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1
Chauchat, Julien, Zhen Cheng, Tim Nagel, Cyrille Bonamy, and Tian-Jian Hsu. "SedFoam-2.0: a 3-D two-phase flow numerical model for sediment transport." Geoscientific Model Development 10, no. 12 (2017): 4367–92. http://dx.doi.org/10.5194/gmd-10-4367-2017.
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Abstract. In this paper, a three-dimensional two-phase flow solver, SedFoam-2.0, is presented for sediment transport applications. The solver is extended from twoPhaseEulerFoam available in the 2.1.0 release of the open-source CFD (computational fluid dynamics) toolbox OpenFOAM. In this approach the sediment phase is modeled as a continuum, and constitutive laws have to be prescribed for the sediment stresses. In the proposed solver, two different intergranular stress models are implemented: the kinetic theory of granular flows and the dense granular flow rheology μ(I). For the fluid stress, laminar or turbulent flow regimes can be simulated and three different turbulence models are available for sediment transport: a simple mixing length model (one-dimensional configuration only), a k − ε, and a k − ω model. The numerical implementation is demonstrated on four test cases: sedimentation of suspended particles, laminar bed load, sheet flow, and scour at an apron. These test cases illustrate the capabilities of SedFoam-2.0 to deal with complex turbulent sediment transport problems with different combinations of intergranular stress and turbulence models.
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Siddiqa, Sadia, M. N. Abrar, M. A. Hossain, and M. Awais. "Dynamics of Two-Phase Dusty Fluid Flow Along a Wavy Surface." International Journal of Nonlinear Sciences and Numerical Simulation 17, no. 5 (2016): 185–93. http://dx.doi.org/10.1515/ijnsns-2015-0044.
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AbstractThis article provides the computational results of laminar, boundary layer flow of a dilute gas-particle mixture over a semi-infinite vertical wavy surface. The governing parabolic partial differential equations are switched into another frame of reference by using primitive variable formulations (PVF). Two-point finite difference scheme is applied to acquire the unknown quantities of the carrier and the particle phase. The results are obtained for the cases: (i) water–metal mixture and (ii) air–metal mixture and are displayed in the form of wall shear stress, wall heat transfer, velocity profile, temperature profile, streamlines and isotherms for different emerging physical parameters. The solutions are compared, as well, with the available data in the literature. Quantitative comparison shows good compatibility between the present and the previous results. For the dusty fluid model it is found that the rate of heat transfer reduces considerably when the amplitude of the sinusoidal waveform increases from 0 to 0.5.
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OURIEMI, MALIKA, PASCALE AUSSILLOUS, and ÉLISABETH GUAZZELLI. "Sediment dynamics. Part 1. Bed-load transport by laminar shearing flows." Journal of Fluid Mechanics 636 (September 25, 2009): 295–319. http://dx.doi.org/10.1017/s0022112009007915.
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We propose a two-phase model having a Newtonian rheology for the fluid phase and friction for the particle phase to describe bed-load transport in the laminar viscous regime. We have applied this continuum model to sediment transport by viscous shearing flows. The equations are shown to reduce to the momentum equation for the mixture and the Brinkman equation for the fluid velocity. This modelling is able to provide a description of the flow of the mobile granular layer. At some distance from threshold of particle motion, where the continuum approach is more realistic as the mobile layer is larger than one particle diameter, there is very little slip between the two phases and the velocities inside the mobile bed have approximately a parabolic profile. When the Poiseuille (or Couette) flow is not significantly perturbed, simple analytical results of the particle flux varying cubically with the Shields number and of the bed-load thickness varying linearly with it can then be obtained. These predictions compare favourably with experimental observations of bed-load transport in pipe flows.
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Huang, Yuanlong, Matthew M. Coggon, Ran Zhao, et al. "The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization." Atmospheric Measurement Techniques 10, no. 3 (2017): 839–67. http://dx.doi.org/10.5194/amt-10-839-2017.
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Abstract. Flow tube reactors are widely employed to study gas-phase atmospheric chemistry and secondary organic aerosol (SOA) formation. The development of a new laminar-flow tube reactor, the Caltech Photooxidation Flow Tube (CPOT), intended for the study of gas-phase atmospheric chemistry and SOA formation, is reported here. The present work addresses the reactor design based on fluid dynamical characterization and the fundamental behavior of vapor molecules and particles in the reactor. The design of the inlet to the reactor, based on computational fluid dynamics (CFD) simulations, comprises a static mixer and a conical diffuser to facilitate development of a characteristic laminar flow profile. To assess the extent to which the actual performance adheres to the theoretical CFD model, residence time distribution (RTD) experiments are reported with vapor molecules (O3) and submicrometer ammonium sulfate particles. As confirmed by the CFD prediction, the presence of a slight deviation from strictly isothermal conditions leads to secondary flows in the reactor that produce deviations from the ideal parabolic laminar flow. The characterization experiments, in conjunction with theory, provide a basis for interpretation of atmospheric chemistry and SOA studies to follow. A 1-D photochemical model within an axially dispersed plug flow reactor (AD-PFR) framework is formulated to evaluate the oxidation level in the reactor. The simulation indicates that the OH concentration is uniform along the reactor, and an OH exposure (OHexp) ranging from ∼ 109 to ∼ 1012 molecules cm−3 s can be achieved from photolysis of H2O2. A method to calculate OHexp with a consideration for the axial dispersion in the present photochemical system is developed.
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Banerjee, R., K. M. Isaac, L. Oliver, and W. Breig. "Features of Automotive Gas Tank Filler Pipe Two-Phase Flow: Experiments and Computational Fluid Dynamics Simulations." Journal of Engineering for Gas Turbines and Power 124, no. 2 (2002): 412–20. http://dx.doi.org/10.1115/1.1445439.
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Extensive flow visualization in an automotive fuel filler pipe made visible by introducing dyes and smoke in water and air, respectively, were conducted for nominal flow rates of 4–18 liters per minute. Video and still cameras were used for imaging. Features of the flow such as laminar-to-turbulent transition, progressive development of strong swirl along filler pipe axis, air entrainment, and mixing with the liquid were observed in the experiments. The experimental observations were supported by computational fluid dynamics (CFD) simulations of the flow which also showed features such as swirl and air entrainment.
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BÉG, O. ANWAR, M. M. RASHIDI, M. AKBARI, and A. HOSSEINI. "COMPARATIVE NUMERICAL STUDY OF SINGLE-PHASE AND TWO-PHASE MODELS FOR BIO-NANOFLUID TRANSPORT PHENOMENA." Journal of Mechanics in Medicine and Biology 14, no. 01 (2014): 1450011. http://dx.doi.org/10.1142/s0219519414500110.
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A computational fluid dynamics (CFD) simulation of laminar convection of Al 2 O 3–water bio-nanofluids in a circular tube under constant wall temperature conditions was conducted, employing a single-phase model and three different two-phase models (volume of fluid (VOF), mixture and Eulerian). The steady-state, three-dimensional flow conservation equations were discretised using the finite volume method (FVM). Several parameters such as temperature, flow field, skin friction and heat transfer coefficient were computed. The computations showed that CFD predictions with the three different two-phase models are essentially the same. The CFD simulations also demonstrated that single-phase and two-phase models yield the same results for fluid flow but different results for thermal fields. The two-phase models, however, achieved better correlation with experimental measurements. The simulations further showed that heat transfer coefficient distinctly increases with increasing nanofluid particle concentration. The physical properties of the base fluid were considered to be temperature-dependent, while those of the solid particles were constant. Grid independence tests were also included. The simulations have applications in novel biomedical flow processing systems.
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Boutopoulos, Ioannis D., Dimitrios S. Lampropoulos, George C. Bourantas, Karol Miller, and Vassilios C. Loukopoulos. "Two-Phase Biofluid Flow Model for Magnetic Drug Targeting." Symmetry 12, no. 7 (2020): 1083. http://dx.doi.org/10.3390/sym12071083.
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Magnetic drug targeting (MDT) is a noninvasive method for the medical treatment of various diseases of the cardiovascular system. Biocompatible magnetic nanoparticles loaded with medicinal drugs are carried to a tissue target in the human body (in vivo) under the applied magnetic field. The present study examines the MDT technique in various microchannels geometries by adopting the principles of biofluid dynamics (BFD). The blood flow is considered as laminar, pulsatile and the blood as an incompressible and non-Newtonian fluid. A two-phase model is adopted to resolve the blood flow and the motion of magnetic nanoparticles (MNPs). The numerical results are obtained by utilizing a meshless point collocation method (MPCM) alongside with the moving least squares (MLS) approximation. The numerical results are verified by comparing with published numerical results. We investigate the effect of crucial parameters of MDT, including (1) the volume fraction of nanoparticles, (2) the location of the magnetic field, (3) the strength of the magnetic field and its gradient, (4) the way that MNPs approach the targeted area, and (5) the bifurcation angle of the vessel.
