| 1. |
- Vanna, Francesco De, et al.
(författare)
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Effect of convective schemes in wall-resolved and wall-modeled LES of compressible wall turbulence
- 2023
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Ingår i: Computers & Fluids. - : Elsevier BV. - 0045-7930 .- 1879-0747. ; 250, s. 105710-
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Tidskriftsartikel (refereegranskat)abstract
- The current study discusses how numerical schemes and discretization approaches affect wall-resolved and wall-modeled LES outcomes. A turbulent boundary layer setup over a flat plate in both super-and hypersonic conditions is used to illustrate the effect of different numerical discretization strategies. In particular, six convective methods are examined, as well as various degrees of hybridization between shock-capturing and centered approaches: The former introducing non-negligible numerical viscosity, the latter being virtually dissipation-free. The analysis reveals that injected numerical viscosity due to upwinding procedures consid-erably alters wall dynamics for both wall-resolved and especially wall-modeled arrangements. In particular, if low-order or pure shock-capturing schemes are used, wall modeling fails in heading the system dynamics due to a strong modulation of main turbulent features. Conversely, realistic turbulence patterns are recovered if hybrid and/or high-order shock-capturing methods are employed. Thus, the paper establishes criteria for selecting a suitable numerical setup in wall-modeled LES, providing suggestions for grid resolution levels and convective scheme selection/hybridization. An overview perspective concerning numerical diffusion coupling with turbulent stresses in wall-resolved and wall-modeled LES is also provided.
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| 2. |
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| 3. |
- Battista, F., et al.
(författare)
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Fractal scaling of turbulent premixed flame fronts : Application to LES
- 2015
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Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X .- 1879-2278. ; 51, s. 78-87
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Tidskriftsartikel (refereegranskat)abstract
- The fractal scaling properties of turbulent premixed flame fronts have been investigated and considered for modeling sub-grid scales in the Large-Eddy-Simulation framework. Since the width of such thin reaction fronts cannot be resolved into the coarse mesh of LES, the extent of wrinkled flame surface contained in a volume is taken into account. The amount of unresolved flame front is estimated via the "wrinkling factor" that depends on the definition of a suitable fractal dimension and the scale at which the fractal scaling is lost, the inner cut-off length e. In this context, the present study considers laboratory experiments and one-step reaction DNS of turbulent premixed jet flames in different regimes of turbulent premixed flames. Fractal dimension is found to be substantially constant and well below that typical of passive scalar fronts. The inner cut-off length shows a clear scaling with the dissipative scale of Kolmogorov for the regimes here considered. These features have been exploited performing Large Eddy Simulations. Good model performance has been found comparing the LES against a corresponding DNS at moderate Reynolds number and experimental data at higher Reynolds numbers.
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| 4. |
- Battista, F., et al.
(författare)
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Turbulence-combustion interaction in H2/CO/air Bunsen flame
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- In last decades, the increasing care to environmental safeguard and costs in the hydrocarbon fuel supplying have prompted in the development of alternative fuels, namely hydrogen based fuels as syngas. Syngas consists in a mixture of hydrogen and carbon monoxide (CO) in different relative concentration, in some cases with small concentration of methane. The aim of this work is to address the dynamics of turbulent hydrogen/carbon-monoxide/air Bunsen flames by means of Direct Numerical Simulation. The main issue is to understand how the thermo-diffusive instabilities occurring in pure hydrogen/air flame [7] are influenced by the presence of the carbon-monoxide. It is well known that the thermo-diffusive instabilities are mainly induced by the high hydrogen diffusivity leading to local quenching and temperature peaks in the flame with consequent increase of pollutant formation (e.g. NOx). The presence of carbon monoxide in the fuel mixture has significant effects in flame dynamics where we observe a damping of the H2/air flame instabilities with less apparent quenching and high temperature peaks.
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| 5. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Breakup of small aggregates in bounded and unbounded turbulent flows
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- Breakup of small tracer-like aggregates is studied by means of numerical simulations in four different flows, namely homogeneous isotropic turbulence, smooth stochastic flow, turbulent channel flow, and developing boundary layer flow. Aggregate breakup occurs when the local hydrodynamic stress σ ∼ ε1/2, where ε is the local energy dissipation, overcomes a given threshold value σcr [or equivalently εcr ∼ σcr2 ] characteristic for a given type of aggregates. Following the aggregate trajectory upon release and detecting the first occurrence of local energy dissipation exceeding the predefined threshold allows for estimating the breakup rate as a function of εcr. Results show that the breakup rate decreases with increasing threshold. For small values of the threshold, this decrease assumes consistent scaling among the different flows which is explained by universal small scale flow properties.
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| 6. |
- Bäbler, Matthäus, et al.
(författare)
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Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 766
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Tidskriftsartikel (refereegranskat)abstract
- Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereby aggregate breakup occurs when the local hydrodynamic stress sigma similar to epsilon(1/2), with epsilon being the energy dissipation at the position of the aggregate, overcomes a given threshold sigma(cr), which is characteristic for a given type of aggregate. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a scaling behaviour among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, the results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.
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| 7. |
- Chiara, Luigi Filippo, et al.
