3D Turbulent Flow Measurements Over Rigid and Flexible Isolated Roughness Elements

The effects of flexibility on the flow over wall-mounted 3D roughness elements immersed in a turbulent boundary layer were experimentally investigated using Union College Water Channel. Two cases were studied: a flexible case manufactured from ABS plastic film and a rigid case manufactured from 316 stainless steel. Both elements had the same geometry resulting in an aspect ratio (ratio between height to width of the element) of 2 where the height was 12 mm, and the width was 6 mm. The incoming flow was set to a free stream velocity of 0.96 m/s, resulting in a Reynolds number of 60,000. Volumetric particle image velocimetry was utilized to capture the dynamics of the flow; the high-speed system consisted of 4 high speed-cameras and a high-speed double-pulsed 40mJ/pulse laser. The time–averaged velocity fields and vortical structures were analyzed to highlight effects of flexibility on the two cases. The results highlight a decrease in size of the velocity deficit region along the streamwise direction for the flexible case relative to the rigid case. In addition, reduction of both upwash and downwash of the flow upstream and downstream of the flexible element relative to the rigid element was observed. Flexibility of the element also led to the formation of inclined arch vortex while switching polarity of the counter-rotating pair of streamwise vortices. POD was utilized for further analysis and to filter small-scale motions in the instantaneous flow fields.