FY2020 Annual Report

Complex Fluids and Flows
Assistant Professor Marco Edoardo Rosti

Abstract

This is the first full fiscal year since the foundation of the Complex Fluids and Flows Unit (CFFU). In this period, the unit welcomed Mr. Mohamed Abdelgawad as PhD student, Dr. Alessandro Monti, Stefano Olivieri and Giovanni Soligo as PostDocs and Mr Luke Collyer-Hoar and Patrick Clark as rotation students.

The Unit research has focused on multiphase systems in laminar and turbulent flows. The unit worked on the turbulence modulation by the presence of a dispersed phase: both particle and fiber suspensions have been considered, together with two-liquid systems. In parallel to this topic, the unit worked on the numerical methods needed to simulate such systems; a new immersed boundary method for rigid objects and an extension of the volume of fluid method for two-liquid systems have been developed, together with the development of a new code able to simulate dense suspensions. The unit research in laminar flows focused on the rheology of multiphase systems, mainly focusing on non-Newtonian fluids and dense suspensions.

In this period, 11 manuscripts were published in international journals, including 1 Scientific Reports and 1 Phsyical Review Letters, with 2 publications highlighted as Editors’ suggestion, 1 as the most read of the journal and 1 broadcasted by international news outlets. Furthermore, several new collaborations have been established, both inside and outside OIST, with 14 international guests invited to present their works in (online) seminars.

1. Staff

Dr. Alessandro Monti, Postdoctoral Researcher
Dr. Stefano Olivieri, Postdoctoral Researcher
Dr. Giovanni Soligo, Postdoctoral Researcher
Mr. Ianto Cannon, Graduate Student
Mr. Mohamed Abdelgawad, Graduate Student
Ms. Megumi Ikeda, Administrative Assistant

2. Collaborations

2.1 Coalescence in turbulent channel flows

Type of collaboration: Joint research
Researchers
- Dr. Daulet Izbassarov, Aalto University
- Mr. Ianto Cannon, OIST
- Prof. Marco E. Rosti, OIST

2.2 Fibers in turbulent flows

Type of collaboration: Joint research
Researchers
- Prof. Andrea Mazzino, University of Genova
- Prof. Luca Brandt, KTH Royal Institute of Technology
- Mr. Stefano Brizzolara, WSL and ETH Zurich
- Prof. Markus Holzner, WSL and ETH Zurich
- Dr. Stefano Olivieri, OIST
- Prof. Marco E. Rosti, OIST

2.3 Grid-induced turbulence

Type of collaboration: Joint research
Researchers
- Prof. Andrea Mazzino, University of Genova
- Prof. Francesco Viola, Gran Sasso Science Institute
- Dr. Stefano Olivieri, OIST
- Prof. Marco E. Rosti, OIST

2.4 Hydrodynamics of clownfish

Type of collaboration: Joint research
Researchers
- Prof. Vincent Laudet, Marine Eco-Evo-Devo Unit, OIST
- Dr. Manon Mercader, Marine Eco-Evo-Devo Unit, OIST
- Dr. Stefano Olivieri, OIST
- Prof. Marco E. Rosti, OIST

2.5 Rheology of dense colloidal suspensions

Type of collaboration: Joint research
Researchers
- Prof. Amy Shen, Micro/Bio/Nanofluidics Unit, OIST
- Dr. Vikram Rathee, Micro/Bio/Nanofluidics Unit, OIST
- Dr. Alessandro Monti, OIST
- Prof. Marco E. Rosti, OIST

2.6 Simulation of expiratory events

Type of collaboration: Joint research
Researchers
- Prof. Andrea Mazzino, University of Genova
- Dr. Mattia Cavaiola, University of Genova
- Dr. Agnese Seminara, CNRS and Université Côte d’Azur
- Dr. Stefano Olivieri, OIST
- Prof. Marco E. Rosti, OIST