(författare)
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Suspensions of deformable particles in Poiseuille flows at finite inertia
- 2020
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Ingår i: Fluid Dynamics Research. - : IOP Publishing. - 0169-5983 .- 1873-7005. ; 52:6
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Tidskriftsartikel (refereegranskat)abstract
- We analyze a suspension of deformable particles in a pressure-driven flow. The suspension is composed of neutrally buoyant initially spherical particles and a Newtonian carrier fluid, and the flow is solved by means of direct numerical simulations, using a fully Eulerian method based on a one-continuum formulation. The solid phase is modeled with an incompressible viscous hyperelastic constitutive relation, and the flow is characterized by three main dimensionless parameters, namely the solid volume fraction, the Reynolds and capillary numbers. The dependency of the effective viscosity on these three quantities is investigated to study the inertial effects on a suspension of deformable particles. It can be observed that the suspension has a shear-thinning behavior, and the reduction in effective viscosity for high shear rates is emphasized in denser configurations. The separate analysis of the Reynolds and capillary numbers reveal that the effective viscosity depends more on the capillary than on the Reynolds number. In addition, our simulations exhibit a consistent tendency for deformable particles to move toward the center of the channel, where the shear rate is low. This phenomenon is particularly marked for very dilute suspensions, where a whole region near the wall is empty of particles. Furthermore, when the volume fraction is increased this near-wall region is gradually occupied, because of higher mutual particle interactions. Deformability also plays an important role in the process. Indeed, at high capillary numbers, particles are more sensitive to shear rate variations and can modify their shape more easily to accommodate a greater number of particles in the central region of the channel. Finally, the total stress budgets show that the relative particle-induced stress contribution increases with the volume fraction and Reynolds number, and decreases with the particle deformability.
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| 8. |
- Costa, Pedro, et al.
(författare)
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Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions
- 2018
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Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 843, s. 450-478
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Tidskriftsartikel (refereegranskat)abstract
- 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 etal. (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 etal. (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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| 9. |
- Costa, Pedro, et al.
(författare)
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Interface-resolved simulations of small inertial particles in turbulent channel flow
- 2020
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 883
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Tidskriftsartikel (refereegranskat)abstract
- We present a direct comparison between interface-resolved and one-way-coupled point-particle direct numerical simulations (DNS) of gravity-free turbulent channel flow laden with small inertial particles, with high particle-to-fluid density ratio and diameter of approximately three viscous units. The most dilute flow considered, solid volume fraction O(10(-5)), shows the particle feedback on the flow to be negligible, whereas differences with respect to the unladen case, notably a drag increase of approximately 10 %, are found for a volume fraction O(10(-4)). This is attributed to a dense layer of particles at the wall, caused by turbophoresis, flowing with large particle-to-fluid apparent slip velocity. The most dilute case is therefore taken as the benchmark for assessing the validity of a widely used point-particle model, where the particle dynamics results only from inertial and nonlinear drag forces. In the bulk of the channel, the first- and second-order moments of the particle velocity from the point-particle DNS agree well with those from the interface-resolved DNS. Close to the wall, however, most of the statistics show major qualitative differences. We show that this difference originates from the strong shear-induced lift force acting on the particles in the near-wall region. This mechanism is well captured by the lift force model due to Saffman (J. Fluid Mech., vol. 22 (2), 1965, pp. 385-400), while other widely used, more elaborate, approaches aiming at extending the lift model for a wider range of particle Reynolds numbers can actually underpredict the magnitude of the near-wall particle velocity fluctuations for the cases analysed here.
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| 10. |
- Costa, Pedro, et al.
(författare)
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Near-wall turbulence modulation by small inertial particles
- 2021
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Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 922
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Tidskriftsartikel (refereegranskat)abstract
- We use interface-resolved simulations to study near-wall turbulence modulation by small inertial particles, much denser than the fluid, in dilute/semi-dilute conditions. We considered three bulk solid mass fractions, , and , with only the latter two showing turbulence modulation. The increase of the drag is strong at , but mild in the densest case. Two distinct regimes of turbulence modulation emerge: for smaller mass fractions, the turbulence statistics are weakly affected and the near-wall particle accumulation increases the drag so the flow appears as a single-phase flow at slightly higher Reynolds number. Conversely, at higher mass fractions, the particles modulate the turbulent dynamics over the entire flow, and the interphase coupling becomes more complex. In this case, fluid Reynolds stresses are attenuated, but the inertial particle dynamics near the wall increases the drag via correlated velocity fluctuations, leading to an overall drag increase. Hence, we conclude that, although particles at high mass fractions reduce the fluid turbulent drag, the solid phase inertial dynamics still increases the overall drag. However, inspection of the streamwise momentum budget in the two-way coupling limit of vanishing volume fraction, but finite mass fraction, indicates that this trend could reverse at even higher particle load.
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| 11. |
- Costa, Pedro, et al.
(författare)
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Universal Scaling Laws for Dense Particle Suspensions in Turbulent Wall-Bounded Flows
- 2016
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 117:13
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Tidskriftsartikel (refereegranskat)abstract
- The macroscopic behavior of dense suspensions of neutrally buoyant spheres in turbulent plane channel flow is examined. We show that particles larger than the smallest turbulence scales cause the suspension to deviate from the continuum limit in which its dynamics is well described by an effective suspension viscosity. This deviation is caused by the formation of a particle layer close to the wall with significant slip velocity. By assuming two distinct transport mechanisms in the near-wall layer and the turbulence in the bulk, we define an effective wall location such that the flow in the bulk can still be accurately described by an effective suspension viscosity. We thus propose scaling laws for the mean velocity profile of the suspension flow, together with a master equation able to predict the increase in drag as a function of the particle size and volume fraction.
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| 12. |
- Dalla Barba, Federico, et al.
(författare)
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An interface capturing method for liquid-gas flows at low-Mach number
- 2021
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Ingår i: Computers & Fluids. - : Elsevier Ltd. - 0045-7930 .- 1879-0747. ; 216
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Tidskriftsartikel (refereegranskat)abstract
- Multiphase, compressible and viscous flows are of crucial importance in a wide range of scientific and engineering problems. Despite the large effort paid in the last decades to develop accurate and efficient numerical techniques to address this kind of problems, current models need to be further improved to address realistic applications. In this context, we propose a numerical approach to the simulation of multiphase, viscous flows where a compressible and an incompressible phase interact in the low-Mach number regime. In this frame, acoustics are neglected but large density variations of the compressible phase can be accounted for as well as heat transfer, convection and diffusion processes. The problem is addressed in a fully Eulerian framework exploiting a low-Mach number asymptotic expansion of the Navier-Stokes equations. A Volume of Fluid approach (VOF) is used to capture the liquid-gas interface, built on top of a massive parallel solver, second order accurate both in time and space. The second-order-pressure term is treated implicitly and the resulting pressure equation is solved with the eigenexpansion method employing a robust and novel formulation. We provide a detailed and complete description of the theoretical approach together with information about the numerical technique and implementation details. Results of benchmarking tests are provided for five different test cases.
