kth.sePublications
Change search
Link to record
Permanent link

Direct link
Alternative names
Publications (10 of 40) Show all publications
Shahmardi, A., Rosti, M. E., Tammisola, O. & Brandt, L. (2021). A fully Eulerian hybrid immersed boundary-phase field model for contact line dynamics on complex geometries. Journal of Computational Physics, 443, 110468-110468, Article ID 110468.
Open this publication in new window or tab >>A fully Eulerian hybrid immersed boundary-phase field model for contact line dynamics on complex geometries
2021 (English)In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 443, p. 110468-110468, article id 110468Article in journal (Refereed) Published
Abstract [en]

We present a fully Eulerian hybrid immersed-boundary/phase-field model to simulate wetting and contact line motion over any arbitrary geometry. The solid wall is described with a volume-penalisation ghost-cell immersed boundary whereas the interface between the two fluids by a diffuse-interface method. The contact line motion on the complex wall is prescribed via slip velocity in the momentum equation and static/dynamic contact angle condition for the order parameter of the Cahn-Hilliard model. This combination requires accurate computations of the normal and tangential gradients of the scalar order parameter and of the components of the velocity. However, the present algorithm requires the computation of averaging weights and other geometrical variables as a preprocessing step. Several validation tests are reported in the manuscript, together with 2D simulations of a droplet spreading over a sinusoidal wall with different contact angles and slip length and a spherical droplet spreading over a sphere, showing that the proposed algorithm is capable to deal with the three-phase contact line motion over any complex wall. The Eulerian feature of the algorithm facilitates the implementation and provides a straight-forward and potentially highly scalable parallelisation. The employed parallelisation of the underlying Navier-Stokes solver can be efficiently used for the multiphase part as well. The procedure proposed here can be directly employed to impose any types of boundary conditions (Neumann, Dirichlet and mixed) for any field variable evolving over a complex geometry, modelled with an immersed-boundary approach (for instance, modelling deformable biological membranes, red blood cells, solidification, evaporation and boiling, to name a few). 

Place, publisher, year, edition, pages
Elsevier BV, 2021
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-320569 (URN)10.1016/j.jcp.2021.110468 (DOI)000687208300007 ()2-s2.0-85107044824 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20221115

Available from: 2022-10-25 Created: 2022-10-25 Last updated: 2024-03-18Bibliographically approved
Brizzolara, S., Rosti, M. E., Olivieri, S., Brandt, L., Holzner, M. & Mazzino, A. (2021). Fiber Tracking Velocimetry for Two-Point Statistics of Turbulence. Physical Review X, 11(3), Article ID 031060.
Open this publication in new window or tab >>Fiber Tracking Velocimetry for Two-Point Statistics of Turbulence
Show others...
2021 (English)In: Physical Review X, E-ISSN 2160-3308, Vol. 11, no 3, article id 031060Article in journal (Refereed) Published
Abstract [en]

We propose and validate a novel experimental technique to measure two-point statistics of turbulent flows. It consists of spreading rigid fibers in the flow and tracking their position and orientation in time and is therefore named “fiber tracking velocimetry.” By choosing different fiber lengths, i.e., within the inertial or dissipative range of scales, the statistics of turbulence fluctuations at the selected length scale can be probed accurately by simply measuring the fiber velocity at its two ends and projecting it along the transverse-to-fiber direction. By means of fully resolved direct numerical simulations and experiments, we show that these fiber-based transverse velocity increments are statistically equivalent to the (unperturbed) flow transverse velocity increments. Moreover, we show that the turbulent energy-dissipation rate can be accurately measured exploiting sufficiently short fibers. The technique is tested against standard particle tracking velocimetry (PTV) of flow tracers with excellent agreement. Our technique overcomes the well-known problem of PTV to probe two-point statistics reliably because of the fast relative diffusion in turbulence that prevents the mutual distance between particles to remain constant at the length scale of interest. This problem, making it difficult to obtain converged statistics for a fixed separation distance, is even more dramatic for natural flows in open domains. A prominent example is oceanic currents, where drifters (i.e., the tracer-particle counterpart used in field measurements) disperse quickly, but at the same time their number has to be limited to save costs. Inspired by our laboratory experiments, we propose pairs of connected drifters as a viable option to solve the issue.