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Wassar, Taoufik, Matthew A. Franchek, Hamdi Mnasri, and Yingjie Tang. "An Explicit Analytical Solution for Transient Two-Phase Flow in Inclined Fluid Transmission Lines." Fluids 6, no. 9 (2021): 300. http://dx.doi.org/10.3390/fluids6090300.
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Due to the complex nonlinearity characteristics, analytical modeling of compressible flow in inclined transmission lines remains a challenge. This paper proposes an analytical model for one-dimensional flow of a two-phase gas-liquid fluid in inclined transmission lines. The proposed model is comprised of a steady-state two-phase flow mechanistic model in-series with a dynamic single-phase flow model. The two-phase mechanistic model captures the steady-state pressure drop and liquid holdup properties of the gas-liquid fluid. The developed dynamic single-phase flow model is an analytical model comprised of rational polynomial transfer functions that are explicitly functions of fluid properties, line geometry, and inclination angle. The accuracy of the fluid resonant frequencies predicted by the transient flow model is precise and not a function of transmission line spatial discretization. Therefore, model complexity is solely a function of the number of desired modes. The dynamic single-phase model is applicable for under-damped and over-damped systems, laminar, and turbulent flow conditions. The accuracy of the overall two-phase flow model is investigated using the commercial multiphase flow dynamic code OLGA. The mean absolute error between the two models in step response overshoot and settling time is less than 8% and 2 s, respectively.
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Luan, Zhaogao, and M. M. Khonsari. "Computational Fluid Dynamics Analysis of Turbulent Flow Within a Mechanical Seal Chamber." Journal of Tribology 129, no. 1 (2006): 120–28. http://dx.doi.org/10.1115/1.2401220.
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Turbulent flow inside the seal chamber of a pump operating at high Reynolds number is investigated. The K−ε turbulence model posed in cylindrical coordinates was applied for this purpose. Simulations are performed using the fractional approach method. The results of the computer code are verified by using the FLUENT and by comparing to published results for turbulent Taylor Couette flow. Numerical results of four cases including two rotational speeds with four flush rates are reported. Significant difference between the laminar and the turbulence flow in the seal chamber is predicted. The behavior of the turbulent flows with very high Reynolds number was also investigated. The physical and practical implications of the results are discussed.
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Hasslberger, Josef, Sebastian Ketterl, Markus Klein, and Nilanjan Chakraborty. "Flow topologies in primary atomization of liquid jets: a direct numerical simulation analysis." Journal of Fluid Mechanics 859 (November 26, 2018): 819–38. http://dx.doi.org/10.1017/jfm.2018.845.
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The local flow topology analysis of the primary atomization of liquid jets has been conducted using the invariants of the velocity-gradient tensor. All possible small-scale flow structures are categorized into two focal and two nodal topologies for incompressible flows in both liquid and gaseous phases. The underlying direct numerical simulation database was generated by the one-fluid formulation of the two-phase flow governing equations including a high-fidelity volume-of-fluid method for accurate interface propagation. The ratio of liquid-to-gas fluid properties corresponds to a diesel jet exhausting into air. Variation of the inflow-based Reynolds number as well as Weber number showed that both these non-dimensional numbers play a pivotal role in determining the nature of the jet break-up, but the flow topology behaviour appears to be dominated by the Reynolds number. Furthermore, the flow dynamics in the gaseous phase is generally less homogeneous than in the liquid phase because some flow regions resemble a laminar-to-turbulent transition state rather than fully developed turbulence. Two theoretical models are proposed to estimate the topology volume fractions and to describe the size distribution of the flow structures, respectively. In the latter case, a simple power law seems to be a reasonable approximation of the measured topology spectrum. According to that observation, only the integral turbulent length scale would be required as an input for the a priori prediction of the topology size spectrum.
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Hague, J., C. T. Ta, M. J. Biggs, and J. A. Sattary. "Small scale model for CFD validation in DAF application." Water Science and Technology 43, no. 8 (2001): 167–73. http://dx.doi.org/10.2166/wst.2001.0491.
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A laboratory model is used to measure the generic flow patterns in dissolved air flotation (DAF). The Perspex model used in this study allows the use of laser Doppler velocimetry (LDV), a non-invasive, high-resolution (±2 mm s−1) laser technique of flow velocity measurement. Measurement of flow velocity in the single-phase situation was first carried out. Air-saturated water was then supplied to the tank and measurements of bubble velocity in the two-phase system were made. Vertical flow re-circulation was observed in the flotation zone. In the bottom of the flotation zone (near the riser) secondary flow re-circulation was observed, but only in the two-phase system. Another phenomenon was the apparent movement of flow across the tank width, which may be due to lateral dispersion of the bubble cloud. Data from preliminary computational fluid dynamics (CFD) models were compared against this measured data in the case of the single-phase system. The CFD model incorporating a k-e model of turbulence was found to give closer agreement with the measured data than the corresponding laminar flow model. The measured velocity data will be used to verify two-phase computational fluid dynamics (CFD) models of DAF.
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Almalowi, Saeed J., and Alparslan Oztekin. "Flow Simulations Using Two Dimensional Thermal Lattice Boltzmann Method." Journal of Applied Mathematics 2012 (2012): 1–12. http://dx.doi.org/10.1155/2012/135173.
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Lattice Boltzmann method is implemented to study hydrodynamically and thermally developing steady laminar flows in a channel. Numerical simulation of two-dimensional convective heat transfer problem is conducted using two-dimensional, nine directional D2Q9 thermal lattice Boltzmann arrangements. The velocity and temperature profiles in the developing region predicted by Lattice Boltzmann method are compared against those obtained by ANSYS-FLUENT. Velocity and temperature profiles as well as the skin friction and the Nusselt numbers agree very well with those predicted by the self-similar solutions of fully developed flows. It is clearly shown here that thermal lattice Boltzmann method is an effective computational fluid dynamics (CFD) tool to study nonisothermal flow problems.
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Eslami, M., M. M. Tavakol, and E. Goshtasbirad. "Laminar Fluid Flow around Two Wall-Mounted Cubes of Arbitrary Configuration." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 224, no. 11 (2010): 2396–407. http://dx.doi.org/10.1243/09544062jmes2026.
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The problem of flow field around multiple bluff bodies mounted on a surface is of great significance in different fields of engineering. In this study, a computational fluid dynamics (CFD) code is developed to calculate three-dimensional (3D) steady state laminar fluid flow around two cuboids of arbitrary size and configuration mounted on a surface in free stream conditions. This study presents the results for two cubes of the same size mounted on a surface in both inline and staggered arrangements. Streamlines are plotted for various combinations of the distance between the two cubes and Reynolds number. Moreover, the effects of different parameters on vortical structures, separation, and reattachment points are discussed. Also, velocity and pressure distributions are plotted in the wake region behind the two cubes. It is clearly shown that how the presence of the second cube changes the flow field and the vortical structures in comparison with the case of a single cube.
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Avila, Kerstin, and Björn Hof. "Second-Order Phase Transition in Counter-Rotating Taylor–Couette Flow Experiment." Entropy 23, no. 1 (2020): 58. http://dx.doi.org/10.3390/e23010058.
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In many basic shear flows, such as pipe, Couette, and channel flow, turbulence does not arise from an instability of the laminar state, and both dynamical states co-exist. With decreasing flow speed (i.e., decreasing Reynolds number) the fraction of fluid in laminar motion increases while turbulence recedes and eventually the entire flow relaminarizes. The first step towards understanding the nature of this transition is to determine if the phase change is of either first or second order. In the former case, the turbulent fraction would drop discontinuously to zero as the Reynolds number decreases while in the latter the process would be continuous. For Couette flow, the flow between two parallel plates, earlier studies suggest a discontinuous scenario. In the present study we realize a Couette flow between two concentric cylinders which allows studies to be carried out in large aspect ratios and for extensive observation times. The presented measurements show that the transition in this circular Couette geometry is continuous suggesting that former studies were limited by finite size effects. A further characterization of this transition, in particular its relation to the directed percolation universality class, requires even larger system sizes than presently available.