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| 13. |
- Fornari, Walter, et al.
(författare)
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Reduced particle settling speed in turbulence
- 2016
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 808, s. 153-167
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Tidskriftsartikel (refereegranskat)abstract
- We study the settling of finite-size rigid spheres in sustained homogeneous isotropic turbulence (1111) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We study semi-dilute suspensions at different Galileo numbers, Ga. The Galileo number is the ratio between buoyancy and viscous forces, and is here varied via the solid-to-fluid density ratio rho(p)/rho(f), The focus is on particles that are slightly heavier than the fluid. We find that in HIT, the mean settling speed is less than that in quiescent fluid; in particular, it reduces by 6 %-60 % with respect to the terminal velocity of an isolated sphere in quiescent fluid as the ratio between the latter and the turbulent velocity fluctuations it is decreased. Analysing the fluid particle relative motion, we find that the mean settling speed is progressively reduced while reducing rho(p)/rho(f) due to the increase of the vertical drag induced by the particle cross-flow velocity. Unsteady effects contribute to the mean overall drag by about 6%-10%. The probability density functions of particle velocities and accelerations reveal that these are closely related to the features of the turbulent flow. The particle mean-square displacement in the settling direction is found to be similar for all Ga if time is scaled by (2a)/u' (where 2a is the particle diameter and a is the turbulence velocity root mean square).
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| 14. |
- Fornari, Walter, et al.
(författare)
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Rheology of Confined Non-Brownian Suspensions
- 2016
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 116:1
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Tidskriftsartikel (refereegranskat)abstract
- We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couette flow using direct numerical simulations. We find that if the width of the channel is a (small) integer multiple of the sphere diameter, the spheres self-organize into two-dimensional layers that slide on each other and the effective viscosity of the suspension is significantly reduced. Each two-dimensional layer is found to be structurally liquidlike but its dynamics is frozen in time.
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| 15. |
- Fornari, Walter, 1989-, et al.
(författare)
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Rheology of extremely confined non-Brownian suspensions
- 2016
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 116:1
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couetteflow using Direct Numerical Simulations.We find that if the width of the channel is a (small) integer multiple of the spherediameter, the spheres self-organize into two-dimensional layersthat slide on each other and the effective viscosity of the suspension issignificantly reduced. Each two-dimensional layer is found to be structurallyliquid-like but its dynamics is frozen in time.
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| 16. |
- Fornari, Walter, 1989-, et al.
(författare)
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Sedimentation of finite-size spheres in quiescent and turbulent environments
- 2016
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 788, s. 640-669
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Tidskriftsartikel (refereegranskat)abstract
- Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yetlittle is known about the behavior of finite-size particles inhomogeneous isotropic turbulence.To fill this gap, we perform Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using anImmersed Boundary Method to accountfor the dispersed rigid spherical particles. The solid volume fractions considered are 0.5-1%,while the solid to fluid density ratio 1.02.The particle radius is chosen to be approximately 6 Komlogorov lengthscales.Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behaviour of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform direct numerical simulations of sedimentation in quiescent and turbulent environments using an immersed boundary method to account for the dispersed rigid spherical particles. The solid volume fractions considered are phi = 0.5-1%, while the solid to fluid density ratio rho(p)/rho(f) = 1.02. The particle radius is chosen to be approximately six Kolmogorov length scales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reductions with respect to a single particle in quiescent fluid are approximately 12 % and 14% for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails arc associated with the intermittent fast sedimentation of particle pairs in drafting kissing tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behaviour observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulae and show that non-stationary effects, related to vortex shedding, explain the increased reduction in mean settling Velocity in a turbulent environment.
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| 17. |
- Fornari, Walter, 1989-, et al.
(författare)
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Settling of finite-size particles in turbulence at different volume fractions
- 2018
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Ingår i: Acta Mechanica. - : Springer Science and Business Media LLC. - 0001-5970 .- 1619-6937. ; 230:2, s. 413-430
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Tidskriftsartikel (refereegranskat)abstract
- We study the settling of finite-size rigid spheres in quiescent fluid and in sustained homogeneous isotropic turbulence (HIT) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We consider semi-dilute and dense suspensions of rigid spheres with solid volume fractions ϕ= 0.5 - 10 % , solid-to-fluid density ratio R= 1.02 , and Galileo number (i.e., the ratio between buoyancy and viscous forces) Ga= 145. In HIT, the nominal Reynolds number based on the Taylor microscale is Re λ ≃ 90 , and the ratio between the particle diameter and the nominal Kolmogorov scale is (2 a) / η≃ 12 (being a the particle radius). We find that in HIT the mean settling speed is less than that in quiescent fluid for all ϕ. For ϕ= 0.5 % , the mean settling speed in HIT is 8 % less than in quiescent fluid. However, by increasing the volume fraction the difference in the mean settling speed between quiescent fluid and HIT cases reduces, being only 1.7 % for ϕ= 10 %. Indeed, while at low ϕ the settling speed is strongly altered by the interaction with turbulence, at large ϕ this is mainly determined by the (strong) hindering effect. This is similar in quiescent fluid and in HIT, leading to similar mean settling speeds. On the contrary, particle angular velocities are always found to increase with ϕ. These are enhanced by the interaction with turbulence, especially at low ϕ. In HIT, the correlations of particle lateral velocity fluctuations oscillate around zero before decorrelating completely. The time period of the oscillation seems proportional to the ratio between the integral lengthscale of turbulence and the particle characteristic terminal velocity. Regarding the mean square particle displacement, we find that it is strongly enhanced by turbulence in the direction perpendicular to gravity, even at the largest ϕ. Finally, we investigate the collision statistics for all cases and find the interesting result that the collision frequency is larger in quiescent fluid than in HIT for ϕ= 0.5 - 1 %. This is due to frequent drafting–kissing–tumbling events in quiescent fluid. The collision frequency becomes instead larger in HIT than in still fluid for ϕ= 5 - 10 % , due to the larger relative approaching velocities in HIT, and to the less intense drafting–kissing–tumbling events in quiescent fluid. The collision frequency also appears to be almost proportional to the estimate for small inertial particles uniformly distributed in space, though much smaller. Concerning the turbulence modulation, we find that the mean energy dissipation increases almost linearly with ϕ, leading to a large reduction of Re λ .
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| 18. |
- Fornari, Walter, 1989-, et al.
(författare)
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The effect of particle density in turbulent channel flow laden with finite-size particles in semi-dilute conditions
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersedin a turbulent channel flow. We performed several Direct Numerical Simulations using an Immersed Boundary Method forfinite-size particles changing the solid to fluid density ratio R, the mass fraction and the volume fraction. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statisticsas much as when varying the volume fraction at constant R and at constant mass fraction.Interestingly, the increase in overall drag found when varying the volume fraction is considerablyhigher than that obtained for increasing density ratios at same volume fraction. The main effect atdensity ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When thedensity ratio R is further increased up to 100 the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas andthe fluid and solid phase statistics drastically change. In this regime, the collisionrate is high and dominated by the normal relative velocity among particles.
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| 19. |
- Fornari, Walter, 1989-, et al.
(författare)
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The effect of polydispersity in a turbulent channel flow laden with finite-size particles
- 2018
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 67, s. 54-64
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Tidskriftsartikel (refereegranskat)abstract
- We study turbulent channel flows of monodisperse and polydisperse suspensions of finite-size spheres by means of Direct Numerical Simulations using an immersed boundary method to account for the dispersed phase. Suspensions with 3 different Gaussian distributions of particle radii are considered (i.e. 3 different standard deviations). The distributions are centered on the reference particle radius of the monodisperse suspension. In the most extreme case, the radius of the largest particles is 4 times that of the smaller particles. We consider two different solid volume fractions, 2% and 10%. We find that for all polydisperse cases, both fluid and particles statistics are not substantially altered with respect to those of the monodisperse case. Mean streamwise fluid and particle velocity profiles are almost perfectly overlapping. Slightly larger differences are found for particle velocity fluctuations. These increase close to the wall and decrease towards the centerline as the standard deviation of the distribution is increased. Hence, the behavior of the suspension is mostly governed by excluded volume effects regardless of particle size distribution (at least for the radii here studied). Due to turbulent mixing, particles are uniformly distributed across the channel. However, smaller particles can penetrate more into the viscous and buffer layer and velocity fluctuations are therein altered. Non trivial results are presented for particle-pair statistics.
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| 20. |
- 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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| 21. |
- 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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| 22. |
- 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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| 23. |
- Lashgari, Iman, et al.
(författare)
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Channel flow of rigid sphere suspensions : Particle dynamics in the inertial regime
- 2016
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Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 78, s. 12-24
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Tidskriftsartikel (refereegranskat)abstract
- We consider suspensions of neutrally-buoyant finite-size rigid spherical particles in channel flow and investigate the relation between the particle dynamics and the mean bulk behavior of the mixture for Reynolds numbers 500 ≤ Re ≤ 5000 and particle volume fraction 0 ≤ Φ ≤ 0.3, via fully resolved numerical simulations. Analysis of the momentum balance reveals the existence of three different regimes: laminar, turbulent and inertial shear-thickening depending on which of the stress terms, viscous, Reynolds or particle stress, is the major responsible for the momentum transfer across the channel. We show that both Reynolds and particle stress dominated flows fall into the Bagnoldian inertial regime and that the Bagnold number can predict the bulk behavior although this is due to two distinct physical mechanisms. A turbulent flow is characterized by larger particle dispersion and a more uniform particle distribution, whereas the particulate-dominated flows is associated with a significant particle migration towards the channel center where the flow is smooth laminar-like and dispersion low. Interestingly, the collision kernel shows similar values in the different regimes, although the relative particle velocity and clustering clearly vary with inertia and particle concentration.
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| 24. |
- Lashgari, Iman, et al.
(författare)
-
Laminar, Turbulent, and Inertial Shear-Thickening Regimes in Channel Flow of Neutrally Buoyant Particle Suspensions
- 2014
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 113:25, s. 254502-
-
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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| 25. |
- Lashgari, Iman, et al.
(författare)
-
Transition and self-sustained turbulence in dilute suspensions of finite-size particles
- 2015
-
Ingår i: Theoretical and Applied Mechanics Letters. - : Elsevier BV. - 2095-0349. ; 5, s. 121-125
-
Tidskriftsartikel (refereegranskat)abstract
- We study the transition to turbulence of channel flow of finite-size particle suspensions at low volume fraction, i.e., Φ ≈ 0.001. The critical Reynolds number above which turbulence is sustained reduces to Re ≈ 1675, in the presence of few particles, independently of the initial condition, a value lower than that of the corresponding single-phase flow, i.e., Re ≈ 1775. In the dilute suspension, the initial arrangement of the particles is important to trigger the transition at a fixed Reynolds number and particle volume fraction. As in single phase flows, streamwise elongated disturbances are initially induced in the flow. If particles can induce oblique disturbances with high enough energy within a certain time, the streaks breakdown, flow experiences the transition to turbulence and the particle trajectories become chaotic. Otherwise, the streaks decay in time and the particles immigrate towards the channel core in a laminar flow.
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| 26. |
- Lashgari, Iman, et al.
