LAUSR

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 [``Numerical investigation of plasma-actuator force-term estimations from flow experiments'', J. Phys. D: Appl. Phys. 48 395203 (2015)], 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.