2.7 Rheology of particles in non-Newtonian liquids

Researchers
- Prof. Massimiliano Villone, University of Naples Federico II
- Prof. Luca Brandt, KTH Royal Institute of Technology
- Prof. Outi Tammisola, KTH Royal Institute of Technology
- Prof. Marco E. Rosti, OIST

2.8 Rheology of droplets in non-Newtonian liquids

Researchers
- Prof. Shu Takagi, The University of Tokyo
- Prof. Marco E. Rosti, OIST

2.9 Fully Eulerian immersed boundary method

Researchers
- Prof. Shu Takagi, The University of Tokyo
- Mr. Naoki Hori, The University of Tokyo
- Prof. Marco E. Rosti, OIST

2.10 Haemorheology of red blood cells

Researchers
- Prof. Naoki Takeishi, Osaka University
- Prof. Luca Brandt, KTH Royal Institute of Technology
- Prof. Marco E. Rosti, OIST

2.11 The effect of porous walls on particle suspensions

Researchers
- Prof. Parisa Mirbod, University of Illinois at Chicago
- Prof. Luca Brandt, KTH Royal Institute of Technology
- Prof. Marco E. Rosti, OIST

2.12 A deep-learning model for turbulent channel flows over porous walls

Researchers
- Prof. Soledad Le Clainche, Universidad Politecnica de Madrid
- Prof. Luca Brandt, KTH Royal Institute of Technology
- Prof. Marco E. Rosti, OIST

2.13 Two dimensional turbulent flows

Researchers
- Prof. Guido Boffetta, University of Torino
- Prof. Stefano Musacchio, University of Torino
- Prof. Andrea Mazzino, University of Genova
- Prof. Marco E. Rosti, OIST

3. Activities and Findings

3.1 Low Reynolds number turbulent flows

We study the laminar and turbulent channel flows over a viscous hyper-elastic wall and show that it is possible to sustain an unsteady chaotic turbulent-like flow at any Reynolds number by properly choosing the wall elastic modulus. We vary the bulk Reynolds number from 2800 to 10 and identify two distinct mechanisms for turbulence production. At moderate and high Reynolds numbers, turbulent fluctuations activate the wall oscillations, which, in turn, amplify the turbulent Reynolds stresses in the fluid. At a very low Reynolds number, the only production term is due to the energy input from the elastic wall, which increases with the wall elasticity. This mechanism may be exploited to passively enhance mixing in microfluidic devices.

*Figure 3.1 : Mean friction Reynolds number as a function of the bulk Reynolds number.*

3.2 Particle laden non-Newtonian flows

We study the effect of spherical particles on non-Newtonian turbulent flows.
First, we study the sedimentation of finite-size particles in quiescent wall-bounded shear-thinning fluids. Overall, we report a twofold effect of shear thinning: first and more important, the substantial increase of the particle sedimentation velocity in the shear-thinning case due to the increase of the shear rate around the particles, which reduces the local viscosity leading to a reduced particle drag; secondly, the shear-thinning fluid reduces the level of particle interactions, causing a reduction of velocity fluctuations and resulting in particles sedimenting at approximately the same speed.
When considering a viscoelastic turbulent channel flow, we 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.

Figure 3.2 : Frictional Reynolds number as a function of the particle volume fraction for different Weissenberg numbers. The inset shows the corresponding drag reduction DR computed with respect to the Newtonian multiphase case with same volume fraction.

