Fig. (5) RMS values of v_rms in the z = 1.3 plane at T = 3.5 when the strong disturbance is continuatively introduced at Re ~ 255. The solid line is our simulation result. The exponential fitting lines from experimental result (Imayama et al. [20]) are given as v_rms ~ exp(αRe), where α is the growth rate. For the dashed line and the dotted line, the values of α are 0.058 and 0.017.