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##### Publications (10 of 201) Show all publications
Zade, S., Fornari, W., Lundell, F. & Brandt, L. (2019). Buoyant finite-size particles in turbulent duct flow. Physical Review Fluids (4), Article ID 024303.
Open this publication in new window or tab >>Buoyant finite-size particles in turbulent duct flow
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, no 4, article id 024303Article in journal (Refereed) Published
##### Abstract [en]

Particle image velocimetry and particle tracking velocimetry have been employed to investigate the dynamics of finite-size spherical particles, slightly heavier than the carrier fluid, in a horizontal turbulent square duct flow. Interface resolved direct numerical simulations (DNSs) have also been performed with the immersed boundary method at the same experimental conditions, bulk Reynolds number Re2H=5600, duct height to particle-size ratio 2H/dp=14.5, particle volume fraction Φ=1%, and particle to fluid density ratio ρp/ρf=1.0035. Good agreement has been observed between experiments and simulations in terms of the overall pressure drop, concentration distribution, and turbulent statistics of the two phases. Additional experimental results considering two particle sizes 2H/dp=14.5 and 9 and multiple Φ=1%, 2%, 3%, 4%, and 5% are reported at the same Re2H. The pressure drop monotonically increases with the volume fraction, almost linearly and nearly independently of the particle size for the above parameters. However, despite the similar pressure drop, the microscopic picture in terms of fluid velocity statistics differs significantly with the particle size. This one-to-one comparison between simulations and experiments extends the validity of interface resolved DNS in complex turbulent multiphase flows and highlights the ability of experiments to investigate such flows in considerable detail, even in regions where the local volume fraction is relatively high.

##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-243895 (URN)10.1103/PhysRevFluids.4.024303 (DOI)000458160100003 ()2-s2.0-85062418601 (Scopus ID)
##### Note

QC 20190215

Available from: 2019-02-09 Created: 2019-02-09 Last updated: 2019-04-04Bibliographically approved
Rosti, M. E., Niazi Ardekani, M. & Brandt, L. (2019). Effect of elastic walls on suspension flow. Physical Review Fluids, 4(6), Article ID 062301.
Open this publication in new window or tab >>Effect of elastic walls on suspension flow
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 6, article id 062301Article in journal (Refereed) Published
##### Abstract [en]

We study suspensions of rigid particles in a plane Couette flow with deformable elastic walls. We find that, in the limit of vanishing inertia, the elastic walls induce shear thinning of the suspension flow such that the effective viscosity decreases as the wall deformability increases. This shear-thinning behavior originates from the interactions between rigid particles, soft walls, and carrier fluids; an asymmetric wall deformation induces a net lift force acting on the particles which therefore migrate towards the bulk of the channel. Based on our observations, we provide a closure for the suspension viscosity which can be used to model the rheology of suspensions with arbitrary volume fraction in elastic channels.

##### Place, publisher, year, edition, pages
American Physical Society, 2019
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-255317 (URN)10.1103/PhysRevFluids.4.062301 (DOI)000473044100001 ()2-s2.0-85069716507 (Scopus ID)
##### Note

QC 20190805

Available from: 2019-08-05 Created: 2019-08-05 Last updated: 2019-08-05Bibliographically approved
Ge, Z., Tammisola, O. & Brandt, L. (2019). Flow-assisted droplet assembly in a 3D microfluidic channel. Soft Matter, 15(16), 3451-3460
Open this publication in new window or tab >>Flow-assisted droplet assembly in a 3D microfluidic channel
2019 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 15, no 16, p. 3451-3460Article in journal (Refereed) Published
##### Abstract [en]

Self-assembly of soft matter, such as droplets or colloids, has become a promising scheme to engineer novel materials, model living matter, and explore non-equilibrium statistical mechanics. In this article, we present detailed numerical simulations of few non-Brownian droplets in various flow conditions, specifically, focusing on their self-assembly within a short distance in a three-dimensional (3D) microfluidic channel, cf. [Shen et al., Adv. Sci., 2016, 3(6), 1600012]. Contrary to quasi two-dimensional (q2D) systems, where dipolar interaction is the key mechanism for droplet rearrangement, droplets in 3D confinement produce much less disturbance to the underlying flow, thus experiencing weaker dipolar interactions. Using confined simple shear and Poiseuille flows as reference flows, we show that the droplet dynamics is mostly affected by the shear-induced cross-stream migration, which favors chain structures if the droplets are under an attractive depletion force. For more compact clusters, such as three droplets in a triangular shape, our results suggest that an inhomogeneous cross-sectional inflow profile is further required. Overall, the accelerated self-assembly of a small-size droplet cluster results from the combined effects of strong depletion forces, confinement-mediated shear alignments, and fine-tuned inflow conditions. The deterministic nature of the flow-assisted self-assembly implies the possibility of large throughputs, though calibration of all different effects to directly produce large droplet crystals is generally difficult.

##### Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2019
##### National Category
Mechanical Engineering
##### Identifiers
urn:nbn:se:kth:diva-252637 (URN)10.1039/c8sm02479k (DOI)000468007600014 ()30958490 (PubMedID)2-s2.0-85064601819 (Scopus ID)
##### Note

QC 20190610

Available from: 2019-06-10 Created: 2019-06-10 Last updated: 2019-06-10Bibliographically approved
Ghosh, S., Loiseau, J.-C., Breugem, W.-P. & Brandt, L. (2019). Modal and non-modal linear stability of Poiseuille flow through a channel with a porous substrate. European journal of mechanics. B, Fluids, 75, 29-43
Open this publication in new window or tab >>Modal and non-modal linear stability of Poiseuille flow through a channel with a porous substrate
2019 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 75, p. 29-43Article in journal (Refereed) Published
##### Abstract [en]

We present modal and non-modal linear stability analyses of Poiseuille flow through a plane channel with a porous substrate modeled using the Volume Averaged Navier-Stokes (VANS) equations. Modal stability analysis shows the destabilization of the flow with increasing porosity of the layer. The instability mode originates from the homogeneous fluid region of the channel for all the values of porosity considered but the governing mechanism changes. Perturbation kinetic energy analysis reveals the importance of viscous dissipation at low porosity values while dissipation in the porous substrate becomes significant at higher porosity. Scaling analysis highlights the invariance of the critical wavenumber with changing porosity. On the other hand, the critical Reynolds number remains invariant at low porosity and scales as Re-c similar to (H/delta)(1.4) at high porosity where delta is the typical thickness of the vorticity layer at the fluid-porous interface. This reveals the existence of a Tollmien-Schlichting-like viscous instability mechanism at low porosity values, and Rayleigh analysis shows the presence of an inviscid instability mechanism at high porosity. For the whole range of porosities considered, the non-modal analysis shows that the optimal mechanism responsible for transient energy amplification is the lift-up effect, giving rise to streaky structure as in single-phase plane Poiseuille flow. The present results strongly suggest that the transition to turbulence follows the same path as that of classical Poiseuille flow at low porosity values, while it is dictated by the modal instability for high porosity values. SAS. All rights reserved.

Elsevier, 2019
##### Keywords
Porous channel flow, Instability
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-245884 (URN)10.1016/j.euromechflu.2018.11.013 (DOI)000458710700003 ()2-s2.0-85059321599 (Scopus ID)
##### Note

QC 20180311

Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-11Bibliographically approved
Rosti, M. E., De Vita, F. & Brandt, L. (2019). Numerical simulations of emulsions in shear flows. Acta Mechanica, 230(2), 667-682
Open this publication in new window or tab >>Numerical simulations of emulsions in shear flows
2019 (English)In: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 230, no 2, p. 667-682Article in journal (Refereed) Published
##### Abstract [en]

We present a modification of a recently developed volume of fluid method for multiphase problems (Ii et al. in J Comput Phys 231(5):2328-2358, 2012), so that it can be used in conjunction with a fractional-step method and fast Poisson solver, and validate it with standard benchmark problems. We then consider emulsions of two-fluid systems and study their rheology in a plane Couette flow in the limit of vanishing inertia. We examine the dependency of the effective viscosity on the volume fraction phi (from 10 to 30%) and the Capillary number Ca (from 0.1 to 0.4) for the case of density and viscosity ratio 1. We show that the effective viscosity decreases with the deformation and the applied shear (shear-thinning) while exhibiting a non-monotonic behavior with respect to the volume fraction. We report the appearance of a maximum in the effective viscosity curve and compare the results with those of suspensions of rigid and deformable particles and capsules. We show that the flow in the solvent is mostly a shear flow, while it is mostly rotational in the suspended phase; moreover, this behavior tends to reverse as the volume fraction increases. Finally, we evaluate the contributions to the total shear stress of the viscous stresses in the two fluids and of the interfacial force between them.