3.3 Numerical methods for multiphase flows

We developed new numerical methods to simulate multiphase flows in laminar and turbulent conditions.
First we proposed an immersed boundary method in which the fluid-structure coupling is achieved in an Eulerian framework. The method is second-order and capable to handle the presence of multiple suspended objects, i.e., a suspension, by including a soft-sphere normal collision model, while the lubrification correction typically added to similar immersed boundary methods in order to capture the sub-grid unresolved lubrification force is here treated implicitly. We show that our methodology can successfully reproduce the rheology of a particle suspension in a shear flow up to a dense regime (with a maximum particle volume fraction around 46%) without any additional correction force. The applicability of this methodology is also tested in a turbulent pressure-driven duct flow at high Reynolds number in the presence of non-negligible inertia and non-uniform shear-rate, showing good agreement with the experimental measurements.
Furthermore, we proposed a numerical approach to the simulation of multiphase, viscous flows where a compressible and an incompressible phase interact in the low-Mach number regime. In this frame, acoustics are neglected but large density variations of the compressible phase can be accounted for as well as heat transfer, convection and diffusion processes. The problem is addressed in a fully Eulerian framework exploiting a low-Mach number asymptotic expansion of the Navier-Stokes equations. A Volume of Fluid approach (VOF) is used to capture the liquid-gas interface, built on top of a massive parallel solver, second order accurate both in time and space. The second-order-pressure term is treated implicitly and the resulting pressure equation is solved employing a robust and novel formulation. We provide a detailed and complete description of the theoretical approach together with information about the numerical technique and implementation details.

*Figure 3.3 : Particle laden turbulent channel flow.*

3.4 Fibers in turbulent flows

A numerical study has been carried out on semi-dilute solutions of rigid fibers dispersed in turbulent flows, a subject of relevance for many environmental and industrial applications. We found that the presence of such dispersed phase totally changes the classic turbulent energy budget. Because of the backreaction of the fibers to the flowing fluid, the intensity of the large-scale motion is damped. The kinetic energy of the flow reduces at large scales while being partially reinjected at small scales causing small-scale mixing. The scenario is robust with respect to changes of the underlying turbulence characteristics. Because we found the same results for both fixed fibers (forming a turbulent porous medium) and freely moving fibers, the two realms of porous media and suspension dynamics now appear much closer than previously thought. Finally, since the resulting phenomenology appears to be ruled by general properties such as inertia and concentration rather than peculiar features of anisotropic particles, we expect a similar scenario to hold also when considering particles of different shapes or droplets.

3.5 Simulation of expiratory events

Violent expiratory events, such as coughing and sneezing, are nontrivial examples of a two-phase mixture of liquid droplets dispersed into an unsteady turbulent airflow. Understanding the physical mechanisms determining the dispersion and evaporation process of respiratory droplets has recently become a priority given the global emergency caused by the SARS-CoV-2 infection. We investigated the dispersion process of virus-containing respiratory droplets by means of high-resolution direct numerical simulations (DNSs) and a comprehensive Lagrangian model for the droplet dynamics. First, we identified the key role of turbulence in the fate of exhaled droplets, showing that the evaporation time is controlled by the combined effect of turbulence and droplet inertia. Then, we demonstrated that available knowledge is largely inadequate to make predictions on the reach of respiratory droplets and on their infectious potential. We found that different initial distributions of droplet size taken from literature and different ambient relative humidity lead to opposite conclusions: (1) most versus none of the viral content settles in the first 1–2 m; (2) viruses are carried entirely on dry nuclei versus on liquid droplets; (3) small droplets travel less than 2.5m versus more than 7.5m.

*Figure 3.5 : Instantaneous visualization of respiratory droplets in the relative humidity generated by cough.*

