This is the flow as it evolves in time. The blue, turquoise, and green lines are trajectories traced out by three fluid particles.  Although at a given instant, streamlines and trajectories are both tangent to the velocity vector, in a nonsteady flow  the trajectories may deviate significantly from the streamlines over even a short interval of time. In this instance, the  trajectories are cycloids and look nothing like the streamlines.

The MATLAB source code for these animations can be found here (warning: somewhat unreadable as of this time) and uses a subroutine written by Zhenhai Wang.

• Durran, D.R., 1999: Numerical Methods for Wave Equations in Geophysical Fluid Dynamics, Springer, 465pp. (Referenced for the second order Runge-Kutta scheme used to create the trajectories)
• Kundu, P.K. and I.M. Cohen, 2002: Fluid Mechanics, Academic Press, 730pp.
• Young, D.F, B.R. Munson, and T.H. Okiishi, 2001: A Brief Introduction to Fluid Mechanics, John Wiley, 517pp.

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