### Video Transcript

The following figure shows the
direction of two different fluid flows over an obstacle. The flow of one fluid is shown by
dashed red lines and the flow of the other fluid by solid black lines. Which lines show the flow that is
turbulent in some places? (A) Both sets of lines, (B) the
solid black lines only, (C) the dashed red lines only, (D) neither set of lines.

Here, we’re trying to recognize
turbulence in the flow of a fluid. Let’s begin by briefly recalling
how we can model some of the properties of fluid flow.

Fluid motion is typically very
complicated to model quantitatively, but we are able to learn about the flow of a
fluid by observing some of its qualitative characteristics. One way we can do this is by
drawing streamlines to represent the motion of different layers of the fluid. That’s what these different sets of
lines on the diagram represent. The two sets of lines help show the
motion of two different fluids. In this question, we want to
recognize whether either set of lines show turbulence in the flow.

Let’s recall that a fluid’s flow is
turbulent when the fluid layers change speed and direction rapidly. Turbulent flow is chaotic, and it’s
characterized by streamlines that are irregular, very curved, or bunched
together. We should also recall that
turbulent flow is the opposite of laminar flow. In laminar flow, the fluid layers
move along nicely together and have a more constant speed and direction with respect
to each other. We can see this in streamlines that
curve only gently and stay equally spaced apart.

Notice that this describes the
patterns shown by both sets of lines in the diagram. Both the red dashed lines and the
black solid lines appear to show laminar flow. Of course, there is this large
obstacle in the fluid’s path. But even as the fluid is redirected
around the obstacle, the layers stay pretty evenly spaced apart with no rapid
changes of direction. Therefore, we can identify option
(D) as the best answer. Neither set of lines show any areas
of turbulence in the fluid flow.