@phdthesis{,
    author = {Eich, Felix},
    title = {Investigation of large-scale structures in turbulent boundary layers},
    editor = {},
    booktitle = {},
    series = {},
    journal = {},
    address = {},
    publisher = {},
    edition = {},
    year = {2021},
    isbn = {},
    volume = {},
    number =  {},
    pages = {},
    url = {},
    doi = {},
    keywords = {Turbulent boundary layers, Turbulence, Large-scale coherent strucutres},
    abstract = {The investigation of large-scale coherent structures in turbulent boundary layers has been subject to intensive research in the past decades. However, the instantaneous spatial organisation of these coherent structures and their statistical impact on the flow is still not completely understood. One difficulty in the experimental investigation are the mutual characteristics of large-scale structures, which can extend multiple boundary-layer thicknesses in length. Another difficulty arises from the large Reynolds number required to obtain largescale structures of sufficient energy for a reliable identification and analysis. The objective of this dissertation is to analyse the effect of Reynolds number, wall distance and streamwise momentum on the turbulent large-scale structures with the aim to better understand the impact of the large-scale structures on the turbulent boundar-layer flow itself. Therefore, velocity fields are measured in multiple wall-parallel planes and in a cross-stream plane normal to the mean flow direction. The results are analysed by means of multi-point statistical methods in order to determine the average sizes, shape and spacing of the largescale turbulent structures. So far, most studies focus on canonical zero-pressure gradient flows. In contrast, most technical flows are subject to pressure gradients. Therefore, the impact of an adverse pressure gradient on large-scale turbulent structures is examined within this study, to investigate the persistence of the large-scale structures and the structure impact on a beginning flow separation. The results show a dependence of the large-scale structure spatial scales on the wall distance and multiple scaling regimes are identified. Furthermore, it was shown by conditioning the statistical analysis on low and high-momentum flow events, that distinct changes in the shape and size of coherent flow structures are present. A conceptual model was then derived based on the statistics that explains the observed scaling of the large-scale structures. The analysis of the flow under the impact of an adverse pressure gradient 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 lowmomentum 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. Focusing on the flow field around the structures, a characteristic vortex structure is identified and quantified. Combining the observations from the scaling analysis and the vortical pattern, a self-sustaining process is described which explains the generation and decay of large-scale structures in the turbulent boundary layer. In a literature review, this process is differentiated to existing models highlighting similarities and controversies in the interpretation of observations.},
    note = {},
    
    school = {Universität der Bundeswehr München},
    
}