Place, publisher, year, edition, pages
American Physical Society (APS), 2021
Keywords
Energy dissipation, Fibers, Ocean currents, Statistics, Turbulence, Velocimeters, Experimental techniques, Fiber length, Fiber tracking, Length scale, Particle-tracking velocimetry, Position and orientations, Rigid fibers, Transverse velocity, Two point statistics, Velocity increments, Velocity measurement
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-312076 (URN)10.1103/PhysRevX.11.031060 (DOI)000704691600001 ()2-s2.0-85116314475 (Scopus ID)
Note

QC 20220511

Available from: 2022-05-11 Created: 2022-05-11 Last updated: 2024-01-17Bibliographically approved
Rosti, M. E., Mirbod, P. & Brandt, L. (2021). The impact of porous walls on the rheology of suspensions. Chemical Engineering Science, 230, Article ID 116178.
Open this publication in new window or tab >>The impact of porous walls on the rheology of suspensions
2021 (English)In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 230, article id 116178Article in journal (Refereed) Published
Abstract [en]

We study the effect of isotropic porous walls on a plane Couette flow laden with spherical and rigid particles. We perform a parametric study varying the volume fraction between 0 and 30%, the porosity between 0.3 and 0.9 and the non-dimensional permeability between 0 and 7.9×10-3 We find that the porous walls induce a progressive decrease in the suspension effective viscosity as the wall permeability increases. This behavior is explained by the weakening of the wall-blocking effect and by the appearance of a slip velocity at the interface of the porous medium, which reduces the shear rate in the channel. Therefore, particle rotation and the consequent velocity fluctuations in the two phases are dampened, leading to reduced particle interactions and particle stresses. Based on our numerical evidence, we provide a closed set of equations for the suspension viscosity, which can be used to estimate the suspension rheology in the presence of porous walls.

Place, publisher, year, edition, pages
Elsevier Ltd, 2021
Keywords
Numerical simulations, Particle suspension, Permeability, Porosity, Rheology, Elasticity, Porous materials, Viscosity, Effective viscosity, Numerical evidence, Parametric study, Particle rotations, Suspension rheology, Suspension viscosity, Velocity fluctuations, Wall permeability, Shear flow
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-285258 (URN)10.1016/j.ces.2020.116178 (DOI)000599717300016 ()2-s2.0-85091940298 (Scopus ID)
Note

QC 20201112

Available from: 2020-11-12 Created: 2020-11-12 Last updated: 2024-01-10Bibliographically approved
Le Clainche, S., Izbassarov, D., Rosti, M. E., Brandt, L. & Tammisola, O. (2020). Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid. Journal of Fluid Mechanics, 888, Article ID A5.
Open this publication in new window or tab >>Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid
Show others...
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 888, article id A5Article in journal (Refereed) Published
Abstract [en]

In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded-unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves.

Place, publisher, year, edition, pages
Cambridge University Press, 2020
Keywords
nonlinear instability, viscoelasticity, turbulent boundary layers
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-268813 (URN)10.1017/jfm.2020.31 (DOI)000511269300001 ()2-s2.0-85079267695 (Scopus ID)
Note

QC 20200221

Available from: 2020-02-21 Created: 2020-02-21 Last updated: 2022-06-26Bibliographically approved
Le Clainche, S., Rosti, M. E. & Brandt, L. (2020). Flow structures and shear-stress predictions in the turbulent channel flow over an anisotropic porous wall. In: Journal of Physics: Conference Series. Paper presented at Fourth Madrid Summer School on Turbulence 10 June to 12 July 2019, Madrid, Spain (pp. 012016). IOP Publishing, 1522(1)
Open this publication in new window or tab >>Flow structures and shear-stress predictions in the turbulent channel flow over an anisotropic porous wall
2020 (English)In: Journal of Physics: Conference Series, IOP Publishing , 2020, Vol. 1522, no 1, p. 012016-Conference paper, Published paper (Refereed)
Abstract [en]

