Lagrangian defocusing particle tracking velocimetry (defocusing PTV, DPTV) measurements are performed in a thin volume above a plasma actuator array that is applied to mimic the effect of wall oscillations by inducing alternating, wall-parallel forcing in opposite directions into the air above the actuator surface for flow control purposes. The aim of the experiments is to capture the plasma-induced flow topology in otherwise quiescent air throughout the oscillation cycle within the measurement volume of 14 mm \(\times\) 1 mm \(\times\) 14 mm, immediately adjacent to the wall-mounted actuator. For this purpose, particle image velocimetry equipment for time-resolved measurements with one camera is used in a DPTV setup, where the out-of-plane particle coordinate is obtained through the diameter of a defocused particle image. Three-dimensional, three-component velocity and acceleration data is extracted by introducing a continuous particle tracking approach and an extended ex situ calibration procedure based on the detection of solid particles directly applied to a wall boundary, for which no prior knowledge of the flow topology or velocity data in the direct vicinity of the wall is required. A novel method for estimating measurement uncertainty in this context is introduced, and the influencing factors are discussed from an application perspective. Through the analysis of Lagrangian particle tracks, both individual flow events and statistical effects within the oscillation cycle can be evaluated. The extraction of phase-resolved flow fields with adaptable spatial resolution shows the forcing effect to be regular across different discharge zones on the plasma actuator array, indicating well-balanced voltage settings and precise manufacturing. Furthermore, the relation between the forcing-induced velocity and acceleration fields is quantitatively assessed, revealing the spatio-temporal transmission characteristics of the applied forcing. In summary, the obtained results demonstrate the applicability of DPTV measurement technique for the flow characterization above a plasma actuator array using the presented modifications.
