[PhD Thesis Presentation] ‐ Ms. Noa Burstein "Flow Instabilities and Vortex Dynamics in Intersecting Flow"

Presenter: Ms. Noa Burstein

Supervisor: Prof. Amy Shen

Unit:  Micro/Bio/Nanofluidics Unit

Title:  Flow Instabilities and Vortex Dynamics in Intersecting Flow

Abstract: 

Inertial and elastic flow instabilities are initiated once critical conditions are met, often resulting in symmetry breaking of the flow field and in the formation of vortices. Predicting and controlling vortex formation and intensity are important for numerous industrial applications (i.e. engineering of bridges, pipelines, airplanes) and in smaller confined environments (i.e. around bends, obstacles and at intersections). However, due to the intermittent character of vortices, it is challenging to create and measure them under uniform and controlled conditions. The aims of this thesis are to experimentally study the effects of confinement and fluid properties on the onset of flow instabilities and vortex dynamics, under controlled and uniform conditions. For this purpose, stationary and stable vortices are formed within microfluidic intersections, imposing different levels of flow confinement as determined by their aspect ratio (α=d/w where d and w are the depth and width of the geometry, respectively). The channels configuration, allow measurements across streamwise planes at high spatial and temporal resolution, using a micro-particle imaging velocimetry (μ-PIV) system. By precisely controlling the Reynolds number (Re= ρUw/η, where U is the average flow velocity, ρ is the density and η is the viscosity of the fluid), symmetry breaking of 4-cells of Dean vortices is initiated at a low-moderate critical value (Rec), resulting in the merging of two co-rotating vortices into a single steady vortex. Similarly, by reducing the Re to Rec* the splitting of a single vortex is induced. The results indicate that the transition type depends on confinement, changing from subcritical to supercritical as α is increased. Merging and splitting of vortices are found to be exponential processes, with a rate that depends on the imposed Re. When imposing Re >> Rec intensification of the steady streamwise vortex occur, until a second critical value (Reus) is met. Above Reus, the flow field becomes unsteady as periodic fluctuations emerge. Additionally, we studied the effect of slight modifications of elasticity on the fluid by the addition of small quantities of flexible polymers. Our results show that polymer additive destabilizes the flow and symmetry breaking occurs at lower Rec. We also find that the stretched polymer’s torque acts counter to the vorticity, reducing vortex intensity. These experiments capture processes that govern turbulent flows and provide important insights into the mechanisms of turbulent drag reduction by polymers, used for reducing energy losses in pipe flows. The conclusions of this thesis are of general importance in fluid dynamics, offering new insights in the field of flow instabilities and vortex dynamics with an unprecedented degree of control.