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Banaei, A. A., Rosti, M. E. & Brandt, L. (2020). Numerical study of filament suspensions at finite inertia. Journal of Fluid Mechanics, 882, Article ID A5.
Open this publication in new window or tab >>Numerical study of filament suspensions at finite inertia
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 882, article id A5Article in journal (Refereed) Published
Abstract [en]

We present a numerical study on the rheology of semi-dilute and concentrated filament suspensions of different bending stiffness and Reynolds number, with the immersed boundary method used to couple the fluid and solid. The filaments are considered as one-dimensional inextensible slender bodies with fixed aspect ratio, obeying the Euler-Bernoulli beam equation. To understand the global suspension behaviour we relate it to the filament microstructure, deformation and elastic energy and examine the stress budget to quantify the effect of the elastic contribution. At fixed volume fraction, the viscosity of the suspension reduces when decreasing the bending rigidity and grows when increasing the Reynolds number. The change in the relative viscosity is stronger at finite inertia, although still in the laminar flow regime, as considered here. Moreover, we find the first normal stress difference to be positive as in polymeric fluids, and to increase with the Reynolds number; its value has a peak for an intermediate value of the filament bending stiffness. The peak value is found to be proportional to the Reynolds number, moving towards more rigid suspensions at larger inertia. Moreover, the viscosity increases when increasing the filament volume fraction, and the rate of increase of the filament stress with the bending rigidity is stronger at higher Reynolds numbers and reduces with the volume fraction. We show that this behaviour is associated with the formation of a more ordered structure in the flow, where filaments tend to be more aligned and move as a compact aggregate, thus reducing the filament-filament interactions despite their volume fraction increases.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2020
Keywords
suspensions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-266939 (URN)10.1017/jfm.2019.794 (DOI)000506238300005 ()
Note

QC 20200203

Available from: 2020-02-03 Created: 2020-02-03 Last updated: 2020-02-03Bibliographically approved
Rosti, M. E., Ge, Z., Jain, S. S., Dodd, M. S. & Brandt, L. (2019). Droplets in homogeneous shear turbulence. Journal of Fluid Mechanics, 876, 962-984
Open this publication in new window or tab >>Droplets in homogeneous shear turbulence
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2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 876, p. 962-984Article in journal (Refereed) Published
Abstract [en]

We simulate the flow of two immiscible and incompressible fluids separated by an interface in a homogeneous turbulent shear flow at a shear Reynolds number equal to 15 200. The viscosity and density of the two fluids are equal, and various surface tensions and initial droplet diameters are considered in the present study. We show that the two-phase flow reaches a statistically stationary turbulent state sustained by a non-zero mean turbulent production rate due to the presence of the mean shear. Compared to single-phase flow, we find that the resulting steady-state conditions exhibit reduced Taylor-microscale Reynolds numbers owing to the presence of the dispersed phase, which acts as a sink of turbulent kinetic energy for the carrier fluid. At steady state, the mean power of surface tension is zero and the turbulent production rate is in balance with the turbulent dissipation rate, with their values being larger than in the reference single-phase case. The interface modifies the energy spectrum by introducing energy at small scales, with the difference from the single-phase case reducing as the Weber number increases. This is caused by both the number of droplets in the domain and the total surface area increasing monotonically with the Weber number. This reflects also in the droplet size distribution, which changes with the Weber number, with the peak of the distribution moving to smaller sizes as the Weber number increases. We show that the Hinze estimate for the maximum droplet size, obtained considering break-up in homogeneous isotropic turbulence, provides an excellent estimate notwithstanding the action of significant coalescence and the presence of a mean shear.

Place, publisher, year, edition, pages
Cambridge University Press, 2019
Keywords
drops, multiphase flow, turbulence simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-257426 (URN)10.1017/jfm.2019.581 (DOI)000480242100001 ()2-s2.0-85070481433 (Scopus ID)
Note

QC 20190902

Available from: 2019-09-02 Created: 2019-09-02 Last updated: 2019-09-02Bibliographically approved
Rosti, M. E., Olivieri, S., Banaei, A. A., Brandt, L. & Mazzino, A. (2019). Flowing fibers as a proxy of turbulence statistics. Meccanica (Milano. Print)
Open this publication in new window or tab >>Flowing fibers as a proxy of turbulence statistics
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2019 (English)In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648Article in journal (Refereed) Published
Abstract [en]

The flapping states of a flexible fiber fully coupled to a three-dimensional turbulent flow are investigated via state-of-the-art numerical methods. Two distinct flapping regimes are predicted by the phenomenological theory recently proposed by Rosti et al. (Phys. Rev. Lett. 121:044501, 2018) the under-damped regime, where the elasticity strongly affects the fiber dynamics, and the over-damped regime, where the elastic effects are strongly inhibited. In both cases we can identify a critical value of the bending rigidity of the fiber by a resonance condition, which further provides a distinction between different flapping behaviors, especially in the under-damped case. We validate the theory by means of direct numerical simulations and find that, both for the over-damped regime and for the under-damped one, fibers are effectively slaved to the turbulent fluctuations and can therefore be used as a proxy to measure various two-point statistics of turbulence. Finally, we show that this holds true also in the case of a passive fiber, without any feedback force on the fluid.

