A numerical investigation of the flow around rectangular cylinders
The viscous flow around rectangles defined by afterbody length, B, and cross-stream dimension, A, is investigated through a hybrid discrete vortex method. For uniform flow conditions the effects of varying the side ratio, BIA, the angle of incidence, a, and the Reynolds number, Re, are all considered. Pulsating flow results are reported for rectangular cylinders with B/A values of 0.62, 1.0, 2.0 and 3.0, a B/A=1.0 cylinder inclined at 45° and a circular cylinder. At a fixed Reynolds number, Re=200, the variation of drag coefficient with side ratio shows CD increasing with decreasing B/A. This contrasts with the known result at higher Reynolds number, 104500 the calculated Strouhal number is dual valued. The 'lock-in' characteristics under pulsating flow are shown to be highly dependent on body geometry. All the cylinders are shown to exhibit an asymmetric resonant mode within which the shedding frequency is controlled at half the forcing frequency and the mean forces increase. Several different shedding patterns are predicted across this asymmetric synchronisation range. A 'quasi-symmetric' mode is also observed for some cylinders
characterised by near wake symmetry and a substantial reduction in mean forces. A pseudo-phase lag is defined which relates a moment of the lift cycle to a moment of the forcing oscillation. This is shown to change across the synchronisation range of each cylinder considered and the change is found to be greater at lower forcing amplitude.