Coherent large scale structures in adverse pressure gradient turbulent boundary layers

Over the last several years, the observation of large scale turbulent flow structures in turbulent boundary layer flows has stimulated intense experimental and numerical investigations with the aim of characterizing not only the topological features of the coherent structures but also their dynamics. Due to the large length of the structures(approximately 7-14 times the boundary layer thickness) it is very difficult to analyze them reliably since both a large field of view and a high spatial resolution are simultaneously required to measure all relevant spatial scales. Moreover, a low uncertainty of the experimental techniques is required as the disturbance of the mean velocity field due to the large scale structures can be quite weak. This holds in particular true for investigations at small Reynolds numbers. As a consequence of the recent technological progress, the required measurement uncertainty can be reached with sophisticated image analysis techniques but the resolution of the structures requires a high number of cameras. Therefore, 16 sCMOS cameras in total were provided by different research organizations in order to perform large scale turbulent boundary layer measurements with appropriate spatial resolution. In the case of adverse pressure gradients (APG) the topology and dynamics of the large scale turbulent flow structures is unknown, as is their significance for the statistical properties of the flow. Therefore the fundamental aim of this project was to resolve and characterize these structures in an APG boundary layer flow at the Laboratoire de Mécanique de Lille (LML) wind tunnel. To access the span-wise / wall normal signature of the structures as well, stereoscopic PIV measurements in span-wise/wall-normal planes were performed at specific stream-wise locations in addition. To complement these large field of view measurements, long-range micro-PIV, time resolved near wall velocity profiles and film-based measurements were performed in order to determine the wall-shear stress and its fluctuations at some specific locations along the model, see Soria et al. (2016).