**Multiphase Microphysics Laboratory Tour**

**Work on Bubbles and Drops**
We are interested in the instability and the transition to turbulence in shear
flows in general, and spatially developing flows in the complex geometries,
multiphase flows and interfacial fluid mechanics in particular. We use combination of tools ranging
from basic modeling, computational fluid mechanics, analytical and linear
stability analysis to study the aforementioned areas of fluid mechanics. We
also intend to extend our research to micro- and bio-fluid mechanics.

## **Lattice Boltzmann simulation
using graphics processing unit**

Lattice Boltzmann simulation of multiphase flows using Graphics Processing Unit (GPU)
is one of the main thrust of our reaserch group. Our present GPU flow solver gives a speed-up
of 25 as compared to single CPU!! The power consumption of a GPU is mere 200
Watts. Several GPUs can be linked in a cluster with further speed-up. We are
currently working in this direction in collaboration with Prof. S. P. Vanka
(University of Illinois at Urbana-Champaign, USA). Thus this novel
implementation can offer us tremendous advantages in terms of the number and
size of problems we are planning to solve.

## **Multiphase flows and interfacial fluid mechanics**

Many fascinating and counter-intuitive phenomena in multiphase fluid mechanics
continue to be discovered despite the long fluid mechanics history in the
areas such as viscous fingering, drop deformation and electro-hydrodynamics.
Multiphase systems can be observed in many naturally occurring phenomena such
as rain, snow, avalanches, clouds, underground water flows, sea waves and in
industrial applications such as turbines, oil and gas wells and pipes, nuclear
reactors and plastics processing. A variety of situations can be consider when
two immiscible fluids are separated by an interface having surface tension, e.g.,
thin films, spreading of liquid on solid, and thermocapillary (Marangoni)
effects. The viscosity and density stratifications, effect of chemical
reaction/heat transfer play important role in multiphase (miscible/immiscible)
flows. This has been an area of extreme interest of our group.

## **Spatially developing flows**

The instabilities of the spatially developing laminar flows are shown
to be fundamentally different from flows that do not vary downstream.
The base flow is also quite complicated and needs to be computed very
accurately, given that the stability is very sensitive to the base flow.
We are interested in studying the linear evolution by considering the
non-parallel stability analysis and subsequently nonlinear effect using
direct numerical simulation. From our earlier studies we found that these
flows, in general, are linearly unstable at surprisingly low Reynolds
number. These results motivate us to study the flow through spatially
varying geometries in small-scale.

## **Micro and bio-fluid mechanics**

Fluid dynamics and the role of the walls at small-scale can be very
different from that at large scales. In large scale flows, the
velocity of a liquid immediately adjacent to a solid is taken equal
to that of the solid. This is called the no-slip boundary condition,
which seems to be confirmed in macroscopic experiments. However, it
is difficult to arrive at such a boundary condition using microscopic
models. It has been noticed that, even in the case of simple liquids, the
no-slip boundary condition is not justified on a microscopic level.
Therefore the no-slip boundary condition is not an exact law but a
statement of what may be expected to happen under normal circumstances. At
microscales, apart from slip, spatially varying walls are frequently encountered.
I would like to study these smallscale flows by direct numerical
simulations using the slip boundary condition, and with a long term aim
to develop codes for microflow. As a first step in this direction, we have
studied the instability in diverging channel flow with wall-slip for small
Knudsen number. We have predicted that a different route to chaos, via
linear instability, takes place at small-scales.

The aforementioned slip condition exists not only in the microflows but also in
the arterial flows due to the hydrophobic nature of the arterial walls. Arterial
flows in general are very complicated as the flow is unsteady and non-Newtonian in
some parts of the arteries. The arteries are also
collapsible and interact with the blood. Thus we would like to
include these features in simulations of blood flow in
arteries and veins.