"' •" •

Figure 3 Vorticity field at different times during the cycle: 6/ R = 0.2, K c = 1.5, Aw = 0.5U0 / R, --clockwise vorticity, ........ counter clockwise vorticity (a)t = 314.16T/2.- (b)t = 314.94T/2.- {c) t = 315.73T/2.- {d) t = 316.51T/2.-.