| 1. |
- Gualtieri, P., et al.
(författare)
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Clustering and turbulence modulation in particle-laden shear flows
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 715, s. 134-162
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent fluctuations induce the common phenomenon known as clustering in the spatial arrangement of small inertial particles transported by the fluid. Particles spread non-uniformly, and form clusters where their local concentration is much higher than in nearby rarefaction regions. The underlying physics has been exhaustively analysed in the so-called one-way coupling regime, i.e. negligible back-reaction of the particles on the fluid, where the mean flow anisotropy induces preferential orientation of the clusters. Turbulent transport in suspensions with significant mass in the disperse phase, i.e. particles back-reacting in the carrier phase (the two-way coupling regime), has instead been much less investigated and is still poorly understood. The issue is discussed here by addressing direct numerical simulations of particle-laden homogeneous shear flows in the two-way coupling regime. Consistent with previous findings, we observe an overall depletion of the turbulent fluctuations for particles with response time of the order of the Kolmogorov time scale. The depletion occurs in the energy-containing range, while augmentation is observed in the small-scale range down to the dissipative scales. Increasing the mass load results in substantial broadening of the energy cospectrum, thereby extending the range of scales driven by anisotropic production mechanisms. As discussed throughout the paper, this is due to the clusters which form the spatial support of the back-reaction field and give rise to a highly anisotropic forcing, active down to the smallest scales. A certain impact on two-phase flow turbulence modelling is expected from the above conclusions, since the frequently assumed small-scale isotropy is poorly recovered when the coupling between the phases becomes significant.
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| 2. |
- Gualtieri, P., et al.
(författare)
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Exact regularized point particle method for particle-laden flows in the two-way coupling regime
- 2012
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Ingår i: Proceedings of the Seventh International Symposium On Turbulence Heat and Mass Transfer. - : Begell House. ; , s. 1342-1351
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Konferensbidrag (refereegranskat)abstract
- In this paper we present a new methodology which is proved to capture the momentum exchange between a carrier turbulent flow and thousands of sub-Kolmogorov inertial particles. The velocity disturbance produced by the disperse phase is described in terms of exact regularized unsteady Stokes solutions. The approach is validated by addressing the motion of a single particle in still fluid and by comparing data of actual turbulent spatially homogeneous flows against results provided by the classical particle-in-cell method.
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| 3. |
- Lambert, Ruth A., et al.
(författare)
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Active suspensions in thin films : nutrient uptake and swimmer motion
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 733, s. 528-557
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Tidskriftsartikel (refereegranskat)abstract
- A numerical study of swimming particle motion and nutrient transport is conducted for a semidilute to dense suspension in a thin film. The steady squirmer model is used to represent the motion of living cells in suspension with the nutrient uptake by swimming particles modelled using a first-order kinetic equation representing the absorption process that occurs locally at the particle surface. An analysis of the dynamics of the neutral squirmers inside the film shows that the vertical motion is reduced significantly. The mean nutrient uptake for both isolated and populations of swimmers decreases for increasing swimming speeds when nutrient advection becomes relevant as less time is left for the nutrient to diffuse to the surface. This finding is in contrast to the case where the uptake is modelled by imposing a constant nutrient concentration at the cell surface and the mass flux results to be an increasing monotonic function of the swimming speed. In comparison to non-motile particles, the cell motion has a negligible influence on nutrient uptake at lower particle absorption rates since the process is rate limited. At higher absorption rates, the swimming motion results in a large increase in the nutrient uptake that is attributed to the movement of particles and increased mixing in the fluid. As the volume fraction of swimming particles increases, the squirmers consume slightly less nutrients and require more power for the same swimming motion. Despite this increase in energy consumption, the results clearly demonstrate that the gain in nutrient uptake make swimming a winning strategy for micro-organism survival also in relatively dense suspensions.
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| 4. |
- Lashgari, Iman, et al.
