Browsing by Author "Van Buren, Tyler"
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Item Comparing steady and unsteady rectangular jets issuing into a crossflow(Journal of Fluid Mechanics, 2022-05-31) Tricouros, Frank A.; Amitay, Michael; Van Buren, TylerThe foundational differences of steady and unsteady jets issued into a laminar boundary layer crossflow are considered. Jets have been used widely for flow control applications, due to their ability to enhance mixing and mitigate separation, but it is unclear what role jet steadiness plays in flow control effectiveness. Here we compare experimentally unsteady (synthetic) and steady rectangular jets issued into a flat-plate laminar boundary layer with varying orifice pitch and skew. The coherent streamwise vortices produced by unsteady jets were shown to be much stronger than those produced by steady jets, despite producing similar flow patterns. These differences are rooted in how vorticity is generated in the orifice, through either a Stokes layer (unsteady) or a Blasius boundary layer (steady). Exploring the time- and phase-averaged vorticity transport equation reveals that the time-varying vorticity term is the reason for the enhanced vortical structure. When considering flow control metrics, we find that the unsteady jet produced greater added momentum in the boundary layer and added vorticity when compared to a momentum-matched steady jet. Both the steady and unsteady jets produced similar jet penetration characteristics.Item Propulsive performance of oscillating plates with time-periodic flexibility(Journal of Fluid Mechanics, 2023-03-22) Yudin, David; Floryan, Daniel; Van Buren, TylerWe use small-amplitude inviscid theory to study the swimming performance of a flexible flapping plate with time-varying flexibility. The stiffness of the plate oscillates at twice the frequency of the kinematics in order to maintain a symmetric motion. Plates with constant and time-periodic stiffness are compared over a range of mean plate stiffnesses, oscillating stiffness amplitudes and oscillating stiffness phases for isolated heaving, isolated pitching and combined leading-edge kinematics. We find that there is a profound impact of oscillating stiffness on the thrust, with a lesser impact on propulsive efficiency. Thrust improvements of up to 35 % relative to a constant-stiffness plate are observed. For large enough frequencies and amplitudes of the stiffness oscillation, instabilities emerge. The unstable regions may confer enhanced propulsive performance; this hypothesis must be verified via experiments or nonlinear simulations.