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Incompressible flow velocity: outlet faster than inlet |
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#1 |
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Xuekun Lu
Join Date: Oct 2015
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Hi,
I just tried a simple constant diameter pipe gas flow. I set the gas as incompressible (constant density), velocity inlet, pressure out at 1 bar. Based on continuity equation, I guess the mass-in should be equal to mass-out. But the simulation results show that, the outlet velocity is larger than inlet, as a consequence, the mass-out is larger than mass-in since the density is constant. Why this can occur? It breaks the continuity equation. Also, the velocity is larger at the outlet than inlet. But I expect it should be no larger than the inlet since the outlet pressure is 1 bar. Why there is no mass flow rate and mass flux in the fluid domain? Can anyone help me with this ? I feel quite confused. Thanks ! |
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#2 | |
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Filippo Maria Denaro
Join Date: Jul 2010
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without details about the formulation, discretization, etc, it is not possible to answer .... just as first try, have you checked you really reached the convergence in the solution? |
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#3 | |
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Xuekun Lu
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Quote:
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#4 | |
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Xuekun Lu
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Quote:
Thanks ! |
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#5 |
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Xuekun Lu
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#6 |
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Filippo Maria Denaro
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did you set uniform velocity at inlet?
how about the mass difference between inlet and outlet? |
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#7 |
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Xuekun Lu
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#8 |
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Filippo Maria Denaro
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how did you estimate the difference between flow rate at inlet and outlet?
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#9 |
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Filippo Maria Denaro
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the fact that the centerline velocity at outlet is greater than at inlet is correct, the acceleration is due to the viscous effect that produce the boundary layer. That does not mean you have different flow rate at the outlet
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#10 |
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Xuekun Lu
Join Date: Oct 2015
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Hi, I didn't estimate that. The mass flow rate is what I want to get out from the results. At the inlet, I just gave it a constant velocity, so I guess the mass flow rate can be calculated by the solver itself at the boundary using the velocity, the diameter and density. I wonder why there is no flux shown in the result.
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#11 |
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Xuekun Lu
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Thanks a lot for your explanation. Based on what you said, does it mean the average velocity at inlet and outlet is the same despite the difference at centerline?
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#12 | |
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Lucky
Join Date: Apr 2011
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You don't even need the code to calculate the mass-flow rate for you, since you already know what it is (you specified it actually when you specified the inlet velocity). |
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#13 | |
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Filippo Maria Denaro
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it is just a simple math issue, you can have the same integral value with infinite different functions .... |
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#14 | |
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Sarang
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The product rho*A*V would be the same throughout the hose. |
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Tags |
gas flow, incompressible flow, pressure and velocity, velocity |
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