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Help with COMSOL - Turbulent flow & Heat transfer Impacting Jet in Low-Re

  1. Nov 25, 2011 #1

    HNB

    User Avatar

    Hello

    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. Oftentimes, 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.

    Please could you 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
    parmeters 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.)
     

    Attached Files:

  2. jcsd
  3. Nov 25, 2011 #2

    HNB

    User Avatar

    And adding last model, using multiphisics for non-isothermal flow, that also does NOT converge :grumpy:
     

    Attached Files:

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