# Thread: Reynolds Number

1. I would like to float this thought experiment just to see peoples' responses to gauge if it is a good example of a concept.

Assume that I have a closed loop of pipe joined to the inlet and outlet of a positive pump. The loop contains water or any similar incompressible liquid with no air pockets.

The loop is at a fixed temperature of say 25 degrees, on the earth's surface.

I turn on the pump. The liquid flows. It attains a certain Reynolds Number and there is a pressure drop along the pipe.

Now I repeat the experiment in zero gravity in a space craft.

Is the Reynold's Number different?

Is the pressure drop different?

2.

3. I might have missed something in what you have written, but I can't see why gravity should change the flow in the pipe you describe, assuming that the pipe is horizontal. Accordingly, it isn't obvious (not to me anyway) why gravity should affect the Reynold's number.

4. You say it has a fixed temperature, but if you are pumping the fluid in a loop, then you are adding pump heat and must have some sort of cooler in the loop to maintain that temperature. Then there could be some natural circulation effect contributing to the flow. The natural circulation would not occur in zero gravity. That's the only difference I can think of.

5. Thanks for the responses. I was hoping to use this example to illustrate that, since Reynolds Number (ρVD/μ ) does not contain 'g', then the it is independent of gravity and use this analogy to explain why, in a similar manner, Froude Number (v2/2gL ) is independent of density, as it does not contain a density term.

6. However, to take a rather silly example, we usually write momentum as m.v (mass x velocity). But we could say that mass is density x volume and put ρV into our expression instead of mass. Then density would appear in the expression for momentum despite the fact that we know that mass would be more appropriate.

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