Volumetric Velocity Measurements in the Wake of a Hemispherical Roughness Element

Plenoptic particle image velocimetry was used to perform instantaneous three-dimensional velocity measurements in the near wake of a wall-mounted hemispherical roughness element at a Reynolds number (based on roughness height) of 4.57x10(3) and boundary layer to roughness height ratio of 2.4. The experiment was performed in a refractive index matched flow facility to mitigate laser reflections from the hemispherical surface. Data gathered from this experiment represented one of the first applications of plenoptic particle image velocimetry. The ensemble-averaged flow is characterized by a separated shear layer and a symmetric recirculation region. In the instantaneous three-dimensional velocity fields, a separated shear layer and recirculation region, both with asymmetric characteristics, are present. Additionally, arch vortices are found that are detached to the hemispherical surface. The proper orthogonal decomposition was applied to both the three-dimensional velocity and three-dimensional vorticity fields with the goal of identifying the structures created by the hemisphere. The velocity modes showed features associated with the overall flow, whereas the vorticity modes were largely associated with the near wake. The most energetic proper orthogonal decomposition modes showed fluctuations in the boundary layer and recirculation region, as well as suggested the existence of shed arch-shaped vortices.