Volumetric characterization of spatially organized features of Reynolds stress anisotropy in the vicinity of submerged model boulders

Experimental characterization of Reynolds stress anisotropy in flows around obstacles is essential to improve understanding of flow physics and provide validation data for numerical models. Recently, techniques enabling spatial visualizations of anisotropy have been developed, but they have been minimally utilized to experimentally examine spatially distributed characteristics of anisotropy. This study reports a characterization of anisotropy based on laboratory volumetric particle tracking velocimetry data obtained in the vicinity of submerged model boulders (i.e., wall-mounted obstacles atop a rough, permeable bed in open-channel flow). Reynolds stress anisotropy was analyzed using two-dimensional (2D) and three-dimensional (3D) visualization methods that are mathematically connected with the Lumley triangle to investigate the hypothesis that anisotropy exhibited spatial organization in this 3D flow field. Multi-planar visualization results depicted spatially organized features in the boulder vicinity via distinct color bands. Some of the anisotropy color bands appeared to originate upstream, wrap around the boulder, detach at the boulder flank, and then extend downstream into the wake. In the upstream region, these specific features corresponded with an approach to two-component turbulence due to the suppression of boulder-normal turbulence. The wake was generally characterized by banded anisotropy regions that originated in the near wake, had a predominantly streamwise orientation, and whose signatures were still visible in the far wake. The complex organization of these bands was investigated via transverse-vertical planes in the wake. In addition, strongly anisotropic behavior was observed via both 2D and 3D visualizations near the expected tip vortex location for a wall-mounted cylinder.