Numerical study of vortical structures around a wall-mounted cubic obstacle in channel flow

Vortical structures around a wall-mounted cubic obstacle in channel flow are studied using numerical simulation. Flows of low-to-moderate Reynolds numbers up to Re=3500 are considered. The objective of this work is to elucidate characteristics of coherent vortical structures produced by the presence of the wall-mounted cubic obstacle, including horseshoe vortex systems upstream of the obstacle, lateral vortices in the vicinity of the two lateral faces of the cube, and hairpin vortices in the near-wake region. As the flow approaches the cube, the adverse pressure gradient produces three-dimensional boundary-layer separation, resulting in the formation of laminar horseshoe vortices. As the Reynolds number increases, the structure of the horseshoe vortex system becomes complex and the number of vortices increases in pairs. The distribution of skin friction on the cube-mounted wall reflects the effect of the horseshoe vortices. Unsteady horseshoe vortex systems are hardly found as long as the upstream flow is fully viscous; they are obtained when the cube is placed in the entrance region of a developing channel flow. The unsteady horseshoe vortex systems are characterized by a repeated process of generation, translation, and mutual merging of the vortices. The laminar wake is characterized by a pair of spiral vortices behind the obstacle; distinct singular points are identified leading to consistent flow topology. In the case of a turbulent wake, however, it is observed that the flow becomes less coherent in the near-wall region downstream of the obstacle. Instead, coherent structures such as lateral vortices and hairpin vortices are found in the vicinity of the two lateral faces of the cube and in the turbulent near-wake region, respectively. Quasiperiodic behaviors of those vortices are noticed and their frequencies are computed. The translating speed of the head portion of a hairpin vortex is lower than the streamwise mean velocity at that location. In the vicinity of the lower wall downstream of the cube, vortical structures of various length scales are identified; they become gradually elongated downstream of the flow reattachment.