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Susanto, Tri Nugroho Hadi. "COMPUTATIONAL FLUID DYNAMICS SIMULATION OF KARTINI REACTOR FUELED PLATE." Indonesian Journal of Physics and Nuclear Applications 4, no. 2 (2019): 33–38. http://dx.doi.org/10.24246/ijpna.v4i2.33-38.
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The purpose of this study is to determine the characteristics of the cooling system on the new design of the Kartini Reactor plate fuel based on numerical calculations (Computational Fluid Dynamics). The fuel plate model was simplified and made in 3D. The model dimensions are 17.3 mm x 68 mm x 900 mm. The space between the two plates called the narrow rectangular channels has a gap of 2 mm. On these simulations a heat flux of 10612,7 watt/m2 was used which was obtained from the MCNP calculation program. Simulations were conducted in a steady state condition and single-phase model laminar flow of an incompressible fluid through the gap between the two fuel plates. This simulation uses UDF (User Define Function) to approach heat flux behaviour that follows the neutron distribution in the reactor core. The simulation results show that the maximum temperature that occur at a flow rate of 0.01 m/s was 43.5 °C.
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Holzner, Markus, Baofang Song, Marc Avila, and Björn Hof. "Lagrangian approach to laminar–turbulent interfaces in transitional pipe flow." Journal of Fluid Mechanics 723 (April 16, 2013): 140–62. http://dx.doi.org/10.1017/jfm.2013.127.
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AbstractTransition in shear flows is characterized by localized turbulent regions embedded in the surrounding laminar flow. These so-called turbulent spots or puffs are observed in a variety of shear flows and in certain Reynolds-number regimes, and they are advected by the flow while keeping their characteristic length. We show here for the case of pipe flow that this seemingly passive advection of turbulent puffs involves continuous entrainment and relaminarization of laminar and turbulent fluid across strongly convoluted interfaces. Surprisingly, interface areas are almost two orders of magnitude larger than the pipe cross-section, while local entrainment velocities are much smaller than the mean speed. Even though these velocities were shown to be small and proportional to the Kolmogorov velocity scale (in agreement with a prediction by Corrsin) in a flow without mean shear before, we find that, in pipe flow, local entrainment velocities are about an order of magnitude smaller than this scale. The Lagrangian method used to study the dynamics of the laminar–turbulent interfaces allows accurate determination of the leading and trailing edge speeds. However, to resolve the highly complex interface dynamics requires much higher numerical resolutions than for ordinary turbulent flows. This method also reveals that the volume flux across the leading edge has the same radial dependence but the opposite sign as that across the trailing edge, and it is this symmetry that is responsible for the puff shape remaining constant.
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Robinet, J. C. "Instabilities in laminar separation bubbles." Journal of Fluid Mechanics 732 (August 30, 2013): 1–4. http://dx.doi.org/10.1017/jfm.2013.355.
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AbstractWall-bounded flows, in their transition from a laminar state to turbulence, pass through a set of particular stages characterized by different physical processes. Among wall-bounded flows, separated flows have a special place because their dynamics can either be noise amplifiers or oscillators. For several years Marxen and co-workers have been studying the evolution of two- and three-dimensional perturbations in the laminar part of a laminar separation bubble. In Marxen et al. (J. Fluid Mech., vol. 728, 2013, p. 58) they study vortex formation and its evolution in laminar–turbulent transition in a forced separation bubble. By the combined use of numerical and experimental methods, different mechanisms of secondary instabilities have been highlighted: elliptic instability of vortex cores and hyperbolic instability responsible for three-dimensionality in the braid region. This work shows, for the first time in laminar separation bubbles, the first nonlinear stages of transition to turbulence of such a flow. However, since this type of flow is very sensitive to various environmental stresses, several scenarios for transition to turbulence remain to be explored.
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Ghoshdastidar, P. S., and Indrajit Chakraborty. "A Coupled Map Lattice Model of Flow Boiling in a Horizontal Tube." Journal of Heat Transfer 129, no. 12 (2007): 1737–41. http://dx.doi.org/10.1115/1.2768102.
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In this work laminar, stratified flow boiling of water is simulated qualitatively by the coupled map lattice (CML) method. The liquid is entering a constant wall temperature horizontal tube (Tw>Tsat at pentrance) in a subcooled condition. A CML is a dynamical system with discrete-time, discrete-space, and continuous states. The procedure basically consists of the following steps: (i) Choose a set of macroscopic variables on a lattice; (ii) decompose the problem into independent components, such as convection, diffusion, phase change, and so on; (iii) replace each component by a simple parallel dynamics on a lattice; and (iv) carry out each unit dynamics successively in each time step until some termination criterion is satisfied. In the present problem, the termination criterion is the laminar-turbulent transition, and hence, the results do not correspond to the steady state. The present modeling by CML is based on the assumption that the flow boiling is governed by (i) nucleation from cavities on the heated surface and migration of vapor into the core, (ii) forced convection, and (iii) phase change in the fluid bulk and mixing. The stirring action of the bubbles is modeled by increasing the fluid momentum and thermal diffusivities by an enhancement factor. The results show that the CML has been able to model flow boiling in a realistic manner.
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Matsumoto, Daichi, Koji Fukudome, and Hirofumi Wada. "Two-dimensional fluid dynamics in a sharply bent channel: Laminar flow, separation bubble, and vortex dynamics." Physics of Fluids 28, no. 10 (2016): 103602. http://dx.doi.org/10.1063/1.4963864.
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PIÑA, E. "Turbulence in incompressible fluids and magnetic fields." Journal of Plasma Physics 59, no. 4 (1998): 719–25. http://dx.doi.org/10.1017/s0022377898006643.
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The Hamiltonian dynamics of a two-degree-of-freedom system is shown to mimic the structure of an incompressible fluid if the fluid streamlines are considered instead of the pathlines used by many authors. This approach is formally equivalent to the determination of magnetic field lines, which satisfy the same mathematical equations. Turbulence in this case is seen as Hamiltonian chaotic dynamics perturbed by time, which plays the role of the codimension parameter of such chaos. Any spatial Lie symmetry leads to a laminar flow, whereas stationary flow could be considered chaotic but not turbulent. The transition from laminar to turbulent flow is associated with the destruction of KAM trajectories produced by time evolution.
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Chapin, V. G., S. Jamme, and P. Chassaing. "Viscous Computational Fluid Dynamics as a Relevant Decision-Making Tool for Mast-Sail Aerodynamics." Marine Technology and SNAME News 42, no. 01 (2005): 1–10. http://dx.doi.org/10.5957/mt1.2005.42.1.1.
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Viscous computational fluid dynamics based on Reynolds averaged Navier-Stokes (RANS) equations have been used to simulate flow around typical mast-sail geometries. It is shown how these advanced numerical methods are relevant to investigate the complexity of such strongly separated flows. Detailed numerical results have been obtained and compared to experimental ones. Comparative analysis has shown that RANS methods are able to capture the main flow features, such as mast-flow separation, recirculation bubble, bubble reattachment through a laminar-turbulent transition process, and trailing-edge separation. A second part has been devoted to the comparative behavior of these flow features through parameters variations to evaluate the qualitative and quantitative capabilities of RANS methods in mast-sail design optimization. The last part illustrates through two examples how RANS methods may be used to optimize the design of mast-sail geometries and evaluate their relative performances.
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Guzmán, A. M., and C. H. Amon. "Dynamical flow characterization of transitional and chaotic regimes in converging–diverging channels." Journal of Fluid Mechanics 321 (August 25, 1996): 25–57. http://dx.doi.org/10.1017/s002211209600763x.
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Numerical investigation of laminar, transitional and chaotic flows in converging–diverging channels are performed by direct numerical simulations in the Reynolds number range 10 < Re < 850. The temporal flow evolution and the onset of turbulence are investigated by combining classical fluid dynamics representations with dynamical system flow characterizations. Modern dynamical system techniques such as timedelay reconstructions of pseudophase spaces, autocorrelation functions, fractal dimensions and Eulerian Lyapunov exponents are used for the dynamical flow characterization of laminar, transitional and chaotic flow regimes. As a consequence of these flow characterizations, it is verified that the transitional flow evolves through intermediate states of periodicity, two-frequency quasi-periodicity, frequency-locking periodicity, and multiple-frequency quasi-periodicity before reaching a non-periodic unpredictable behaviour corresponding to low-dimensional deterministic chaos.Qualitative and quantitative differences in Eulerian dynamical flow parameters are identified to determine the predictability of transitional flows and to characterize chaotic, weak turbulent flows in converging–diverging channels. Autocorrelation functions, pseudophase space representations and Poincaré maps are used for the qualitative identification of chaotic flows, assertion of their unpredictable nature, and recognition of the topological structure of the attractors for different flow regimes. The predictability of transitional flows is determined by analysing the autocorrelation functions and by representing their attractors in the reconstructed pseudophase spaces. The transitional flow behaviour is examined by the geometric visualization of the evolution of the attractors and Poincaré maps until the appearance of a strange attractor at the onset of chaos. Eulerian Lyapunov exponents and fractal dimensions are quantitative parameters to establish the onset of chaos, the persistence of chaotic flow behaviour, and the long-term persistent unpredictability of chaotic Eulerian flow regimes. Lastly, three-dimensional simulations for converging–diverging channel flow are performed to determine the effect of the spanwise direction on the route of transition to chaos.
