Flexible Roughness Elements in a Turbulent Boundary Layer

This research consists of experimentally observing the effects of flexible roughness elements in a turbulent boundary layer using the Union College water channel. Both flexible and rigid elements were experimentally observed for greater understanding of the effects of flexibility; the experiments were performed at a Reynolds number of 60,000. The flexible elements were manufactured from an ABS plastic film while the rigid elements were manufactured from 316 stainless steel; all elements have the same geometry of a rectangular flat plate. Within addition to the effects of flexibility, the effects of the aspect ratio (the ratio between height and width) of the elements were also investigated. Aspect ratios of 0.5, 1, 2, and 4 were considered. The flow was measured using a 2D particle image velocimetry (PIV) setup, which consisted of one 16 MP high speed camera, and a 200 mJ/pulse double pulsed laser. The time-averaged velocity field and turbulence statistics were analyzed. The results highlight a minor reduction in turbulence in the wake of the flexible elements. However, the flexible elements caused the turbulence to follow a more upward trajectory in their wake relative to the wake of the rigids elements. The data suggests this may be due to the differences in wall-normal velocity between rigid and flexible cases. Along with this change in turbulence, the flexible elements experienced a significantly smaller recirculation zone.