Questions about Nonlinear elasticity, PROM

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
25/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Stale
Tech stack
cpp
Domain
hpc

Research direction

Start with examples/prom/nonlinear_elasticity_global_rom.cpp and the supplied nonlinear_elasticity_global_rom command. Trace how the position and velocity errors are calculated and how rvdim, rxdim, and hdim are used; done means explaining the reported discrepancies or identifying a reproducible correction.

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Description

I'm working with the nonlinear elasticity example:

https://github.com/LLNL/libROM/blob/master/examples/prom/nonlinear_elasticity_global_rom.cpp
https://www.librom.net/examples.html

My command line is

./nonlinear_elasticity_global_rom --mesh "../data/beam-hex-nurbs.mesh"  --online -dt 0.01 -tf 5.0 -s 14 -vs 5 -hyp -rvdim 40 -rxdim 10 -hdim 71 -nsr 200 -sc 4.0

I have the following questions:

  1. I get relative error of ROM position at t_final as 0.00677831 which agrees well with the error reported in your example.
  2. I get relative error of ROM velocity at t_final as 9.58099. Why is this so high? After looking at the code, it seems to be 950% error.
  3. What do 'rvdim' , 'rxdim', and 'hdim' mean? Are 'rvdim' and 'rxdim' the number of basis vectors used to approximate the velocity and displacement respectively? And is 'hdim' the number of basis vectors used to approximate the non-linear term?

Edit: Changing the rvdim to 80 in the command line above increases the errors. The position error is 0.00749 and velocity error is 12.33 both of which have increased inspite of number of basis vectors increasing, and I fail to understand this.

Thanks,

Nachiket

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