(författare)
-
Transition to Turbulence in the Presence of Finite Size Particles
- 2015
-
Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 211-217
-
Konferensbidrag (refereegranskat)abstract
- We study the transition from laminar to turbulent flow in a channel seeded with finite-size neutrally buoyant particles. A fixed ratio of 10 between the channel height and the particle diameter is considered. The flow is examined in the range of Reynolds numbers 500 ≤ Re ≤; 5000 and the particle volume fractions 0.001 ≤ Φ ≤; 0.3. We report a non-monotonic behavior of the threshold value of the Reynolds number above which the flow becomes turbulent, in agreement with previous experimental studies. The mean square velocity fluctuations and Reynolds shear stress of the fluid phase are reduced by increasing the particle volume fraction at a fixed Re=1500, while the mean square velocities of the solid phase are enhanced monotonically suggesting a transition from fluid to particle dominated dynamics at high volume fraction.
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| 27. |
- Lashgari, Iman, et al.
(författare)
-
Turbulent channel flow of a dense binary mixture of rigid particles
- 2017
-
Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 818, s. 623-645
-
Tidskriftsartikel (refereegranskat)abstract
- We study turbulent channel flow of a binary mixture of finite-sized neutrally buoyant rigid particles by means of interface-resolved direct numerical simulations. We fix the bulk Reynolds number and total solid volume fraction, Re-b = 5600 and Phi = 20 %, and vary the relative fraction of small and large particles. The binary mixture consists of particles of two different sizes, 2h/d(l) = 20 and 2h/d(s) = 30 where h is the half-channel height and d(l) and d(s) the diameters of the large and small particles. While the particulate flow statistics exhibit a significant alteration of the mean velocity profile and turbulent fluctuations with respect to the unladen flow, the differences between the mono-disperse and bi-disperse cases are small. However, we observe a clear segregation of small particles at the wall in binary mixtures, which affects the dynamics of the near-wall region and thus the overall drag. This results in a higher drag in suspensions with a larger number of large particles. As regards bi-disperse effects on the particle dynamics, a non-monotonic variation of the particle dispersion in the spanwise (homogeneous) direction is observed when increasing the percentage of small/large particles. Finally, we note that particles of the same size tend to cluster more at contact whereas the dynamics of the large particles gives the highest collision kernels due to a higher approaching speed.
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| 28. |
- Magaletti, F., et al.
(författare)
-
The sharp-interface limit of the Cahn-Hilliard/Navier-Stokes model for binary fluids
- 2013
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 714, s. 95-126
-
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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| 29. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Asymmetric invasion in anisotropic porous media
- 2021
-
Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 104:4
-
Tidskriftsartikel (refereegranskat)abstract
- We report and discuss, by means of pore-scale numerical simulations, the possibility of achieving a directional-dependent two-phase flow behavior during the process of invasion of a viscous fluid into anisotropic porous media with controlled design. By customising the pore-scale morphology and heterogeneities with the adoption of anisotropic triangular pillars distributed with quenched disorder, we observe a substantially different invasion dynamics according to the direction of fluid injection relative to the medium orientation, that is depending if the triangular pillars have their apex oriented (flow aligned) or opposed (flow opposing) to the main flow direction. Three flow regimes can be observed: (i) for low values of the ratio between the macroscopic pressure drop and the characteristic pore-scale capillary threshold, i.e., for Δp0/pc≤1, the fluid invasion dynamics is strongly impeded and the viscous fluid is unable to reach the outlet of the medium, irrespective of the direction of injection; (ii) for intermediate values, 1<Δp0/pc≤2, the viscous fluid reaches the outlet only when the triangular pillars are flow-opposing oriented; (iii) for larger values, i.e., for Δp0/pc>2, the outlet is again reached irrespective of the direction of injection. The porous medium anisotropy induces a lower effective resistance when the pillars are flow-opposing oriented, suppressing front roughening and capillary fingering. We thus argue that the invasion process occurs as long as the pressure drop is larger then the macroscopic capillary pressure determined by the front roughness, which in the case of flow-opposing pillars is halved. We present a simple approximated model, based on Darcy's assumptions, that links the macroscopic effective permeability with the directional-dependent front roughening, to predict the asymmetric invasion dynamics. This peculiar behavior opens up the possibility of fabrication of porous capillary valves to control the flow along certain specific directions.
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| 30. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Asymmetric Two-phase Flows Resistance in Homogeneous and Heterogeneous Anisotropic Porous Microstructure
- 2019
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Two-phase flows in porous media exhibit anomalous behaviours at low capillary numbers due to the complex mechanism of interaction between flow spatial configuration and topology of the microstructure. In this study, we investigate the asymmetrical nature of the two-phase flow resistance induced by the anisotropic features of the porous microstructure. We perform pore-scale direct numerical simulations of two-phase flows in porous media composed of solid particles with different shapes and orientations, using the Lattice-Boltzmann method. The results indicate that the infiltration of a fluid into a single pore is regulated by the topological traits of the pore, including its anisotropy. These traits determine a geometrical characteristic length of the pore ℓp quantifying the flow resistance, which is directional-dependent: if the capillary length ℓγ=γ/pc (i.e. the ratio between surface tension and capillary pressure) falls below the characteristic pore length ℓγ<ℓp, pore infiltration occurs, otherwise the fluid remains trapped. We extend the analysis to heterogeneous anisotropic microstructure in order to investigate the effect of the spatial configuration of the pores on the global flow resistance. *This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 790744.
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| 31. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Characterization of fluid-mechanic efficiency of porous electrodes using X-ray computed tomography and Lattice-Boltzmann simulations
- 2017
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Flow batteries are a promising solution for storing the energy produced by carbon-free intermittent renewable sources. They are constituted by liquid electrolytes flowing and reacting into carbon felt porous electrodes. In order to enhance their performances it is necessary to maximise electrolyte dispersion and reaction together and to minimize the flow resistance through the porous media [1]. It is well reported in the literature how peculiar and anomalous can be the dispersion behaviour in natural porous media [2] and in fibrous materials [3]; however, it is still badly known how much it affects the performances of real electrodes of flow batteries. To clarify this issue, real carbon felt electrodes have been reconstructed by means of X-ray computed tomography (CT). In particular, a metrological micro-CT system has been used to obtain accurate three-dimensional reconstructions, which have then been used as input for Lattice-Boltzmann flow simulations coupled with a Lagrangian Particle Tracking algorithm, in order to investigate the dispersion and reaction behaviour of tracers through the medium microstructures, see e.g. Fig.1. For each material, macroscopic dispersion, reaction efficiencies and flow resistance have been evaluated from the underlying microscopic statistics of flow and tracer trajectories. From these analyses, the overall fluid-mechanic efficiency of each material has been evaluated and compared allowing the identification of the optimal porous microstructure for flow battery applications, which shows the highest dispersion and reaction rate together with the lowest pressure drop. [1] Wang B., Kuo J., Bae S.C. and Granick S., 2012. Nature Materials, 11(6): 481-5. [2] Alotto P., Guarnieri M. and Moro F., 2014. Renewable and Sustainable Energy Reviews, 29, pp.325-335. [3] Maggiolo D., Picano F. and Guarnieri M., 2016. Physics of Fluids, 28(10), p.102001.
