An extended experimental method is presented in which the micro-pillar shear-stress sensor (MPS 3 ) and high-speed stereo particle-image velocimetry measurements are simultaneously performed in turbulent channel flow to conduct concurrent time-resolved measurements of the two-dimensional wall-shear stress (WSS) distribution and the velocity field in the outer flow. The extended experimental setup, which involves a modified MPS 3 measurement setup and data evaluation compared to the standard method, is presented and used to investigate the footprint of the outer, large-scale motions (LSM) onto the near-wall small-scale motions. The measurements were performed in a fully developed, turbulent channel flow at a friction Reynolds number R e τ = 969 . A separation between large and small scales of the velocity fluctuations and the WSS fluctuations was performed by two-dimensional empirical mode decomposition. A subsequent cross-correlation analysis between the large-scale velocity fluctuations and the large-scale WSS fluctuations shows that the streamwise inclination angle between the LSM in the outer layer and the large-scale footprint imposed onto the near-wall dynamics has a mean value of Θ ¯ x = 16.53 ∘ , which is consistent with the literature relying on direct numerical simulations and hot-wire anemometry data. When also considering the spatial shift in the spanwise direction, the mean inclination angle reduces to Θ ¯ x z = 13.92 ∘ .
The flow features developing in the presence of a two-dimensional forward-backward facing step of small width are visualized. The flow visualizations were conducted by means of Temperature-Sensitive Paint (TSP). The tests were conducted in the Laminarwasserkanal (laminar water channel) at the Institute for Aerodynamics and Gas dynamics (IAG) in Stuttgart. A step Reynolds number Re k from 199 up to 1890 enables the investigation of laminar, transitional and turbulent reattachment of the flow. The variations of flow separation and impinging structures related to reattachment are well captured by the TSP technique. The TSP method requires a temperature difference between the model surface and the flow to visualize flow phenomena. This temperature difference alters the flow structures around the forward-backward facing step for low Reynolds numbers by inducing buoyancy forces in the boundary layer and recirculation zone.
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