[DrZoidberg]

Dear CFD Community,

I try to simulate a mold filling process and I need the transient pressure distribution on the wall (to calculate some mechanical loads there). Liquid aluminum alloy is injected through narrow gates at high speeds. In my case it is 30m/s at the inlet. The cavitiy is vented via an pressure-outlet (please see figures below). I tried a variety of Fluent parameters so far as well as fine and coarse meshes. The results below are the most promising so far.

However, there are very high (unphysical) local pressures especially after the narrow gates. I understand that the compressed gas want to expand so v increases and p drops. As soon as the low pressure cell that was filled with compressible phase1 gets filled with incompressible phase2 the pressure drops badly. This results in negative static pressure up to -1e6Pa! This would give an negative absolute pressure. I enclosed some screenshots of the flow before that event. Unfortunately, I do not have the -1e6Pa results at hand atm, but the location is the same and the pressure drops lower with every timestep.

[I read that might be an indicator for cavitation or does not matter at all for imcompressible, single-phase fluids, becasue in the latter case only grad(p) matters. However in VOF the actual pressure matters for phase1. The state of art/literature for aluminum casting does not report such problems or even cavitation as far as I know. Furthermore these kind of simulations are quite common in industry and seem to work fine if one reviews the publications in metal casting journals.]

The settings are:
VOF implicit, phase1=air ideal-gas, phase2=aluminum melt (2500kg/m³, 850K,0.0017Pas, surf tension coefficient 0.75N/m), Contiuum Surface Modelling was deactivated (impossible to converge, as soon as it hits a wall - but due to high Re and We it should not be of major importance, especially if bubble/porosity formation is not of interest)

Viscous Model is kwSST with Production Limiter

Operating density 0kg/m³ (bc of compressible gas as UserGuide suggests), operating pressure=101325Pa outside of the cavity

SIMPLEC with strong URFs (~0.3 on momentum, 0.3-0.6 others) and first order formulation, timestep 1e-6s, Compressive and Modified Body Force Weighted scheme

convergence residuals reach down to 1e-4 for conti and 1e-5...1e-6 for others depending on the flow event.

Does anybody has experience with such kind of negative pressure problems in Fluent VOF? I would very much appreciate if you would like to share your thoughts.

Best regards

[DrZoidberg]

Hello again,

I tried to enclose figures here, but the website did not respond anymore. I will try to upload them one by one as replies to this thread.


Figure for Setup:

[DrZoidberg]

Volume fraction phase-2

[DrZoidberg]

Velocity magnitude

[DrZoidberg]

Static pressure (overview, detailed see below)

[DrZoidberg]

Detailed view of the diffusor geometry with negative pressure spot that is evolving over the flowtime to 'more negative' numbers.

Please note:

1) The color spec: red = 0Pa...blue = -1.5e5Pa which is already equal to an unphysical p_abs=p_op+p_solver = -0.5e5Pa

2) In this view, the location of the inlet is on the right and the location of the outlet is on the left side of the image.

[DrZoidberg]

Hello again, and sorry for the confusion. The problem was that Fluent's CAS/DAT files were loaded in CFDpost (which was made for CFX, which maps its results differently). So, it was a user fault in postprocessing.


Looking at the results in Fluent or via a CDAT file in CFDpost, then there are no negative pressures.