Place, publisher, year, edition, pages
Springer Netherlands, 2019
Keywords
Dispersed flows, Fiber, Multiphase flows, Turbulence, Multiphase flow, Numerical methods, Dispersed flow, Flapping behavior, Phenomenological theory, Resonance condition, Three-dimensional turbulent flow, Turbulence statistics, Turbulent fluctuation, Two point statistics, Fibers
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-263285 (URN)10.1007/s11012-019-00997-2 (DOI)2-s2.0-85068170932 (Scopus ID)
Note

QC 20191105

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2020-01-09Bibliographically approved
Takeishi, N., Rosti, M. E., Imai, Y., Wada, S. & Brandt, L. (2019). Haemorheology in dilute, semi-dilute and dense suspensions of red blood cells. Journal of Fluid Mechanics, 872, 818-848
Open this publication in new window or tab >>Haemorheology in dilute, semi-dilute and dense suspensions of red blood cells
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2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 872, p. 818-848Article in journal (Refereed) Published
Abstract [en]

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) in a wall-bounded shear flow. The flow is assumed as almost inertialess. The suspension of RBCs, modelled as biconcave capsules whose membrane follows the Skalak constitutive law, is simulated for a wide range of viscosity ratios between the cytoplasm and plasma, D 0 : 1-10, for volume fractions up to D 0 : 41 and for different capillary numbers (Ca). Our numerical results show that an RBC at low Ca tends to orient to the shear plane and exhibits so-called rolling motion, a stable mode with higher intrinsic viscosity than the so-called tumbling motion. As Ca increases, the mode shifts from the rolling to the swinging motion. Hydrodynamic interactions (higher volume fraction) also allow RBCs to exhibit tumbling or swinging motions resulting in a drop of the intrinsic viscosity for dilute and semi-dilute suspensions. Because of this mode change, conventional ways of modelling the relative viscosity as a polynomial function of cannot be simply applied in suspensions of RBCs at low volume fractions. The relative viscosity for high volume fractions, however, can be well described as a function of an effective volume fraction, defined by the volume of spheres of radius equal to the semi-middle axis of a deformed RBC. We find that the relative viscosity successfully collapses on a single nonlinear curve independently of except for the case with Ca > 0 : 4, where the fit works only in the case of low/ moderate volume fraction, and fails in the case of a fully dense suspension.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
blood flow, capsule/cell dynamics, suspensions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-255171 (URN)10.1017/jfm.2019.393 (DOI)000471976300003 ()2-s2.0-85072027261 (Scopus ID)
Note

QC 20190904

Available from: 2019-09-04 Created: 2019-09-04 Last updated: 2019-10-04Bibliographically approved
Alghalibi, D., Rosti, M. E. & Brandt, L. (2019). Inertial migration of a deformable particle in pipe flow. Physical Review Fluids, 4(10), Article ID 104201.
Open this publication in new window or tab >>Inertial migration of a deformable particle in pipe flow
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 10, article id 104201Article in journal (Refereed) Published
Abstract [en]

We perform fully Eulerian numerical simulations of an initially spherical hyperelastic particle suspended in a Newtonian pressure-driven flow in a cylindrical straight pipe. We study the full particle migration and deformation for different Reynolds numbers and for various levels of particle elasticity, to disentangle the interplay of inertia and elasticity on the particle focusing. We observe that the particle deforms and undergoes a lateral displacement while traveling downstream through the pipe, finally focusing at the pipe centerline. We note that the migration dynamics and the final equilibrium position are almost independent of the Reynolds number, while they strongly depend on the particle elasticity; in particular, the migration is faster as the elasticity increases (i.e., the particle is more deformable), with the particle reaching the final equilibrium position at the centerline in shorter times. Our simulations show that the results are not affected by the particle initial conditions, position, and velocity. Finally, we explain the particle migration by computing the total force acting on the particle and its different components, viscous and elastic.

Place, publisher, year, edition, pages
American Physical Society, 2019
Keywords
deformable particle, hyper-elastic
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-263656 (URN)10.1103/PhysRevFluids.4.104201 (DOI)000489589700003 ()2-s2.0-85074434642 (Scopus ID)
Funder
EU, European Research Council, ERC-2013-CoG-616186
Note

QC 20191115

Available from: 2019-11-07 Created: 2019-11-07 Last updated: 2019-11-15Bibliographically 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.

