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) The solid black lines only, (B)
the dashed red lines only, (C) both sets of lines, (D) neither set of lines.
In this question, we are shown a
diagram displaying the direction of two different fluids as they flow over an
obstacle. One fluid’s flow is shown by the
set of solid black lines, and the other is shown by the dashed red lines. We are asked to figure out which
set of lines shows a flow that is turbulent in some places. Let’s start by reminding ourselves
what laminar flow and turbulent flow are and how we can determine the type of flow
using the lines representing the flow direction.
When we say “flow,” we are talking
about the movement of a fluid, liquid or gas, through an area. Remember that gases and liquids are
each made up of many individual particles that are free to move independently from
one another. The flow of a fluid can be
described as either laminar or turbulent. Laminar flow occurs when all the
particles are moving uniformly in both direction and speed, without much deviation
from each other. In contrast, turbulent flow occurs
when there is a significant change of direction and speed of the particles as they
flow.
Let’s look at an example of laminar
flow. Notice that the orange lines
showing the direction of flow are relatively straight and do not change much in
direction or spacing. These are the characteristics we
would expect from the laminar flow of a fluid. It’s worth mentioning that, as in
this example, the lines do not have to be exactly straight and parallel in order to
have laminar flow so long as any changes are small and occur gradually.
Now, let’s have a look at an
example of turbulent flow. In this sketch of turbulent flow,
we can see that the directional lines are changing considerably and not all flowing
in the same direction. If there is turbulent flow, we
should expect to see something like this, in which there are areas where the lines
are not uniform.
With this in mind, let’s take a
look at the sets of lines in the figure given to us in the question. As the dashed red lines flow over
the object, we see that they stay smooth and move with the others around them. There is no big change in direction
as they flow over the object. But taking a look at the black
lines, we can see that the line closest to the object changes direction and flows in
a circle before continuing on. This part of the diagram shows us
that the black lines’ flow is turbulent in the area to the right of the object.
Since the dashed red lines move
smoothly past the object, we can see that they show laminar flow. For the black lines, since there is
a region where the direction of a line changes considerably, we have identified this
as an area of turbulent flow. This means that option (A) is the
correct answer. The lines that show a flow that is
turbulent in some places are the solid black lines only.
