Navier Stokes: only one boundary condition workring

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scorpdaddy
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Joined: 16 Nov 2015, 04:22
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Navier Stokes: only one boundary condition workring

Post by scorpdaddy »

Coming up the curve on Elmer. Trying to simulate 3 bodies, a simple heat exchange: 2 air bodies counter-flowing through a metal box. Drawn in Freecad. Exported as step - step file attached. Meshed with Gmsh with mesh to feature size set to 0.33. Ran Netgen optimization. Then exported msh file. Setup Elmer using "curved pipe" tutorial as guide. Seems like the boundary conditions are not setup right. Also, the mesh has more surfaces per body then the "curved pipe" example did. At every face where one solid meets another, both solids have a surface at that face- i.e. two identical surfaces, one attached to each body. Have tried attaching boundary conditions to either one of the two identical surfaces.

In short there should be cold flow into one end, warm flow into the other end, and heat exchange going on as they pass through the metal box. Flow doesn't look right on one of the air bodies. Temperatures are not as expected either. Have tried slowing air flow to get more noticeable heat transfer. No good. Tried using one NS+Heat equation for both air bodies and one for the metal box, per the "curved pipe" tutorial. No good. Have tried using 2 separate equations, e.g. one per air body, as well as the metal box heat equation. No good. I suspect (1) the way the model is drawn may not work in Elmer with dual identical surfaces between each body, and (2) the boundary conditions are not setup correctly.

This is not the end goal of the project. The metal interface between the air bodies is going to get more complex. But in order to get up the curve, this is a very simplified model.

Any tips will help get up the curve, TY.
Attachments
hex test 3.zip
Elmer Project folder less mesh file
(858.88 KiB) Downloaded 304 times
one third.zip
gmsh mesh with 1/3 feature size mesh
(449.07 KiB) Downloaded 289 times
all.step
original model with 3 solids
(78.37 KiB) Downloaded 306 times
mzenker
Posts: 1999
Joined: 07 Dec 2009, 11:49
Location: Germany

Re: Navier Stokes: only one boundary condition workring

Post by mzenker »

Hi,

you seem to have have duplicated interfaces between your bodies, so there cannot be any flow between them. gmsh cannot resolve those duplicated faces (except for very simple cases - you might try "cut and merge faces" in tools->options->geometry->general before you open your geometry).
Solution: either one of 1) or 2) below.
1) Prepare your geometry so that all interfaces are unique (e.g. with gmsh directly)
2) Use Salomé, import the geometry (e.g. in STEp format) and do a partition (see http://www.elmerfem.org/elmerwiki/index ... e_to_Elmer). Then either do the meshing in Salomé or export the partitioned geometry as brep, import in gmsh and mesh.

HTH,

Matthias
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