Superstructures in a Turbulent Boundary Layer Under the Influence of an Adverse Pressure Gradient Investigated by Large-Scale PIV

Abstract

The flow of a turbulent boundary layer under the influence of an adverse pressure gradient was investigated using a large-scale, high-resolution PIV setup. Wall-normal vector fields with a length of 1.7m (ultimately over 3.0m) and a resolution below one mm were stitched from multiple camera views. Reθ was measured to be 4,600 at the end of a long flat plate preceding the pressure gradient.Within this dataset long, stretched regions with positive or negative streamwise velocity fluctuations can be observed in singular snapshots. These so-called ‘super-structures’ (or ‘very large-scale motions’) were already identified in undisturbed turbulent boundary layers [1], pipe flows [2], and in flows under the influence of an adverse pressure gradient [3, 4]. These measurements either used PIV with a limited measurement region or probe-measurements assuming frozen turbulence. As already described in other publications, a reliable capture of very long flow henomena proved to be problematic due to spanwise meandering. The structures move in and out of the twodimensional measurement plane used in the presented experiment, reducing their perceived extent. However, in many snapshots the meandering is of a low scale, so that large parts of the structures could be imaged. In order to characterize the occurring flow structures, two-point-correlations were performed, yielding the average structure size in the course of the adverse pressure gradient for different heights above the wall. Average sizes of 1.6–3.2 δ were found, with an increase in structure length with the advance of the pressure gradient. Especially smaller-scale structures near the model wall increase in size as they are driven from the wall,being stretched in the process. Further measures to identify singular super-structures instead of applying statistical approaches—which are biased by the huge number of small structures and the meandering of the large ones—are discussed.