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| 32. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Directional-dependent invasion dynamics in anisotropic porous media with customised disorder
- 2021
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We show possibility of achieving a directional-dependent two-phase flow behaviour during the process of invasion of a viscous fluid into anisotropic porous media with customised pore-scale morphology and heterogeneity. Via pore-scale numerical simulations, we observe a substantially different invasion dynamics according to the medium orientation relative to the direction of fluid injection, i.e. with flow-aligned or flow-opposing oriented pillars. The porous medium anisotropy induces a lower effective resistance when the pillars are flow-opposing oriented, suppressing front roughening and capillary fingering, while promoting transverse invasion with respect to the direction of fluid injection. We argue that fluid infiltration occurs as long as the pressure drop is larger then the macroscopic capillary pressure determined by the front roughness. We present a simple approximated model, based on Darcy's assumptions, that links the macroscopic effective permeability with the directional-dependent front roughening. The model correctly predicts an intermediate flow regime, defined by a specific range of values of the ratio between the macroscopic pressure drop and the medium characteristic pore-scale capillary threshold, within which the injected viscous fluid reaches the outlet only whith flow-opposing oriented pillars. The prediction of the observed directional-dependent fluid conductance is important for e.g. the fabrication of porous materials that act as capillary valves to control the flow along certain specific directions. This work is supported by the Horizon 2020 research and innovation programme, Grant agreement No 790744, and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), Grant Numbers 2019-01261. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at C3SE and HPC2N partially funded by the Swedish Research Council through Grant agreement no. 2018-05973.
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| 33. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Flow and dispersion in anisotropic porous media: A lattice-Boltzmann study
- 2016
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1089-7666 .- 1070-6631. ; 28:10
-
Tidskriftsartikel (refereegranskat)abstract
- Given their capability of spreading active chemical species and collecting electricity, porous media made of carbon fibers are extensively used as diffusion layers in energy storage systems, such as redox flow batteries. In spite of this, the dispersion dynamics of species inside porous media is still not well understood and often lends itself to different interpretations. Actually, the microscopic design of efficient porous media, which can potentially and effectively improve the performances of flow batteries, is still an open challenge. The present study aims to investigate the effect of fibrous media micro-structure on dispersion, in particular the effect of fiber orientation on drag and dispersion dynamics. Several lattice-Boltzmann simulations of flows through differently oriented fibrous media coupled with Lagrangian simulations of particle tracers have been performed. Results show that orienting fibers preferentially along the streamwise direction minimizes the drag and maximizes the dispersion, which is the most desirable condition for diffusion layers in flow batteries’ applications.
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| 34. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Particle based method and X-ray computed tomography for pore-scale flow characterization in VRFB electrodes
- 2019
-
Ingår i: Energy Storage Materials. - : Elsevier BV. - 2405-8297. ; 16, s. 91-96
-
Tidskriftsartikel (refereegranskat)abstract
- Porous electrodes are pivotal components of Vanadium Redox Flow Batteries, which influence the power density, pressure drop losses, activation overpotentials, limit current density, bulk and contact resistance, and ohmic losses. The quantification of the fluid-mechanic efficiency of porous electrodes based on their real geometry is a useful measure, as it primarily affects the mass transport losses and the overall battery performances. Although several studies, both numerical and experimental, have been devoted to the electrode enhancement, most analyses are carried out under the simplifying assumption of linear, macrohomogeneous and isotropic behavior of the fluid mechanics in the porous material. We present an original approach built on the Lattice-Boltzmann Method and Lagrange Particle Tracking that makes use of pore-scale accurate geometrical data provided by X-ray computed tomography with the aim of studying the dispersion and reaction rates of liquid electrolyte reactants in the flow battery porous electrode. Following this methodology, we compare the fluid-dynamic performances provided by a commonly used carbon felt and an unconventional material, that is, a carbon vitrified foam. Surprisingly, results unveil the possibility of achieving higher fluid-mechanic efficiencies with the foam electrode, whose intrinsic microstructure promotes higher reaction rate.
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| 35. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Pore trapping mechanisms in two-phase flows through fuel cells porous media
- 2019
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The peak performances in fuel cells are strongly limited by flooding of the electrodes. Flooding occurs as a consequence of the intense electrochemical reaction in the cell, when, at high current density, a high amount of water is produced and trapped in the gas diffusion layers (GDLs). The trapped water blocks the pores of the fibrous GDLs and impedes the uniform diffusion of fuel gas along the cell area. Therefore, a fast removal of water from the porous layer is demanded. How to enhance such a mechanism in order to improve the water management remains still an open question. In the present study we address this problem with an innovative bottom-up approach: by means of Lattice-Boltzmann simulations, we investigate the effects of the microstructure at the pore-scale on the two-phase flows dynamics, in order to optimise the GDLs design and obtain an efficient water management at the macroscale. Results show that, during imbibition, the wetting phase can be trapped at the pore throat, impeding liquid removal. The trapping mechanism is primarily governed by the thermodynamic energy barriers induced by the pore expansions, in a complementary way to Haines jump during drainage. This undesired phenomenon is exacerbated in hydrophobic media during imbibition. These findings suggest a possible new route for innovative design of gas diffusion layers in fuel cells applications.
