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This work details the application of dielectric barrier discharge (DBD) actuators to the control of transition in subcritical Reynolds number (250 ≤ Re ≤ 1000) circular pipe flows for the purposes of increasing mixing or momentum and heat transfer. Primary flow measurements were made using a single hot-wire anemometer and these were augmented using smoke filament visualization with high-speed photography. A sensitive balance was used to calibrate the body force generated by the actuator as a function of input power. Several distinct actuator configurations were considered and a downselection indicated that the introduction of swirl produced the largest coherent oscillations and purely turbulent fluctuations. Using the swirl actuator, a detailed parametric study was conducted where operation parameters such as duty cycle, input power, frequency, momentum, etc. were systematically varied. The largest coherent oscillations occurred at a reduced frequency of approximately 0.08 while peak noncoherent disturbances occurred around 0.34. High-speed flow visualization indicated a complex three-dimensional flow structure within the actuated flow regime, with regions of localized reverse flow or vortex breakdown. Several diameters downstream, however, the flow became essentially axi-symmetric in a phase-averaged sense and a fairly simple theoretical model could be used to describe the basic flow mechanisms.

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