Session W11: General Fluid Dynamics: Rotating Flows (10:00am - 10:45am CST)

A Fluid Mechanic's Analysis of the Tea-Cup Singularity

In 1926 Einstein published a short paper explaining the meandering of rivers. He famously began the paper by discussing the secondary flow generated in a stirred tea cup -- the flow now widely known to be responsible for the collection of tea leaves at the center of a stirred cup of tea. In 2014, Luo and Hou presented detailed numerical evidence of a finite-time singularity in a rotating, incompressible, inviscid flow. The key driving mechanism of that singularity is the secondary tea-cup flow. The present work is not aimed at proving the existence of a singularity in this flow, nor is it aimed at generating more highly resolved numerical evidence for the singularity than already exists. Rather, I will assume that the flow simulated by Luo and Hou genuinely develops a singularity. My goal is to understand, from a fluid-mechanics perspective, why.   

Numerical investigation into the role of fluid dynamics in a novel device for graphene exfoliation.

In this study a novel experimental rig is used to perform liquid phase exfoliation of graphite in order to produce graphene. During this process, graphite particles are exposed to high shear rates, forming few-layer materials through processes such as peeling and fragmentation. The rig is comprised of two concentric cylinders, the inner of which is rotating. Through this design, two distinct flow regimes are present, the well-studied Taylor-Couette flow in the narrow gap between the cylinders, and a rotating film domain within the hollow inner cylinder. Each of these domains is studied using numerical simulations, including LES of the Taylor-Couette flow and DNS of the rotating film. Through this approach, strain maps can be constructed in each case, allowing one to identify the extent and location of exfoliation. Further factors such as the impact of the peristaltic pump and residence time are also taken into account in order to determine the ideal operating points for graphene production.   

Nonlinear wall modes in rapidly rotating Rayleigh-Benard convection

Wall-localised dynamics has seen a recent upsurge in interest in the geophysical fluids community. For example, recent work from condensed-matter physics has highlighted how well-known robust geophysical waves are direct analogues behaviours found in exotic quantum materials; e.g., topological insulators that only conduct current on their boundary. Also, for example, numerous laboratory experiments of thermal convection show definite signatures of wall instabilities, which can confound heat-flux measurements and interact with magnetism when present. This talk will present new simulation results and theoretical understanding of wall-mode convection in a regime without the bulk convection. We speculate that the vigorous wall dynamics are nonlinearly unstable analogues of topologically protected waves. Even so, we find that with sufficient thermal driving, wall modes can undergo secondary instabilities to semi-turbulent puffs that can seed an ostensibly stable interior. Theoretically, we explain how wall instability differs from traditionally understood notions of buoyancy-driven dynamics in a rapidly rotating system.   

Investigations of near wake in flow past rotating cylinders.

Investigations of near wake region of the flow field past rotating cylinders whose axis is aligned with the flow are presented from a computational and experimental point of view. Cases covered include single cylinders and cylinders whose front or the rear segment spins while attached or separated. The time-averaged equations of motion along with the modeled form of transport equations for the turbulence kinetic energy and the scalar form of turbulence dissipation are solved by the use of finite-volume scheme. The energy spectrum is modified to incorporate the effects of swirl and rotation using a parametric characterization of the model coefficients. Experimental investigations from a subsonic wind tunnel cover a data for a range of spin rates and free stream flow conditions for flow past axially aligned cylinders with spinning segments. The results are discussed in the context of projectile design with such free spinning segments in smart munitions to provide effective control, stability and target guidance.   

Characterization of Swirl Number Effect on a Cyclone Vortex Combustion Chamber Flow Field.

An innovative characteristic of the cyclone vortex combustion chamber is the tangential injection of oxidizer near the nozzle that gives rise to a distinct vortical-helical motion over the entire length of the chamber, which reverses its direction at the head wall and forms a core vortex traveling down the chamber before exiting through the nozzle. The primary advantages of the vortex combustion chamber are prolonged fuel residence time, increased fuel/oxidizer mixing length and thermal shielding of chamber walls. It has been determined that the internal flow field of such Cyclone Vortex Chamber is highly swirl driven due to the tangential injection of the oxidizer, also significant dissipation of tangential momentum is undesirable as the stability of the internal flow field is dependent on the coherency of the vortex structure. The effect of Swirl number (i.e ratio of tangential momentum flux to axial momentum flux) has been characterized. It has been observed that varying the swirl number has a significant effect, not only on the magnitude of the normalized velocity but also on its spatial distribution, thus making the swirl number an important design parameter for the cyclonic chamber.