| dc.description.abstracteng | Clouds play an important yet poorly understood role in weather forecasting and climate
change. The objective in the present work is to establish a laboratory-scale experiment
for simulating clouds in the Earth's atmosphere. The experiments are conducted in a
moist Rayleigh-B enard convection (RBC) system. In this thesis we investigate three
di erent problems associated with moist convection.
In the rst problem, we investigate the e ects of phase change on the Rayleigh Taylor
instability (RTI) in a thin lm. We use Sulphur Hexa
ouride (SF6) as the working
uid
at conditions where SF6 exits in both liquid and vapor phases. We report on the
patterns formed at the cold top plate due to the condensation of moist SF6 from the
bottom plate. We observe two di erent regimes in this experiment. In regime 1, the
bottom plate was covered with a layer of liquid SF6. We show that the condensed liquid
layer at the top plate forms hexagonal patterns if the imposed temperature di erence
is su ciently large. These patterns drip periodically into the liquid pool at the bottom
plate. In regime 2, we eliminate the liquid SF6 layer on the bottom plate by adjusting
the pressure in the convection cell. We show that the liquid layer at the top plate is
stable if the evaporative e ects below the liquid layer is su ciently large. We show that
under appropriate conditions, the liquid layer at the top plate form hexagonal surface
pattern with nearly no dripping.
In the second problem, we report results from a moist convecting cloud chamber with
a SF6-Helium binary mixture as the working
uid, where SF6 models the moist component
in the Earth's atmosphere (water vapor), and He models the dry components
(Nitrogen, Oxygen etc.). We observe that under appropriate conditions, micro-droplets
nucleate in the wake of a large cold drop falling through a supersaturated SF6-He atmosphere.
We show that the micro-droplets are formed in the cold wake of the large drop
through homogeneous nucleation. We extend our results to the atmospheric clouds,
and our model calculations suggest that under supersaturated conditions, falling hailstones/
graupel and large rain drops may signi cantly enhance the nucleation of cloud
droplets in their wake. We also show that under appropriate conditions a stable horizontal
layer of cloud micro-droplets was established in the convection chamber. The
layer was formed between the supersaturated and sub-saturated volumes in the chamber. In the third problem, we examine the possibility of a novel secondary ice nucleation
mechanism in deep convective clouds. These experiments are inspired by the wake
nucleation experiments in the SF6-He binary mixture. The experiment is conducted
in a cloud chamber using a mixture of air and water vapor as the working
uid. In
this experiment, we investigate the heterogeneous nucleation of micro-droplets and ice
crystals in the wake of a warm drop. We show that the evaporative supersaturation
attained in the wake of the warm drop was su cient to activate water droplets and ice
nuclei. We model the
ow eld behind the warm drop and use that to calculate the
growth of a droplet from a nucleus to an activated droplet behind the drop. We extend
this model to atmospheric clouds and conduct a detailed study on various parameters
that a ects the activation of water droplets and ice crystals. Our analysis shows that in
the wake of a hailstone/graupel in the wet growth regime, the ice crystal concentration
increases from 1 per liter to 5 per liter at a temperature of 15 C. This may partly
explains the enhanced ice concentrations observed in deep convective systems. Based
on these results we propose a new technique for cloud engineering.
We also conduct a preliminary investigation of three additional problems. First, we
examine the role of humidity in the fragmentation of drops during free fall conditions.
Our observations suggest that in a supersaturated environment, the critical Weber number
for a drop to become unstable may increase. Similarly, in a sub-saturated environment
the critical Webber number may decrease. Second, we examine the dynamics
of chimney formation in Leidenfrost drops of various sizes. We observe that the number
of chimneys in a drop increases with the size of the drop. Above a critical size,
the chimneys also grow in size by merging with other chimneys. Third, we propose a
new technique for visualizing the
ow structure of the di usive wall layer in RBC using
Helium bubbles. | de |