Now that we have discussed how one can mathematically describe flow fields (Eulerian versus Lagrangian) as well as reference frames for balances (system versus control volume), it is important to think of visualization of flows. (We already covered (briefly) fluid property fields and showed that for scalar properties, contour plots are sufficient up to 2D).

Three well-accepted flow "visualizers" are the following:

A **Streamline** is a line that is tangent to the
instantaneous velocity field. Imagine "connecting the dots" from a
vector field plot of the velocity profile.

A **Pathline** is a line that is traced out by "watching" the
flow of a particular fluid element. Imagine a long-time exposure
image of a firefly's motion.

A **Streakline** is a line that is generated by "tagging"
particles that have visited a particular location. Imagine
injecting a continuous stream of dye into a flowing fluid.

There are a couple of interesting things to note regarding these lines:

- Since the direction of a streamline is wholly determined by the
instantaneous velocity field, and no point may have two different
velocities at the same time,
*streamlines cannot cross each other*(but they*can*converge to a point, if the velocity goes to zero at that point, which we will call a**stagnation point**) - Our firefly could easily change his mind and "double-back", so
pathlines
*can*cross each other - Similarly, someone could "stir" our fluid up and cause the ink
streams to cross each other; streaklines
*can*cross each other as well

Identify and differentiate between streaklines, streamlines, and pathlines

Determine when these three types of lines are the same; when are they different? (HINT: what would need to be true about the underlying flow, for the firefly to "change its mind", or our ink streams to follow different trajectories from each other?)

It is interesting to visualize how/why these lines are different using an example.

Given a time-dependent velocity field such as $v_x = sin(t)$ and $v_y = 1$ it is instructive to look at how each of these visualization lines look/evolve: