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Moist Rayleigh Benard Convection

dc.contributor.advisorBodenschatz, Eberhard Prof. Dr.
dc.contributor.authorPrabhakaran, Prasanth
dc.titleMoist Rayleigh Benard Convectionde
dc.contributor.refereeBodenschatz, Eberhard Prof. Dr.
dc.subject.gokPhysik (PPN621336750)de
dc.description.abstractengClouds 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
dc.contributor.coRefereeTilgner, Andreas Prof. Dr.
dc.subject.engsecondary nucleationde
dc.subject.engpattern formationde
dc.subject.engLeidenfrost effectde
dc.subject.engflow visualizationde
dc.subject.engdrop fragmentationde
dc.affiliation.instituteFakultät für Physikde

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