Experimental Study of Energy Contribution by Coherent Structures in a Turbulent Boundary Layer

An experimental study was carried out to investigate a contribution of large coherent structures in turbulent flow. Both dynamometer and Clauser fitting method were adopted to calculate the frictional velocity near the wall having uncertainty. In order to quantify coherent structures, Particle Image Velocimetry (PIV) system was used to reveal their characteristics connecting through boundary layer. Variable Interval Time Averaging (VITA) technique scanned through flow fluctuations at the edge of boundary layer, whether there was a large deviation from the background flow. According to the presence of large scale structures, the flow field was divided into weak and strong cases to validate their existence. Changing a free-stream velocity from 0.5 m/s to 1.0 m/s increased the Reynolds stress more than 3 times. Proper Orthogonal Decomposition (POD) analysis was also applied to compare the energy budgets between them.