Springer, 2019
##### National Category
Other Engineering and Technologies
##### Identifiers
urn:nbn:se:kth:diva-245931 (URN)10.1007/s00707-018-2265-5 (DOI)000459141100015 ()2-s2.0-85055982638 (Scopus ID)
##### Note

QC 20190314

Available from: 2019-03-14 Created: 2019-03-14 Last updated: 2019-03-14Bibliographically approved
Watteaux, R., Sardina, G., Brandt, L. & Iudicone, D. (2019). On the time scales and structure of Lagrangian intermittency in homogeneous isotropic turbulence. Journal of Fluid Mechanics, 867, 438-481, Article ID 025301(R).
Open this publication in new window or tab >>On the time scales and structure of Lagrangian intermittency in homogeneous isotropic turbulence
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 867, p. 438-481, article id 025301(R)Article in journal (Refereed) Published
##### Abstract [en]

We present a study of Lagrangian intermittency and its characteristic time scales. Using the concepts of flying and diving residence times above and below a given threshold in the magnitude of turbulence quantities, we infer the time spectra of the Lagrangian temporal fluctuations of dissipation, acceleration and enstrophy by means of a direct numerical simulation in homogeneous and isotropic turbulence. We then relate these time scales, first, to the presence of extreme events in turbulence and, second, to the local flow characteristics. Analyses confirm the existence in turbulent quantities of holes mirroring bursts, both of which are at the core of what constitutes Lagrangian intermittency. It is shown that holes are associated with quiescent laminar regions of the flow. Moreover, Lagrangian holes occur over few Kolmogorov time scales while Lagrangian bursts happen over longer periods scaling with the global decorrelation time scale, hence showing that loss of the history of the turbulence quantities along particle trajectories in turbulence is not continuous. Such a characteristic partially explains why current Lagrangian stochastic models fail at reproducing our results. More generally, the Lagrangian dataset of residence times shown here represents another manner for qualifying the accuracy of models. We also deliver a theoretical approximation of mean residence times, which highlights the importance of the correlation between turbulence quantities and their time derivatives in setting temporal statistics. Finally, whether in a hole or a burst, the straining structure along particle trajectories always evolves self-similarly (in a statistical sense) from shearless two-dimensional to shear bi-axial configurations. We speculate that this latter configuration represents the optimum manner to dissipate locally the available energy.

##### Place, publisher, year, edition, pages
Cambridge University Press, 2019
##### Keywords
topological fluid dynamics, turbulence simulation
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-249843 (URN)10.1017/jfm.2019.127 (DOI)000462506900001 ()2-s2.0-85063598590 (Scopus ID)
##### Note

QC 20190426

Available from: 2019-04-26 Created: 2019-04-26 Last updated: 2019-04-26Bibliographically approved
Zade, S., Lundell, F. & Brandt, L. (2019). Turbulence modulation by finite-size spherical particles in Newtonian and viscoelastic fluids. International Journal of Multiphase Flow, 112, 116-129
Open this publication in new window or tab >>Turbulence modulation by finite-size spherical particles in Newtonian and viscoelastic fluids
2019 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 112, p. 116-129Article in journal (Refereed) Published
##### Abstract [en]

We experimentally investigate the influence of finite-size spherical particles in turbulent flows of a Newtonian and a drag reducing viscoelastic fluid at varying particle volume fractions and fixed Reynolds number. Experiments are performed in a square duct at a Reynolds number Re2H of nearly 1.1 × 104, Weissenberg number Wi for single phase flow is between 1 and 2 and results in a drag-reduction of 43% compared to a Newtonian flow (at the same Re2H). Particles are almost neutrally-buoyant hydrogel spheres having a density ratio of 1.0035 ± 0.0003 and a duct height 2H to particle diameter dp ratio of around 10. We measure flow statistics for four different volume fractions ϕ namely 5, 10, 15 and 20% by using refractive-index-matched Particle Image Velocimetry (PIV). For both Newtonian Fluid (NF) and Visceolastic Fluid (VEF), the drag monotonically increases with ϕ. For NF, the magnitude of drag increase due to particle addition can be reasonably estimated using a concentration dependent effective viscosity for volume fractions below 10%. The drag increase is, however, underestimated at higher ϕ. For VEF, the absolute value of drag is lower than NF but, its rate of increase with ϕ is higher. Similar to particles in a NF, particles in VEF tend to migrate towards the center of the duct and form a layer of high concentration at the wall. Interestingly, relatively higher migration towards the center and lower migration towards the walls is observed for VEF. The primary Reynolds shear stress reduces with increasing ϕ throughout the duct height for both types of fluid.

