NOVA: Turbulent Flow

[justadream]

NOVA Physics page 175 #27


“Consider water flowing in a pipe. Which of the following would tend to reduce the likelihood of turbulent flow?”

1. Increase Flow Rate
2. Make the Joints in the Pipe Smooth
3. Raise the temperature
4. Increase Radius

Answer: B

Why wouldn’t raising the temperature work? That increases the volume (reducing the density), thus decreasing the Reynolds #.



NOVA’s reasoning for B is “making the joints smooth does not seem to do anything at first glance, but perhaps this would remove obstacles that would create turbulence”


I somewhat can see that intuitively but how come there’s no way to account for that in any equation?

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[Cawolf]

Whenever questions are posted, I always answer them before looking at the answer to see what I would say. In this case I agree with the solutions manual.

My first thought on seeing a temperature increase is that the internal energy would increase - making turbulence more likely.

I see what you are saying about the change in density with heating, but I don't know if it would be a meaningful enough change in density. The density of liquid water goes from almost 1000 kg/m^3 at just past freezing to 959 kg/m^3 just before boiling. Another factor to consider is that the viscosity will decrease with increased temperature, perhaps balancing the change in density?

As far as the joint goes; that seems pretty apparent as an object in the path of flow will cause turbulence by nature.

 

[Shirafune]

As far as the joint goes; that seems pretty apparent as an object in the path of flow will cause turbulence by nature.

I second this. I think the most intuitive answer would be to make the pipe smoother. I wouldn't play around with speculation because it seems like your reasoning for (B) is not convincing enough, at least not for me. Cawolf makes a great point about density not changing appreciably and the fact that a decrease in viscosity would also be seen, although this change also probably not be very noticeable.

I'm not as familiar with fluid dynamics, but according to Wiki, "Flow in which the kinetic energy dies out due to the action of fluid molecular viscosity is called laminar flow." It stands to reason, then, that when kinetic energy is increased greatly and viscosity remains relatively the same (if not decrease), laminar flow will no longer be possible as the kinetic energy will overcome the liquid's viscosity.

 

[popopopop]

This is exactly how a cold air intake works on a car. Air flows like molasses inside the intake and cylinder head. People get the head of their car ported (smoothen out), route shorter intake pipes, and angle them towards the front (or even route them outside the engine bay) of the car for "cold" (dense is more correct) air, instead of the heated portion under the hood.