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Casting help!

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Old   October 20, 2009, 01:33
Default Casting help!
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Sean
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Hi all, new to the forum.

First off, some details on my simulations:
  • The material used is temperature and strain dependent which effects it's viscosity. All of the correct input data for this alloy has been input into the program.
  • During our casting process if we do a short shot (not enough material), then we see that the material is flowing along the walls first before filling through the middle of the cavity.
  • Compared to conventional die-casting, this material's injection speed is much lower. This is due to the viscosity change being gradual rather than becoming solidified at a specific temperature. Even though the velocity is less than that of traditional die-casting, I am sure that the turbulence is still quite high.
  • When running Flow3D I am never seeing a turbulence or mixing effect even if I set to turbulent flow. The results continue to show as laminar flow (the stuff taught in fluid dynamics through a pipe).
So far I have not been able to see this effect using the software. I am able to get down to a mesh size of 0.25mm but I am wondering if that is not small enough to view this tunneling effect.

Do you have any suggestions for me to try and see this?
Does this have anything to do with tweaking the numerics or physics tabs to provide these results.

Thanks in advance!
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Old   October 20, 2009, 01:43
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Sean
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A few more things to add:

I have tried using two different type of boundary conditions:

  1. Pressure (I can get metal pressure results directly from the casting machine)
  2. Velocity (this is set and can provide a stable input)
Whenever metal pressure is used, it seems to provide results that are not realistic. For example, the velocity of the material at the start of the simulation is very high because all of the pressure is acting on only the amount of material that has entered the domain.

On the other hand, the velocity may be effecting my simulation because no matter what the viscosity of the material is in the simulation (considering it is always changing based on temperature and strain), the velocity boundary condition will continue to push the material through the mesh no matter what. Is there any way to "cap off" the pressure that is available in the simulation while using the velocity boundary condition?

Thanks again.
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Old   October 21, 2009, 00:13
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michael barkhudarov
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Hi,

Sounds like a pretty complex process to model.

When modeling turbulence using a k-epsilon or any other turbulence model that is based on time-averaged quantities, you will not see any additional mixing and vortexing. Turbulence is modeled by averaging all fluctuations and presenting you with the mean flow. So it looks like laminar flow, but the viscosity is much higher than the molecular value, which enhances the momentum diffusion thus accounting for turbulent fluctuations.

There is no way to combine pressure and velocity BC at the inlet. You will have to define your BC at a reasonable location, where the flow conditions are known best at all time during the filling process. You are right that using the machine pressure does not make sense when there is no or little metal in the cavity.

Regarding the 'tunneling' effect. Why do you think it flows along the walls first? Could it be an artefact of the short shots? For example, could metal solidify at the walls quickly, while the metal in the core drains elsewhere thus creating the effect of tunneling?
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