Application of Hpiv Data of Turbulent Duct Flow for Turbulence Modeling

Holographic PIV is used to measure the three-dimensional velocity distribution of fully developed turbulent flow within a square duct at ReH = 1.2 × 10. The data is used for mapping the structure of the filtered, three-dimensional vorticity, strain-rate and subgrid-scale (SGS) stress tensor distributions. Visualizations of iso-vorticity surfaces show structures that are only slightly elongated, as opposed to the long and thin "worms" observed in DNS for unfiltered turbulence. The structure of these fields is studied further by measuring absolute and relative orientations of characteristic vectors. The fluctuating vorticity has a preferred direction aligned at about 45° to the mean flow. The preferential alignment of the vorticity with the intermediate eigenvector of the strain-rate tensor observed previously in DNS data is also found in this high Reynolds number shear flow. The PDF of the intermediate eigenvalue shows some differences compared to DNS results. Two of the SGS stress eigenvectors are nearly randomly aligned compared to the most extensive and intermediate eigenvectors of the filtered strain-rate tensor. In contrast, the most extensive stress eigenvector has a significant correlation with the direction of the most compressive strain-rate eigendirection. These results help to shed light on the relative success of eddy viscosity models in predicting the SGS dissipation, but their failure in predicting the actual stresses. In addition, the observed preferential alignments of the filtered vorticity with two stress eigendirections may point to new modeling approaches based on vorticity.