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| 36. |
- Maggiolo, Dario, 1985, et al.
(författare)
-
Solute transport and reaction in porous electrodes at high Schmidt numbers
- 2020
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 896:A13
-
Tidskriftsartikel (refereegranskat)abstract
- We present lattice Boltzmann pore-scale numerical simulations of solute transport and reaction in porous electrodes at a high Schmidt number, Sc=10^2. The three-dimensional geometry of real materials is reconstructed via X-ray computed tomography. We apply a volume-averaging upscaling procedure to characterise the microstructural terms contributing to the homogenised description of the macroscopic advection–reaction–dispersion equation. We firstly focus our analysis on its asymptotic solution, while varying the rate of reaction. The results confirm the presence of two working states of the electrodes: a reaction-limited regime, governed by advective transport, and a mass-transfer-limited regime, where dispersive mechanisms play a pivotal role. For all materials, these regimes depend on a single parameter, the product of the Damköhler number and a microstructural aspect ratio. The macroscopic dispersion is determined by the spatial correlation between solute concentration and flow velocity at the pore scale. This mechanism sustains reaction in the mass-transfer-limited regime due to the spatial rearrangement of the solute transport from low-velocity to high-velocity pores. We then compare the results of pre-asymptotic transport with a macroscopic model based on effective dispersion parameters. Interestingly, the model correctly represents the transport at short characteristic times. At longer times, high reaction rates mitigate the mechanisms of heterogeneous solute transport. In the mass-transfer-limited regime, the significant yet homogeneous dispersion can thus be modelled via an effective dispersion. Finally, we formulate guidelines for the design of porous electrodes based on the microstructural aspect ratio.
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| 37. |
- Niazi Ardekani, Mehdi, et al.
(författare)
-
Drag reduction in turbulent channel flow laden with finite-size oblate spheroids
- 2017
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 816, s. 43-70
-
Tidskriftsartikel (refereegranskat)abstract
- We study suspensions of oblate rigid particles in a viscous fluid for different values of the particle volume fractions.Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle-particle and particle-wall interactions. With respect to the single phase flow, we show that in flows laden with oblate spheroids the drag is reduced and the turbulent fluctuations attenuated.In particular, the turbulence activity decreases to lower values than those obtained by only accounting for the effective suspension viscosity.To explain the observed drag reduction we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of oblate particles.These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.
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| 38. |
- Niazi Ardekani, Mehdi, et al.
(författare)
-
Heat transfer in laminar Couette flow laden with rigid spherical particles
- 2018
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 834, s. 308-334
-
Tidskriftsartikel (refereegranskat)abstract
- We study heat transfer in plane Couette flow laden with rigid spherical particles by means of direct numerical simulations. In the simulations we use a direct-forcing immersed boundary method to account for the dispersed phase together with a volume-of-fluid approach to solve the temperature field inside and outside the particles. We focus on the variation of the heat transfer with the particle Reynolds number, total volume fraction (number of particles) and the ratio between the particle and fluid thermal diffusivity, quantified in terms of an effective suspension diffusivity. We show that, when inertia at the particle scale is negligible, the heat transfer increases with respect to the unladen case following an empirical correlation recently proposed in the literature. In addition, an average composite diffusivity can be used to approximate the effective diffusivity of the suspension in the inertialess regime when varying the molecular diffusion in the two phases. At finite particle inertia, however, the heat transfer increase is significantly larger, smoothly saturating at higher volume fractions. By phase-ensemble-averaging we identify the different mechanisms contributing to the total heat transfer and show that the increase of the effective conductivity observed at finite inertia is due to the increase of the transport associated with fluid and particle velocity. We also show that the contribution of the heat conduction in the solid phase to the total wall-normal heat flux reduces when increasing the particle Reynolds number, so that particles of low thermal diffusivity weakly alter the total heat flux in the suspension at finite particle Reynolds numbers. On the other hand, a higher particle thermal diffusivity significantly increases the total heat transfer.
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| 39. |
- Nowbahar, Arash, et al.
(författare)
-
Turbophoresis attenuation in a turbulent channel flow with polymer additives
- 2013
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 732, s. 706-719
-
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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| 40. |
- Olivieri, S., et al.
(författare)
-
The effect of the Basset history force on particle clustering in homogeneous and isotropic turbulence
- 2014
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 26:4, s. 041704-
-
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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| 41. |
- Picano, Francesco, et al.
(författare)
-
DNS of turbulent channel flows laden with finite-size particles at high volume fractions
- 2020
-
Ingår i: 14th European Turbulence Conference, ETC 2013. - : Zakon Group LLC.
-
Konferensbidrag (refereegranskat)abstract
- Suspensions are often found in different processes and applications, e.g. sediment transport in environments or pharmaceutical engineering. The laminar regime in the semi-dilute or dense cases, non vanishing volume fraction, is usually characterized by the sometime spectacular rheological properties induced by the suspended phase. Much less is known about dissipation and mixing in the turbulent regime. The aim of the present work is to investigate the turbulent channel flow of a fluid laden with rigid spherical particles at a fixed bulk Reynolds number Reh = U0h/ν = 2800. The particle radius is selected to be 18 times smaller than the channel half-width. Fully-resolved Direct Numerical Simulations with particle tracking and Immersed Boundary Method are presented for values of the volume fraction up to φ = 0.2. As expected for “large†particles, the overall drag increases with the volume fraction. We show that the presence of the particles deeply changes flow behavior, as already evident from the mean velocity profile with the canonical regions, buffer- or log-layer, strongly altered.
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| 42. |
- Picano, Francesco, et al.
(författare)
-
Editorial
- 2020
-
Ingår i: Meccanica (Milano. Print). - : SPRINGER. - 0025-6455 .- 1572-9648. ; 55:2, s. 295-297
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 43. |
- Picano, Francesco, et al.
