Streaklines in a Perturbed Shear Flow

Streaklines are numerically obtained for a shear flow perturbed by an unamplifying traveling sinusoidal wave. It is shown that, if a dye is injected from near the critical layer, the streaklines have an appearance of amplification and rolling as if to indicate that the flow develops into discrete vortices. The rolling-up of a streakline cannot therefore constitute a positive identification of the presence of discrete vortices. Away from the critical layer, on the other hand, the streakline appear to show an alternating amplification and damping. Apparent wavelength and wave velocity are both incorrect except in the critical layer. Further, any information due to pathlines or obtained by tracing marked particles is equally improper in regard to the wave motion. These results strongly suggest that a great deal of caution must be exercised in interpreting flow phenomena based upon flow-visualization techniques.