General Mechanical

General Mechanical

What is Ansys doing with Element Birth and Death Elements?

    • solani
      Subscriber

      Hello everybody,

      I wanted to do a transient thermal simulation about an geometry element that is removed and later on re-introduced (with a different temperature) in the simulation. I use Ansys Mechanical.

      To get the simulation done I use the element birth and death technique: '

      Step 1: Thermal analysis with active element,

      Step 2 "without" this element, more or less the same simulation

      When I checked the solution for the removed element I found something weird:

    • BenjaminStarling
      Subscriber
      I am not sure if I can come up with an intuitive response as I primarily work with structures, and do limited thermal. But in a structural analysis when an element is killed, it's stiffness is set to near 0 through the use of the ESTIF variable, which is set on the ESTIF command, this defaults to 1e-6. For a thermal analysis the conductivity is the analogue to stiffness, and is what is set to a near 0 value. The other effect of element death is that element loads are set to zero, and when they are born again they are reintroduced at this 0 value. For a structural analysis this is the strain of the element, and I believe for thermal this is heat energy.
      To answer why you can still see a temperature result after element death.
      Temperature is analogous to Displacement between thermal and structural analysis. When using birth/death the elements never truly dissapear, they are still in the solution, just with the modifications mentioned above. So in a structural analysis Displacement on these nodes/elements will still be available but can be interpreted in two ways.
      The displacements are nonsense, we have removed the body/material from the structure therefore we do not care what the displacements are, we can imagine that they are either 0, or infinite or any value in between.
      The displacements are representative of the displacement of the bodies/materials neighbouring the dead elements. If we imagine a body with no stiffness, and no loads applied, attached to a structure that does have stiffness and loads applied, this body will just follow/conform to the structure. This is the temperature I believe that you are seeing after the element death has occured, it is the temperature driven by the surrounding elements, but this may be incorrect (again, not super familiar with thermal analysis).

    • solani
      Subscriber
      Thank you for your fast an very detailed answer. It helps me a lot. So I just ignore these temperatures and hope that they dont effect my simulation in a bad way...
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