This article identifies the main coherent structures driving the flow dynamics in the turbulent channel flow over anisotropic porous walls. Two different cases have been analyzed where the drag increases or decreases with respect to a channel with isotropic porous walls. Higher order dynamic mode decomposition (HODMD) is applied to analyze these data, identifying 20 and 15 high amplitude modes in the drag increasing (DI) and drag reducing (DR) cases, respectively, which well reflects the largest flow complexity in the former case. The frequency of 13 modes and the three-dimensional structure of the modes are similar in the DR and DI cases, suggesting the need of using more complex analyses to deepen our physical insight of these flows. The spatio-temporal HODMD analysis identifies a periodic solution along the spanwise direction (as imposed by the boundary conditions). The wavenumbers related to the modes with highest amplitude are β = 0 and β = 3 (Lz = 2 3 π ). The rollers, groups of spanwise correlated structures, are mostly identified in the DI case near the wall, with β = 0, while the presence of the streaks, streamwise correlated structures are mostly identified in the DR case. Although, in areas far away from the wall it is possible to identify these two types of structures with β = 3 in both cases, depending on the temporal frequency of the DMD modes, the rollers and the streaks are related to high and low frequency DMD modes, respectively. Finally, a model is constructed to predict the temporal evolution of the wall shear, using the 6 most relevant DMD modes interacting near the channel wall: 6 low frequency modes for DR and 3 low and 3 high frequency modes for DI. In the DR case the wall shear is predicted for almost 300 time units with relative error ∼ 2%, however, this error is larger in the DI case, ∼ 6%, suggesting the need of using a larger number of modes to represent this more complex flow.

Place, publisher, year, edition, pages
IOP Publishing, 2020
Keywords
Anisotropy, Channel flow, Drag reduction, Rollers (machine components), Shear stress, Turbulence, Wall flow, Coherent structure, High-frequency mode, Higher-order dynamics, Low-frequency modes, Temporal evolution, Temporal frequency, Three-dimensional structure, Turbulent channel flows, Shear flow
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-301681 (URN)10.1088/1742-6596/1522/1/012016 (DOI)2-s2.0-85086637095 (Scopus ID)
Conference
Fourth Madrid Summer School on Turbulence 10 June to 12 July 2019, Madrid, Spain
Note

QC 20210915

Available from: 2021-09-15 Created: 2021-09-15 Last updated: 2024-01-10Bibliographically approved
Rosti, M. E., Olivieri, S., Banaei, A. A., Brandt, L. & Mazzino, A. (2020). Flowing fibers as a proxy of turbulence statistics. Meccanica (Milano. Print), 55, 357-370
Open this publication in new window or tab >>Flowing fibers as a proxy of turbulence statistics
Show others...
2020 (English)In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 55, p. 357-370Article 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, 2020
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)000512775600006 ()2-s2.0-85068170932 (Scopus ID)
Note

QC 20191105

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2022-06-26Bibliographically approved
Rosti, M. E. & Brandt, L. (2020). Increase of turbulent drag by polymers in particle suspensions. Physical Review Fluids, 5(4), Article ID 041301.
Open this publication in new window or tab >>Increase of turbulent drag by polymers in particle suspensions
2020 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, no 4, article id 041301Article in journal (Refereed) Published
Abstract [en]

We study the effect of spherical particles on the turbulent flow of a viscoelastic fluid and find that the drag reducing effect of polymer additives is completely lost for semidense suspensions, with the drag increasing more than for suspensions in Newtonian fluids. This different behavior is due to three separate effects. First, polymer stretching is reduced by the presence of rigid particles, thus canceling the drag reducing benefit of the viscoelastic fluid. Second, drag increase is provided by the growth of the particle and polymeric shear stresses with the particles, due to larger shear rates in the vicinity of the particle surface. Third, particles migrate towards the wall due to the shear-thinning property of the fluid, thus enhancing the particle near-wall layer and further increasing the drag.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2020
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-272812 (URN)10.1103/PhysRevFluids.5.041301 (DOI)000524339600001 ()2-s2.0-85085986400 (Scopus ID)
Note

