The TANGO mission represents a significant leap forward in the global monitoring of greenhouse gas emission plumes, particularly CO2 and NO2. Comprising two satellites, TANGO-Carbon and TANGO-Nitro, the mission is designed to operate from 2027 to 2031, offering unprecedented capabilities through stereo (temporally separated) and high spatial res- olution emission plume images. These capabilities create a unique framework that facilitates the development of novel estimation methods for emission rates of greenhouse gas emitters. Conventional methods for estimating emission rates of CO2 and NO2 rely on combining gas concentration measurements with wind velocity fields derived from meteorological data, often introducing substantial uncertainties due to the inherent inaccuracies in wind velocity estimation. To address this challenge, this research explores alternative methods enabled by TANGO’s innovative framework, which allows for the direct measurement of emission plume velocity from concentration data. By eliminating the dependency on uncertain me- teorological inputs, these methods promise to reduce the uncertainty in emission rate estimates.
Through detailed simulations and analyses, this research demonstrates that the TANGO mission can effectively establish a framework for directly measuring emission plume velocities. By simulating the data products of the TANGO satellites using Large-Eddy Simulations and applying advanced methods such as traditional Correlation Image Velocimetry (CIV) and Computer Vision Correlation Image Velocimetry (CVision-CIV), wind velocity fields were successfully estimated from plume imagery. These estimates were found to be of promising precision across a range of conditions, including varying wind velocities, emission rates of greenhouse gasses, and levels of measurement noise in the simulations. Results revealed that the CVision-CIV method outperforms the traditional CIV method, especially in scenarios with low signal-to-noise ratios. Wind velocity fields directly estimated from plume imagery were implemented in the Cross-sectional Flux Method to estimate CO2 emission rates. The emission rate estimates indicate that direct plume velocity measure- ments provide a more accurate estimate of emission source rates than conventional methods, which rely on indirect wind velocity estimates from meteorological data. The use of wind velocity fields obtained through the CVision-CIV method resulted in CO2 emission rate estimates with ±20% accuracy in most scenarios, particularly under optimal SNR conditions. Additionally, the study highlights the impact of mission parameters such as image resolu- tion and measurement noise on the accuracy of wind velocity estimations. It was found that higher image resolution and longer time intervals between measurements enhance the precision of wind velocity field estimates by reducing the relative effects of measurement noise.
In summary, this research demonstrates that direct estimation of wind velocities from emission plume imagery, as enabled by the TANGO mission’s advanced capabilities, can accurately be performed and significantly enhance the accuracy of emission rate estimates. The improved wind velocity estimation methods proposed in this thesis offer a promising advancement in remote sensing techniques for greenhouse gas monitoring.

This thesis presents a novel 3D tracking system for studying microorganism locomotion in complex fluid environments, addressing the limitations of current 2D and 3D techniques . The multi-camera microscopy system developed allows for high-resolution tracking of microswimmers like Chlamydomonas reinhardtii, providing new insights into their helical swimming patterns and interactions. The work includes the study of pairwise interactions and the impact of non-Newtonian fluids on motility, revealing key differences in swimming behavior and fluid dynamics in different media type.

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Fluid flows such as rowing and animal flight involve both acceleration and vortex shedding. Some of such flows also involve complex fluids, as in blood flow from the left atrium into the left ventricle in a human heart. Among the complexities of such fluids, we are interested in the viscoelastic nature. The effect of acceleration in vortex dynamics and forces has been studied for a long time. However, there have been discrepancies in the origin of the unsteady forces and modelling them. It is still not very well understood how unsteady forces scale in laminar, highly separated flows? On the other hand, the presence of viscoelasticity alters the vortex dynamics and hence the forces. It has been well studied how the viscoelasticity may affect steady state forces. However, the change in vortex dynamics and forces due to presence of viscoelasticity during acceleration is not well understood. This experimental study aims to add to the literature on unsteady force modelling, effect of viscoelasticity in unsteady forces and vortex dynamics.

