Measuring Tollmien–Schlichting waves using phase-averaged particle image velocimetry

This article addresses the direct experimental measurement of Tollmien–Schlichting waves on a flat plate, when the laminar boundary layer is excited by velocity perturbations; the free stream velocity was 16 m/s, the excitation frequency 250 Hz. The two-dimensional velocity field in proximity of the flat plate was captured using a conventional PIV system; however, the image recording was phase locked with the disturbance source and ensemble averaging was used to obtain characteristics of the Tollmien–Schichting waves. In particular, after subtraction of the mean velocity, the characteristics of the excited waves in terms of streamlines were extracted, revealing that the investigated waves represented velocity deviations with an order of magnitude of 1 % of the undisturbed free stream flow. This study is a prelude to the use of the same technique to visualize the effect of dielectric barrier discharge plasma actuators on the suppression of such Tollmien–Schlichting waves, which is difficult using other measurement techniques.