A Study of Flow Separation in Micro and Milli-Channels

Abstract

Flow separation is detrimental in aerodynamics and is the focus of many studies that aim to delay or prevent it using active flow control methods. In this thesis, flow separation was induced in micro and milli-scale channels. Results show the prevalence of characteristic regimes for flows over cylindrical features, including attached flow, separated flow, the formation of a separation bubble/recirculation zone, growth of this recirculation zone, followed by flow unsteadiness and a von K´arm´an vortex street, as the flow Reynolds number is increased. Channel parameters, such as the shape of the separation-inducing feature and the confinement of the channel, defined as the cross sectional area for the flow to travel in a channel, affect the flow by changing the effects of wall shear and boundary layers on the flow passing over these features. For flow over a backwards facing step, a novel phenomenon was observed: the free shear layer that forms after flow separates over the step edge reattaches to the vertical wall of the step instead of the channel wall for lower flow Reynolds numbers, an occurrence that is not seen in macro-scale backward-facing step experiments or any of the current literature on the topic. Finally, flow over a cylinder in 3D-printed milli-channels shows helical streamlines, similar to those seen in the production of Dean vortices. Such motion increases with Reynolds number, which influences the dynamics of flows in confined micro and milli-scale systems. Overall, the study of flow separation in small scale microfluidic and 3D printed channels presents an interesting problem to study the effects of boundary layer and confinement on flow separation.