Place, publisher, year, edition, pages
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
Villone, M. M., Rosti, M. E., Tammisola, O. & Brandt, L. (2019). Numerical simulations of oscillatory shear flow of particle suspensions at finite inertia. Rheologica Acta, 58(11-12), 741-753
Open this publication in new window or tab >>Numerical simulations of oscillatory shear flow of particle suspensions at finite inertia
2019 (English)In: Rheologica Acta, ISSN 0035-4511, E-ISSN 1435-1528, Vol. 58, no 11-12, p. 741-753Article in journal (Refereed) Published
Abstract [en]

We perform immersed-boundary-method numerical simulations of oscillatory shear flow of suspensions of mono-disperse non-colloidal rigid spherical particles in a Newtonian liquid from the dilute to the concentrated regime. Both small and large amplitude oscillatory shear flow (SAOS and LAOS, respectively) are studied and the effects of particle concentration, fluid inertia, particle-to-fluid density ratio, and deformation amplitude on the measured apparent viscoelastic moduli of the suspensions are quantified. In the SAOS regime, a non-zero storage modulus G '-values significantly change with inertia, but depend on the volume fraction of the solid phase only for suspensions of particles denser than the fluid. On the other hand, the loss modulus G '' increases with both inertia and particle concentration. In the LAOS regime, the moduli are only weakly dependent on the deformation amplitude for a dilute suspension, whereas non-monotonic variations are observed at high concentrations.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Rheology, Suspensions, Oscillatory shear flow, Inertia, Numerical simulations
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-265771 (URN)10.1007/s00397-019-01177-5 (DOI)000496233500001 ()2-s2.0-85075210703 (Scopus ID)
Note

QC 20200108

Available from: 2020-01-08 Created: 2020-01-08 Last updated: 2020-01-10Bibliographically approved
De Vita, F., Rosti, M. E., Caserta, S. & Brandt, L. (2019). On the effect of coalescence on the rheology of emulsions. Journal of Fluid Mechanics, 880, 969-991
Open this publication in new window or tab >>On the effect of coalescence on the rheology of emulsions
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 880, p. 969-991Article in journal (Refereed) Published
Abstract [en]

We present a numerical study of the rheology of a two-fluid emulsion in dilute and semidilute conditions. The analysis is performed for different capillary numbers, volume fractions and viscosity ratios under the assumption of negligible inertia and zero buoyancy force. The effective viscosity of the system increases for low values of the volume fraction and decreases for higher values, with a maximum for approximately 20% concentration of the disperse phase. When the dispersed fluid has lower viscosity, the normalised effective viscosity becomes smaller than 1 for high enough volume fractions. To single out the effect of droplet coalescence on the rheology of the emulsion we introduce an Eulerian force which prevents merging, effectively modelling the presence of surfactants in the system. When the coalescence is inhibited the effective viscosity is always greater than 1 and the curvature of the function representing the emulsion effective viscosity versus the volume fraction becomes positive, resembling the behaviour of suspensions of deformable particles. The reduction of the effective viscosity in the presence of coalescence is associated with the reduction of the total surface of the disperse phase when the droplets merge, which leads to a reduction of the interface tension contribution to the total shear stress. The probability density function of the flow topology parameter shows that the flow is mostly a shear flow in the matrix phase, with regions of extensional flow when the coalescence is prohibited. The flow in the disperse phase, instead, always shows rotational components. The first normal stress difference is positive, except for the smallest viscosity ratio considered, whereas the second normal difference is negative, with their ratio being constant with the volume fraction. Our results clearly show that the coalescence efficiency strongly affects the system rheology and that neglecting droplet merging can lead to erroneous predictions.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
emulsions, multiphase flow, rheology
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-266940 (URN)10.1017/jfm.2019.722 (DOI)000506235800035 ()2-s2.0-85073786964 (Scopus ID)
Note

QC 20200203

Available from: 2020-02-03 Created: 2020-02-03 Last updated: 2020-02-03Bibliographically 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
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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
Izbassarov, D., Rosti, M. E., Niazi Ardekani, M., Sarabian, M., Hormozi, S., Brandt, L. & Tammisola, O. (2018). Computational modeling of multiphase viscoelastic and elastoviscoplastic flows. International Journal for Numerical Methods in Fluids, 88(12), 521-543
Open this publication in new window or tab >>Computational modeling of multiphase viscoelastic and elastoviscoplastic flows
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2018 (English)In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363, Vol. 88, no 12, p. 521-543Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2018
Keywords
elastoviscoplastic multiphase systems, FENE-P model, immersed boundary method, level-set method, Oldroyd-B model, Saramito model
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-240127 (URN)10.1002/fld.4678 (DOI)000450028200001 ()2-s2.0-85052317831 (Scopus ID)
Funder
Swedish Research Council, VR 2014-5001Swedish Research Council, VR2017-4809Swedish Research Council, VR2013-5789
Note

QC 20181214

Available from: 2018-12-12 Created: 2018-12-12 Last updated: 2019-05-03Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-9004-2292

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