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Deepak, N. R., S. L. Gai, and A. J. Neely. "A computational investigation of laminar shock/wave boundary layer interactions." Aeronautical Journal 117, no. 1187 (2013): 27–56. http://dx.doi.org/10.1017/s0001924000007740.
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AbstractHypersonic laminar flow past a compression corner has been numerically investigated using time-accurate computational fluid dynamics (CFD) approach. Two flow conditions were considered relevant to high and low enthalpy conditions with a total specific enthalpy of 19MJ/kg and 2·8MJ/kg. The Mach number and unit Reynolds number per metre were 7·5, 9·1 and 3·10 × 105and 32·2 × 105respectively. These free stream conditions provided attached, incipiently separated and fully separated flows for ramp angles between θw= 5° to 24°. A grid independence study has been carried out to estimate the sensitivity of heat flux and skin friction in the strong interaction regions of the flow. The investigation was carried out assuming the flow to be laminar throughout and high temperature effects such as thermal and chemical nonequilibrium are studied using Park’s two temperature model with finite rate chemistry. A critical comparison has been made with existing steady state computational and experimental data and the study has highlighted the importance of high temperature effects on the flow separation and reattachment.
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Cheloii, Navid Ahmadi, Omid Ali Akbari, and Davood Toghraie. "Computational fluid dynamics and laminar heat transfer of water/Cu nanofluid in ribbed microchannel with a two-phase approach." International Journal of Numerical Methods for Heat & Fluid Flow 29, no. 5 (2019): 1563–89. http://dx.doi.org/10.1108/hff-05-2018-0243.
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Purpose This study aims to numerically investigate the heat transfer and laminar forced and two-phase flow of Water/Cu nanofluid in a rectangular microchannel with oblique ribs with angle of attacks equal to 0-45°. This simulation was conducted in the range of Reynolds numbers of 5-120 in volume fractions of 0, 2 and 4 per cent of solid nanoparticles in three-dimensional space. Design/methodology/approach This study investigates the effect of the changes of angle of attack of rectangular rib on heat transfer and hydrodynamics of two-phase flow. This study was done in three-dimensional space and simulation was done with finite volume method. SIMPLEC algorithm and second-order discretization of equations were used to increase the accuracy of results. The usage of nanofluid, application of rips with different angles of attacks and using the two-phase mixture method is the distinction of this paper compared with other studies. Findings The results of this research revealed that the changing angle of attack of ribs is an effective factor in heat transfer enhancement. On the other hand, the existence of rib on the internal surfaces of a microchannel increases friction coefficient. By increasing the volume fraction of nanoparticles, due to the augmentation of fluid density and viscosity, the pressure drop increases significantly. For all of the angle of attacks studied in this paper, the maximum rate of performance evaluation criterion has been obtained in Reynolds number of 30 and the minimum amount of performance evaluation criterion was been obtained in Reynolds numbers of 5 and 120. Originality/value Many studies have been done in the field of heat transfer in ribbed microchannel. In this paper, the laminar flow in the ribbed microchannel Water/Cu nanofluid in a rectangular microchannel by using two-phase mixture method is numerically investigated with different volume fractions (0-4 per cent), Reynolds numbers (5-120) and angle of attacks of rectangular rib in the indented microchannel (0-45°).
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Arul Prakash, M., K. Mayilsamy, and P. Rajesh Kanna. "Numerical Simulation of Two Dimensional Laminar Wall Jet Flow over Solid Obstacle." Applied Mechanics and Materials 592-594 (July 2014): 1935–39. http://dx.doi.org/10.4028/www.scientific.net/amm.592-594.1935.
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A Computational Fluid Dynamics code was developed to study the flow characteristics of two dimensional laminar incompressible flow. Stream function-vorticity formulation was used for solving two dimensional continuity and momentum equations. The unsteady vorticity transport equation is solved by alternate direction implicit scheme. The stream function equation is solved by the successive over relaxation method. A computational code in c-language was developed to solve the tridiagonal system of algebraic equations. Two dimensional flow through a channel with rectangular block at the bottom wall was considered for the validation. The streamline patterns obtained for different Reynolds number shows good agreement with published results. The code was modified to simulate an incompressible laminar wall jet flow around a solid obstacle. Simulations were carried out for different Reynolds numbers. Contour plots of Stream line, u-velocity and v-velocity were obtained. The variations of flow patterns and the development of vortices were studied and reported.
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Elsaady, Wael, S. Olutunde Oyadiji, and Adel Nasser. "A review on multi-physics numerical modelling in different applications of magnetorheological fluids." Journal of Intelligent Material Systems and Structures 31, no. 16 (2020): 1855–97. http://dx.doi.org/10.1177/1045389x20935632.
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Magnetorheological fluids involve multi-physics phenomena which are manifested by interactions between structural mechanics, electromagnetism and rheological fluid flow. In comparison with analytical models, numerical models employed for magnetorheological fluid applications are thought to be more advantageous, as they can predict more phenomena, more parameters of design, and involve fewer model assumptions. On that basis, the state-of-the-art numerical methods that investigate the multi-physics behaviour of magnetorheological fluids in different applications are reviewed in this article. Theories, characteristics, limitations and considerations employed in numerical models are discussed. Modelling of magnetic field has been found to be rather an uncomplicated affair in comparison to modelling of fluid flow field which is rather complicated. This is because, the former involves essentially one phenomenon/mechanism, whereas the latter involves a plethora of phenomena/mechanisms such as laminar versus turbulent rheological flow, incompressible versus compressible flow, and single- versus two-phase flow. Moreover, some models are shown to be still incapable of predicting the rheological nonlinear behaviour of magnetorheological fluids although they can predict the dynamic characteristics of the system.
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Spalton, T., Rachel A. Tomlinson, A. E. Garrard, and S. B. M. Beck. "Streaming Birefringence - A Step Forward." Applied Mechanics and Materials 13-14 (July 2008): 23–28. http://dx.doi.org/10.4028/www.scientific.net/amm.13-14.23.
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An investigation into three dimensional fluid flow has been conducted which combines the use of Computational Fluid Dynamics (CFD) simulations with the experimental phenomenon of Streaming Birefringence. A versatile flow channel was designed and built for use in conjunction with a circular polariscope. The experimental liquid used was an aqueous solution of a dye, commercially known as Milling Yellow NGS with the addition of Sodium Chloride. To extract the flow fields, six image phase stepping photoelasticity was used over backward and forward steps, and flows around a cylinder, and full-field fringe data were obtained. This method needs laminar flow regimes and the Reynolds number of the flow was around 10. To allow direct comparisons of the CFD solutions with the optical results, a macro (UDF) was written to interpret the flow field results from a (FLUENT6) CFD simulation. This integrated the shear stresses across the flow field and banded the results into fringes. A good correlation between the simulated fringes and the shearstrain rate was obtained from these observations.
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Sundararaj, Aldin Justin, B. C. Pillai, Austin Lord Tennyson, Allison Edward, and Bhaskar Gupta. "Numerical Investigation of Convective Heat Transfer of Refined Kerosene-Alumina Nanofluid Under Laminar and Turbulent Regime." Advanced Science Letters 24, no. 8 (2018): 5543–47. http://dx.doi.org/10.1166/asl.2018.12145.
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The study reports Computational Fluid Dynamics (CFD) investigations of the convective heat transfer coefficient of Al2O3/refined kerosene nanofluids. The study was carried out under laminar and turbulent regime in a circular tube under uniform and constant heat flux on the wall. The study was carried out for Re 500 to 5500 for base refined kerosene and with alumina added with 0.01% and 0.05% volume concentration in the base refined kerosene. The size of the alumina nanoparticle was 35 nm. Different computational models of Ansys-Fluent were used for the study. For laminar flows, laminar viscous models and K-Epsilon model for turbulence modelling was used. Energy model was used to define convective heat transfer and a discrete phase model to study particle behaviour and flow pattern in the tube. Multi-phase model with two phase refined kerosene suspended with alumina nano particles were used for the study. Experimental and simulation results showed that as the Reynolds number and the particle concentration increased there was an enhancement in the thermal performance of nanofluids which was found to be higher than that of the base fluid. The convective heat transfer increased by 14% for volume concentration of 0.05% and Reynolds number of 5500.
