Dielectric-barrier discharge plasma actuators for turbulent friction-drag manipulation via spanwise oscillations

Ein Plasmaaktuator wird über instationäre Betriebsmodi angesteuert, um wandnahe Fluidoszillationen zu erzeugen. Das Ziel ist es, spannweitig oszillierende Wände zugunsten einer Verringerung des turbulenten Reibungswiderstands nachzuahmen. Da der Aktuator keine beweglichen Teile besitzt, könnte er sich als nicht-mechanischer Ersatz der oszillierenden Wand eignen. Die Kombination von Betriebsmodus und zugrundeliegender Elektrodenanordnung ist eine Neuerung, welche die spannweitige Homogenität der Strömung solcher virtuellen Wandoszillationen verbessert. Die mechanische Charakterisierung wird mittels eines planaren Feldmessverfahrens durchgeführt, um sowohl die induzierten Strömungstopologien als auch die Effekte von Volumenkraft und „virtueller Wandgeschwindigkeit“, d.h. Reaktion des Fluids, aufzuzeigen. Daraus wird zur Bewertung und Optimierung der Leistungsfähigkeit des Aktuators ein universelles Diagramm hinsichtlich aktuatorspezifischer Parameter abgeleitet. Da die berechnete Volumenkraft die Art der Kraftausübung gut widerspiegelt, kann diese modellhaft zu verbesserten numerischen Simulationen der Aktuatorik dienen. Ferner wird eine neue Vorgehensweise für die Bestimmung der elektrischen Leistung von Aktuatoren mit mehreren Hochspannungselektroden bereitgestellt, welche die potenzielle Abschätzung des Nettogewinns in aktiven Kontrollszenarien ermöglicht. Zuletzt wird die unmittelbare Auswirkung der oszillatorischen Kraftausübung auf den Reibungswiderstand in der Querebene einer voll entwickelten turbulenten Kanalströmung mittels einer stereoskopischen Feldmesstechnik untersucht. Im Wesentlichen verbleibt die Strömung im sich entwickelnden Stadium und erfährt auf dem Aktuator eine Erhöhung des Reibungswiderstands, während sich dieser stromab des Aktuators verringert

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

On the interplay of body-force distributions and flow speed for dielectric-barrier discharge plasma actuators

The dielectric-barrier discharge plasma actuator is a well-established device commonly operated in boundary-layer airflows for active flow control. In the present experimental investigation, their ability to cause momentum transfer to the surrounding fluid is analyzed by means of spatio-temporal body-force distributions in both quiescent air and external airflow conditions. The work is motivated by the limitation to quiescent-air operating conditions of frequent previous efforts. Available analytical velocity-information-based force derivation approaches are contrasted to investigate the actuator performance under conditions of their area of application. Results of body force in quiescent air, in agreement with literature, confirm the major taken assumption for Navier–Stokes-based body-force formulations—a negligible pressure gradient. However, the previous circumstance turns out as an invalid assumption for plasma actuation encountering an external airflow. These outcomes coincide with the findings in the numerical work of (2015 Numerical investigation of plasma-actuator force-term estimations from flow experiments J. Phys. D: Appl. Phys.48 395203), following the recommendation to apply a vorticity-equation-based approach under such conditions. Furthermore, the shape of the spatio-temporal body-force distribution is observed to undergo changes when the airflow speed increases. On the other hand, the integral force magnitude is found to remain approximately constant. Moreover, the choice of phase resolution of the discharge cycle has an implication on the accuracy of the temporal force evolution, therefore, clarifying the importance of a priori defining the type of body-force analysis in an experiment; i.e. integral force magnitude, time-averaged or time-resolved evaluation. As a promising finding of utmost importance for the actuator performance, the actuator remains as effective as in quiescent air under presence of the external airflow, which immediately renders the actuator fluid-mechanic efficiency to increase for increasing airflow speed

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