Minimizing the Distortions Induced by Mean Shear within 3D Reconstructions of Turbulent Flows from Time-Resolved sPIV Measurements

The efficacy of reconstructing a 3D volume of time-evolving 3 component velocity from planar experimental measurements is explored within strongly shear-distorting turbulent flows. A common approach to convert the temporal dimension into the streamwise spatial dimension is Taylor’s frozen turbulence hypothesis where the mean velocity is imposed as the convective velocity. In flows with a strong mean shear-rate the instantaneous turbulence structure is distorted when a traditional Taylor’s hypothesis method is used to reconstruct 3D volumes. In the current study, we compare existing methods that extend the classical Taylor’s hypothesis approach to retain time-locality in the convective velocity in order to accurately reconstruct a 4D (time-resolved) velocity field for accurate analysis of turbulence structure. Specifically, we analyze a local mean convective velocity approach (Pinton & Labbe 1994) as well as an instantaneous convective velocity approach (Fratantonio et al 2021) using time-resolved sPIV measurements in transverse and longitudinal planes within the near-wall surface layer of a canonical flat-plate turbulent boundary layer at Re_θ=7,700. The reconstruction methods are evaluated based on their ability to preserve both the statistical properties of the flow and the instantaneous structure of the turbulence eddies as well as the streamwise extent to which these methods can be applied.