Steady Bi-dimensional Crossflow Plasma Jets in Turbulent Channel Flows

In this study, the possibility of reducing the friction drag exerted by turbulent flows by means of wall-mounted plasma actuators is experimentally investigated. Two large plasma actuators (PAs) arrays were operated in a channel-flow facility. They were conceived to replicate, the flow control approach investigated by Mahfoze and Laizet (Int J Heat Fluid Flow 66:83–94, 2017) by means of numerical simulations. Namely, steady and relatively largely spaced (378 wall units) actuators were lain down such to induce stationary crossflow-directed fluid motions. Different actuation parameters (actuators’ configurations and supplied voltages) and flow Reynolds numbers were tested. Flow static pressure measurements were performed along with the actuators mechanical and electrical characterization. The resulting values of drag manipulation and actuation efficiency are reported. The tested flow actuation led to overall higher values of flow friction drag, whereas values overcoming the value of 30% of drag reduction were measured at the more downstream actuation positions. The discrepancy with the above reference is deemed to be mainly due to the finite flow actuation hereby considered. Nevertheless, a slightly different Reynolds number was here considered while the actuators effect was measured to be considerably weaker

Characterization of very-large-scale motions in high-Re pipe flows

Very-large-scale structures in pipe flows are characterized using an extended Proper Orthogonal Decomposition (POD)-based estimation. Synchronized non-time-resolved Particle Image Velocimetry (PIV) and time-resolved, multi-point hot-wire measurements are integrated for the estimation of turbulent structures in a pipe flow at friction Reynolds numbers of 9500 and 20000. This technique enhances the temporal resolution of PIV, thus providing a time-resolved description of the dynamics of the large-scale motions. The experiments are carried out in the CICLoPE facility. A novel criterion for the statistical characterization of the large-scale motions is introduced, based on the time-resolved dynamically-estimated POD time coefficients. It is shown that high-momentum events are less persistent than low-momentum events, and tend to occur closer to the wall. These differences are further enhanced with increasing Reynolds number

On durable materials for dielectric-barrier discharge plasma actuators

In the current experimental investigation various electrode and dielectric materials for dielectric-barrier discharge plasma actuators have been studied in quiescent air under consideration of actuator degradation during long-term operation. The performance variation of the different actuators was initially monitored via alteration of the electrical power consumption P̄a during 6-hour continuous operation. While some material combinations led to premature failure, certain dielectrics such as quartz-glass and aluminum oxide maintained constant performance. The latter was selected for screen-printing of electrodes, so as to obtain reproducible actuator geometries. These actuators were deployed in 10-hour continuous operation. Besides P̄a, the cold capacitance C0 was tracked for each actuator, in order to assess the degradation process of the actuator. Among the tested metals for the screen-printed electrodes, copper showed the best endurance characteristics and, thus, is recommended for both comparable laboratory experiments and durability in AFC application. Admixtures of platinum in the electrode material are to be avoided because of heavy oxidation under ozone exposition.The quantitative outcomes supported by the P̄a and C0 measurements were qualitatively supported by visual inspection of the actuators and of the discharge light emission. On a final note, the screen-printed copperaluminum-oxide actuator configuration, featuring both good durability and reproducibility, is a recommended combination