the effect of the density and viscosity ratios between phases. Two problems, namely, a rising bubble in a liquid medium and impact dynamics between a liquid drop and a solid surface are adopted. As the density ratio increases, the single grid computation becomes substantially more time-consuming; with the present problems, an increase of factor 10 in density ratio results in approximately a three-fold increase in CPU time. Overall, the multigrid technique speeds up the computation and furthermore, the impact of the density ratio on the CPU time required is substantially reduced. On the other hand, the impact of the viscosity ratio does not play a major role on the convergence rates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1108/09615530410511658
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  • 2
    Electronic Resource
    Electronic Resource
    Evaluation of geometric conservation law using pressure-based fluid solver and moving grid technique (2004)
    Bradford : Emerald
    International journal of numerical methods for heat & fluid flow 14 (2004), S. 851-865 
    Details
    ISSN: 0961-5539
    Source: Emerald Fulltext Archive Database 1994-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: The geometric conservation law (GCL) is an important concept for moving grid techniques because it directly regulates the treatments of the fluid flow and grid movement. With the grid movement at every time instant, the Jacobian, associated with the volume of each element in curvilinear co-ordinates, needs to be updated in a conservative manner. In this study, alternative GCL schemes for evaluating the Jacobian have been investigated in the context of a pressure-based Navier-Stokes solver, utilizing moving grid and the first-order implicit time stepping procedure as well as the PISO scheme. GCL-based on first and second-order, implicit as well as time-averaged, time integration schemes were considered. Accuracy and conservative properties were tested on steady-state, laminar flow inside a 2D channel and time dependent, turbulent flow around a 3D elastic wing; both treated with moving grid techniques. It seems that the formal order of accuracy is not a decisive indicator. Instead, the speed of grid movement and the interplay between the flow solver and the GCL treatments make a more noticeable impact.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1108/09615530410546254
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  • 3
    Electronic Resource
    Electronic Resource
    Effects of convection and electric field on thermofluid transport in horizontal high-pressure mercury arcs (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 1712-1719 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A three-dimensional theoretical model based on first principles has been developed to predict the characteristics of mass, momentum, energy, and electrostatic potential transport in high-pressure mercury arcs confined in a quartz arctube. The model is utilized to systematically investigate the impact of convection on the transport process by including and excluding the gravity effect. Strong three-dimensional convection flows with multiple contrarotating vortices have been identified. These vortices substantially change the energy balance within the arc, causing highly nonuniform gas temperature distribution and lowering the maximum gas temperature. Geometrical modifications of the arctube such as wall contour curvature and electrode offsets do not change the strength of convection but can produce better overall temperature uniformity within the arctube by accommodating the upward-moving tendency of temperature contours caused by convection. In agreement with the experimental measurement, the model predicts that the arctube curvature can cause large differences of wall temperature profiles, including shape, level, and locations of peak values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.346097
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  • 4
    Electronic Resource
    Electronic Resource
    Double-diffusive flow in enclosures (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2592-2607 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Numerical calculations are conducted for two-dimensional steady-state double-diffusive flow in enclosures, where both the temperature and solute gradients are imposed horizontally, and the two buoyancy effects can either augment or counteract each other. Important controlling parameters including the thermal and solutal Grashof numbers, Schmidt number, and Prandtl number, are varied and new insights into the solutions of both the augmenting and counteracting modes gained. Even though the relative directions of the two buoyancy mechanisms reverse between the augmenting and counteracting modes, qualitative similarities in convection characteristics emerge. For both modes, with fixed thermal and solutal Grashof numbers, convection strength increases as the ratio between the Schmidt and Prandtl numbers, i.e., the Lewis number, becomes higher. With appropriate combinations of the Grashof number and buoyancy ratio, multiple-cell flow patterns also appear in both modes, where the length scale disparity due to a large Lewis number causes the convection to be largely controlled by the temperature field within the primary convection cell, and by the solute field either in the region across the cell interface or in the secondary cells. Effects of irregular geometry of the enclosure on the transport processes are also assessed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858200