(författare)
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Laminar, Turbulent, and Inertial Shear-Thickening Regimes in Channel Flow of Neutrally Buoyant Particle Suspensions
- 2014
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 113:25, s. 254502-
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this Letter is to characterize the flow regimes of suspensions of finite-size rigid particles in a viscous fluid at finite inertia. We explore the system behavior as a function of the particle volume fraction and the Reynolds number (the ratio of flow and particle inertia to viscous forces). Unlike single-phase flows, where a clear distinction exists between the laminar and the turbulent states, three different regimes can be identified in the presence of a particulate phase, with smooth transitions between them. At low volume fractions, the flow becomes turbulent when increasing the Reynolds number, transitioning from the laminar regime dominated by viscous forces to the turbulent regime characterized by enhanced momentum transport by turbulent eddies. At larger volume fractions, we identify a new regime characterized by an even larger increase of the wall friction. The wall friction increases with the Reynolds number (inertial effects) while the turbulent transport is weakly affected, as in a state of intense inertial shear thickening. This state may prevent the transition to a fully turbulent regime at arbitrary high speed of the flow.
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| 5. |
- Magaletti, F., et al.
(författare)
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The sharp-interface limit of the Cahn-Hilliard/Navier-Stokes model for binary fluids
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 714, s. 95-126
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Tidskriftsartikel (refereegranskat)abstract
- The Cahn-Hilliard model is increasingly often being used in combination with the incompressible Navier-Stokes equation to describe unsteady binary fluids in a variety of applications ranging from turbulent two-phase flows to microfluidics. The thickness of the interface between the two bulk fluids and the mobility are the main parameters of the model. For real fluids they are usually too small to be directly used in numerical simulations. Several authors proposed criteria for the proper choice of interface thickness and mobility in order to reach the so-called 'sharp-interface limit'. In this paper the problem is approached by a formal asymptotic expansion of the governing equations. It is shown that the mobility is an effective parameter to be chosen proportional to the square of the interface thickness. The theoretical results are confirmed by numerical simulations for two prototypal flows, namely capillary waves riding the interface and droplets coalescence. The numerical analysis of two different physical problems confirms the theoretical findings and establishes an optimal relationship between the effective parameters of the model.
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| 6. |
- Nowbahar, Arash, et al.
(författare)
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Turbophoresis attenuation in a turbulent channel flow with polymer additives
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 732, s. 706-719
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Tidskriftsartikel (refereegranskat)abstract
- Turbophoresis occurs in wall-bounded turbulent flows where it induces a preferential accumulation of inertial particles towards the wall and is related to the spatial gradients of the turbulent velocity fluctuations. In this work, we address the effects of drag-reducing polymer additives on turbophoresis in a channel flow. The analysis is based on data from a direct numerical simulation of the turbulent flow of a viscoelastic fluid modelled with the FENE-P closure and laden with particles of different inertia. We show that polymer additives decrease the particle preferential wall accumulation and demonstrate with an analytical model that the turbophoretic drift is reduced because the wall-normal variation of the wall-normal fluid velocity fluctuations decreases. As this is a typical feature of drag reduction in turbulent flows, an attenuation of turbophoresis and a corresponding increase in the particle streamwise flux are expected to be observed in all of these flows, e. g. fibre or bubble suspensions and magnetohydrodynamics.
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| 7. |
- Olivieri, S., et al.
(författare)
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The effect of the Basset history force on particle clustering in homogeneous and isotropic turbulence
- 2014
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 26:4, s. 041704-
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Tidskriftsartikel (refereegranskat)abstract
- We study the effect of the Basset history force on the dynamics of small particles transported in homogeneous and isotropic turbulence and show that this term, often neglected in previous numerical studies, reduces the small-scale clustering typical of inertial particles. The contribution of this force to the total particle acceleration is, on average, responsible for about 10% of the total acceleration and particularly relevant during rare strong events. At moderate density ratios, i.e., sand or metal powder in water, its presence alters the balance of forces determining the particle acceleration.
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| 8. |
- Picano, Francesco, et al.
