Turbulence structure of boundary layers perturbed by isolated and tandem roughness elements

The modification to the turbulence structure of a boundary layer perturbed by an isolated cylindrical roughness element and by two tandem cylindrical roughness elements was experimentally investigated using particle image velocimetry at a Reynolds number of 56,800. In addition to the unperturbed flow and isolated cylinder cases, 9 tandem arrangements of two cylinders were studied at various degrees of sheltering, achieved by varying the streamwise spacing and height ratio of upstream cylinder to downstream cylinder. Three centre-to-centre spacings (2d 4d, and 6d) were considered along with three height ratios (1, 0.75, and 0.5) at each spacing; here, d denotes the cylinders' diameter. Quadrant analysis and proper orthogonal decomposition (POD) were used to characterise the modification to the turbulence structure due to the roughness element(s). Important changes to the quadrant contributions are observed in the near wake; however, these changes are significantly relaxed in the far wake. Most notably, contributions from the first and third quadrants are relatively large below the tip of the downstream cylinder. In some cases, these contributions balance or dominate over contributions from the second and fourth quadrants in limited regions, which impacts the turbulent kinetic energy production. Furthermore, the POD analysis suggests an enhancement of small-scale flow motions and a weakening of the large scales due to the cylinder(s). The localised disturbance to the POD modes in heights below the cylinder(s) suggests that large-scale flow motions maintain their organisation above the perturbed flow region. Overall, the quadrant and POD analyses indicate that the turbulence structure, in the near wake, is highly sensitive to both the streamwise spacing and the height ratio, demonstrating the potential for flow control using two adjacent roughness elements as a passive control mechanism.