Elsevier, 2019
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-243840 (URN)10.1016/j.ijmultiphaseflow.2018.12.015 (DOI)2-s2.0-85058816573 (Scopus ID)
##### Note

QC 20190215

Available from: 2019-02-07 Created: 2019-02-07 Last updated: 2019-04-03Bibliographically approved
Shahmardi, A., Zade, S., Niazi Ardekani, M., Poole, R. J., Lundell, F., Rosti, M. E. & Brandt, L. (2019). Turbulent duct flow with polymers. Journal of Fluid Mechanics, 859, 1057-1083
Open this publication in new window or tab >>Turbulent duct flow with polymers
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 859, p. 1057-1083Article in journal (Refereed) Published
##### Abstract [en]

We have performed direct numerical simulation of the turbulent flow of a polymer solution in a square duct, with the FENE-P model used to simulate the presence of polymers. First, a simulation at a fixed moderate Reynolds number is performed and its results compared with those of a Newtonian fluid to understand the mechanism of drag reduction and how the secondary motion, typical of the turbulent flow in non-axisymmetric ducts, is affected by polymer additives. Our study shows that the Prandtl's secondary flow is modified by the polymers: the circulation of the streamwise main vortices increases and the location of the maximum vorticity moves towards the centre of the duct. In-plane fluctuations are reduced while the streamwise ones are enhanced in the centre of the duct and dumped in the corners due to a substantial modification of the quasi-streamwise vortices and the associated near-wall low- and high-speed streaks; these grow in size and depart from the walls, their streamwise coherence increasing. Finally, we investigated the effect of the parameters defining the viscoelastic behaviour of the flow and found that the Weissenberg number strongly influences the flow, with the cross-stream vortical structures growing in size and the in-plane velocity fluctuations reducing for increasing flow elasticity.We have performed direct numerical simulation of the turbulent flow of a polymer solution in a square duct, with the FENE-P model used to simulate the presence of polymers. First, a simulation at a fixed moderate Reynolds number is performed and its results compared with those of a Newtonian fluid to understand the mechanism of drag reduction and how the secondary motion, typical of the turbulent flow in non-axisymmetric ducts, is affected by polymer additives. Our study shows that the Prandtl's secondary flow is modified by the polymers: the circulation of the streamwise main vortices increases and the location of the maximum vorticity moves towards the centre of the duct. In-plane fluctuations are reduced while the streamwise ones are enhanced in the centre of the duct and dumped in the corners due to a substantial modification of the quasi-streamwise vortices and the associated near-wall low- and high-speed streaks; these grow in size and depart from the walls, their streamwise coherence increasing. Finally, we investigated the effect of the parameters defining the viscoelastic behaviour of the flow and found that the Weissenberg number strongly influences the flow, with the cross-stream vortical structures growing in size and the in-plane velocity fluctuations reducing for increasing flow elasticity.

##### Place, publisher, year, edition, pages
Cambridge University Press, 2019
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-240129 (URN)10.1017/jfm.2018.858 (DOI)000451519800001 ()2-s2.0-85057589811 (Scopus ID)
##### Funder
Swedish Research Council, 2014-5001EU, European Research Council, ERC-2013-CoG-616186Swedish e‐Science Research Center
##### Note

QC 20181213

Available from: 2018-12-12 Created: 2018-12-12 Last updated: 2018-12-13Bibliographically approved
Niazi Ardekani, M., Rosti, M. E. & Brandt, L. (2019). Turbulent flow of finite-size spherical particles in channels with viscous hyper-elastic walls. Journal of Fluid Mechanics, 873, 410-440, Article ID PII S0022112019004130.
Open this publication in new window or tab >>Turbulent flow of finite-size spherical particles in channels with viscous hyper-elastic walls
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 873, p. 410-440, article id PII S0022112019004130Article in journal (Refereed) Published
##### Abstract [en]