(författare)
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Leray-alpha Regularization of the Smagorinsky-Closed Filtered Equations for Turbulent Jets at High Reynolds Numbers
- 2012
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Ingår i: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 89:4, s. 627-650
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Tidskriftsartikel (refereegranskat)abstract
- The article reports on blending of the Leray-alpha regularization with the conventional Smagorinsky subgrid-scale closure as an option for large-eddy-simulation of turbulent flows at very high Reynolds number on coarse meshes. The model has been tested in the self-similar far-field region of a jet at a range of Reynolds numbers spanning over two decades (4x10(3), 4x10(4) and 4x10(5)) on two very coarse meshes of 2x10(5) and 3x10(4) mesh cells. The results are compared with the well-resolved DNS for Re-D = 4 x 10(3) on 15 million cells and experimental data for higher Re numbers. While the pure Leray-alpha can fail badly at high Re numbers on very coarse meshes, a blending of the two strategies by adding a small amount of extra-dissipation performs well even at a huge jet Reynolds number of Re-D = 4 x 10(5) on a very coarse mesh (2x10(5) cells), despite the ratio of the typical mesh spacing to the Kolmogorov length exceeding 300. It is found that the main prerequisite for successful LES, both for the classic Smagorinsky and the blended Leray-alpha/Smagorinsky model, is to resolve the shear-length L-s = root epsilon/delta(3) (where is the shear-rate modulus), defined by the constraint Delta/L-s < 1, where Delta is the typical mesh-cell size. For the mixed Leray-alpha/Smagorinsky model the regularization parameter should also be related to the shear-length rather than the local mesh size or Reynolds number, for which we propose a guide criterion alpha = 0.15 divided by 0.3 L-s .
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| 9. |
- Picano, Francesco, et al.
(författare)
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Shear Thickening in Non-Brownian Suspensions : An Excluded Volume Effect
- 2013
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:9, s. 098302-
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Tidskriftsartikel (refereegranskat)abstract
- Shear thickening appears as an increase of the viscosity of a dense suspension with the shear rate, sometimes sudden and violent at high volume fraction. Its origin for noncolloidal suspension with non-negligible inertial effects is still debated. Here we consider a simple shear flow and demonstrate that fluid inertia causes a strong microstructure anisotropy that results in the formation of a shadow region with no relative flux of particles. We show that shear thickening at finite inertia can be explained as an increase of the effective volume fraction when considering the dynamically excluded volume due to these shadow regions.
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| 10. |
- Sardina, Gaetano, et al.
(författare)
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Large Scale Accumulation Patterns of Inertial Particles in Wall-Bounded Turbulent Flow
- 2011
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Ingår i: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 86:3-4, s. 519-532
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent internal flow in channel and pipe geometry with a diluted second phase of inertial particles is studied numerically. Direct numerical simulations (DNS) are performed at moderate Reynolds number (Re (tau) a parts per thousand aEuro parts per thousand 200) in pipe and two channels-a smaller one similar in size to previous studies and a 3 x 3-times larger one-and Eulerian statistics pertaining to the particle concentration are evaluated. This simulation box constitutes the largest domain used for particle-laden flows so far. The resulting two-point correlations of the particle concentration show that in the smaller channel the particles organize in thin, streamwise elongated patterns which are very regular and long. The spanwise spacing of these structures is 120 and 160 plus units for the channel and pipe, respectively. Only in the larger box, the streamwise extent is long enough for the particle streaks to decorrelate, thus allowing the particles to move more freely. The influence of the box size on the characteristics of the turbophoresis is clearly shown; a 10% increase of the near-wall correlation is observed for particles with Stokes number St (+) = 50. It is thus shown that the box dimensions are an important factor in correctly assessing the motion of inertial particles, and their relation to the underlying velocity field. In addition the binning size effects on the correlation statistics of particle concentration are exploited. In particular the spanwise correlation peak values appear very sensitive to the adopted binning size, although the position of these peaks is found almost independent. Hence to allow a significant comparison between data of different configurations it is necessary to adopt the same binning spacing in inner variable.
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| 11. |
- Sardina, Gaetano, et al.
