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Problems with dynamic mesh settings while mesh motion

    • Midjan Jusic
      Subscriber

      I attempted to move the 'moving cylinder' downwards to press the water beneath it through the pressure outlet. To achieve this, I adjusted the dynamic mesh settings several times and experimented with various variations (Layering, Smoothing, Remeshing).

      I created two dynamic mesh zones:

      • 'moving cylinder' as a rigid body
      • 'interior' as deforming

      While the translation of the 'moving cylinder' is successful, the remeshing process does not work correctly. The mesh above the 'moving cylinder' stretches instead of generating new layers. I also tried using triangles. How can I avoid this problem?

       

      Thanks!

    • Federico
      Ansys Employee

      This looks like a classic Layering case.

      Make sure you specify the ideal cell height for each layer in the Meshing Options tab from the Dynamic Mesh Zones dialog box.

      Layering will split layers that are 1.4 times this ideal height and collapse those that are 0.2. The default is height is 0, which means to remeshing for the layer, which I suspect is why you're seeing the stretching you are showing. 

    • Federico
      Ansys Employee

      You can read more on this here: 50.9. Dynamic Mesh Task Page (ansys.com)

    • Midjan Jusic
      Subscriber

      Thank you for these answers. I have one more question regarding this. In the case I described, do I need to create more than one dynamic mesh zone? Or would a single dynamic mesh zone be sufficient for the moving cylinder?

      Another question concerns how I can lower the cylinder depending on pressure. I've attempted this using the pressure-inlet boundary condition, but it doesn't result in any movement.

       

      Thank you.

      • Federico
        Ansys Employee

        Yes exactly, you can do with only the cylinder if you define the proper Cell Height in the Meshing Options as per my previous post.

    • Midjan Jusic
      Subscriber

      Thank you. But does that mean, that my cell height has to be exact the same size as my mesh element size? The element size of my mesh is 9 mm. I specified the cell height in dynamic mesh also as 9 mm. Still the problem with stretching mesh above the cylinder.

      • Federico
        Ansys Employee

        The cell height you specify is the ideal cell height for the adjacent cell layer. 

        Fluent will collapse cells with height < 0.2*[ideal cell height], and split cells with height > 1.4*[ideal cell height]

        0.2 and 0.4 are correspondingly the collapse and split factors by default. You can change these settings in the Layering settings.

    • Midjan Jusic
      Subscriber

      Hello, thank you for the answer. I tried it again. I still have problems with splitting cells above the cylinder. Collapsing below the cylinder works perfect. 

      If I build the model like here:

       

      everything works. But I need the area above the cylinder to define a pressure-inlet, because I want to move the cylinder by giving pressure in the pressure-inlet.

       

      Question 1:

      Do you have any idea, why the collapsing process works but the splitting not? The cells are still stretching above the cylinder. I tried various cell heights and splitting parameters.

       

      Question 2:

      If I definied a body force with -9.81 m/s in Y direction because I have water in my model. How can I make sure, that the cylinder does not fall downwards with -9.81 m/s? I want to move the cylinder with pressure at pressure-inlet. 

       

      Thanks a lot!

    • Federico
      Ansys Employee

      1) Do you have 2 different adjacent zones on either side of your cylinder? If so, make sure you define a cell height for both sides. In the picture I shared, there was only 1 adjacent zone.

    • Federico
      Ansys Employee

      2) You will need to enable the 6DOF solver to have the cylinder move in response to pressure forces. 12.6. Using Dynamic Meshes (ansys.com)

    • Midjan Jusic
      Subscriber

      Hello, I think I discovered the reason for the stretching but I'm still looking for a solution:

      I just have one interior-face for the mesh. The area above and below the cylinder is just one zone in my case. I think the cell-row above and below the cylinder is one row, instead of two. If  the cylinder moves downwards and the cell-row below collapses (due to the collapsing factors), the cell-row above the cylinder collapses also. Thats why it seams like a "stretching-process". If I try to separate the cell zones in Ansys fluent, the two "not-touching" cylinder lines also separate. After that I cant combine them anymore, because they dont touch each other.

       

      Do you know how I can separate the cell zone in two cell zones (one above, one below) in the mesher, bevor I start to configurate the set-up? 

       

      Thanks!

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