What is my research about?

I study the physics of multiphase flows. In particular I focus on particles as they are carried by a fluid. It plays a major role in a multitude of industrial processes such as slurry transport, water treatment plants, land reclamation projects and fluidized beds. It also plays a prominent role in the dynamics of environmental processes such as volcanic eruptions, sediment transport in rivers and rain.

What is the main challenge to overcome?

It is very difficult to study multiparticle systems using conventional methods. It is challenging to measure the properties of these systems optically in an experiment due to their opaque nature. Presently X-rays, MRI’s and ultrasound techniques have had significant success, but are limited in their measurements.

In my work, I simulate this system on a cluster of computers (like 1000-4000 computers). The physics of these systems are described by a set of fundamental equations which are solved in parallel on all the computers. This is necessary to process the large number of equations to be solved. This provides detailed measurements at each point in the fluid and on the surface of every particle.

Analyzing these detailed datasets allows me to describe the governing mechanisms in these systems. I also provide correlations that are applicable to other researchers and aids in the development of analysis tools in industry.

How are the results computed?

We define a region in space (domain) using a high resolution mesh. The Navier-Stokes equations that describe conservation of momentum are solved at each point on this mesh. The Newton-Euler equations describe the mechanics of particle interaction. We solve these on a separate mesh attached to the surface of the particle. The Immersed Boundary Method is used to couple the interaction between the fluid and solid phase.

To achieve a high resolution, we typically have to solve a few billion equations at each step of the calculation. This can take from a couple of weeks to a month to complete. Our code is written in FORTRAN and we run our calculations on the computing facility Cartesius, SurfSARA.

How to make use of these results?

I work in fundamental research where I am interested in even the smallest of interactions. However, using all this information in everyday operations is overwhelming and impractical. For daily operations, the industry requires an easily accessible tool that runs on a desktop computer and provides results with reasonable accuracy.

I meet this challenge by developing models and/or correlations from that data which can be applied to a wide range of problems. The wealth of information from our simulations provides us unique insights into the dynamics of the underlying phenomenon.

What is the focus of my work?

I work on sedimentation of suspensions and mass transport in pipes. My work on sedimentation focuses on studying the role of concentration and fluid inertia on the behavior of sedimenting suspensions. My research on sediment transport is focused on the interaction of turbulence and gravity. In both these studies, first, we are interested in characterizing the regimes where the properties of these systems show a distinct difference and second, to describe the scaling of different properties as function different governing parameters. We are particularly interested in the underlying mechanics that dictate these dynamics.

What else does my work involve?

My other responsibilities include supervising students and duties as a teaching assistant for a course on multiphase flows. In addition to my work I travel to conferences and attend summer schools. I collaborate often with my experimental colleagues to validate the results of my simulations. I was a board member of Panta Rhei for a year, where I organized weekly seminars for researchers in my group and monthly seminars for guest speakers. The last of my duties as a board member was to ensure a fresh pot of coffee every morning.

Sedimentation

Macroscopic properties of sedimenting suspensions have been studied and charterized extensively. However, the particle–particle and particle–fluid interactions that dictate these macroscopic trends have been challenging to study. We examine the effect of concentration on the structure and dynamics of sedimenting suspensions by performing numerical simulations based on an Immersed Boundary Method of mono-disperse sedimenting suspensions of spherical particles.

Our simulations reproduce the macroscopic trends observed in experiments and are in good agreement with semi-empirical correlations in literature. From our studies, we observe, first, a change in trend in the mean settling velocities, the dispersive time scales and the structural arrangement of particles in the sedimenting suspension at different concentrations, indicating a gradual transition from a dilute to a dense regime. Second, we observe the vertical propagation of kinematic waves as fluctuations in the local horizontally-averaged concentration of the sedimenting suspension.

Read the publication here.