Scalar field inconsistency at geometry face border?
Posted: 16 May 2019, 10:45
Hi Elmer community,
as I found very competent help here for my few past questions, I'd like to tap this ressource once more for a Problem that I don't get fixed on my own, as it seems. I am performing a Navier-Stokes calculation of a simple case, and I get inconsistent results that seemingly depend on the way, how the geometry model is made.
I am preparing the model in Salome 8.3.0 from a Step Import and also produce the mesh there. The Simulation deals with flow in narrow channels, so the "thickness" of the model is sheet-like in large Areas. It gets meshed in Salome just fine, with the element size in thickness direction in the range of sheet thickness. Across such a sheet area, the elements often are flattened. Mesh density is increased at geometry edges. It also solves nicely in Elmer 8.2 in appropriate time, although stability of the solution could be higher. I Attribute this however to the narrowness of the geometry and the fact, that I am simulating water (viscosity in the range of mPas).
The results are viewed in Paraview 5.2. It Shows that from three Areas that should behave in principle identical (e.g. area 2 in the Picture), one (area number 1) is off and Shows a steep drop in the scalar field along a line that corresponds with a face edge in the geometry model, that also lies directly in x-z-plane. Also generally it seems that the Solutions has "steps" in the scalar field at face transitions of the geometry model, although the faces are transitioning tangentially.
Among the possible Solutions paths I have investigated are:
- increasing mesh density: this is currently limited to the amount of Memory in my machine. I am getting an upgrade there in the next few days - for right now I have to live with approx. 1-2 Million Elements. In this range of element numbers, there is no Change of the phenomenon. An artificially coarse mesh does not help either.
- healing the model in salome: the face edges have sometimes lower precision than 1e-6, but healing does not really help there. It does not Change this phenomenon, and meshing and solving still works without Errors thrown....
- I also tried to build the model from scratch in Salome not using Step import, which works as well, and all edges have proper Maximum precision and no Errors. But still the phenomenon remains, in some cases is even more pronounced, depending on meshing Details.
- another time I rotated my models such that no face edges are lying on the principal axis directions, but it also did not Change. The face edges remains problematic.
- using a 120° sector model and apply symmetry. This makes the phenomen seemingly go away (did not Play too much with mesh variants), but mid-term, this is no Course of Action for me as I will Need to apply modifications to the model that make it unsymmetric by Definition.
I hope someone else may have some additional idea what I could try, or even state a Focus as to where apply tweaks, i.e. in which of the Software, Salome or Elmer? (I think Paraview just Shows whatever the other two produce, so I exclude it as possible source of the Problem).
Thanks in advance for your contributions - let me know if I missed an important aspect to explain.
Thomas.
as I found very competent help here for my few past questions, I'd like to tap this ressource once more for a Problem that I don't get fixed on my own, as it seems. I am performing a Navier-Stokes calculation of a simple case, and I get inconsistent results that seemingly depend on the way, how the geometry model is made.
I am preparing the model in Salome 8.3.0 from a Step Import and also produce the mesh there. The Simulation deals with flow in narrow channels, so the "thickness" of the model is sheet-like in large Areas. It gets meshed in Salome just fine, with the element size in thickness direction in the range of sheet thickness. Across such a sheet area, the elements often are flattened. Mesh density is increased at geometry edges. It also solves nicely in Elmer 8.2 in appropriate time, although stability of the solution could be higher. I Attribute this however to the narrowness of the geometry and the fact, that I am simulating water (viscosity in the range of mPas).
The results are viewed in Paraview 5.2. It Shows that from three Areas that should behave in principle identical (e.g. area 2 in the Picture), one (area number 1) is off and Shows a steep drop in the scalar field along a line that corresponds with a face edge in the geometry model, that also lies directly in x-z-plane. Also generally it seems that the Solutions has "steps" in the scalar field at face transitions of the geometry model, although the faces are transitioning tangentially.
Among the possible Solutions paths I have investigated are:
- increasing mesh density: this is currently limited to the amount of Memory in my machine. I am getting an upgrade there in the next few days - for right now I have to live with approx. 1-2 Million Elements. In this range of element numbers, there is no Change of the phenomenon. An artificially coarse mesh does not help either.
- healing the model in salome: the face edges have sometimes lower precision than 1e-6, but healing does not really help there. It does not Change this phenomenon, and meshing and solving still works without Errors thrown....
- I also tried to build the model from scratch in Salome not using Step import, which works as well, and all edges have proper Maximum precision and no Errors. But still the phenomenon remains, in some cases is even more pronounced, depending on meshing Details.
- another time I rotated my models such that no face edges are lying on the principal axis directions, but it also did not Change. The face edges remains problematic.
- using a 120° sector model and apply symmetry. This makes the phenomen seemingly go away (did not Play too much with mesh variants), but mid-term, this is no Course of Action for me as I will Need to apply modifications to the model that make it unsymmetric by Definition.
I hope someone else may have some additional idea what I could try, or even state a Focus as to where apply tweaks, i.e. in which of the Software, Salome or Elmer? (I think Paraview just Shows whatever the other two produce, so I exclude it as possible source of the Problem).
Thanks in advance for your contributions - let me know if I missed an important aspect to explain.
Thomas.