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.

In an effort to make water electrolysis more efficient and simple, flow-through electrolyzers eliminate the need for a gas-separating membrane between the electrodes. However, replacing the membrane with a forced flow field brings new challenges and considerations. In this research, the design of a flow-through electrolyzer is optimized for minimum voltage losses due to bubble formation, pressure drop, and electrolytic resistance. These parameters are all influenced by the potassium hydroxide concentration, the inter-electrode gap and the feed flow rate. The effect on the power dissipation of a flow-through electrolyzer is analytically modelled and validated using experiments, where the design variables are varied. Additionally, the effect on the resistance in the electrolyte of keeping the gasses dissolved in the electrolyte is studied using IV curves of degassed electrolyte. The gasses in the electrolyte were purged with a vacuum chamber. This degassed electrolyte showed a reduction in overpotential of 150 mV at max. However, this reduction in overpotential was outweighed by the energy requirements to degas the electrolyte. Besides degassing, the effect of suppressing bubble formation by different flow rates was investigated. It was found that there was no noticeable reduction in overpotential between the state where bubbles are suppressed and bubbles were formed. In addition to the effect of keeping the gasses dissolved, an analytical model was constructed to describe the necessary flow rates to mitigate the electrical resistance due to bubbles in the electrolyte. In the analytical model, solubility plays a big role in determining the necessary flow rate, as solubility is related to the emergence of bubbles. Contrarily, experiments showed the effect of solubility was found to be rather low. By varying the gap width, it was shown that the shear rate at the wall is a better indicator for bubble removal and therefore reduction in resistance due to bubbles. Furthermore, the shape of the discharge channel was changed to promote uniform flow across the electrodes. The uniform flow should make the product removal at every part of the electrolyzer equal, such that there are no stagnant zones where gas accumulation could build up. This new discharge channel shape was analysed using COMSOL Multiphysics by comparing it with a conventional straight discharge channel. The variable discharge channel outperformed the straight discharge channel in creating a uniform flow across the electrodes for Euler numbers bigger than 10, meaning that the inertial forces are negligible compared to the pressure drop. During experiments, the electrolyzer with a variable discharge channel was tested. This electrolyzer configuration did not perform as well as expected from the theory and simulations, having a higher pressure drop and electric resistance than the conventional electrolyzer. The reason why the variable discharge channel performed poorly was inconclusive. Lastly, the performance of the various variables was evaluated using the total power dissipation as a function of the current density. This took both the pressure drop across the system and the electrical power consumption of the electrolyzer into account. It was found that using a high electrolyte of 6M potassium hydroxide (KOH) together with a small inter-electrode gap gave the lowest energy dissipation per kg produced hydrogen gas. However, increasing the KOH concentration increases the viscosity and thereby the pressure drop. The theoretical optimum for KOH concentration was calculated to be 5M for this flow-through electrolyzer.

This research work presents the observations of the updated quasi-steady aerodynamic model of the Atalanta project, a flapping wing Micro Air Vehicle (MAV) project, for additional velocity conditions. The main focus of this research work is to update the existing quasi-steady model of Q.Wang developed for hovering conditions since its performance under additional velocity conditions remains unexplored. This work thus examines the influence of the additional velocities on the computed aerodynamic loads such as lift, drag, and also the influence on the passive pitching motion. To incorporate the additional velocity into the quasi-steady model, the resultant translational velocity is computed by the vector addition of the kinematic velocity of the wing and the additional velocity. This is done by transforming the additional velocity in the inertial frame of reference to the co-rotating frame of reference, followed by the proper vector addition of the translational velocity components to compute the resultant translational velocity to be used in the aerodynamic load calculations. Observations reveal that the generated aerodynamic lift and drag, and the passive pitching motion vary depending on the orientation and the magnitude of the additional velocity with respect to the kinematic motion of the wing, and that the significance of influence of the additional velocity magnitude depends on the flapping frequency and the elastic hinge stiffness. The results observed are analyzed to understand the influence of various additional velocity conditions on the computed aerodynamic loads and the passive pitching motion. The updated model provides valuable insights into the behaviour of lift, drag, and passive pitching motion under various additional velocity conditions which can be used as a basis for approaching the solution of forward flight. These observations contribute to a better understanding of the aerodynamic quasi-steady model of the Atalanta project and pave the way for future research and experimental validation of the model.

In recent times, soft matter has gained significant interest among researchers in the fields of biomechanics and biomedicine, especially in areas like soft robotics and biopolymers, due to its remarkable ability to undergo substantial deformations. Soft robotics often requires materials that can flexibly adapt to or mimic the movements of living organisms, requiring properties of flexibility and easy deformability. Biopolymers, naturally occurring in the human body, such as within brain tissue or blood clots, have gained attention due to their tendency to exhibit extensive deformation even under minimal loads. Consequently, there has been a growing emphasis on investigating stress-strain responses associated with these materials in recent years. This research particularly centers on a specific deformation phenomenon known as the Poynting effect. The Poynting effect is related to the transverse stress or strain response when subjected to simple shear, revealing that the application of simple shear strain does not solely result in simple shear stress. This intriguing phenomenon captures our attention, primarily because it challenges intuitive expectations. Within the scope of this thesis, we employ two distinct deformation gradient tensors to analyze stress-strain responses under two separate boundary conditions: constant gap and constant normal stress boundary conditions. We also introduce a methodology for predicting the sign of the Poynting effect under conditions of small yet finite strain. Finally, we validate our analysis through simulation experiments. We have modified Meng's original network-theory-based model, which is rooted in an energy density function derived from the force-extension relationship of a single chain. Our objective was to create a model with nearly incompressible properties in order to investigate the impact of compressibility. To determine the direction of the Poynting effect, we directly computed the stress and strain responses of a cube under shear forces. Later, we developed a method for predicting the direction of the Poynting effect without the need for precise stress and strain calculations. Our results demonstrate the successfulness of this prediction method. The simulation outcomes reveal a closer alignment of the four-variable tensor, suggesting that our chosen boundary conditions more closely resemble those used in numerical solutions. Additionally, it is worth noting that the specific geometries we employed in our study, namely the cylinder and cube, did not have a discernible influence on determining the direction of the Poynting effect, especially within the context of the selected model and material parameters.

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.