Effect of spacing on the 3D flow around a pair of cubic roughness elements embedded in a turbulent channel flow resolved using tomographic holography

The effect of spacing on the flow structure around a pair of roughness cubes embedded in the inner part of a turbulent channel flow at Re_τ=2500 is measured using tomographic holography. The cube height, a=1 mm, corresponds to 4% of the half channel height and 90 wall units. The spacings between cubes are 1.0a, 1.6a, and 2.5a. The boundary layer separates upstream of cube, causing formation of horseshoe vortex that rolls up in front of the cube, and then wraps around it, forming a pair of counter-rotating “legs”. A vortical “canopy”, dominated by wall-normal vorticity along the cube sides and spanwise vorticity above it, covers the entire cube and part of the near wake behind it, appearing as an arch surrounding the recirculation region. The canopy is asymmetric with respect to the cube center due to influence of the neighboring canopy. The high-speed jetting between the cubes deforms the adjacent canopy, realigning the vertical vorticity in the axial direction. The resulting large streamwise structures engulf the adjacent horseshoe vortex legs. This process occurs earlier with decreasing cube spacing, presumably because of the corresponding faster jetting between cubes. Conversely, behind the cubes, the streamwise momentum deficit increases with decreasing spacing.