Incompressible flow velocity: outlet faster than inlet
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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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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Thanks ! |
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did you set uniform velocity at inlet?
how about the mass difference between inlet and outlet? |
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Yes, the inlet velocity is constant for the whole boundary surface. There is no mass value from the results, as you can see it is totally grey. |
how did you estimate the difference between flow rate at inlet and outlet?
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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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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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it is just a simple math issue, you can have the same integral value with infinite different functions .... |
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The product rho*A*V would be the same throughout the hose. |
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