I am trying to simulate a mixing process of Helium jet in atmospheric air. He is coming out from a tube of 4.6 mm inner dia with a Re number of 2500 where the air is considered stagnant.
To set the model, I use an inlet velocity BC with a velocity of 66.5 m/s for He jet inflow in the domain. I also set a zero pressure outlet BC at a sufficient distance from the inlet to allow enough space for mixing.
The model is in a steady state. To capture the mixing, I use the species transport model available in Fluent. It looks like the model works fine.
However, I found the velocity goes unexpectedly high (~1200 m/s max) in the domain when the solution is converged.
If anyone has any suggestions, it would be highly appreciated.
I dont know the accuracy of your mesh and numerics but i will explain in way you understand what you are observing. You know continuity has to hold locally (in a particular cell) as well for the given problem as a whole. Just to understand, Let us consider a crossection anywhere in a tube with laminar flow. The flow near the boundary layer would be subjected viscous stresses and the velocity that region would be smaller that what you prescribed at the inlet. However, the flow towards the centerline would be faster due low viscous stresses and to compensate the low flow rate in the near wall regions. The velocity would higher than your inlet velocity. In case of turbulet flow, viscous and turblunet stress will yield low and high velocity zones. Therefore, velocity magnitude will fluctuate.
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