| 1. |
- Fukagata, K., et al.
(författare)
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Anomalous velocity fluctuations in particulate turbulent channel flow
- 2001
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Ingår i: International Journal of Multiphase Flow. - 0301-9322 .- 1879-3533. ; 27:4, s. 701-719
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Tidskriftsartikel (refereegranskat)abstract
- Gas-particle turbulent channel flow at Re-tau = 644, loaded with copper particles at a mass flow ratio of 2%, is studied numerically by large eddy simulation (LES) coupled with Lagrangian particle tracking (LPT), Inter-particle collisions and correction of drag force in the vicinity of walls are accounted for. Focus is made on the influence of particle wall boundary conditions and their influence on the statistical structure of the flow, It is shown that accordance with experimental data can be improved if a mechanism which can suppress the direct re-entrainment of particles after the impact at the wall is present. Present result shows that inter-particle collisions may play an important role in the re-distribution of particle momentum among different components even at low mass loading conditions,
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| 2. |
- Nourgaliev, R. R., et al.
(författare)
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The lattice Boltzmann equation method : theoretical interpretation, numerics and implications
- 2003
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Ingår i: International Journal of Multiphase Flow. - 0301-9322 .- 1879-3533. ; 29:1, s. 117-169
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Forskningsöversikt (refereegranskat)abstract
- During the last ten years the lattice Boltzmann equation (LBE) method has been developed as an alternative numerical approach in computational fluid dynamics (CFD). Originated from the discrete kinetic theory, the LBE method has emerged with the promise to become a superior modeling platform, both computationally and conceptually, compared to the existing arsenal of the continuum-based CFD methods. The LBE method has been applied for simulation of various kinds of fluid flows under different conditions. The number of papers on the LBE method and its applications continues to grow rapidly, especially in the direction of complex and multiphase media. The purpose of the present paper is to provide a comprehensive, self-contained and consistent tutorial on the LBE method, aiming to clarify misunderstandings and eliminate some confusion that seems to persist in the LBE-related CFD literature. The focus is placed on the fundamental principles of the LBE approach. An excursion into the history, physical background and details of the theory and numerical implementation is made. Special attention is paid to advantages and limitations of the method, and its perspectives to be a useful framework for description of complex flows and interfacial (and multiphase) phenomena. The computational performance of the LBE method is examined, comparing it to other CFD methods, which directly solve for the transport equations of the macroscopic variables.
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| 3. |
- Abbasi Hoseini, A., et al.
(författare)
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Finite-length effects on dynamical behavior of rod-like particles in wall-bounded turbulent flow
- 2015
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 76, s. 13-21
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Tidskriftsartikel (refereegranskat)abstract
- Combined Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) measurements have been performed in dilute suspensions of rod-like particles in wall turbulence. PIV results for the turbulence field in the water table flow apparatus compared favorably with data from Direct Numerical Simulations (DNS) of channel flow turbulence and the universality of near-wall turbulence justified comparisons with DNS of fiber-laden channel flow. In order to examine any shape effects on the dynamical behavior of elongated particles in wall-bounded turbulent flow, fibers with three different lengths but the same diameter were used. In the logarithmic part of the wall-layer, the translational fiber velocity was practically unaffected by the fiber length l. In the buffer layer, however, the fiber dynamics turned out to be severely constrained by the distance z to the wall. The short fibers accumulated preferentially in low-speed areas and adhered to the local fluid speed. The longer fibers (l/z > 1) exhibited a bi-modal probability distribution for the fiber velocity, which reflected an almost equal likelihood for a long fiber to reside in an ejection or in a sweep. It was also observed that in the buffer region, high-speed long fibers were almost randomly oriented whereas for all size cases the slowly moving fibers preferentially oriented in the streamwise direction. These phenomena have not been observed in DNS studies of fiber suspension flows and suggested l/z to be an essential parameter in a new generation of wall-collision models to be used in numerical studies.
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| 4. |
- Ahmed, Zaheer, et al.
(författare)
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Effects of soluble surfactant on lateral migration of a bubble in a pressure driven channel flow
- 2020
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Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 126
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Tidskriftsartikel (refereegranskat)abstract
- The effects of soluble surfactant on the lateral migration of a bubble in a pressure-driven channel flow are examined by interface-resolved numerical simulations. The interfacial and bulk surfactant concentration evolution equations are solved fully coupled with the incompressible Navier-Stokes equations. A non-linear equation of state is used to relate interfacial surface tension to surfactant concentration at the interface. Extensive computations are performed to investigate the bubble dynamics for a wide range of parameters. It is found that surfactant dramatically changes the bubble dynamics. In the clean case, the bubble position depends on its deformability, characterized by the Eotvos (Eo) and the capillary (Ca) numbers. The spherical bubble moves towards the wall, while the deformable one migrates away from it. On the other hand, in the presence of the surfactant, even the spherical bubble moves away from the wall. It is also found that the contaminated bubble stays away from the wall for Eo = 0.1 and Eo = 1.5 while it migrates towards the wall for 0.1 < Eo < 1.5. Also, at high Eo, the onset of path instability is observed for both the clean and the contaminated cases. However, adding surfactant to the system triggers the path instability earlier and amplifies the oscillations afterwards.
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| 5. |
- Alghalibi, Dhiya, et al.
