Transition from pipe flow to open channel flow

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Discussion Overview

The discussion revolves around the transition from pipe flow to open channel flow, specifically focusing on the conditions under which this transition occurs and the dynamics involved in fluid flow between different states. Participants explore scenarios involving fluid dynamics in pipes and the implications of pressure and density differences in related systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether it is possible to predict the velocity at which flow transitions from pipe flow to open channel flow, suggesting that the transition may depend on the presence of air in the pipe.
  • Another participant clarifies that if the pipe is filled with liquid without a source of air, the transition is not a function of velocity, indicating that air must be entrained into the flow for a transition to occur.
  • Another viewpoint suggests that the original question may relate to the flow rate dropping below a certain threshold, leading to insufficient liquid to maintain a full pipe.
  • Several participants introduce a related problem involving a horizontal pipe filled with a high-density fluid connected to a reservoir of lower-density fluid, questioning how to calculate the time for the high-density fluid to be replaced.
  • One participant posits that if both fluids are at the same pressure, there may be no flow, while another participant requests more details to clarify the scenario.
  • Further, a participant uses an analogy of a tipped glass to illustrate fluid dynamics, questioning how the water can flow out despite equal pressure at the interface, and seeks to understand the timing of this flow in a long tube scenario.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for the transition from pipe flow to open channel flow, with no consensus reached on the specific mechanics or calculations involved. The discussion remains unresolved regarding the implications of pressure and density in the related scenarios presented.

Contextual Notes

Participants have not fully defined the assumptions regarding fluid properties, flow conditions, or the specific configurations of the systems discussed, which may affect the interpretations of the problems posed.

heb26
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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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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 referring 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.
 
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?
 
ariespc said:
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?
 
ariespc said:
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?

So the pipe is not closed on one end, but rather open to the atmosphere, and on the other end the pipe is connected (and open) to a reservoir with a lower density fluid...correct?

Is the reservoir top open to the atmosphere?

CS
 

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