Time-Resolved 3D Measurements of the Flow Surrounding 2D Isolated and Tandem Roughness Elements

Fluid flow over two-dimensional roughness elements was measured using volumetric particle image velocimetry. The roughness was immersed in a turbulent boundary layer, and two roughness configurations were considered: a single element, and two elements with the upstream element 0.75 times the height of the downstream element and separated by one element width. The Reynolds number of the flow was found to be ~66,000, based on the boundary layer thickness and freestream velocity. Time-averaged velocity, Reynolds shear stress, and turbulent kinetic energy profiles were considered for each arrangement in order to compare with previous planar particle image velocimetry measurements. Agreement with the planar measurements was achieved, supporting the calibration of the volumetric particle image velocimetry system and suggesting that measurements made further from the center plane would accurately describe the flow. The flow over the elements was found to be highly three-dimensional, supporting the need for the volumetric measurements. Time-resolved measurements of the system were also made to better view the vortices being shed by the elements and their interaction. Vortices were identified using multiple vortex identification methods and observed forming at the rear of the elements and travelling downstream. The frequency of this vortex shedding was analyzed to determine the Strouhal number.