(författare)
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Sedimentation of finite-size particles in quiescent wall-bounded shear-thinning and Newtonian fluids
- 2020
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Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 129
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Tidskriftsartikel (refereegranskat)abstract
- We study the sedimentation of finite-size particles in quiescent wall-bounded Newtonian and shear-thinning fluids by interface resolved numerical simulations. The suspended phase consists of Non-Brownian rigid spherical particles with particle to fluid density ratio ρp/ρf=1.5 at three different solid volume fractions Φ=1%, 5% and 20%. Firstly, to focus on the effect of shear-thinning on the particle dynamics and interactions, the Archimedes number is increased for a single particle to have the same settling speed in the Newtonian fluid as in the shear-thinning fluid. Secondly, we consider fixed Archimedes and vary the shear-thinning properties of the fluid. Overall, we report a twofold effect of shear thinning. First and more important, the substantial increase of the particle sedimentation velocity in the shear-thinning case due to the increase of the shear rate around the particles, which reduces the local viscosity leading to a reduced particle drag. Secondly, the shear-thinning fluid reduces the level of particle interactions, causing a reduction of velocity fluctuations and resulting in particles sedimenting at approximately the same speed. Moreover, the mean settling velocities decrease with the particle concentration as a consequence of the hindering effect. Particles tend to sediment in the middle of the channel, preferentially positioning in the wake of neighbouring particles or aside them, resulting in lower levels of fluid velocity fluctuations in the gravity direction in the shear-thinning fluid.
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| 6. |
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| 7. |
- Altimira, Mireia, et al.
(författare)
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Numerical investigation of throttle flow under cavitating conditions
- 2015
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Ingår i: International Journal of Multiphase Flow. - Oxford : Elsevier BV. - 0301-9322 .- 1879-3533. ; 75, s. 124-136
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Tidskriftsartikel (refereegranskat)abstract
- The present paper shows the importance of the resolution of large unsteady flow structures in numerical simulations of cavitating flows. Three-dimensional simulations of the flow through a throttle geometry representative for fuel injectors have been performed to characterise the inception and development of cavitation, adopting the implicit Large Eddy Simulation approach. The two-phase flow has been handled by the Volume of Fluid method; whilst the simplified Rayleigh equation has been adopted to handle bubble dynamics. The mathematical model has been solved in the open source C++ toolbox OpenFOAM 2.0.1. Results obtained with the mathematical model are compared with those from RANS-based simulations and validated against experimental measurements. The performed Large Eddy Simulations not only are able to reproduce vortex cavitation, but also give further insight into the complex interaction between cavitation and turbulence through the assessment of the different terms of the vorticity equation.
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| 8. |
- Andric, Jelena, 1979, et al.
(författare)
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Ballistic deflection of fibres in decelerating flow
- 2016
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 85, s. 57-66
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the motion of inertial, rod-like fibres in the decelerating flow of a wedge-shaped channel with non-creeping fibre-flow interactions. We consider the trajectories of isolated fibres to identify the conditions for which these trajectories deflect from the streamlines of the flow as well as a rectilinear path. We carry out analytical and numerical studies under the assumption of an infinite fibre hydrodynamic resistance to transverse flow, and we expand the numerical study by taking into account a finite transverse hydrodynamic resistance. The analytical analysis identifies a longitudinal ballistic number Bℓ and a transverse ballistic number Bt as two dimensionless parameters that govern the fibre dynamics. It is found that Bℓ is the product of the Stokes number Stℓ in the longitudinal direction of the fibre and the channel opening angle β. As anticipated, a fibre moves along the streamlines in the viscosity-dominated regime (Bℓ « 1, Bt « 1), while it moves in a straight line without being rotated in the inertia-dominated regime (Bt » 1). The focus of the present study is on the intermediate regime (Bℓ » 1, Bt
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| 9. |
- Anglart, Henryk, et al.
(författare)
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Experimental investigation of post-dryout heat transfer in annulus with spacers
- 2007
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 33:8, s. 809-821
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Tidskriftsartikel (refereegranskat)abstract
- Experimental investigations of post-dryout heat transfer in 10 x 22.1 annulus test section with space. rs were carried out in the high-pressure two-phase flow loop at the Royal Institute of Technology (KTH). The test section was manufactured of Inconel 600 to withstand high temperatures. Several thermocouples were installed on tube and rod surfaces to measure the local wall temperature. Measurements were performed for mass flow rate in range from 500 to 2000 kg m(-2) s(-1), with inlet subcooling equal to 10 and 40 K, heat flux in a range from 480 to 1380 kW m(-2) and for the system pressure of 7 MPa. Uniform axial power distributions were applied on rod and tube walls. Using different distributions of heat flux between walls, post-dryout was achieved either on the inner or on the outer wall. The experimental results indicate a very strong influence of spacers on post-dryout heat transfer. For low mass flow rates the wall superheat was significantly reduced downstream of spacers, even though the whole distance between spacers was still under post-dryout conditions when heat flux was high enough. At high mass flow rates and under investigated range of heat flux the dryout patches were effectively quenched downstream of spacers.
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| 10. |
- Ardekani, Mehdi Niazi, et al.
(författare)
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Numerical study of the sedimentation of spheroidal particles
- 2016
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 87, s. 16-34
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Tidskriftsartikel (refereegranskat)abstract
- The gravity-driven motion of-rigid particles in a viscous fluid is relevant in many natural and industrial processes, yet this has mainly been investigated for spherical particles. We therefore consider the sedimentation of non-spherical (spheroidal) isolated and particle pairs in a viscous fluid via numerical simulations using the Immersed Boundary Method. The simulations performed here show that the critical Galileo number for the onset of secondary motions decreases as the spheroid aspect ratio departs from 1. Above this critical threshold, oblate particles perform a zigzagging motion whereas prolate particles rotate around, the vertical axis while having their broad side facing the falling direction. Instabilities of the vortices in the wake follow when farther increasing the Galileo number. We also study the drafting kissing-tumbling associated with the settling of particle pairs. We find that the interaction time increases significantly for non-spherical particles and, more interestingly, spheroidal particles are attracted from larger lateral displacements. This has important implications for the estimation of collision kernels and can result its increasing clustering in suspensions of sedimenting spheroids.
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