Dear Koki
Reducing the time step merely delays the problem. Petsc runs into error (error code 55: Out of memory) after some time steps are executed. Please find attached the revised code. I have reduced the time step to 10 ns and try to run the simulation for around 30000 time steps.  I would really appreciate your help on this. The simulation essentially should trigger elastic waves that travel along the pipe. That does not seem to happen with the results obtained for the converged time steps.

P.S. I tried to provide an initial value for the solver as the solution from the previous time step. That also delays the convergence issue but does not solve the problem.
--
Abhishek Venketeswaran
NETL Research Associate - ORISE
National Energy Technology Laboratory
Department of Energy
abhishek.venketeswaran@netl.doe.gov
Work: 412-386-4833
Mobile: 716-507-7890
www.netl.doe.gov

From: Sagiyama, Koki <k.sagiyama@imperial.ac.uk>
Sent: Friday, January 14, 2022 6:01 AM
To: Venketeswaran, Abhishek (FN) <Abhishek.Venketeswaran@netl.doe.gov>; firedrake <firedrake@imperial.ac.uk>
Subject: [EXTERNAL] Re: Elastic Wave Propagation in Pipe Simulation Failing
 
Hi Abhishek,

I think the explicit time integrator (central difference for acceleration + evaluation at $u_n$ for stress) requires a small enough timestep for stability. If I run PipeKMVExptrac.py with mus=1e-6 and Nsteps  = 40, I see the solution diverge, but if I use mus=5e-7 and Nsteps  = 80, I do not.

Please let us know if it does not resolve the issue.

Thanks,
Koki
From: firedrake-bounces@imperial.ac.uk <firedrake-bounces@imperial.ac.uk> on behalf of Venketeswaran, Abhishek (FN) <Abhishek.Venketeswaran@netl.doe.gov>
Sent: Wednesday, January 12, 2022 10:31 PM
To: firedrake <firedrake@imperial.ac.uk>
Subject: [firedrake] Elastic Wave Propagation in Pipe Simulation Failing
 
Hi,
I have spent considerable time trying to debug my code and am not able to find the bug in the code. I am essentially trying to simulate 3D elastic wave propagation in a pipe fixed on both ends and having an ultrasonic collar triggering ultrasonic impulse on a portion of the outer pipe segment. I would ideally like to use the KMV elements (similar to the spyro package). I generated a tetrahedral mesh of the pipe using GMsh and labeled the ends of the pipe using tag "1" and the collar segment as "2". Next I wrote the UFL expressions for 2nd order central difference for acceleration term and the elasticity as well as traction terms. The code runs but the displacement at certain nodes diverges to infinity and results in nonsensical results. I have tried multiple things including
  1. specifying the collar load as displacement as well as traction
  2. trying out KMV and CG element
  3. trying out different KSP methods and preconditioners
  4. Formulating the problem using semi-implicit HHT as well as second order explicit difference method
  5. Ensuring the Gamma value of the tetrahedral mesh is atleast 0.2
I would really appreciate  it if someone could help me out with this.

--
Abhishek Venketeswaran
NETL Research Associate - ORISE
National Energy Technology Laboratory
Department of Energy
abhishek.venketeswaran@netl.doe.gov
Work: 412-386-4833
Mobile: 716-507-7890

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