Question Video: Graphically Recognizing Steady and Turbulent Flow over an Obstacle | Nagwa Question Video: Graphically Recognizing Steady and Turbulent Flow over an Obstacle | Nagwa

Question Video: Graphically Recognizing Steady and Turbulent Flow over an Obstacle Physics • Second Year of Secondary School

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

03:05

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.

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