Wake flow structure and hydrodynamic characteristics of flow around a C-shaped cylinder with variable attack angle at low Reynolds numbers

A numerical investigation is conducted on the flow over a C-shaped cylinder in the low Reynolds number range of Re = 40-160. The effect of attack angle (alpha) ranging from 0 degrees to 180 degrees is examined simultaneously. Wake evolution and vortex structure as well as the hydrodynamic characteristics are analyzed. Seven flow patterns are identified based on the location of boundary layer separation points and the evolution of near-wall vortices. The boundary layer separation points lock on the two ends of the C-shaped cylinder, resulting in the typical Karman vortex street (Pattern I). A separation point shifts to the curved surface in Pattern II-1 and Pattern II-2, and a quasi-stagnation vortex (QS) is formed within the groove in Pattern II-2. In Pattern III-1 and Pattern III-2, the QS fills the groove. The subordinate vortex is observed in the groove close to the lower end (Pattern IV). The complicated vortex merging occurs around the lower end in Pattern V. The separation points lock on the two ends, exhibiting a pair of counter-rotating vortex shedding downstream of the two ends (Pattern VI). No vortex shedding is found in Pattern VII. Additionally, the characteristic parameters and the hydrodynamic coefficients are related, and they are associated with the flow pattern partition. Four types of vortex street are identified in the wake of the C-shaped cylinder, including no vortex street, 2S vortex mode and decayed vortex street, 2S vortex mode and secondary vortex street (2S-SVS), and P + S vortex mode and secondary vortex street in vortex evolution (P + S-SVS).