I'm doing a thermal analysis with NX Thermal (not Nastran). As I'm searching the convergence temperature of a steady-state-problem I'm playing with the number of finite elements in a component of the system.
The problem is that if I go with the element length under 0.05 mm (for example 0.04mm) the simulation seems to have problems and does not converge. Over the iterations the model solid maximum temperature is jumping around in the range of nearly 0 to 6000 °C; also the minimum temperature is even below 0 (impossible... the model is cooled with fluid at 22°C). I know that the max temperature should be around 160 - 170 °C.
What can I do to get a converging model again?
I guess you need to use 2D elements in stead of 3D elements of such small elements.I had the same issue when I wanted to mesh a fluid with small gaps of 30 µm. I solved that by removing the small gaps in the fluid. After doing that I could mesh the fluid again with 3D elements.Another way could be to mesh the part with a 2D mesh and connect the surrounding parts with a thermal coupling.
The element size is important regarding accuracy results, but if for reducing element size it caused that the mesh gets higly distorted then solution could diverge. Also, you do not mention what type of Finite Elements you are using: 3-D TET/HEX, or 2-D elements, this is important to understand the problem.
Mesh quality is critical, then take a look to mesh distorsion, inspect your model, check for Jacobian, Aspect Ratio & TET collapse ratio, and see if mesh mass the minimum threshold quality requirements. Activate the flag "Auto Fix Falied Elements" during Meshing process, the software NX AdvSim uses different strategies to repair element quality issues depending on the quality criteria that the element failed:
If the software removes an element, it maintains the connectivity of the nodes on the surrounding elements such that the mesh remains contiguous. The software removes elements that fail Tet Collapse when:
The mesh quality is the trick!. Avoit to use at all TET elements and mesh with HEXAEDRAL elements, you will see a great improvement in both mesh quality & accuracy results!!.