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Kozubková, Milada, Jana Jablonská, Marian Bojko, František Pochylý, and Simona Fialová. "Multiphase Flow in the Gap Between Two Rotating Cylinders." MATEC Web of Conferences 328 (2020): 02017. http://dx.doi.org/10.1051/matecconf/202032802017.
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The research of liquids composed of two (or more) mutually immiscible components is a new emerging area. These liquids represent new materials, which can be utilized as lubricants, liquid seals or as fluid media in biomechanical devices. The investigation of the problem of immiscible liquids started some years ago and soon it was evident that it will have a great application potential. Recently, there has been an effort to use ferromagnetic or magnetorheological fluids in the construction of dumpers or journal bearings. Their advantage is a significant change in dynamic viscosity depending on magnetic induction. In combination with immiscible liquids, qualitatively new liquids can be developed for future technologies. In our case, immiscible fluids increase the dynamic properties of the journal hydrodynamic bearing. The article focuses on the stability of single-phase and subsequently multiphase flow of liquids in the gap between two concentric cylinders, one of which rotates. The aim of the analysis was to study the effect of viscosity and density on the stability/instability of the flow, which is manifested by Taylor vortices. Methods of experimental and mathematical analysis were used for the analysis in order to verify mathematical models of laminar and turbulent flow of immiscible liquids.
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Guo, Yuan, and Xiong (Bill) Yu. "Analysis of surface erosion of cohesionless soils using a three-dimensional coupled computational fluid dynamics – discrete element method (CFD–DEM) model." Canadian Geotechnical Journal 56, no. 5 (2019): 687–98. http://dx.doi.org/10.1139/cgj-2016-0421.
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A fluid–solid interaction model has been implemented by coupling two numerical methods — computational fluid dynamics (CFD) and discrete element method (DEM) — that capture the mesoscale behaviors of the fluid–solid system. The model is first validated by comparing the results of simulations with two types of experiments: free settling of a single sphere in water and formation of angle of repose of particles under water, which show its capability in modeling the behaviors of both particle phase and fluid phase. The verified model is then used to study factors affecting the soil erodibility, where case studies are designed for soil particles deposited inside a pipe and subsequently subjected to water flow–induced surface erosion. Influencing factors for soil erodibility, including particle diameter and interparticle bond, are analyzed. For cohesionless soils without bond strength, the critical shear stress is found to be linearly related to particle size; while for soils with bond strength, simulation results show that interparticle bonding largely decelerates the erosion process and causes a much lower erosion rate. To further the understanding of soil surface erosion under turbulent flow, the “k–ε” turbulence model has been successfully implemented for the fluid phase. Comparison between the laminar model and the turbulence model shows turbulence significantly accelerates the erosion process.
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Pajcin, Miroslav, Aleksandar Simonovic, Toni Ivanov, Dragan Komarov, and Slobodan Stupar. "Numerical analysis of a hypersonic turbulent and laminar flow using a commercial CFD solver." Thermal Science 21, suppl. 3 (2017): 795–807. http://dx.doi.org/10.2298/tsci160518198p.
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Computational fluid dynamics computations for two hypersonic flow cases using the commercial ANSYS FLUENT 16.2 CFD software were done. In this paper, an internal and external hypersonic flow cases were considered and analysis of the hypersonic flow using different turbulence viscosity models available in ANSYS FLUENT 16.2 as well as the laminar viscosity model were done. The obtained results were after compared and commented upon.
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Yang, Xiao Guang, Hong Xing Dong, and Xing Hua Zhang. "Simulating Pipe Reactor of Biological Growth in Single-Phase Laminar Flow Based on Computational Fluid Dynamics and Reaction Dynamics by OpenFOAM." Advanced Materials Research 557-559 (July 2012): 2279–82. http://dx.doi.org/10.4028/www.scientific.net/amr.557-559.2279.
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Pipe reactor simulation of biological growth was investigated based on computational fluid dynamics (CFD) and reaction dynamics in single-phase laminar incompressible flow with the open-source software, OpenFOAM. OpenFOAM supplied a kind of open structure, which made it convenient to add a suitable physical model with CFD equations according to the problem. In order to compute mass transfer and biological growth as a reaction with fluid flow, the component conservation equation was listed with Navier-Stokes equations. With the equations, a pipe reactor was simulated by our solver which was developed based on OpenFOAM. By the simulation, the pressure, velocity and component mass fraction can be obtained at different time and position, which is important to analyze equipments in chemical engineering. Although some details need to be considered such as the definition of reaction term, the boundary conditions, etc., reactor simulation with OpenFOAM has showed very large potential.
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Rahmani, Ramin K., Theo G. Keith, and Anahita Ayasoufi. "Three-Dimensional Numerical Simulation and Performance Study of an Industrial Helical Static Mixer." Journal of Fluids Engineering 127, no. 3 (2005): 467–83. http://dx.doi.org/10.1115/1.1899166.
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In many branches of processing industries, viscous liquids need to be homogenized in continuous operations. Consequently, fluid mixing plays a critical role in the success or failure of these processes. Static mixers have been utilized over a wide range of applications such as continuous mixing, blending, heat and mass transfer processes, chemical reactions, etc. This paper describes how static mixing processes of single-phase viscous liquids can be simulated numerically, presents the flow pattern through a helical static mixer, and provides useful information that can be extracted from the simulation results. The three-dimensional finite volume computational fluid dynamics code used here solves the Navier-Stokes equations for both laminar and turbulent flow cases. The turbulent flow cases were solved using k-ω model and Reynolds stress model (RSM). The flow properties are calculated and the static mixer performance for different Reynolds numbers (from creeping flows to turbulent flows) is studied. A new parameter is introduced to measure the degree of mixing quantitatively. Furthermore, the results obtained by k-ω and RSM turbulence models and various numerical details of each model are compared. The calculated pressure drop is in good agreement with existing experimental data.
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King, M. J., T. David, and J. Fisher. "An Initial Parametric Study on Fluid Flow Through Bileaflet Mechanical Heart Valves Using Computational Fluid Dynamics." Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 208, no. 2 (1994): 63–72. http://dx.doi.org/10.1243/pime_proc_1994_208_267_02.
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The effect of leaflet opening angle on flow through a bileaflet mechanical heart valve has been investigated using computational fluid dynamics (CFD). Steady state, laminar flow for a Newtonian fluid at a Reynolds number of 1500 was used in the two-dimensional model of the valve, ventricle, sinus and aorta. This computational model was verified using one-dimensional laser Doppler velocimetry (LDV). Although marked differences in the flow fields and energy dissipation of the jets downstream of the valve were found between the CFD predictions and the three-dimensional experimental model, both methods showed similar trends in the changes of the flow fields as the leaflet opening angle was altered. As the opening angle increased the area of recirculating fluid downstream of the leaflets, the pressure drop across the valve and the volumetric flow rate through the outer orifice decreased. For opening angles greater than 80° the jet through the outer orifice recombined with the central jet downstream of the leaflet; for an opening angle of 78° the jet through the outer orifice impinged on the aortic wall before recombining with the central jet. This study suggests that the opening angle has a marked effect on the flow downstream of the bileaflet mechanical heart valve and that valves with opening angles greater than 80° are preferable.
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Akolekar, Harshal D., Fabian Waschkowski, Yaomin Zhao, Roberto Pacciani, and Richard D. Sandberg. "Transition Modeling for Low Pressure Turbines Using Computational Fluid Dynamics Driven Machine Learning." Energies 14, no. 15 (2021): 4680. http://dx.doi.org/10.3390/en14154680.
