TBR Physics Section VII, Example 7.8a Problem

[Vanguard23]

Page 67 of my book, it covers fluid flow.
The question: Which of the following will decrease the chance of turbulent blood flow in a vein?

A. Narrowing the vein( I thought this would be correct as per the equation provided).
B. Thinning the blood without changing the density(ie, lowering the viscocity)
C. Increasing the absolute pressure on each end of the vein by the same amount.
D. Lowering the blood density without thinning it(Correct Answer).

Now my problem here is the fact that in the Solution provided, it says this about A:
Choice A WIDENS the vein and increases the Reynolds number. This will increase the chance of turbulent flow, making it an incorrect choice.

Now, that logic is fine if the Question stem didn't say "Narrowing the vein" whereas the solution says widen.
Faux pas on my part?

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[BerkReviewTeach]

Page 67 of my book, it covers fluid flow.
The question: Which of the following will decrease the chance of turbulent blood flow in a vein?

A. Narrowing the vein( I thought this would be correct as per the equation provided).
B. Thinning the blood without changing the density(ie, lowering the viscocity)
C. Increasing the absolute pressure on each end of the vein by the same amount.
D. Lowering the blood density without thinning it(Correct Answer).

Now my problem here is the fact that in the Solution provided, it says this about A:
Choice A WIDENS the vein and increases the Reynolds number. This will increase the chance of turbulent flow, making it an incorrect choice.

Now, that logic is fine if the Question stem didn't say "Narrowing the vein" whereas the solution says widen.
Faux pas on my part?

You are correct, so that must be a typo in the book. I think you must have an older version of the book, because that is question 7.10a in my book and the answer choices are a bit different:

A. Widening the vein
B. Thinning the blood without changing the density
C. Increasing the absolute pressure on each end of the vein by the same amount.
D. Lowering the blood density without thinning it.

Quoted Solution (from the book)
You can apply the Reynolds number equation to each choice, crossing out choices that either increase the chance of turbulence or leave that chance unchanged. Choice A widens the vein and increases the Reynolds number. This will increase the chance of turbulent flow, making it an incorrect choice. One way to remember this relationship is to watch smoke rising from a candle. As it leaves the candle, it flows upward in a smooth (non-turbulent) narrow band. This band widens as it rises and becomes turbulent. So, narrow paths (or objects) are less likely to encounter turbulence. Wider paths (or objects) are more likely to do so.

Choice B lowers blood’s viscosity, increasing the Reynolds number. This will make turbulence more likely. Choice B is incorrect. Choice C creates a greater pressure everywhere around the vein, but this changes nothing. The onset of turbulence depends upon a sufficiently large flow rate. Flow rate in turn depends upon a pressure difference. If that difference does not change, the flow rate does not change. This rules out choice C, leaving only choice D.

If this were the MCAT, you'd now move on to the next problem. However, if you'd like to know why D is correct, then notice that lowering the blood density would decrease the Reynolds number. This means that turbulence is less likely. Odds are good that the passage text would contain an equation as uncommon and unattractive as the Reynolds number equation. In fact, any time you are given an equation like this in the passage, use it to help organize your thoughts.

The best answer is choice D.

 

[Vanguard23]

It's a 2009 "blue cover" copy; No idea why mine says Narrowing in the question stem instead of widening.

 

[BerkReviewTeach]

It's a 2009 "blue cover" copy; No idea why mine says Narrowing in the question stem instead of widening.

That's a pisser, because mine is the Fall 2009 beta version of the one that's coming out sometime soon (supposedly next month). I assumed the typos that were corrected in mine all came from the version you have, and that the typos would be the same in both. The text sections are quite similar except for the E and M section, some additional sample questions, and B-answer explanations.

 

[neurosnap]

Page 67 of my book, it covers fluid flow.
The question: Which of the following will decrease the chance of turbulent blood flow in a vein?

A. Narrowing the vein( I thought this would be correct as per the equation provided).
B. Thinning the blood without changing the density(ie, lowering the viscocity)
C. Increasing the absolute pressure on each end of the vein by the same amount.
D. Lowering the blood density without thinning it(Correct Answer).

Now my problem here is the fact that in the Solution provided, it says this about A:
Choice A WIDENS the vein and increases the Reynolds number. This will increase the chance of turbulent flow, making it an incorrect choice.

Now, that logic is fine if the Question stem didn't say "Narrowing the vein" whereas the solution says widen.
Faux pas on my part?

The way I interpreted A was you were narrowing the vein in a specific spot. The image that came to mind was pinching a hose; when you pinch the middle of the hose, it constricts one part but widens both sides of the pinched area, which would then increase the pressure on both sides, therefore increasing reynolds number.

I could be completely wrong. I think maybe the way they worded choice A more than likely led to people becoming confused, which is why they changed it.

 

[BellJS]

Narrowing the vein would increase the effect of viscous forces. I'm having trouble describing why narrowing or widening the vein wouldn't change the velocity through the section, but anyway:

If I was to think about this I would think about it in terms of trying to get the viscous forces to dominate over the inertial forces ( which only D would look like).