QC 20200428

Available from: 2020-04-28 Created: 2020-04-28 Last updated: 2022-06-26Bibliographically approved
Sarabian, M., Rosti, M. E., Brandt, L. & Hormozi, S. (2020). Numerical simulations of a sphere settling in simple shear flows of yield stress fluids. Journal of Fluid Mechanics, 896, Article ID A17.
Open this publication in new window or tab >>Numerical simulations of a sphere settling in simple shear flows of yield stress fluids
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 896, article id A17Article in journal (Refereed) Published
Abstract [en]

We perform three-dimensional numerical simulations to investigate the sedimentation of a single sphere in the absence and presence of a simple cross-shear flow in a yield stress fluid with weak inertia. In our simulations, the settling flow is considered to be the primary flow, whereas the linear cross-shear flow is a secondary flow with amplitude 10 % of the primary flow. To study the effects of elasticity and plasticity of the carrying fluid on the sphere drag as well as the flow dynamics, the fluid is modelled using the elastoviscoplastic constitutive laws proposed by Saramito (J. Non-Newtonian Fluid Mech., vol. 158 (1-3), 2009, pp. 154-161). The extra non-Newtonian stress tensor is fully coupled with the flow equation and the solid particle is represented by an immersed boundary method. Our results show that the fore-aft asymmetry in the velocity is less pronounced and the negative wake disappears when a linear cross-shear flow is applied. We find that the drag on a sphere settling in a sheared yield stress fluid is reduced significantly compared to an otherwise quiescent fluid. More importantly, the sphere drag in the presence of a secondary cross-shear flow cannot be derived from the pure sedimentation drag law owing to the nonlinear coupling between the simple shear flow and the uniform flow. Finally, we show that the drag on the sphere settling in a sheared yield stress fluid is reduced at higher material elasticity mainly due to the form and viscous drag reduction.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2020
Keywords
particle, fluid flows
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-276864 (URN)10.1017/jfm.2020.316 (DOI)000536845400001 ()2-s2.0-85085750298 (Scopus ID)
Note

QC 20200623

Available from: 2020-06-23 Created: 2020-06-23 Last updated: 2022-06-26Bibliographically approved
De Vita, F., Rosti, M. E., Caserta, S. & Brandt, L. (2020). Numerical simulations of vorticity banding of emulsions in shear flows. Soft Matter, 16(11), 2854-2863
Open this publication in new window or tab >>Numerical simulations of vorticity banding of emulsions in shear flows
2020 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 16, no 11, p. 2854-2863Article in journal (Refereed) Published
Abstract [en]

Multiphase shear flows often show banded structures that affect the global behavior of complex fluids e.g. in microdevices. Here we investigate numerically the banding of emulsions, i.e. the formation of regions of high and low volume fractions, alternated in the vorticity direction and aligned with the flow (shear bands). These bands are associated with a decrease of the effective viscosity of the system. To understand the mechanism of experimentally observed banding, we have performed interface-resolved simulations of the two-fluid system. The experiments were performed starting with a random distribution of droplets, which under the applied shear, evolve in time resulting in a phase separation. To numerically reproduce this process, the banded structures are initialized in a narrow channel confined by two walls moving in opposite directions. We find that the initial banded distribution is stable when droplets are free to merge and unstable when coalescence is prevented. In this case, additionally, the effective viscosity of the system increases, resembling the rheological behavior of suspensions of deformable particles. Droplet coalescence, on the other hand, allows emulsions to reduce the total surface of the system and, hence, the energy dissipation associated with the deformation, which in turn reduces the effective viscosity.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2020
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-271929 (URN)10.1039/c9sm01898k (DOI)000521354400018 ()32107513 (PubMedID)2-s2.0-85082095925 (Scopus ID)
Note

QC 20200420

Available from: 2020-04-20 Created: 2020-04-20 Last updated: 2022-06-26Bibliographically approved
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 ()2-s2.0-85079828455 (Scopus ID)
Note

QC 20200203

Available from: 2020-02-03 Created: 2020-02-03 Last updated: 2022-06-26Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-9004-2292

Search in DiVA

Show all publications