LAGRANGIAN STATISTICS OF SUPERSTRUCTURES IN A TURBULENT BOUNDARY LAYER WITH PRESSURE GRADIENTS

In the log-law region of turbulent boundary layers, streamwise elongated flow regions of high- and low-momentum can extent up to several boundary layer thicknesses. They are often referred to as superstructures. These structures contain a relatively large portion of the layer's turbulent kinetic energy and have been shown to interact with the near-wall features. In the last few decades these structures have been extensively analyzed for the zero-pressure gradient turbulent boundary layer condition. However by comparison, the structural characteristics for adverse pressure gradient turbulent boundary layer flows are much less studied. Therefore, the three-dimensional dynamics of turbulent superstructures in a turbulent boundary layer flow are investigated in the Atmospheric Wind Tunnel Munich (AWM) using a novel multi-camera 3D time-resolved Lagrangian particle tracking approach. In this study, Lagrangian statistics will be used to characterize the dynamics and interaction of turbulent superstructures within a zero pressure gradient (ZPG) turbulent boundary layer at Re_τ = 5460 or Re_θ = 13 300 that then flows over a curved plate subjected to a favorable (FPG) and strong adverse (APG) pressure gradient, which eventually separates. The main research aim is to determine if mass and momentum transfer between the superstructures exists. It was found that the dispersion of single particles along trajectories in the log-law layer are capable of moving more than the average Eulerian superstructure spacing in the spanwise direction. Furthermore, single particle dispersion structure functions indicate that, on average, the maximum dispersion in the spanwise direction is due to trajectories originating in the log-law layer. This implies that the highly energetic turbulent superstructures are responsible for a significant amount of the dispersion or mixing present in the turbulent boundary layer.