A generalized k-epsilon model for turbulence modulation in fluid-particle flows

A large amount of published data show that particles with diameter above 10\% of the turbulence integral length scale ($D/l >0.1$) tend to increase the turbulent kinetic energy of the carrier fluid above the single-phase value, and smaller particles tend to suppress it. A revised phenomenological model of the $k-\epsilon$ type was developed to reproduce these effects with the correct asymptotic limit of no turbulence modulation for small particles, and augmentation for larger diameter solids. Particle-kinetic theory was used to derive the work exchanged between the particles and the fluid due to both drag and added mass forces to accommodate any particle/fluid density ratios including bubbles, droplets and heavy solids. For the larger particles, we devised a new model for vortex shedding induced by the slip between the particles and the turbulent flow, due to particle inertia. Simple approximate formulae for the turbulence modulation were obtained through asymptotic analysis. The overall effect for solid particles is that augmentation for large diameter solids is due to vortex shedding, and turbulence suppression for small diameters is due to mainly to turbulent drag forces and extra fluid dissipation. The transition from suppression to augmentation around $D/l = 0.1$ is a robust feature for a wide range of particle Reynolds and Stokes numbers, but we could not prove this to be a general relation on a theoretical basis. Indeed, bubbles and droplets may not display turbulence augmentation at all for the larger diameters due to moderate turbulence levels needed to prevent breakup, and the velocity difference between particles and fluid may therefore be too low for vortex shedding to occur. On the basis of the model we find that some data for solids in vertical gas flow show very large turbulence augmentation that can only be due to gravitational settling.Comment: 19 pages 8 figure

Reduction of the effective shear viscosity in polymer solutions due to crossflow migration in microchannels: Effective viscosity models based on DPD simulations

Molecular dynamics simulations (dissipative particle dynamics–DPD) were developed and used to quantify wall-normal migration of polymer chains in microchannel Poseuille flow. Crossflow migration due to viscous interaction with the walls results in lowered polymer concentration near the channel walls. A larger fraction of the total flow volume becomes depleted of polymer when the channel width h decreases into the submicron range, significantly reducing the effective viscosity. The effective viscosity was quantified in terms of channel width and Weissenberg number Wi, for 5% polymer volume fraction in water. Algebraic models for the depletion width δ(Wi, h) and effective viscosity μe(δ/h, Wi) were developed, based on the hydrodynamic theory of Ma and Graham and our simulation results. The depletion width model can be applied to longer polymer chains after a retuning of the polymer persistence length and the corresponding potential/thermal energy ratio.submittedVersio

Effects of polymer adsorption on the effective viscosity in microchannel flows: phenomenological slip layer model from molecular simulations

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Techno-economic feasibility of hybrid hydro-FPV systems in Sub-Saharan Africa under different market conditions

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Calcium Ii Phase Relations And Chromospheric Dynamics

. Velocity-velocity and velocity-intensity phase relations from radiative-hydrodynamics calculations by Carlsson & Stein (1992, 1994) are compared with spatially resolved internetwork observations of the H, K, 8498 A, 8542 A and 8662 A lines of ionized calcium. Several of the computed phase relations agree with observations even though the simulations are dominated by propagating shock waves and the observations have been interpreted as evidence for standing waves in the chromosphere. In particular, V(8542) -- V(8498) is close to zero in agreement with observations in spite of a factor of nine difference in opacity. This is explained as due to the Doppler shift of the 8498 line core not being a measure of the velocity at the line core formation height. V(3968) -- V(8542) shows a propagating signature contrary to recent observations (Fleck et al., 1994). We speculate that part of the discrepancy is due to depth-dependent microturbulence and effects of seeing, limited instrument resolu..

A DPD study of asphaltene aggregation: The role of inhibitor and asphaltene structure in diffusion-limited aggregation

The kinetic effects of DBSA (dodecyl benzene sulfonic acid) and a linear amphihile on asphaltene aggregation was investigated, using dissipative particle dynamics molecular simulations. The simulation results indicated that without inhibitor, diffusion-limited asphaltene aggregation can be initiated by a kinetic/diffusive capture process between polar side chain groups rather than by interaction between polyaromatic rings. The most likely reason for this is that the side chains have higher diffusive mobility than the more massive aromatic ring structures. The DBSA acidic head groups adhered to the asphaltene side chain polar groups (the basic functional groups), resulting in lowered mobility of the side chain/DBSA complexes, thereby suppressing asphaltene aggregation initiation. A more mobile amphiphilic inhibitor without the aromatic ring gave a higher asphaltene aggregation rate. Adsorption of asphaltenes on a solid surface was suppressed with DBSA, due to an adsorbed mono-layer of DBSA that occupied a significant fraction of the surface area.This is the manuscript of an article published by Taylor & Francis in Journal of Dispersion Science and Technology 2017, available online: http://www.tandfonline.com/10.1080/01932691.2016.117297

A DPD study of asphaltene aggregation: The role of inhibitor and asphaltene structure in diffusion-limited aggregation

The kinetic effects of DBSA (dodecyl benzene sulfonic acid) and a linear amphihile on asphaltene aggregation was investigated, using dissipative particle dynamics molecular simulations. The simulation results indicated that without inhibitor, diffusion-limited asphaltene aggregation can be initiated by a kinetic/diffusive capture process between polar side chain groups rather than by interaction between polyaromatic rings. The most likely reason for this is that the side chains have higher diffusive mobility than the more massive aromatic ring structures. The DBSA acidic head groups adhered to the asphaltene side chain polar groups (the basic functional groups), resulting in lowered mobility of the side chain/DBSA complexes, thereby suppressing asphaltene aggregation initiation. A more mobile amphiphilic inhibitor without the aromatic ring gave a higher asphaltene aggregation rate. Adsorption of asphaltenes on a solid surface was suppressed with DBSA, due to an adsorbed mono-layer of DBSA that occupied a significant fraction of the surface area

On the interfacial roughness scale in turbulent stratified two-phase flow: 3D lattice Boltzmann numerical simulations with forced turbulence and surfactant

Numerical 3D simulations of turbulent, stratified two-phase shear flow with a surfactant laden interface were used to test and develop a phenomenological interfacial roughness scale model where the energy required to deform the interface (buoyancy, interfacial tension, and viscous work) is proportional to the turbulent kinetic energy adjacent to the interface. The turbulence was forced in the upper and lower liquids in the simulations, to emulate the interfacial dynamics without requiring (prohibitively) large simulation domains and Reynolds numbers. The addition of surfactant lead to an increased roughness scale (for the same turbulent kinetic energy) due to the introduction of interfacial dilatational elasticity that suppressed horizontal motion parallel to the interface, and enhanced the vertical motion. The phenomenological roughness scale model was not fully developed for dilatational elasticity in this work, but we proposed a source term that represents surfactant induced pressure fluctuations near the interface. This source term should be developed further to account for the relation between surfactant density fluctuations and turbulence adjacent to the interface. We foresee that the roughness scale model can be used as a basis for more general interfacial closure relations in Reynolds averaged turbulence models, where also mobile surfactant is accounted for