(författare)
-
Leray-alpha Regularization of the Smagorinsky-Closed Filtered Equations for Turbulent Jets at High Reynolds Numbers
- 2012
-
Ingår i: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 89:4, s. 627-650
-
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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| 44. |
- Picano, Francesco, et al.
(författare)
-
Shear Thickening in Non-Brownian Suspensions : An Excluded Volume Effect
- 2013
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:9, s. 098302-
-
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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| 45. |
- Picano, Francesco, et al.
(författare)
-
Turbulent channel flow of dense suspensions of neutrally buoyant spheres
- 2015
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 764, s. 463-487
-
Tidskriftsartikel (refereegranskat)abstract
- Dense particle suspensions are widely encountered in many applications and in environmental flows. While many previous studies investigate their rheological properties in laminar flows, little is known on the behaviour of these suspensions in the turbulent/inertial regime. The present study aims to fill this gap by investigating the turbulent flow of a Newtonian fluid laden with solid neutrally-buoyant spheres at relatively high volume fractions in a plane channel. Direct numerical simulation (DNS) are performed in the range of volume fractions Phi=0-0.2 with an immersed boundary method (IBM) used to account for the dispersed phase. The results show that the mean velocity profiles are significantly altered by the presence of a solid phase with a decrease of the von Karman constant in the log-law. The overall drag is found to increase with the volume fraction, more than one would expect if just considering the increase of the system viscosity due to the presence of the particles. At the highest volume fraction investigated here, Phi = 0.2, the velocity fluctuation intensities and the Reynolds shear stress are found to decrease. The analysis of the mean momentum balance shows that the particle-induced stresses govern the dynamics at high Phi and are the main responsible of the overall drag increase. In the dense limit, we therefore find a decrease of the turbulence activity and a growth of the particle induced stress, where the latter dominates for the Reynolds numbers considered here.
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| 46. |
- Rudski, Lawrence G., et al.
(författare)
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Stressing the Cardiopulmonary Vascular System: The Role of Echocardiography
- 2018
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Ingår i: Journal of the American Society of Echocardiography. - : MOSBY-ELSEVIER. - 0894-7317 .- 1097-6795. ; 31:5, s. 527-
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Forskningsöversikt (refereegranskat)abstract
- The cardiopulmonary vascular system represents a key determinant of prognosis in several cardiorespiratory diseases. Although right heart catheterization is considered the gold standard for assessing pulmonary hemodynamics, a comprehensive noninvasive evaluation including left and right ventricular reserve and function and cardiopulmonary interactions remains highly attractive. Stress echocardiography is crucial in the evaluation of many cardiac conditions, typically coronary artery disease but also heart failure and valvular heart disease. In stress echocardiographic applications beyond coronary artery disease, the assessment of the cardiopulmonary vascular system is a cornerstone. The possibility of coupling the left and right ventricles with the pulmonary circuit during stress can provide significant insight into cardiopulmonary physiology in healthy and diseased subjects, can support the diagnosis of the etiology of pulmonary hypertension and other conditions, and can offer valuable prognostic information. In this state-of-the-art document, the topic of stress echocardiography applied to the cardiopulmonary vascular system is thoroughly addressed, from pathophysiology to different stress modalities and echocardiographic parameters, from clinical applications to limitations and future directions.
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| 47. |
- Sardina, Gaetano, et al.
(författare)
-
Continuous Growth of Droplet Size Variance due to Condensation in Turbulent Clouds
- 2015
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 115:18
-
Tidskriftsartikel (refereegranskat)abstract
- We use a stochastic model and direct numerical simulation to study the impact of turbulence on cloud droplet growth by condensation. We show that the variance of the droplet size distribution increases in time as t(1/2), with growth rate proportional to the large-to-small turbulent scale separation and to the turbulence integral scales but independent of the mean turbulent dissipation. Direct numerical simulations confirm this result and produce realistically broad droplet size spectra over time intervals of 20 min, comparable with the time of rain formation.
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| 48. |
- Sardina, Gaetano, et al.
(författare)
-
Effects of polymer additives on turbophoresis in a turbulent channel flow
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
-
Konferensbidrag (refereegranskat)abstract
- Turbophoresis is the migration of inertial particles towards the wall in a wall-bounded flow induced by turbulence. In this work, we analyze the effects of drag reducing polymer additives on turbophoresis in a turbulent channel flow. The numerical data set is obtained from a direct numerical simulation (DNS) of a turbulent channel flow of a viscoelastic fluid and laden with particles of different inertia. The results indicate that polymer additives decrease the turbophoretic drift. We establish that turbophoresis is reduced because of the smaller wall-normal variation of wall-normal fluid velocity fluctuations that occurs in all drag reducing flows. Hence a reduction of turbophoresis should be a common feature of all drag reducing flows such s fiber, bubble suspensions and MHD.
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| 49. |
- Sardina, Gaetano, 1982, et al.
(författare)
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Estimating time and length scales of indirect transmission of respiratory diseases
- 2021
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Ingår i: Bulletin of the American Physical Society. ; 66
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The recent pandemic has highlighted several open questions across many disciplines. One of the main epidemiological uncertainty relates to the mechanisms of pathogen transmission between individuals. In particular, a dichotomy exists between direct (short-range) transmission and indirect (long-range) transmission. Direct transmission is associated with larger droplets falling immediately in the proximity of the infected person and spreading the virus with deposition on other individuals or objects. On the contrary, indirect transmission is related to smaller droplets that evaporate faster and can remain suspended in the air for longer times as aerosol nuclei reaching longer distances from the emission point. Here, we will use Direct Numerical Simulations (DNS) to assess the indirect transmission in a poorly ventilated room to quantify the relevant resident times and distances of respiratory droplets. We will show that droplets with an initial diameter of less than 30 microns can remain suspended for more than an hour, reaching distances even of the order of 10 meters. Assuming typical values of viral loads, we can explain the occurrence of superspreading events and evaluate the efficiency of non-pharmaceutical interventions such as social distances and masks.
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| 50. |
- 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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