In this thesis the dynamics of bubbles and drops dispersed in a turbulent Taylor-Couette (TC) flow is studied.
‘The work led to a variety of efficient and versatile numerical tools,’ Vamsi Spandan Arza says. ‘Numerical simulations give in depth understanding of phenomena normally too complex for experimentation. The models and theoretical insights can be used for a range of application areas.’
Vamsi started by understanding the influence of small (sub-Kolmogorov) spherical bubbles, on the overall drag in TC flow. ‘It was observed before that bubbles reduce drag,’ Vamsi says. ‘This can be observed by the torque of the cylinder system falling down considerably.’
This efficiency gain was an inspirational thrive for Vamsi’s research. For example in pipe flows and in maritime applications further simulations and theory building seem promising.
Within the Physics of Fluids (PoF) Group three PhD’s work on TC research and experiments involving bubbles. ‘The collaboration within the Group is a fruitful one,’ Vamsi shares. ‘The experimentalists provide us with data; we help them with modelling and provide inspiring - sometimes quite contra-intuitive - scenarios in complex TC flows.'
As a strategy, during his PhD work, Vamsi Spandam Arza first focused on small bubbles, then on deformable bubbles and complex shaped bubbles successively.
‘One of the first complex additions to the numerical model was the implementation of a two-way coupling between the carrier fluid and the dispersed bubbles,‘ he says. ‘This coupling is crucial to understand the influence of the bubbles on the coherent flow structures that arise in TC flows.’
It appeared that stronger buoyancy in comparison to the centripetal forces was effective in reducing drag (low Froude numbers).
Then deformation dynamics were introduced in the form of a phenomenological sub-grid model.
‘The time evolution of the shape tensor depends strongly not only on the local strain-rate, vorticity in the flow and capillary number, but also on its relative spatial position. Additionally, the viscosity ratio is an important parameter to determine the overall shape of the drops,’ Vamsi concluded.
He further improved the numerical model, in order to account for momentum exchange between the carrier fluid and the dispersed ellipsoidal phase. Also the influence of deformable ellipsoidal bubbles on a carrier turbulent flow, was studied. ‘We found that preferential accumulation of deformed bubbles in TC flow leads to increase in drag reduction effects,’ thus Vamsi.
In more complex simulations which involved finite size bubbles, Vamsi reached ‘almost a 100 times increase in the complexity’ as compared to current solvers.
‘Interestingly enough we found that the drag reduction increases with increasing bubble deformability but it is not related to any preferential accumulation by the bubbles,’ he says. ‘We have taken a step forward in understanding the underlying physical mechanisms and relevant parameters behind drag reduction induced by bubbles and drops in TC flow.’
The numerical techniques and algorithms developed in this thesis can easily be extended to other flow systems and can also be combined with each other, to explore flow regimes that have never been studied before.
For example, the reception of his work at the Surf-Sara Visualization Challenge 2016 was great. ‘We worked on this complex flow solver in collaboration with the University of Rome,’ he says. ‘New insights were gained on how the functioning of the valves influences the flow structures within heart chambers.’
‘The surgeons – providing us with omnifarious data – were happy with the numerical simulations and visualizations we were able to offer them. Using our novel fluid-structure interaction algorithms, we performed simulations of blood flows inside the left ventricle of the human heart with prosthetic mechanical mitral valves. We studied the flow behaviour for different valve types and configurations, in order to understand its effect on the overall flow dynamics inside a human heart.’
Vamsi found the programming part of his PhD work challenging.
‘I had no previous experiences on this type of work, and little reference in literature could be found on the topic,’ he says. ‘Along the way quite different approaches passed, each of them taking quite some time and effort to construct, program and test. As, in simulations, one is not able to test the phenomena in the real world, it takes quite some time to be able to see what actually happens within the numerical methods. On the other hand, it is even the more rewarding if surprising and valuable results occur.’
Vamsi plans to stay into academic research after his PhD Defense.
‘The freedom of thinking and performing research is very important to me,’ so Vamsi. ‘In a post-doc follow-up I strive to extend the insights towards different problems and application areas, for example in flexible filaments and membranes.’
‘Above that I want to take my research to a next level, including mass transfer into the analyses. I am well-informed in this research topic already, and I have built up quite some experience in working with the models and analyses. So the chances of reaching results are good. Hereby, it is a big advantage to collaborate and exchange ideas with Mesa+ colleagues. One can always learn something new.’