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Axisymmetric Flow: Incorrect Pressure at Axis and Sudden Pressure Drop at Outlet BC

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Old   March 26, 2020, 18:53
Default Axisymmetric Flow: Incorrect Pressure at Axis and Sudden Pressure Drop at Outlet BC
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I appreciate if any person can offer some ideas on the following issues?

I am running an axisymmetric simulations of a nozzle. However, the pressure along a large portion of the axis is always equals to the value I set at the outlet boundary. Does anyone knows how to solve the problem? In particular, the pressure value of the outlet (to the right of the figure) is 2.5 MPa, the inlet pressure is 10 MPa.

Moreover, for some reasons, the values of pressure on the nodes immediately before the outlet BC at much higher (at about 9 MPa), then, it suddenly drops to the boundary value in only a distance of one mesh cell. Does this have anything to do with the weird pressure distribution on the axis? Any idea how to correct these issues?
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File Type: jpg Pressure on Axis.jpg (30.1 KB, 8 views)
File Type: jpg Pressure vs x.jpg (42.9 KB, 5 views)
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Old   March 27, 2020, 03:27
Default Axis Boundary
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Could you confirm if axis is really defined as axis type under boundary condition? It should not be defined as any other type. Furthermore, it should really be aligned with the x-axis to behave like an axis. This is not a requirement for axisymmetric scenario, such as, an annulus, but for the cases that have their domain touching the axis.
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Old   March 27, 2020, 17:14
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Hi,

Thanks for your reply.

Yes, the axis is defined using the axis type and perfectly aligned along the x-axis. There are no nodes lying below the x-axis.

When I plot the pressure contour using cell values, the pressure-field looks continuous. I do not understand the discrepancy. Should the contours from both not be the same? When should I look at node values versus cell values.

Do you think this is related with the suddenly drop of pressure (other other flow variables) at the outlet boundary?
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Old   March 28, 2020, 15:46
Default Node values
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Fluent calculates everything at cell centers. Values at the nodes are interpolated using gradients. Therefore, there is always a difference. However, that cannot explain what you observed.

Though the details of the mesh show minimum y as 0.0, still why are there two lines near the axis in the image you shared? Or it could be just visual.
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Old   March 28, 2020, 15:57
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The two lines should be just the resolution. For some reason the image downloaded from here looks kind of blur when zoomed in.

I actually exported the ASCII data file and the minimum y-coordinate for each given x-coordinate are actually 0.00000. There are no nodes below the x-axis.
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Old   March 28, 2020, 16:08
Default Node on negative y
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That's good. Fluent does not run if it detects even a single node on negative y. However, it is not must to have any element lying on x-axis. E.g., if the geometry represents annular region, all nodes will have y > 0 and none with y=0.

Is the solution properly converged then? And if you plot y-velocity, does it show any negative velocity near the axis? As per the pressure field, it should show that. That implies the solution is not fully converged. If that is the case, try using some other numerical scheme.
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Old   March 30, 2020, 17:59
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Hello,

Based on the residual curves, they are all at least 3 orders of magnitude smaller than the initial values. Moreover, when I checked the averaged values at the nozzle exit (the second vertical line from the right in the mesh), they are pretty constant even I ran the simulation for 800000 iterations. That is why I am not sure that this is an issue with not sufficiently converged.

Per you suggestions, it seems that there is slight negative radial velocity at the beginning of the nozzle. But that does not correspond the region where P_axis = P_out.

Is there a general rule on Node Values versus Cell Values. The simulation is about liquid water being evaporated to water vapour. I am treating the vapour as ideal gas as the real gas law does not work.

I am using Coupled scheme with 2nd order Upwind for all variables. This is the only possible scheme for the simulation as anything else would crash.
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File Type: jpg Velocity.jpg (33.9 KB, 2 views)
File Type: jpg Convergence.jpg (39.7 KB, 2 views)

Last edited by cwl6750084; March 30, 2020 at 18:01. Reason: Include Figures
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Old   March 31, 2020, 04:04
Default Multiphase
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If the simulation is multiphase, operating conditions will have a lot of effect. Do check the following

1. Value and direction of gravity. It must be either -9.81 or 9.81 in x-direction. Gravity cannot be in y-direction for axisymmetric case.

2. Reference location for the operating pressure should be the highest point in the domain. If gravity is -9.81, then maximum value of x in the domain should be used as reference location and vice-versa.

3. Operating density should be the density of the gas phase.
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Old   April 3, 2020, 02:56
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Quote:
Originally Posted by vinerm View Post
If the simulation is multiphase, operating conditions will have a lot of effect. Do check the following

1. Value and direction of gravity. It must be either -9.81 or 9.81 in x-direction. Gravity cannot be in y-direction for axisymmetric case.

2. Reference location for the operating pressure should be the highest point in the domain. If gravity is -9.81, then maximum value of x in the domain should be used as reference location and vice-versa.

3. Operating density should be the density of the gas phase.
I used 9.81 for gravity in the position x-direction and the minimum value of x (inlet) for operating condition. However, I used 0 for the operating condition per the instruction in Fluent for compressible phase (water vapour). It seems that the pressure field does not match the outlet BC because the domain is not long enough, though I do not know why - as I thought the boundaries are always matched.

However, the nodal pressure along the axis are still very low (almost equals to the outlet pressure and I am not sure what else can alter that).
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Old   April 6, 2020, 05:55
Default Operating Conditions
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If you are using ideal or real gas, then operating density as well as operating pressure, both should be set to 0. And the pressure at the outlet should have some positive value.
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