Experimental Investigation of Low-frequency Unsteadiness in Trailing-edge Separation

The low-frequency unsteadiness related to flow separation on wings is known for producing force-fluctuations. The present investigation explored the low-frequency unsteadiness of the turbulent separated flow near the trailing-edge of a NACA 4418 airfoil at a chord-based Reynolds number of 720,000 and angle-of-attack of 9.7°. Time-resolved planar particle image velocimetry was performed at the trailing edge section in a streamwise-wall-normal measurement plane at the midspan and a streamwise-spanwise plane parallel to the airfoil surface. The turbulent separation bubble was found to breathe at a low frequency with a Strouhal number on the order of 0.01. The spectral analysis of the flow unsteadiness and space-time correlations between the separation line and near-wall streaks suggested that the streaks were most likely responsible for the breathing motion. Proper orthogonal decomposition analysis revealed that the breathing motion consisted of two types of energetic motions: large-scale expansion and contraction, and small-scale undulation of the separation line.