Mapping of the flow structure and hydrodynamic properties of a round-ended cylinder

This paper reports the numerical results of flow past a round-ended cylinder with various incidence angles in a low Reynolds number range of Re = 60-160. Mapping of the flow structure and hydrodynamic properties is examined in the incidence angle range of alpha = 0 degrees-90 degrees with increment of 15 degrees. Three wake patterns are identified, including the steady and symmetric wake without vortex shedding (Pattern I), the Karman vortex street (Pattern II), and the Karman vortex street with the occurrence of subordinate vortex (Pattern III). The reattachment of boundary layers results in the occurrence of a subordinate vortex and hence the non-single-frequency fluctuation of hydrodynamic coefficients (C-L and C-D, which are lift and drag coefficients, respectively). Elliptical and figure-eight C-L-C-D curves are observed, depending on the frequency ratios of the two coefficients and their weights. Non-zero time-averaged C-L occurs when 0 degrees < alpha < 90 degrees, due to the asymmetric boundary layer separation. The backward migration of boundary layer separation point contributes to the reduction of frictional drag and the vortex formation length. The shortening of the vortex formation length results in the enhanced fluctuations of C-L and C-D.