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Lupo, G., Niazi Ardekani, M., Brandt, L. & Duwig, C. (2019). An Immersed Boundary Method for flows with evaporating droplets. International Journal of Heat and Mass Transfer, 143, Article ID 118563.
Open this publication in new window or tab >>An Immersed Boundary Method for flows with evaporating droplets
2019 (English)In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 143, article id 118563Article in journal (Refereed) Published
Abstract [en]

We present a new Immersed Boundary Method (IBM) for the interface resolved simulation of spherical droplet evaporation in gas flow. The method is based on the direct numerical simulation of the coupled momentum, energy and species transport in the gas phase, while the exchange of these quantities with the liquid phase is handled through global mass, energy and momentum balances for each droplet. This approach, applicable in the limit of small spherical droplets, allows for accurate and efficient phase coupling without direct solution of the liquid phase fields, thus saving computational cost. We provide validation results, showing that all the relevant physical phenomena and their interactions are correctly captured, both for laminar and turbulent gas flow. Test cases include fixed rate and free evaporation of a static droplet, displacement of a droplet by Stefan flow, and evaporation of a hydrocarbon droplet in homogeneous isotropic turbulence. The latter case is validated against experimental data, showing the feasibility of the method towards the treatment of conditions representative of real life spray fuel applications.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Spray, Fuel, Evaporation, Phase change, Immersed boundary, Multiphase, Direct numerical simulation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-261933 (URN)10.1016/j.ijheatmasstransfer.2019.118563 (DOI)000487564400090 ()2-s2.0-85070927066 (Scopus ID)
Note

QC 20191015

Available from: 2019-10-15 Created: 2019-10-15 Last updated: 2019-10-15Bibliographically approved
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: 2019-09-03Bibliographically 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., 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
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
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.

Place, publisher, year, edition, pages
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.

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
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.

Place, publisher, year, edition, pages
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: 2019-09-03Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-4346-4732

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