(författare)
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Self-similar transport of inertial particles in a turbulent boundary laye
- 2012
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 706, s. 584-596
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Tidskriftsartikel (refereegranskat)abstract
- Results are presented from a direct numerical simulation of a particle-laden spatially developing turbulent boundary layer up to Re-theta = 2500. The peculiar feature of a boundary-layer flow seeded with heavy particles is the variation of the local dimensionless parameters defining the fluid-particle interactions along the streamwise direction. Two different Stokes numbers can be defined, one using inner flow units and the other with outer units. Since these two Stokes numbers exhibit different decay rates in the streamwise direction, we find a decoupled particle dynamics between the inner and the outer region of the boundary layer. Preferential near-wall particle accumulation is similar to that observed in turbulent channel flow, while different behaviour characterizes the outer region. Here the concentration and the streamwise velocity profiles are found to be self-similar and to depend only on the local value of the outer Stokes number and the rescaled wall-normal distance. These new results are powerful in view of engineering and environmental applications and corresponding flow modelling.
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| 12. |
- Sardina, Gaetano, et al.
(författare)
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Statistics of Particle Accumulation in Spatially Developing Turbulent Boundary Layers
- 2014
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Ingår i: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 92:1-2, s. 27-40
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of a Direct Numerical Simulation of a particle-laden spatially developing turbulent boundary layer up to Re (theta) = 2500. Two main features differentiate the behavior of inertial particles in a zero-pressure-gradient turbulent boundary layer from the more commonly studied case of a parallel channel flow. The first is the variation along the streamwise direction of the local dimensionless parameters defining the fluid-particle interactions. The second is the coexistence of an irrotational free-stream and a near-wall rotational turbulent flow. As concerns the first issue, an inner and an outer Stokes number can be defined using inner and outer flow units. The inner Stokes number governs the near-wall behavior similarly to the case of channel flow. To understand the effect of a laminar-turbulent interface, we examine the behavior of particles initially released in the free stream and show that they present a distinct behavior with respect to those directly injected inside the boundary layer. A region of minimum concentration occurs inside the turbulent boundary layer at about one displacement thickness from the wall. Its formation is due to the competition between two transport mechanisms: a relatively slow turbulent diffusion towards the buffer layer and a fast turbophoretic drift towards the wall.
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| 13. |
- Sardina, Gaetano, et al.
(författare)
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Statistics of particle accumulation in spatially developing turbulent boundary layers
- 2012
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Ingår i: THMT-12. Proceedings of the Seventh International Symposium On Turbulence Heat and Mass Transfer. - : Begell House. ; , s. 1715-1723
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Konferensbidrag (refereegranskat)abstract
- A Direct Numerical Simulation of a particle-laden spatially developing turbulent boundary layer up to Reθ = 2500 has been performed. The peculiar feature of a boundary layer flow seeded with heavy particles is the variation of the local dimensionless parameters defining the fluid-particle interactions along the streamwise direction. An inner and an outer Stokes number can be defined using inner and outer flow units. These two parameters show different decay rates in the streamwise direction so that it is possible to find a decoupled particle dynamics between the inner and the outer region of the boundary layer. Preferential near-wall particle accumulation is similar to that observed in turbulent channel flow, while a self-similar behavior characterizes the outer region.
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| 14. |
- Troiani, G., et al.
(författare)
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Turbulent consumption speed via local dilatation rate measurements in a premixed bunsen jet
- 2013
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 160:10, s. 2029-2037
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Tidskriftsartikel (refereegranskat)abstract
- The mean local reaction rate related to the average expansion across the front and computed from the mean velocity divergence is evaluated in this work. Measurements are carried out in a air/methane premixed jet flame by combined PIV/LIF acquisitions. The procedure serves the purpose of obtaining values of a turbulent flame speed, namely the local turbulent consumption speed SLC, as a function of the position along the bunsen flame. With the further position that the flamelet assumption provides a proportionality between turbulent burning speed normalized with the laminar unstretched one and the turbulent to average flame surface ratio, the proportionality constant, i.e., the stretching factor becomes available. The results achieved so far show the existence of a wide region along which the bunsen flame front has a constant stretching factor which apparently depends only on the ratio between turbulent fluctuations and laminar flame speed and on the jet Reynolds number.
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