# Transition from pipe flow to open channel flow

## Main Question or Discussion Point

I'm working on a fluid flow problem that doesn't seem very complex but the solution is eluding me at the moment. Given a length of smooth tubing of known ID and roughness with water running through it, is it possible to predict at what velocity the flow will transition from "pipe" flow (fully wetted perimeter) to "open channel" flow (less than fully wetted perimeter with some open surface)?

Any assistance is appreciated.

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Q_Goest
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Hi heb,
... is it possible to predict at what velocity the flow will transition from "pipe" flow (fully wetted perimeter) to "open channel" flow (less than fully wetted perimeter with some open surface)?
I assume you're refering to a gas that gets into a pipe otherwise filled with liquid as opposed to two phase flow. If that's correct, then it's not a function of velocity. A pipe without a source of air can't entrain air regardless of velocity. The air has to get 'sucked in' at some point.

If you're talking about two phase flow, then the thermodynamics of the pipe is responsible for converting liquid to gas, not velocity.

FredGarvin
I took the OP's question to mean that the flowrate has dropped sufficiently to not have enough "stuff" to keep the pipe full.

Related problem

I have a closely related question: I have a horizontal pipe open at one end filled with a high density fluid. The open end is connected to a large reservoir containing a lower density fluid. Both fluids are at the same pressure. How can I calculate the time it takes for the high density fluid to run out and be replaced by the lower density fluid?

stewartcs
I have a closely related question: I have a horizontal pipe open at one end filled with a high density fluid. The open end is connected to a large reservoir containing a lower density fluid. Both fluids are at the same pressure. How can I calculate the time it takes for the high density fluid to run out and be replaced by the lower density fluid?
Assuming my mental picture of your setup is correct, since the pressure is the same it won't flow. A little more detail or a drawing might help clear that up though.

CS

I'm not sure how to post a figure, but think of a glass full of water that gets tipped on its side. The pressure at the interface is equal to atmospheric pressure in both fluids (water and air), and yet the water flows out of the glass and is replaced by air. What's the explanation? How long does it take for the water to flow out if the glass is a long tube?

stewartcs