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  • 5
    Electronic Resource
    Electronic Resource
    Computation of unsteady laminar flow over a flexible two-dimensional membrane wing (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 7 (1995), S. 2175-2184 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Computations of the harmonically forced, unsteady viscous flow over a flexible, two-dimensional membrane wing are presented. The aeroelastic problem is nondimensionalized and a set of six basic dimensionless parameters is derived that govern the physical problem. The computational investigation is facilitated by distinguishing three distinct classes of problems—the constant tension, elastic, and inextensible membrane problems—which are associated with limiting cases of the dimensionless parameter set. A pressure-based method for the incompressible Navier–Stokes equations written in general time-dependent curvilinear coordinates is adopted as the flow solver. The computations were performed at a Reynolds number of 4×103, which is near the upper limit of the laminar flow regime. The periodic appearance and collapse of recirculation zones, along with an attendant adjustment in membrane configuration, results in an aeroelastic response, which may not be characterized as a simple harmonic response at the free-stream forcing frequency. Ostensibly, the computations are designed to simulate the behavior of a marine sail in a wind gust; however, the simulation of a harmonically forced free-stream flow also proves to be a useful vehicle for demonstrating some of the more generic features of membrane wing mechanics. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868467
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  • 6
    Electronic Resource
    Electronic Resource
    Study of compressibility modifications to the k−cursive-epsilon turbulence model (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 2769-2788 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In modeling the effect of compressibility on the turbulence structure, additional physical mechanisms arise in production and dissipation of turbulent kinetic energy. Several proposed treatments dealing with the dilatation dissipation and the pressure dilatation correlation are discussed in the context of the k−cursive-epsilon two-equation model. Modifications accounting for the turbulent mass flux, enthalpic production, and baroclinic torque are also assessed along with the nonequilibrium treatment. These compressibility models are evaluated with the aid of experimental data for supersonic flow over an axisymmetric afterbody. With the free-stream Mach number of 2.46, the compressibility and nonequilibrium modifications give marginally better results for the turbulence structure and offer added insight into the mean and fluctuating flow fields. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869468
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  • 7
    Electronic Resource
    Electronic Resource
    Computation of aerodynamic coefficients for a flexible membrane airfoil in turbulent flow: A comparison with classical theory (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 3346-3353 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the present paper an aeroelastic model of flexible membrane wing aerodynamics which incorporates the Reynolds-averaged Navier–Stokes equations is presented. The Reynolds stresses are prescribed by the k–ω shear-stress transport eddy-viscosity model recently proposed by Menter. The computed coefficients are compared with classical inviscid membrane airfoil theory and with a portion of the available experimental data for membrane wings. The results indicate that classical potential-based membrane airfoil theory can provide a meaningful description of membrane wing aerodynamics only for a small range of incidence angles near ideal and then only for membrane airfoils with small excess length ratios. For larger excess lengths and incidence angles viscous effects dominate the aerodynamics. The agreement of the computed results with the experimental data is mixed. The current status of the available experimental data for membrane airfoils is also reviewed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869122
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  • 8
    Electronic Resource
    Electronic Resource