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Existing Reynolds Averaged Navier–Stokes-based transition models do not accurately predict separation induced transition for low pressure turbines. Therefore, in this paper, a novel framework based on computational fluids dynamics (CFD) driven machine learning coupled with multi-expression and multi-objective optimization is explored to develop models which can improve the transition prediction for the T106A low pressure turbine at an isentropic exit Reynolds number of Re2is=100,000. Model formulations are proposed for the transfer and laminar eddy viscosity terms of the laminar kinetic energy transition model using seven non-dimensional pi groups. The multi-objective optimization approach makes use of cost functions based on the suction-side wall-shear stress and the pressure coefficient. A family of solutions is thus developed, whose performance is assessed using Pareto analysis and in terms of physical characteristics of separated-flow transition. Two models are found which bring the wall-shear stress profile in the separated region at least two times closer to the reference high-fidelity data than the baseline transition model. As these models are able to accurately predict the flow coming off the blade trailing edge, they are also able to significantly enhance the wake-mixing prediction over the baseline model. This is the first known study which makes use of `CFD-driven’ machine learning to enhance the transition prediction for a non-canonical flow.
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Miao, Sha, Kelli Hendrickson, and Yuming Liu. "Slug generation processes in co-current turbulent-gas/laminar-liquid flows in horizontal channels." Journal of Fluid Mechanics 860 (December 3, 2018): 224–57. http://dx.doi.org/10.1017/jfm.2018.868.
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We theoretically and computationally investigate the physical processes of slug-flow development in concurrent two-phase turbulent-gas/laminar-liquid flows in horizontal channels. The objective is to understand the fundamental mechanisms governing the initial growth and subsequent nonlinear evolution of interfacial waves, starting from a smooth stratified flow of two fluids with disparity in density and viscosity and ultimately leading to the formation of intermittent slug flow. We numerically simulate the entire slug development by means of a fully coupled immersed flow (FCIF) solver that couples the two disparate flow dynamics through an immersed boundary (IB) method. From the analysis of spatial/temporal interface evolution, we find that slugs develop through three major cascading processes: (I) stratified-to-wavy transition; (II) development and coalescence of long solitary waves; and (III) rapid channel bridging leading to slugging. In Process I, relatively short interfacial waves form on the smooth interface, whose growth is governed by the Orr–Sommerfeld instability. In Process II, interfacial waves evolve into long solitary waves through multiple resonant and near-resonant wave–wave interactions. From instability analysis of periodic solitary waves, we show that these waves are unstable to their subharmonic disturbances and grow in amplitude and primary wavelength through wave coalescence. The interfacial forcing from the turbulent gas–laminar liquid interactions significantly precipitates the growth of instability of solitary waves and enhances coalescence of solitary waves. In Process III, we show by an asymptotic analysis that interfacial waves achieve multiple-exponential growth right before bridging the channel, consistent with observations in existing experiments. The present study provides important insights for effective modelling of slug-flow dynamics and the prediction of slug frequency and length, important for design and operation of (heavy-oil/gas) pipelines and production facilities.
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Adiguzel, Ozkan, Mehmet Gokhan Gokcen, and Ali Bahadir Olcay. "Evaluation of the Effect of Needle Tilting Angle on Irrigant Flow in the Root Canal Using Side-Vented Needle by an Unsteady Computational Fluid Dynamics Model." International Dental Research 6, no. 1 (2016): 1. http://dx.doi.org/10.5577/intdentres.2016.vol6.no1.1.
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Aim: The Irrigant flow dynamics has strong influence on the root canal cleaning effectiveness. The aim of this study was to evaluate the effect of needle tilting angle on irrigant flow inside a prepared root canal during final irrigation with a side-vented needle using a validated Computational Fluid Dynamics (CFD) model.
 Methodology: To analyze the irrigant flow a CFD model with tilting angles of 0 and 2 degrees was created. The irrigant flow in the apical root canal was simulated. Computations were carried out for two selected flow rates of 0.26 and 0.78 mL/s to evaluate the velocity and turbulence quantities along the solution domain.
 Results: In addition to velocity and pressure distribution at the apex, wall shear stress distribution, vorticity and turbulent intensity results were obtained for needle tilting angle of 0 and 2 degrees. In the case of turbulent flows where the flow rate was higher, irrigation is better; however, higher apical pressures were observed for both tilting angles. Although the effect of tilting angle of two degrees for laminar flow was slightly better than zero degrees, the effect of tilting was significant for the turbulent flow case. Wall shear stress distribution, vorticity and turbulent intensity results were consistent with each other.
 Conclusions: A small tilting angle of 2 degrees had an effect on irrigation effectiveness which could be clearly observed from the wall shear stress, vorticity and velocity distribution results. The velocity distribution results obtained at the symmetry plane should be evaluated with the wall shear stress values together to observe the complete fluid dynamics structure inside the root canal. 
 How to cite this article: Adiguzel O, Gokcen MG, Olcay AB. Evaluation of the Effect of Needle Tilting Angle on Irrigant Flow in the Root Canal Using Side-Vented Needle by an Unsteady Computational Fluid Dynamics Model. Int Dent Res 2016;6:1-8. 
 Linguistic Revision: The English in this manuscript has been checked by at least two professional editors, both native speakers of English.
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Osborn, Eric A., Aleksandr Rabodzey, C. Forbes Dewey, and John H. Hartwig. "Endothelial actin cytoskeleton remodeling during mechanostimulation with fluid shear stress." American Journal of Physiology-Cell Physiology 290, no. 2 (2006): C444—C452. http://dx.doi.org/10.1152/ajpcell.00218.2005.
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Fluid shear stress stimulation induces endothelial cells to elongate and align in the direction of applied flow. Using the complementary techniques of photoactivation of fluorescence and fluorescence recovery after photobleaching, we have characterized endothelial actin cytoskeleton dynamics during the alignment process in response to steady laminar fluid flow and have correlated these results to motility. Alignment requires 24 h of exposure to fluid flow, but the cells respond within minutes to flow and diminish their movement by 50%. Although movement slows, the actin filament turnover rate increases threefold and the percentage of total actin in the polymerized state decreases by 34%, accelerating actin filament remodeling in individual cells within a confluent endothelial monolayer subjected to flow to levels used by dispersed nonconfluent cells under static conditions for rapid movement. Temporally, the rapid decrease in filamentous actin shortly after flow stimulation is preceded by an increase in actin filament turnover, revealing that the earliest phase of the actin cytoskeletal response to shear stress is net cytoskeletal depolymerization. However, unlike static cells, in which cell motility correlates positively with the rate of filament turnover and negatively with the amount polymerized actin, the decoupling of enhanced motility from enhanced actin dynamics after shear stress stimulation supports the notion that actin remodeling under these conditions favors cytoskeletal remodeling for shape change over locomotion. Hours later, motility returned to pre-shear stress levels but actin remodeling remained highly dynamic in many cells after alignment, suggesting continual cell shape optimization. We conclude that shear stress initiates a cytoplasmic actin-remodeling response that is used for endothelial cell shape change instead of bulk cell translocation.
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Nguyen, Thanh Trung, and Buddhima Indraratna. "Hydraulic behaviour of parallel fibres under longitudinal flow: a numerical treatment." Canadian Geotechnical Journal 53, no. 7 (2016): 1081–92. http://dx.doi.org/10.1139/cgj-2015-0213.
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Modelling fluid flow through fibrous porous materials has gained increasing attention from industry and research communities. Analytical and numerical methods are commonly used to predict the hydraulic characteristics of fibrous material during fluid flow, although to date most techniques have been conducted using the same assumption that the geometric features of fibres remain unchanged. In other words, the mutual interaction between fibre elements and fluid is ignored, which undermines the actual working condition of fibres. This paper therefore presents a potential numerical approach that is capable of capturing the behaviour of a fluid–solid system. Individual fibres are simulated by the discrete element method (DEM) coupled with the concept of computational fluid dynamics (CFD), whereby the information contained in each phase is constantly exchanged and updated with other phases. In comparison with conventional solutions, including the Kozeny–Carman (K–C) fluid flow principle and other valid studies, the results show an acceptable agreement in predicting the hydraulic conductivity of a fibrous system. Subjected to laminar longitudinal flow, fibre motion is also evaluated with respect to varying bond stiffness and flow velocity. The study indicates the potential of the proposed technique in modelling drainage and filtration that is based on the hydraulic behaviour of fibrous porous geomaterials.
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Costa, Pedro, Francesco Picano, Luca Brandt, and Wim-Paul Breugem. "Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions." Journal of Fluid Mechanics 843 (March 22, 2018): 450–78. http://dx.doi.org/10.1017/jfm.2018.117.
