Interactions of Wall Turbulence with Patches of Flexible Roughness

The interaction of a zero-pressure gradient boundary layer with patches of hair-like, high aspect ratio, flexible, micro-pillars was studied systematically. These micro-structures are inspired by naturally occurring sensory structures such as those seen in fishes, insects, etc. Experiments were conducted in a zero-pressure gradient boundary layer facility. Patches of these micro-pillars, each comprising micro-pillars of systematically varied parameters were manufactured and studied. The fetch of these patches was greater than the boundary layer thickness in both the streamwise and spanwise directions. The individual micro-pillar parameters that were varied were the inner normalized height, the aspect ratio, and the Youngs Modulus of the micro-pillar material. These parameters collectively varied the stiffness of each micro-pillar that comprised an array. In addition, the streamwise and spanwise spacing between micro-pillars, within an array, were varied. The flow developing over these arrays and the smooth wall case were compared with each other. In general, the turbulent intensity profiles showed a shift in the inner peak away from the wall. This inner peak had moved over the height of the micro-pillars. An increase in the small-scale energy in the near-wall region was also observed. This increase was more pronounced in the case of the micro-pillars with lower stiffness.