To begin with, shear and extensional rheological experiments were performed to identify a weakly elastic fluid. An experimental setup was built with a linear traverse, PIV system and force sensor. The experiments were performed with an accelerating flat plate (aspect ratio of two), in the identified viscoelastic fluid and also in a viscosity matched Newtonian fluid for one-one comparison. To understand the vortex dynamics, we use FTLE fields, Lamb-Oseen model, and Q-criteria. We quantify the vortex formation time and other properties of a vortex ring.

Overall, we observe that both the vortex growth rate and the decay rate are enhanced by the presence of viscoelasticity. We extend the idea of optimal vortex formation to two time-scales instead of one. One, when the plate no longer provides energy and the other, when the vortex is filament free without any more addition of coherent fluid parcels. Furthermore, a limit for optimal vortex formation is proposed to indicate a completely different type of vortex dynamics at lower accelerations. The drag reduction and enhancement due to the presence of viscoelasticity qualitatively agrees well with the trend in literature. In terms of unsteady forces, we try to interpret an equivalent mass for potential flow's added mass in separated Newtonian flows. We propose a model using wake's mass, and it reasonably agrees with our experimental results in Newtonian cases. We use FTLE ridges and vortex-frame streamlines to estimate wake mass, along with a literature driven model for third dimension. Furthermore, we also propose that the added mass in separated flows is time varying and eventually reaches a constant value. Moreover, the effect of viscoelasticity is primarily observed in unsteady forces for acceleration less than optimal vortex formation limit.

Considering the abundance of viscoelastic fluids in nature, growing attention has been received for the study of microorganisms in viscoelastic fluids. In Newtonian fluids, the flow of microorganisms is governed by the viscosity alone. But in viscoelastic fluids, the addition of the elastic component complicates the situation. The flow of the organism is now dictated by a combination of the two forces- viscous and elastic. Understanding the behaviour of these organisms is not only important because of the presence of such environments in nature, but also because of the potential pharmaceutical applications that such studies might lead to.
This thesis focuses on the experimental study of the swimming of Chlamydomonas reinhardtii, a model motile swimmer that swims at low Reynolds numbers. The main objective of the thesis is to characterize these differences in motility and hydrodynamic interactions of these cells in Newtonian and viscoelastic fluids of varying viscosities. This is accomplished by observing the motion of a dilute suspension of Chlamydomonas reinhardtii in 3D using four cameras. The cells are subsequently tracked using an in-house 3D particle tracking code that incorporates a recursive divide and conquer strategy to reconstruct the trajectories.
The experiments showed a drop in velocity in viscoelastic fluids as compared to its Newtonian counterparts, validating the results from existing literature. The cells also maintained their helical motion previously observed in TRIS, although with a drop in radius and pitch in viscoelastic fluids. The ratio of the radius to the pitch, however, remained constant for all the cases, indicating a tendency to retain its overall motion.
The study of cell-wall interactions is of prime importance because of the presence of confined surfaces encountered in nature. The concentration profile of the cells was observed to be similar in all the fluids, showing a non-uniform distribution with large concentration at the boundaries. The overall trajectories near the wall in TRIS revealed that the cells tend to arrive at steep angles in the contact region and leave at shallow angles, a phenomenon termed as asymmetric reflection. When the viscosity increased, however, this behaviour became less apparent as the incoming angles became less steep, while the outgoing angles remained shallow. For the viscoelastic case, the behaviour appears to become more symmetric with increase in viscoelasticity. Additionally, the data also points to a less steep drop in velocity in the contact region in solutions of higher viscosity as compared to the less viscous solutions. Though this might indicate a greater influence of hydrodynamics near the wall for more viscous fluids, more data is required to observe this behaviour deeply.
The obtained results show a clear effect of viscoelasticity on the motility and kinematics of the cells, confirming results and predictions from existing literature. For cell wall interactions, differences are certainly discerned, not only for the viscoelastic cases but also for the more viscous cases. More experiments on these fluids are recommended to give a clear indication of the change in wall interactions in fluids of higher viscosity and viscoelasticity.