Per the explanation of this question and the correct answer, I understand that Pipe A opens to air at ground level where the air is moving slower, therefore exerting more pressure. I understand that Pipe B opens above ground where the air is moving quicker and exerting less pressure. I don't quite understand how those 2 situations affect the underground tunnel and the direction of the air flow.
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For this question, they give us Bernoulli's equation right in the question stem. Therefore, let's consider what variables we're concerned about in the equation.
Bernoulli's equation says P + 1/2ρv2 + ρgy = constant. The constants 1/2 and g, and the variable ρ aren't really something we're considering here (the density of the air, it tells us, should be assumed to be constant). So what we're actually concerned about is P, v, and y. Let's consider what these represent here. P is the pressure of the fluid, v is the velocity of the fluid, and y is the height (altitude) of the fluid. In the question, we are told that both the velocity of air and the altitude of the air are higher at Pipe B. If this is true, the only way for the sum (P + v + y) to stay constant is if the pressure at Pipe B is much lower. This should also make some intuitive sense to you because pressure goes down with increasing altitude. So, P(A) > P(B).
How does this affect the air in the tunnel? Well, remember that air (and any fluid, for that matter) will follow a pressure gradient, going from the area of high pressure to the area of low pressure. We just identified Pipe A's opening as that point of high pressure and Pipe B's opening as the point of low pressure. Thus, the air will flow down Pipe A, across the tunnel (a left-to-right direction), and up and out of Pipe B.
This is the same phenomenon as what is occurring when you have two windows open in different rooms of your apartment, and may sometimes experience an "air tunnel". The air is rushing because it's following a pressure gradient in the building.
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