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

Abstract

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 Reϭ3500 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.

Simple diffusion hopping model with convection

“…The wake region is typified by the presence of a recirculation flow consisting of vortex flow patterns. Similar vortices have also been observed for particle flux around wallmounted cubic obstacles in flow channels [52][53][54][55][56], around geographical mountains or hills [57], around islands in shallow water [58,59], and airflows around buildings [60,61]. In this case study we examine the effect of a particle flux with varying Re on the velocity field in the vicinity of a square obstacle of side l placed at the center of a flow channel.…”

A numerical study of coherent flow structures around a cubic bluff body using large eddy simulation

“…They pointed out that when the flow passes a square cylinder, the large-scale eddies depend strongly upon the small-scale perturbations, and thus it is difficult to determine how much resolution is adequate in calculating the pressure and the force in the structure. Hwang et al [7] used the direct numerical simulation (DNS) to study the vortical structures around a wallmounted cubic obstacle in laminar flow in a channel. The moderate Reynolds numbers vary until 3500.…”

Performance of two equation turbulence models for prediction of flow and heat transfer over a wall mounted cube

“…Hwang et al [2] studied numerically the vortical structures around a wall mounted cubic obstacle in a laminar channel flow from low to moderate Reynolds numbers up to 3500. They concluded that flow becomes turbulent in the near wake region at low Reynolds numbers itself.…”