- #1

Freixas

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To create a flow in the tube, I've been told I need a difference in pressure. I've also been told that the pressure of the fluid exiting the tube will be equal to the atmospheric pressure. To get a flow, the pressure of the fluid at the entrance needs to be higher than the pressure at the exit.

For beginners to fluid dynamics, the term "pressure" is problem. Sometimes it means total pressure and sometimes static pressure. For the exit pressure, I'm pretty sure the rule is for static pressure. When I mentioned needing a pressure difference to get a flow, I'm less sure, but I think we're talking about total pressure.

The continuity equation tells me that if the fluid is incompressible and the tube has a fixed diameter, the velocity of the flow will be equal at every point. Then the Bernoulli equation let's me calculate the static pressure at any point given the velocity. Since the velocities are equal, the static pressures will also be equal all the way along the tube.

Yet people talk about a "pressure gradient" in the tube. And a pressure gradient makes sense if one thinks about what might be happening at the molecular level. Do the pressure gradients exist only very close to the entrance and exit of the tube?

One final puzzle piece. Another text talked about a frictional flow. In this example, it was clear that the author was saying that the difference in static pressure between two points in a tube was because of energy lost due to friction--there was no mention of any other pressure gradient other than the one caused by friction.

Hopefully, someone can quickly straighten me out. I'd appreciate the help.