Fluid Mechanics and Waves Seminar - Spring 2019

 
Fish and birds moving in groups are thought to benefit from hydrodynamic or aerodynamic interactions between individuals, yet the role of these interactions is not fully understood. To better understand these effects, we devise a robotic `school' of flapping swimmers whose formations and motions come about from flow interactions. Surprisingly, we find that the flows naturally generated during swimming can also prevent collisions and separations, allowing even uncoordinated individuals with different flapping motions to travel together. Other benefits include freeloading by a `lazy' follower who keeps up with a faster-flapping leader by surfing on its wake. We formulate a reduced-order model which produces remarkable agreement with all experimentally observed modes by relating the follower’s thrust to its flapping speed relative to the wake flow.

The plasmas arising in astrophysics and fusion experiments are frequently far from local thermodynamic equilibrium. This fact prevents the use of fluid equations and necessitates so-called kinetic partial differential equations. In general, these equations are 6-dimensional (plus time), which has historically motivated particle-based methods because of their favorable scaling with dimension. By far the most widely used the the particle-in-cell (PIC) method. We show, however, that PIC still suffers from the curse of dimensionality due to the presence of a spatial grid. We propose the use of the sparse grid combination technique in concert with PIC to rectify this circumstance. We show that the use of sparse grids together with particle methods yields tremendous benefits for statistical resolution. Finally, we present recent work leveraging our new understanding of sparse grids for high-order finite volume schemes. Results from numerical experiments are presented that demonstrate the effectiveness of the schemes along with opportunities for improvement.

Cells, the building blocks of eukaryotic life, move. This is what enables single cells to divide, groups of cells to organize into tissues, and tissues to become organisms as complex as ourselves. On cellular scales this motion is actuated by the cytoskeleton, a collection of dynamic polymer filaments and molecular scale motors that convert chemical energy into mechanical work. Much progress has been made in identifying the key molecular players in cytoskeletal assemblies, yet how they self-organize into the complex cellular scale machinery that underlies life remains largely unknown. In this talk, I will discuss our recent progress in deciphering the physics of the cytoskeleton. I will discuss our advances in the context of the physics of the mitotic spindle, which moves chromosomes during cell division, and the cell cortex, which actuates cellular deformations. Finally I will give a perspective on our advances towards designing similar systems in the lab.

Flow can be used as a microstructural designer to provide soft materials and chemicals with specific functionalities. Likewise, these materials are often harnessed in continuous operations and the effect of flow on their microstructure and functionality is a key-step towards materials development, environmental remediation and soft robotics realization. Here, I will share my research experience in exploiting various flow conditions and physico-chemical interactions to design new soft materials with tailored properties and understand how to control these parameters to achieve a unique realization in many applications.

The study of drop coalescence is of fundamental importance in understanding different natural and industrial processes such as rain drop formation in clouds, emulsion stability, ink-jet printing, viscous sintering, and oil desalting. When two liquid drops come into contact, a topological transformation takes place. A microscopic liquid bridge forms between them and rapidly expands until the two drops merge into a single bigger drop. A typical sequence of the process is shown Figure 1. Numerous studies have been devoted to the investigation of the coalescence singularity in the case where the drops coalesce in a medium of negligible vapor pressure such as vacuum or air. However, coalescence of liquid drops may also take place in a medium of relatively high vapor pressure (condensable vapor phase), where the effect of the surrounding vapor phase should not be neglected, such as the merging of drops in clouds. Lattice Boltzmann (LB) numerical simulations of the dynamics of two liquid drops coalescing in a non-condensable gas and in a condensable vapor phase will be presented. Attention is paid to the effects of initial conditions and the vapor pressure of the surrounding medium on the formation and growth dynamics of the liquid bridge. The results show that the onset of the coalescence occurs earlier and the expansion of the bridge initially proceeds faster when the coalescence takes place in a saturated vapor compared to the coalescence in a non-condensable gas. The initially faster evolution of the coalescence process in the saturated vapor is caused by the vapor transport through condensation during the early stages of the coalescence.

Figure 1: Sequence of the coalescence process of two droplets

To stimulate the production of oil and gas from unconventional hydrocarbon reservoirs, it is customary to use hydraulic stimulation. By injecting fluids into the formation at high pressures, one induces fractures in the rock that create highly conductive paths to the wellbore. Granular materials are generally mixed with the fluids to keep the fracture open after the stimulation. These materials are called “proppant”. One single well requires thousands of tons of proppant. The well production depends very much on the placement and stability of the proppant. In this talk, I will review a number of challenges faced by the industry related to the phenomenology of granular materials during hydraulic stimulation. I will present experimental and simulation results intended to model proppant transport. Some of these results show that earlier studies in the area may have underestimated the importance of a realistic scaling of the experimental cell, injection perforations and fluid flow rate.