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%0 Thesis
%A Sun, Weiqi
%T Evolution of synthetic streamwise vortices in turbulent boundary layers
%I Rheinisch-Westfälische Technische Hochschule Aachen
%V Dissertation
%C Aachen
%M RWTH-2025-08395
%P 1 Online-Ressource : Illustrationen
%D 2025
%Z Cotutelle-Dissertation. - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2025. - Dissertation, University of Melbourne, 2025
%X Turbulent boundary layers (TBLs) with embedded streamwise vortices are ubiquitous in nature and industrial applications. Considerable effort has been devoted to this topic for the past several decades to deepen the understanding of boundary dynamics, and then develop potential flow control strategies. There is, however, a limited understanding of the evolution characteristics of embedded streamwise vortices and the underlying physical mechanisms regarding their wall-normal domain of influence. The present study investigates the evolution of synthetic streamwise vortices of different size in low-friction-Reynolds-number TBLs using high-resolution planar Particle Image Velocimetry (PIV) measurements and wall-resolved Large Eddy Simulation (LES). The evolution of small synthetic vortices in TBLs at a friction Reynolds number of 440 is first explored using the LES. A comparative study of the properties of the small synthetic and naturally occurring streamwise vortex pairs (SVPs) in the near-wall region is performed. Analyses employing a triple decomposition of velocity and pressure and its extension to the incompressible Navier-Stokes equation are used to better understand the coherent and turbulence fields associated with the synthetic and naturally occurring SVPs. Overall, it is observed that the near-wall synthetic and natural streamwise vortices have similar interactions with the surrounding turbulent field, and exhibit similarities in statistical structure. It is found that the ensemble-averaged natural and synthetic SVPs are identified to own similar spatial scales and evolution characteristics such that when they are inner scaled they exhibit similar magnitudes at comparable streamwise stations. These similarities suggest that embedded small synthetic streamwise vortices are self-contained in the near-wall region and directly interact with the structures in this area to influence the associated turbulent transport. A detailed investigation of the evolution of the small, mesoscale and relatively large synthetic vortices in TBLs at a friction Reynolds number of 273 is then performed using PIV measurements and LES. These varying-size vortices are generated from geometrically similar miniature vortex generators of different heights. Analysis employing a triple decomposition of velocity found that the wall-normal domains of influence of these vortices are different and show dependence on the wall-normal peak location of the total primary Reynolds shear stress component. It is found that the evolution of the synthetic vortices in TBLs is scale-dependent and indicated by the wall-normal range of the positive (momentum source) and negative (momentum sink) total turbulent inertia. The ratio of the gradient of the viscous stress (of a canonical TBL) to the total turbulent inertia acts as a diagnostic of the wall-normal domain of influence of the synthetic vortices. Furthermore, the analysis of the wall-normal domains of influence shows the existence of an 'escape' scale for the synthetic vortices that can be estimated by the above ratio profile. Finally, the evolution of the mesoscale and relatively large synthetic vortices under significant constraints on their spanwise size is examined via PIV measurements. It is found that these constrained mesoscale and relatively large synthetic vortices tend to have a more limited domain of influence. This is characterized by the modified momentum transport signatures in TBLs due to the constraints on their spanwise scale.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2025-08395
%U https://publications.rwth-aachen.de/record/1019509