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We use interface-resolved numerical simulations to study finite-size effects in turbulent channel flow of neutrally buoyant spheres. Two cases with particle sizes differing by a factor of two, at the same solid volume fraction of 20 % and bulk Reynolds number are considered. These are complemented with two reference single-phase flows: the unladen case, and the flow of a Newtonian fluid with the effective suspension viscosity of the same mixture in the laminar regime. As recently highlighted in Costa et al. (Phys. Rev. Lett., vol. 117, 2016, 134501), a particle–wall layer is responsible for deviations of the mesoscale-averaged statistics from what is observed in the continuum limit where the suspension is modelled as a Newtonian fluid with (higher) effective viscosity. Here we investigate in detail the fluid and particle dynamics inside this layer and in the bulk. In the particle–wall layer, the near-wall inhomogeneity has an influence on the suspension microstructure over a distance proportional to the particle size. In this layer, particles have a significant (apparent) slip velocity that is reflected in the distribution of wall shear stresses. This is characterized by extreme events (both much higher and much lower than the mean). Based on these observations we provide a scaling for the particle-to-fluid apparent slip velocity as a function of the flow parameters. We also extend the scaling laws in Costa et al. (Phys. Rev. Lett., vol. 117, 2016, 134501) to second-order Eulerian statistics in the homogeneous suspension region away from the wall. The results show that finite-size effects in the bulk of the channel become important for larger particles, while negligible for lower-order statistics and smaller particles. Finally, we study the particle dynamics along the wall-normal direction. Our results suggest that single-point dispersion is dominated by particle–turbulence (and not particle–particle) interactions, while differences in two-point dispersion and collisional dynamics are consistent with a picture of shear-driven interactions.
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Cai, Jian-Chen, Jie Pan, Shi-Ju E, Wei-Dong Jiao, and Dong-Yun Wang. "A preliminary study of the pressure and shear stress on a plane surface beneath a circular cylinder in turbulent flow fields." Journal of Naval Architecture and Marine Engineering 14, no. 1 (2017): 9–24. http://dx.doi.org/10.3329/jname.v14i1.27967.
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This paper studies the fluctuating forces on a plane surface beneath a circular cylinder in the subcritical flow regime using two-dimensional computational fluid dynamics (CFD). The turbulent flow fields were calculated via numerical solutions of the NavierStokes (NS) equations without a turbulence model (laminar flow computation), large eddy simulation (LES), and Reynolds-Averaged N-S equations (RANS) approach with the shear-stress transport (SST) turbulence model. The primary goal is to evaluate the performance of 2-D turbulence simulation with different approaches and to have preliminary knowledge of the forces on the plane which is important in studying scours and flow-induced vibration in ocean engineering. Results show that although a coarse mesh scheme can only obtain potential flows, the laminar approach with high mesh resolution can adequately simulate turbulent flows at moderate Reynolds numbers. Spatially, the fluctuating forces on the plane surface due to the flow are significant within three times the cylinder diameter in the downstream, and within one cylinder diameter in the upstream of the cylinder. The pressure fluctuations are approximately two orders of magnitude larger than the shear stress fluctuations. In the frequency domain, the fluctuating forces are significant under twice the vortex-shedding frequency. Within one cylinder diameter in the downstream and upstream regions of the cylinder, the pressure fluctuations on the plane surface are well correlated, while the shear stress is not so well correlated.
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KLEWICKI, J., R. EBNER, and X. WU. "Mean dynamics of transitional boundary-layer flow." Journal of Fluid Mechanics 682 (July 19, 2011): 617–51. http://dx.doi.org/10.1017/jfm.2011.253.
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The dynamical mechanisms underlying the redistribution of mean momentum and vorticity are explored for transitional two-dimensional boundary-layer flow at nominally zero pressure gradient. The analyses primarily employ the direct numerical simulation database of Wu & Moin (J. Fluid Mech., vol. 630, 2009, p. 5), but are supplemented with verifications utilizing subsequent similar simulations. The transitional regime is taken to include both an instability stage, which effectively generates a finite Reynolds stress profile, −ρuv(y), and a nonlinear development stage, which progresses until the terms in the mean momentum equation attain the magnitude ordering of the four-layer structure revealed by Wei et al. (J. Fluid Mech., vol. 522, 2005, p. 303). Self-consistently applied criteria reveal that the third layer of this structure forms first, followed by layers IV and then II and I. For the present flows, the four-layer structure is estimated to be first realized at a momentum thickness Reynolds number Rθ = U∞ θ/ν ≃ 780. The first-principles-based theory of Fife et al. (J. Disc. Cont. Dyn. Syst. A, vol. 24, 2009, p. 781) is used to describe the mean dynamics in the laminar, transitional and four-layer regimes. As in channel flow, the transitional regime is marked by a non-negligible influence of all three terms in the mean momentum equation at essentially all positions in the boundary layer. During the transitional regime, the action of the Reynolds stress gradient rearranges the mean viscous force and mean advection profiles. This culminates with the segregation of forces characteristic of the four-layer regime. Empirical and theoretical evidence suggests that the formation of the four-layer structure also underlies the emergence of the mean dynamical properties characteristic of the high-Reynolds-number flow. These pertain to why and where the mean velocity profile increasingly exhibits logarithmic behaviour, and how and why the Reynolds stress distribution develops such that the inner normalized position of its peak value, ym+, exhibits a Reynolds number dependence according to $y_m^+ {\,\simeq\,} 1.9 \sqrt{\delta^+}$.
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Scanlon, T. J., M. T. Stickland, and A. Oldroyd. "A numerical analysis of vortex shedding within a confined channel flow." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 213, no. 5 (1999): 477–90. http://dx.doi.org/10.1243/0954406991522716.
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The numerical analysis of two-dimensional laminar vortex shedding from a rectangular cylinder within a confined channel flow is presented. This study, carried out using a computational fluid dynamics (CFD) code based on the SIMPLEST algorithm, considers the influence of numerical diffusion on the prediction of the vortex shedding frequency. The computational analysis compares the commonly used first-order accurate UPWIND scheme with the well-known third-order scheme QUICK and its derivative SMART, used for the discretization of convective transport. For the temporal differencing, a fully implicit scheme has been adopted. Plots of Strouhal number against Reynolds number suggest that the implementation of a higher-order scheme is beneficial for the accurate capture of the vortex shedding transient in unsteady flows of this nature.
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Viré, Axelle, Adriaan Derksen, Mikko Folkersma, and Kumayl Sarwar. "Two-dimensional numerical simulations of vortex-induced vibrations for a cylinder in conditions representative of wind turbine towers." Wind Energy Science 5, no. 2 (2020): 793–806. http://dx.doi.org/10.5194/wes-5-793-2020.
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Abstract. Vortex-induced vibrations (VIVs) of wind turbine towers can be critical during the installation phase, when the rotor–nacelle assembly is not yet mounted on the tower. The present work uses numerical simulations to study VIVs of a two-dimensional cylinder in the transverse direction under flow conditions that are representative of wind turbine towers both from a fluid dynamics and structural dynamics perspective. First, the numerical tools and fluid–structure interaction algorithm are validated by considering a cylinder vibrating freely in a laminar flow. In that case, both the motion amplitude and frequency are shown to agree well with previous results from the literature. Second, VIVs are modelled in the turbulent supercritical regime using unsteady Reynolds-averaged Navier–Stokes equations. In this context, the turbulence model is first validated against flow past a stationary cylinder with a high Reynolds number. Then, the results from forced vibrations are validated against experimental results for a range of reduced frequencies and velocities. It is shown that the behaviour of the aerodynamic damping changes with the frequency ratio and can therefore lead to either self-limiting or self-exciting VIVs when the cylinder is left to freely vibrate. Finally, results are shown for a freely vibrating cylinder under realistic flow and structural conditions. While a clear lock-in map is identified and shows good agreement with published numerical and experimental data, the work also highlights the unsteady nature of the aerodynamic forces and motion under certain operating conditions.
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Prasertlarp, Supasit, and Sompong Putivisutisak. "Numerical Simulation of Fluid Mixing in Micro-Mixers." Key Engineering Materials 659 (August 2015): 671–75. http://dx.doi.org/10.4028/www.scientific.net/kem.659.671.
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A 3-D numerical simulation is performed to study the flow dynamics and mixing characteristics between two different kinds of fluid within T-shaped micro-mixers. Water and ethanol are selected as the mixing fluids due to its application in calibrating the ultrasound imaging equipment. The present work focuses on the effects of inlet velocity and aspect ratio of the mixing channel. The Reynolds number is varied from 0.1 to 300 and the aspect ratio in the range between 0.2 and 1. The flow of interest is laminar, steady and without chemical reaction. It is found that at low Reynolds number, the stratified flow character is presented. As the velocity inlet increases, the mixing efficiency is decreased. However, for the Reynolds number greater than 100 the mixing efficiency is increased due to the buildup vortex structure. Furthermore, when increasing the Reynolds number, the pressure drop significantly increases. Thus, it is seen that both the inlet velocity and aspect ratio significantly affect the mixing efficiency and pressure drop.
