General Mechanical

General Mechanical

Why do load vectors change over time with constant applied heat flux?

    • centzminger
      Subscriber

      I'm having trouble interpreting the data I'm exporting from Ansys Mechanical. I can get the stiffness matrix, load vector, and mapping files from Ansys. I have a simple cube with a heat flux constraint on top and a cooling convection constraint on the bottom.


      The heat flux has a time varying load curve with load steps at seconds [1 5 10 50 100 500]:


      1000W from 0 to 50 sec,


      1000 to 0W from 50 to 100 sec,


      0W from 100 to 500 sec


      I am trying to export the stiffness and load matrices from Ansys Mechanical to solve myself in Matlab using model order reduction techniques. Through my efforts, I've found that the load vector varies over time. I thought that this may be what was causing the errors in my simulations and have tried to export the load vector data at every load step so that I may interpolate between the values. I used the GET, VEC, and EXPORT commands in APDL using an inserted Commands block to obtain the load vector at each load step.


      It is my understanding that the load vector is the applied loads to the model in a rudimentary sense. For a 2D two node element with one degree of freedom at each node, you get a 2x2 stiffness matrix and a 2x1 load vector where the load vector is the applied force at that node. Then for a global assembled stiffness matrix, you combine all the stiffness matrices and equations together to make a big nxn matrix where n is the number of degrees of freedom. Why does the exported load vector change values if the applied heat flux is constant from time 0 to 50 seconds? My material properties and geometric properties are unchanging with time or temperature.


      More so to my confusion, I found that the stiffness matrix exported with both the heat flux and convection constraints applied was not equal to the stiffness matrix with just an applied convection constraint. It is my understanding that the stiffness matrix depends on the material and geometric properties and is not dependent on the input heat fluxes or other constraints.


      Could someone help explain these concepts to me a bit more clearly? Is there some documentation for how the load vectors and stiffness matrix are created? Attached I have Fi and Fc, Ansys Output, and my Matlab Output. Fi is the output load vectors at each load step, where each column is a different load vector with both the heat flux and the convection constraints being applied. Fc are the vectors for convection only. I’m currently only using the last exported load vectors, Fi(:, 6) and Fc(:, 6) for my Matlab simulation along with the exported stiffness matrices. Does that make the other load vectors irrelevant? 


      References:


      http://homepages.cae.wisc.edu/~suresh/ME964Website/M964Notes/Notes/introfem.pdf


      https://www.sharcnet.ca/Software/Ansys/17.0/en-us/help/ans_cmd/Hlp_C_HBMAT.html


      …and other APDL command documentation…


      Let me know if I can provide code, data, or clarifications, thanks!

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