    Hydrodynamics of a compound drop with application to leukocyte modeling (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 10 (1998), S. 760-774 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We study the dynamics of a compound liquid drop which is comprised of an outer membrane surface, a shell layer, and a core. The deformation due to an imposed extensional flow and the subsequent recovery are investigated computationally employing a combined Eulerian–Lagrangian technique. The numerical method allows for large viscosity and capillarity differences between layers. The present study reports several findings which provide direct insight into developing a dynamic model for leukocytes. A compound drop behaves like a homogeneous, simple liquid drop if the core is sufficiently deformed and the time scale of the core, related to the combination of its viscosity and capillarity, is comparable to that of the shell layer. Disparate time scales between the core and shell layer result in a rapid initial recoil of the drop during which the shell fluid is the primary participant in the hydrodynamics, followed by a slower relaxation period during which the core and shell layer interact with each other. Consequently, the apparent viscosity of the drop depends not only on the rheological properties of the drop, but also on the flow dynamics surrounding it. The findings obtained with the three-layer compound drop model can explain several main characteristics of leukocytes reported in the literature. Furthermore, our study suggests that unless the presence and possible deformation of the nucleus are explicitly accounted for, neither Newtonian nor non-Newtonian models for leukocytes can adequately predict the hydrodynamics of leukocytes. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869601
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  • 9
    Electronic Resource
    Electronic Resource
    Effects of Nucleus on Leukocyte Recovery (1999)
    Springer
    Annals of biomedical engineering 27 (1999), S. 648-655 
    Details
    ISSN: 1573-9686
    Keywords: Leukocyte model ; Computational cellular dynamics ; Cell recovery
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract The rheological properties of a leukocyte significantly affect its biological and mechanical characteristics. To date, existing physical models of leukocyte are not capable of quantitatively explaining the wide range of deformation and recovery behaviors observed in experiment. However, a compound drop model has gained some success. In the present work, we investigate the effect of nucleus size and position, and the relative rheological properties of cytoplasm and nucleus, on cell recovery dynamics. Two nucleus sizes corresponding to that of neutrophil and lymphocyte are considered. Direct comparison between numerical simulations and experimental observation is made. Results indicate that the time scale ratio between the nucleus and cytoplasm plays an important role in cell recovery characteristics. Comparable time scales between the two cell components yield favorable agreement in recovery rates between numerical and experimental observations; disparate time scales, on the other hand, result in recovery behavior and cell shapes inconsistent with experiments. Furthermore, it is found that the nucleus eccentricity exhibits minimum influence on all major aspects of the cell recovery characteristics. The present work offers additional evidence in support of the compound cell model for predicting the rheological behavior of leukocytes. © 1999 Biomedical Engineering Society. PAC99: 8717-d, 8719Tt
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1114/1.214
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  • 10
    Electronic Resource
    Electronic Resource
    Three-dimensional analysis of the flow in a curved hydraulic turbine draft tube (1986)
    Chichester : Wiley-Blackwell
    International Journal for Numerical Methods in Fluids 6 (1986), S. 861-882 
    Details
    ISSN: 0271-2091
    Keywords: Three Dimensional Flow ; Turbine Draft Tube ; Curvilinear Co-ordinates ; Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: The three-dimensional turbulent flow in a curved hydraulic turbine draft tube is studied numerically. The analysis is based on the steady Reynolds-averaged Navier-Stokes equations closed with the κ-ε model. The governing equations are discretized by a conservative finite volume formulation on a non-orthogonal body-fitted co-ordinate system. Two grid systems, one with 34 × 16 × 12 nodes and another with 50 × 30 × 22 nodes, have been used and the results from them are compared. In terms of computing effort, the number of iterations needed to yield the same degree of convergence is found to be proportional to the square root of the total number of nodes employed, which is consistent with an earlier study made for two-dimensional flows using the same algorithm. Calculations have been performed over a wide range of inlet swirl, using both the hybrid and second-order upwind schemes on coarse and fine grids. The addition of inlet swirl is found to eliminate the stalling characteristics in the downstream region and modify the behaviour of the flow markedly in the elbow region, thereby affecting the overall pressure recovery noticeably. The recovery factor increases up to a swirl ratio of about 0·75, and then drops off. Although the general trends obtained with both finite difference operators are in agreement, the quantitative values as well as some of the fine flow structures can differ. Many of the detailed features observed on the fine grid system are smeared out on the coarse grid system, pointing out the necessity of both a good finite difference operator and a good grid distribution for an accurate result.
    Additional Material: 21 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/fld.1650061202
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