Discussion Forum

Hello all !


I am trying to validate my model by comparing a simple axisymetric 2D impacting flow of air on a surface.
The geometry is simple: a cylindrical (diameter d=26.5e-3[m]) jet of air (debit Speed Ud=13.46[m/s]) is impacting a surface at distance H=0.2833[m]. Flow temp is Tj=20°C Wall temp is Tp=30°C

The input flow profile is either
- a uniform flow with speed Ud and standard length scales It=0.05 and Lt=0.01
- an established flow profile Ue, ke, epe
- a non-established flow profile Ueb, keb, epeb

The model is very capricious, I'm on it since 2 monthes and it seems to converge quite rarely...

1) First, I tried to get the turbulent low-Re flow physic to work. The first file shows my first approach, with a thin wall at the side of the jet. In that case, the model converge to a solution where the flow in the dead zone is homogenous, and the shape of the turbulent viscosity seems correct. But if I try to refine the mesh, then it doesn't converge anymore, and the 'thin wall' solution is not really practical either.

2) The second file shows a model which converges to a solution where on can see a (toric) rotation structure in the flow of the dead zone. As a result, the shape of the turbulent viscosity looks quite bad. Here the mesh is quite dense, and the model does not converge anymore if I try to enlarge it in the zones that aren't that interesting. You will see a lot of unused meshes and functions that come from many tries I made to make this damn *thing* converge. Many times, the rotation structure goes mad on itself and creates a wirlpool with growing turbulent energy that makes the model diverge.

3) Based on that 2nd model, I tried a multiphysics model adding a heat transfer physics (ht+spf) or using non-isothermal Low-Re turbulent flow (nitf). Whatever I try (mesh, BC, etc.), there is no convergence.

So could anyone help me to understand:
- what makes my first model converge only with large mesh and diverge if I refine the mesh?
- where do those rotational structures come from? Are they realistic physics, or artefacts? Maybe the incoming flow from the outputs BC adds turbulent energy to the system? But changing BC outputs to open frontiers and specifying k=0 and ep=0 does not seem to help.
- Is there anything wrong in my model (in Discretization or Stabilization prameters for instance)
- And finally, are my solver configuration parameters correct?

Thanks a lot in advance!


(All parameters are taken from: "Etude comparative de modeles à bas nombre de Reynolds dans la prédiction d'un écoulement à point de stagnation" (in French) R. Hadef, B. Leduc. 5, 2002, Int. Comm. Heat Mass Transfer, Vol. 29, pp. 683-695.)