We study single-phase and particle-laden turbulent channel flows bounded by two incompressible hyper-elastic walls with different deformability at bulk Reynolds number $5600$ . The solid volume fraction of finite-size neutrally buoyant rigid spherical particles considered is $10\,\%$ . The elastic walls are assumed to be of a neo-Hookean material. A fully Eulerian formulation is employed to model the elastic walls together with a direct-forcing immersed boundary method for the coupling between the fluid and the particles. The data show a significant drag increase and the enhancement of the turbulence activity with growing wall elasticity for both the single-phase and particle-laden flows when compared with the single-phase flow over rigid walls. Drag reduction and turbulence attenuation is obtained, on the other hand, with highly elastic walls when comparing the particle-laden flow with the single-phase flow for the same wall properties; the opposite effect, drag increase, is observed upon adding particles to the flow over less elastic walls. This is explained by investigating the near-wall turbulence, where the strong asymmetry in the magnitude of the wall-normal velocity fluctuations (favouring positive $v<^>{\prime }$ ), is found to push the particles towards the channel centre. The particle layer close to the wall contributes to turbulence production by increasing the wall-normal velocity fluctuations, so that in the absence of this layer, smaller wall deformations and in turn turbulence attenuation is observed. For a moderate wall elasticity, we increase the particle volume fraction up to $20\,\%$ and find that particle migration away from the wall is the cause of turbulence attenuation with respect to the flow over rigid walls. However, for this higher volume fractions, the particle induced stress compensates for the decreasing Reynolds shear stress, resulting in a higher overall drag for the case with elastic walls. The effect of the wall elasticity on the overall drag reduces significantly with increasing particle volume fraction.

##### Place, publisher, year, edition, pages
Cambridge University Press, 2019
##### Keywords
multiphase flow, particle, fluid flows
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-255295 (URN)10.1017/jfm.2019.413 (DOI)000472633900001 ()
##### Note

QC 20190729

Available from: 2019-07-29 Created: 2019-07-29 Last updated: 2019-07-29Bibliographically approved
Fukada, T., Fornari, W., Brandt, L., Takeuchi, S. & Kajishima, T. (2018). A numerical approach for particle-vortex interactions based on volume-averaged equations. International Journal of Multiphase Flow, 104, 188-205
Open this publication in new window or tab >>A numerical approach for particle-vortex interactions based on volume-averaged equations
2018 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 104, p. 188-205Article in journal (Refereed) Published
##### Abstract [en]

To study the dynamics of particles in turbulence when their sizes are comparable to the smallest eddies in the flow, the Kolmogorov length scale, efficient and accurate numerical models for the particle-fluid interaction are still missing. Therefore, we here extend the treatment of the particle feedback on the fluid based on the volume-averaged fluid equations (VA simulation) in the previous study of the present authors, by estimating the fluid force correlated with the disturbed flow. We validate the model against interface-resolved simulations using the immersed-boundary method. Simulations of single particles show that the history effect is well captured by the present estimation method based on the disturbed flow. Similarly, the simulation of the flow around a rotating particle demonstrates that the lift force is also well captured by the proposed method. We also consider the interaction between non-negligible size particles and an array of Taylor–Green vortices. For density ratios ρd /ρc ≥ 10, the results show that the particle motion captured by the VA approach is closer to that of the fully-resolved simulations than that obtained with a traditional two-way coupling simulation. The flow disturbance is also well represented by the VA simulation. In particular, it is found that history effects enhance the curvature of the trajectory in vortices and this enhancement increases with the particle size. Furthermore, the flow field generated by a neighboring particle at distances of around ten particle diameters significantly influences particle trajectories. The computational cost of the VA simulation proposed here is considerably lower than that of the interface-resolved simulation.

Elsevier, 2018
##### Keywords
History effect, Particle-laden flow, Particle-vortex interaction, Volume-averaged equation
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-227530 (URN)10.1016/j.ijmultiphaseflow.2018.02.019 (DOI)000432643700015 ()2-s2.0-85043509672 (Scopus ID)
##### Funder
EU, European Research Council, ERC-2013-CoG-616186Swedish Research CouncilSwedish e‐Science Research CenterSwedish National Infrastructure for Computing (SNIC)
##### Note

QC 20180517

Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-06-13Bibliographically approved
##### Identifiers
ORCID iD: orcid.org/0000-0002-4346-4732

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