Learning Forum

[junbae929]

Thank you for your interest.

I am currently performing a simulation using compressible flow and the VOF model.

I am encountering a situation where the absolute pressure becomes negative near the outlet of a narrow flow passage, particularly where the fluid exits into an air-filled region. The geometry can be considered similar to a nozzle, and the negative pressure appears near the nozzle exit boundary, approximately in the red-circled region in the figure.

The operating pressure is set to atmospheric pressure, and I have also confirmed that the pressure being examined in post-processing is absolute pressure, not static/gauge pressure.

Due to the nozzle geometry, I understand that an air vortex is formed in the indicated region and that the local pressure can decrease. However, I am having difficulty understanding why the absolute pressure becomes negative.

I tested the following two cases:

• A case where the air density was defined as a pressure-dependent density using a UDF
• A case where the air density was fixed as a constant

In both cases, negative absolute pressure appeared at approximately the same location.

In the first case, where the air density depends on pressure, I also observed that the convergence deteriorates or the solution diverges as the nozzle velocity increases.

Therefore, my questions are:

1. Why can negative absolute pressure occur in this type of simulation?
2. Why is the solver able to continue the calculation even when negative absolute pressure has occurred?

For reference, I am using:

• VOF: Implicit
• Time step size: 1×10−5 s
• CFL number: below 5

From a numerical stability perspective, I considered these settings to be fairly conservative.

In addition, since I wanted to model compressible flow without solving the energy equation, I selected the following approach/settings. However, I did not expect it to result in negative absolute pressure.

I would appreciate any explanation regarding the physical or numerical reason for this behavior.

[skulkarn]

Hello,

The image is not seen or has been removed. Could you please re-upload the image again? What are the pressure limits you have set in the Controls?