4. Publications

4.1 Journals

M. E. Rosti, L. Brandt, "Increase of turbulent drag by polymers in particle suspensions" DOI: 10.1103/PhysRevFluids.5.041301
D. Alghalibi, M. E. Rosti, L. Brandt, "Sedimentation of finite-size particles in quiescent wall-bounded shear-thinning and Newtonian fluids" DOI: 10.1016/j.ijmultiphaseflow.2020.103291
M. Sarabian, M. E. Rosti, L. Brandt, and S. Hormozi, "Numerical simulations of a sphere settling in simple shear flows of yield stress fluids" DOI: 10.1017/jfm.2020.316
S. Olivieri, A. Akoush, L. Brandt, M. E. Rosti, and A. Mazzino, "Turbulence in a network of rigid fibers DOI: 10.1103/PhysRevFluids.5.074502
M. E. Rosti, L. Brandt, "Low Reynolds number turbulent flows over elastic walls DOI: 10.1063/5.0018770
S. Olivieri, L. Brandt, M. E. Rosti, A. Mazzino, "Dispersed Fibers Change the Classical Energy Budget of Turbulence via Nonlocal Transfer" DOI: 10.1103/PhysRevLett.125.114501
L. F. Chiara, M. E. Rosti, F. Picano, L. Brandt, "Suspensions of deformable particles in Poiseuille flows at finite inertia" DOI: 10.1088/1873-7005/abc606
M. E. Rosti, S. Olivieri, M. Cavaiola, A. Seminara, A. Mazzino, "Fluid dynamics of COVID-19 airborne infection suggests urgent data for a scientific design of social distancing" DOI: 10.1038/s41598-020-80078-7
M. E. Rosti, P. Mirbod, L. Brandt, "The impact of porous walls on the rheology of suspensions DOI: 10.1016/j.ces.2020.116178
F. Dalla Barba, N. Scapin, A. D. Demou, M. E. Rosti, F. Picano, L. Brandt, "An interface capturing method for liquid-gas flows at low-Mach number" DOI: 10.1016/j.compfluid.2020.104789
M. E. Rosti, M. Cavaiola, S. Olivieri, A. Seminara, A. Mazzino, "Turbulence role in the fate of virus-containing droplets in violent expiratory events" DOI: 10.1103/PhysRevResearch.3.013091
I. Banerjee, M. E. Rosti, T. Kumar, L. Brandt, A. Russom, "Analogue tuning of particle focusing in elasto-inertial flow" DOI: 10.1007/s11012-021-01329-z

4.2 Books and other one-time publications

Nothing to report

4.3 Oral and Poster Presentations

Invited Talks

M. E. Rosti, "Progress towards Fiber Tracking Velocimetry to measure turbulent flows" Symposium of Japan Consortium for Theoretical and Applied Mechanics, Japan, Online (2020.09.01)
M. E. Rosti, "Spreading of polydisperse droplets in a turbulent puff of saturated exhaled air" Joint Center for Advanced High Performance Computing (JCAHPC) seminar, Japan, Online (2020.10.15)

Contributed Talks

I. Cannon, "Droplet coalescence in turbulent channel flows" 34th Computational Fluid Dynamics Symposium of Japan Society of Fluid Mechanics, Online (2020.12.21)
S. Olivieri, "Flexible fiber suspensions in homogeneous isotropic turbulence: a numerical study" 34th Computational Fluid Dynamics Symposium of Japan Society of Fluid Mechanics, Online (2020.12.21)
M. Abdelgawad, "Single droplet deformation in Couette flow" 34th Computational Fluid Dynamics Symposium of Japan Society of Fluid Mechanics, Online (2020.12.22)
S. Olivieri, "Fluttering wings for energy harvesting: a numerical study using a parallel immersed boundary method" 14th WCCM & ECCOMAS Congress, Online (2021.01.11)
M. E. Rosti, "Droplets in shear flows" BICTAM-CISM Symposium on Dispersed Multiphase Flows, Online (2021.03.02)

Seminars

M. E. Rosti, "Turbulence in a network of rigid fibers" Poromechanics Lab, University of Oxford, UK, Online (2020.06.04)
M. E. Rosti, "The fluid dynamics of social distancing" OIST Faculty Lunchtime Seminar, Okinawa, Japan (2020.10.20)
S. Olivieri, "Fluid-structure interaction for flow energy harvesting" OIST Internal Seminar Series, Okinawa, Japan (2020.11.27)
M. E. Rosti, "Science Talk - Transcending Borders" OIST Science Challenge, Okinawa, Japan, Online (2021.03.16)

5. Intellectual Property Rights and Other Specific Achievements