Question Video: Determining Where Turbulent Flow Can Be Seen | Nagwa Question Video: Determining Where Turbulent Flow Can Be Seen | Nagwa

Question Video: Determining Where Turbulent Flow Can Be Seen Physics • Second Year of Secondary School

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The figure shows the direction of fluid flow over an obstacle. Within which of the regions shown, if any, is there some turbulent flow?

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Video Transcript

The following figure shows the direction of fluid flow over an obstacle. Within which of the regions shown, if any, is there some turbulent flow? (A) Region 1 only, (B) region 2 only, (C) region 3 only, (D) all of the regions shown, (E) none of the regions shown.

Studying this example of fluid flow, we want to pick which of these four regions, if any of them, show some turbulent flow. The idea is that there is a flowing fluid whose movement is represented by these streamlines. And as this fluid flows from left to right, it passes by this obstacle. Region 1, we see, is just upstream of the obstacle, region 2 is at the obstacle, region 3 is just downstream of it, and region 4 is farther downstream.

To figure out which, if any, of these regions show us turbulent flow, let’s recall that turbulence arises due to friction between adjacent layers of flowing fluid. In a smooth or laminar flow situation, these layers stay separate from one another. However, if the friction between them is great enough, then the layers begin to mix. This mixture is a sign of turbulence. If we look at the streamlines in our diagram, we see that at no point does the fluid flowing in any given layer, say this one here, mix with the fluid flowing in the layers adjacent to it. No matter which layer we pick and which of our four regions we choose, we don’t see any signs of mixing of adjacent fluid layers. Even though these layers are diverted due to the obstacle, they still remain intact without mixing with one another.

If there was turbulent flow in any of these four regions, it would be evidenced by something like we see here. The streamlines, which we can take as boundaries between adjacent layers of fluid, would be disrupted and not follow a smooth left-to-right flow pattern. Because we don’t see this anywhere in our diagram, we can’t say that we see any signs of turbulent flow. For our answer then, we’ll choose option (E). In none of the regions shown do we see any signs of turbulent flow.

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