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Qiao, Lei, Jun-Qiang Bai, Jia-Kuan Xu, Jing-Lei Xu, and Yang Zhang. "Modeling of Supersonic/Hypersonic Boundary Layer Transition Using a Single-Point Approach." International Journal of Nonlinear Sciences and Numerical Simulation 19, no. 3-4 (2018): 263–74. http://dx.doi.org/10.1515/ijnsns-2017-0011.
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AbstractDuring the process of aerodynamic shape design of supersonic and hypersonic space planes, laminar flow design and boundary layer transition prediction play important roles in aero-thermal numerical simulations and aero-thermal protection design. Therefore, in this study, a computational fluid dynamics compatible transition closure model for high speed laminar-to-turbulent transitional flows is formulated with consideration of the analysis results from stability theory. The proposed model contains two transport equations to describe the transition mechanism using local variables. Specifically, the eddy viscosity of laminar fluctuations and intermittency factor are chosen to be the characteristic parameters and modeled by transport equations. Accounting for the dominant instability modes at supersonic/hypersonic conditions, the first- and second- modes are modeled using local variables through the analysis of laminar self-similar boundary layers. Then, the present transition model is applied with compressibility corrected $k$-$\omega$ shear stress transport turbulence model. Thus, as the main significance of the current work, the present model is enabled to capture the overshoot phenomena as well as predict the transition onset position. Finally, comparisons between the predictions using the present model and the wind tunnel experimental results of several well-documented flow cases are provided to validate the proposed transition turbulence model.
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Pirozzoli, Sergio, Paolo Orlandi, and Matteo Bernardini. "The fluid dynamics of rolling wheels at low Reynolds number." Journal of Fluid Mechanics 706 (July 20, 2012): 496–533. http://dx.doi.org/10.1017/jfm.2012.273.
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AbstractWe study the fluid dynamics of rolling wheels at Reynolds number ${\mathit{Re}}_{D} \leq 1000$ (where ${\mathit{Re}}_{D} $ is the Reynolds number based on the wheel diameter), with the objective of characterizing the various regimes of steady and unsteady motion. Regardless of the Reynolds number, the flow is found to separate approximately $1{0}^{\ensuremath{\circ} } $ upstream of the apex of the wheel, where a saddle point in the pseudo-streamtrace pattern is observed. Under the flow conditions here essayed, the drag coefficient steadily decreases with ${\mathit{Re}}_{D} $, and the lift coefficient remains strictly positive. The positive lift provided by the rolling wheel is associated with the presence of a strong (positive) peak of the static pressure in the upstream proximity of the contact point with the ground, which we interpret as the result of the impingement of flow particles entrained in the boundary layer that develops on the front part of the wheel. Steady laminar flow is observed up to ${\mathit{Re}}_{D} \approx 300$, which is characterized by a three-dimensional wake whose length increases with the Reynolds number. Unsteadiness is first observed at ${\mathit{Re}}_{D} \approx 400$, under which conditions the flow retains planar symmetry, and is characterized by the quasi-periodic shedding of hairpin vortices. Transition to three-dimensional flow happens at ${\mathit{Re}}_{D} \approx 500$, in which case a sinuous mode of instability in the wheel wake is established, which modulates the shedding of the hairpins, and which causes the onset of a non-zero side force. At the highest Reynolds number considered here (${\mathit{Re}}_{D} = 1000$) the wake exhibits some characters of turbulence, with wide-band frequency spectra, and its topology entirely changes, becoming split into two parts, and being much shortened compared to the lower-${\mathit{Re}}_{D} $ cases. Despite the limitation of the study to low Reynolds numbers we find that, once significant three-dimensionality and scale separation are established in the wheel wake, the nature of the flow becomes qualitatively similar to fully developed turbulent flow. In perspective, this observation opens interesting avenues for the prediction of unsteady flow around rotating tyres at moderate computational cost.
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Khayat, Roger E., and Byung Chan Eu. "Generalized hydrodynamics and linear stability analysis of cylindrical Couette flow of a dilute Lennard–Jones fluid." Canadian Journal of Physics 71, no. 11-12 (1993): 518–36. http://dx.doi.org/10.1139/p93-081.
Full textAbstract:
Linear stability analysis is carried out for cylindrical Couette flow of a Lennard–Jones fluid in the density range from the dense liquid to the dilute gas regime. Generalized hydrodynamic equations are used to calculate marginal stability curves and compare them with those obtained by using the Navier–Stokes–Fourier equations for compressible fluids and also for incompressible fluids. In the low Reynolds or Mach number regime, if the Knudsen number is sufficiently low, the marginal stability curves calculated by the generalized hydrodynamic theory coincide, within numerical errors, with those based on the Navier–Stokes theory. But there are considerable deviations between them in the regimes beyond those mentioned earlier, since nonlinear effects manifest themselves in the laminar mean flow through the nonlinear dissipation term and normal stresses. There are three marginal stability curves obtained in contrast to the Navier–Stokes theory, which yields only two. The previously observed phase-transition-like behavior in fluid variables and the slip phenomenon are found to occur beyond the hydrodynamic stability point. The disturbance entropy production associated with the Taylor–Couette vortices is calculated to first order in disturbances in flow variables and is found to decrease as the number of vortices increases and thereby the dynamic structure is progressively more organized.
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Banik, Anirban, Tarun Kanti Bandyopadhyay, and Sushant Kumar Biswal. "Computational Fluid Dynamics (CFD) Simulation of Cross-flow Mode Operation of Membrane for Downstream Processing." Recent Patents on Biotechnology 13, no. 1 (2019): 57–68. http://dx.doi.org/10.2174/1872208312666180924160017.
Full textAbstract:
Background: Membrane filtration process produced good quality of permeate flux due to which it is used in different industries like dairy, pharmaceutical, sugar, starch and sweetener industry, bioseparation, purification of biomedical materials, and downstream polishing etc. The cross-flow mode of operation has also been used to improve the quality of the Rubber Industrial effluent of Tripura, India. </P><P> Method: The Computational Fluid Dynamics (CFD) simulation of the cross-flow membrane is done by using ANSYS Fluent 6.3. The meshing of the geometry of the membrane is done by Gambit 2.4.6 and a grid size of 100674, the number of faces is 151651 and number of nodes being 50978 has been selected for the simulation purpose from the grid independence test. We have revised and included all patents in the manuscripts related to the membrane filtration unit. </P><P> Results: Single phase Pressure-Velocity coupled Simple Algorithm and laminar model is used for the simulation of the developed model and Fluent 6.3 used for the prediction of pressure, pressure drop, flow phenomena, wall shear stress and shear strain rate inside the module is studied for cross flow membrane. </P><P> Conclusion: From the study, it has been found that CFD simulated results hold good agreement with the experimental values.
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Ó Náraigh, Lennon, Prashant Valluri, David M. Scott, Iain Bethune, and Peter D. M. Spelt. "Linear instability, nonlinear instability and ligament dynamics in three-dimensional laminar two-layer liquid–liquid flows." Journal of Fluid Mechanics 750 (June 10, 2014): 464–506. http://dx.doi.org/10.1017/jfm.2014.274.
Full textAbstract:
AbstractWe consider the linear and nonlinear stability of two-phase density-matched but viscosity-contrasted fluids subject to laminar Poiseuille flow in a channel, paying particular attention to the formation of three-dimensional waves. A combination of Orr–Sommerfeld–Squire analysis (both modal and non-modal) with direct numerical simulation of the three-dimensional two-phase Navier–Stokes equations is used. For the parameter regimes under consideration, under linear theory, the most unstable waves are two-dimensional. Nevertheless, we demonstrate several mechanisms whereby three-dimensional waves enter the system, and dominate at late time. There exists a direct route, whereby three-dimensional waves are amplified by the standard linear mechanism; for certain parameter classes, such waves grow at a rate less than but comparable to that of the most dangerous two-dimensional mode. Additionally, there is a weakly nonlinear route, whereby a purely spanwise wave grows according to transient linear theory and subsequently couples to a streamwise mode in weakly nonlinear fashion. Consideration is also given to the ultimate state of these waves: persistent three-dimensional nonlinear waves are stretched and distorted by the base flow, thereby producing regimes of ligaments, ‘sheets’ or ‘interfacial turbulence’. Depending on the parameter regime, these regimes are observed either in isolation, or acting together.