In turbulent boundary layers, streamwise elongated regions of high- and low-momentum in the log-law layer that can extent up to several boundary layer thicknesses are often referred to as turbulent 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 flow structures. In the last few decades extensive research on zero-pressure gradient (ZPG) turbulent boundary layers has been done, however by comparison, the structural characteristics for adverse pressure gradient turbulent (APG) boundary layer flows are much less studied despite their strong significance aero-hydrodynamic vehicle design. 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 multi-camera 3D time-resolved Lagrangian particle tracking approach. In this study, Lagrangian and Eulerian statistics will be used to characterize the dynamics and interaction of turbulent superstructures within a zero pressure gradient (ZPG) turbulent boundary layer at Reτ = 5000 or Reθ = 14 000 that then flows over a curved plate subjected to a favorable (FPG) and strong adverse (APG) pressure gradient, which eventually separates. An Eulerian analysis, using multi-point correlations of 3D velocity fields, found that the average superstructure topology is modulated by decelerating flow in the APG region when compared to the ZPG region, however the basic shape and spanwise pattern is preserved. Looking into the behavior of individual trajectories, 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, the mean square of the single particle dispersion indicates that the maximum dispersion in the spanwise direction comes from particles released at the wall-normal location corresponding to the so-called "second-peak/plateau" region in the streamwise normal Reynolds stress.

A large-scale 3D Lagrangian particle tracking (LPT) investigation of a turbulent boundary layer (TBL) flow developing across different pressure gradient regions is presented in this study. Three high-speed multi-camera imaging systems, LED illumination and helium-filled soap bubbles (HFSB) tracers have been adopted to produce time-resolved sequences of particle images over a large volume encompassing approximately 3 m in the streamwise direction, 0:8 m in the spanwise direction and 0:25 m in the wall-normal direction. Individual tracers have been reconstructed and tracked within the imaged volume by means of the Shake-The-Box algorithm (STB, Schanz et al. (2016)); the FlowFit data assimilation algorithm (Gesemann et al. (2016)) has been used to evaluate the spatial velocity gradients and to interpolate the scattered LPT results onto a regular grid. Thanks to the large size of the investigated volume and to the time-resolved nature of the recorded images, the entire spatial extent of the large-scale coherent motions within the logarithmic region of the TBL (i.e. superstructures) could be captured and their dynamics investigated during their development over several boundary layer thickness in the streamwise direction, from the zero pressure gradient region (ZPG) to the adverse pressure gradient region (APG). Two free-stream velocities were investigated, namely 7 and 14m=s, corresponding to Ret ~ 3,000 and 5,000 respectively. The results confirm the location and scale of the elongated high- and low-momentum structures in the logarithmic region, as well as their meandering in the spanwise direction. Two-point correlation statistics show that the width and spacing of the superstructures are not affected by the transition from the ZPG to the APG region. The analysis of the instantaneous flow realizations from both a Lagrangian and Eulerian perspective indicates the presence of significant fluid particle elements exchange across the interfaces of the large-scale structures.

A turbulent boundary layer under the impact of a strong adverse pressure gradient up to flow separation is investigated experimentally at Reynolds numbers up to Re = 12,000, with the aim to analyse and characterise the interaction between large-scale coherent turbulent structures and a separated flow region. It is shown that the dynamics of the separation line in space and time are strongly modulated by the low frequency large-scale flow motions. Using a conditional comparison of mean flow parameters, it is shown that high-momentu m largescale motions are able to shi f t the point of separation downstream while the opposite is true for low-momentu m large-scale motions. This interaction has a significant impact on the temporal dynamics of the separation line. The spatial frequencies observable along the line of separation on the other hand are determined by the spanwise arrangement of high and low-momentum large-scale motions. Finally, it is demonstrated that the separated region does not have a major influence on the mean bounda r y layer thickness in contrast to the mean flow velocity and spatial scales of coherent structures which are visibly influenced in the vicinity of the separated region.

This study compares the predicted synthetic flow fields generated based on the representative structures of the attached eddy model to experimental data captured using Particle Image Velocimetry of a turbulent boundary layer. To this end, wall-parallel and cross-stream planar fields are analyzed qualitatively and quantitatively by examining instantaneous flow features and by statistical two-point correlation functions, respectively. Our results reveal that although single-point flow statistics are in good agreement with the experimental data, a comparison of instantaneous flow fields and multipoint statistics between the attached eddy model and experiments shows differences in the spatial coherence. Based on these observations, a modification to the placement of the representative eddies in the attached eddy model is proposed that incorporates the meandering of these flow structures, which has been extensively reported in turbulent boundary layers. Our results reveal that this subtle modification provides a superior spatial representation of a turbulent boundary layer from the attached eddy model by reducing periodic effects and the overestimated spatial coherence. Similar improvements are also reported for the spatial representation of the span wise velocity component.

The investigation of large-scale coherent structures in turbulent boundary layers has become an established field of research in the last decades. Most studies considered the canonical zero pressure gradient (ZPG) boundary layer flow case. Within this study, a turbulent boundary layer under the impact of a strong adverse pressure gradient up to separation is investigated experimentally up to Reτ = 12000. The analysis of the acquired PIV velocity fields shows a persistence of large-scale coherent structures from the preceding zero pressure gradient section through the adverse pressure gradient region until the flow separation takes place. Furthermore, the interaction between the coherent turbulent structures and the flow separation is characterised. Using conditional comparison of mean flow parameters, it is shown that high-momentum structures are able to shift the point of separation downstream while the opposite is true for low-momentum large-scale structures. This interaction has a significant impact on the dynamics of the separation line. It is also demonstrated that the separated region does not have a major influence on the mean boundary layer thickness but the mean flow velocity and spatial scales of coherent structures are visibly influenced.

We report on an experimental undertaking with the goal of detecting so-called turbulent superstructures within a turbulent boundary layer and describing their temporal development while propagating from a region of zero pressure gradient (ZPG) into a region of adverse pressure gradient (APG). The flow was characterized by performing Lagrangian Particle Tracking (LPT) of Helium-filled soap bubbles (HFSBs) using the Shake-The-Box (STB, Schanz et al. 2016) algorithm. This work will present first results, but will concentrate on the description of the experimental efforts and the evaluation approaches that were necessary to successfully conduct this investigation. The tracking of a large number of tracer particles (several 100,000) within an extensive volume (nearly three meters in length) was made possible by the use of a system consisting of 12 cameras, forming three successive volumetric systems. Calibration and evaluation methods for fusing the different systems into one large camera system are described. Challenges in calibrating camera systems with a large field-of-view due to small-scale distortion effects introduced by the observation windows are discussed and correction methods are introduced. Methods for achieving a perfect transition between overlapping camera systems are introduced. For higher flow velocities, a double illumination approach was created in order to limit the pixel displacement between frames. To the knowledge of the authors, this is the first time such an approach was realized for time-resolved image sequences.

We report on an experimental undertaking with the goal of detecting so-called turbulent superstructures within a turbulent boundary layer and describing their temporal development while propagating from a region of zero pressure gradient (ZPG) into a region of adverse pressure gradient (APG). The flow was characterized by performing Lagrangian Particle Tracking (LPT) of Helium-filled soap bubbles (HFSBs) using the Shake-The-Box algorithm.