Divergence detected in AMG solver: pressure coupling

- February 18, 2019 at 9:55 am
  
  tayyaba.bano
  Subscriber
  
  Hi
  
  I am doing 2 way FSI simulation of a flap.Case is 3D and flow is laminar .I started simulation with pressure based transient flow using time step 1e-4. Coupled scheme is used with second order implicit transient formulation.
  
  Case worked well until 1800 coupling steps then suddenly I got the error
  
  "Divergence detected in AMG solver, pressure coupling"
  
  I want to know the reason for this type of error, if any one could guide me please. Is there any problem in solver or discretization scheme?
  
  Waiting for a kind response.
  
  Kind regards
  
  Tayyaba
- February 18, 2019 at 10:38 am
  
  DrAmine
  Ansys Employee
  
  Might be due to many reasons: you need to debug! Just check the results before that point and store cell residuals so that you find the regions which might resulted to divergence. Check if the displacement sent to Fluent from Mechanical are pretty large and here you need to assess for more stability (implicit update might be helpful here)
- February 18, 2019 at 12:43 pm
  
  tayyaba.bano
  Subscriber
  
  Thank you very much for your kind reply.
  
  Implicit update means change in scheme?
  
  Regards
  
  Tayyaba
- February 18, 2019 at 1:01 pm
  
  DrAmine
  Ansys Employee
  
  Check the documentation. In Transient run do not reduce the URF to smaller value (best to be one not lower then 0.. You need to monitor the force on the FSI walls in Fluent to monitor its evolution of the number of steps.
  
  You have in Fluent the Solution Stabilization: Dynamic Mesh>System Coupling boundary (Solver Options).
  
  And for general Dynamic Mesh Motion: Implicit Update
- February 18, 2019 at 2:46 pm
  
  tayyaba.bano
  Subscriber
  
  Thanks once again.
  
  But I was working with URF as 0.1 in data transfer 'Transient structural' as a participant and for 'Fluent flow' as a participant I used URF as 1.
  
  And yes I have already used this solution stabilization option in some other simulation but still the question is which method to choose?Either volume based or coefficient based? and the right value of scale factor.
  
  Also does 'double precision' effect this type of error? as I did not make it on.
  
  Regards
  
  Tayyaba
- February 18, 2019 at 3:37 pm
  
  DrAmine
  Ansys Employee
  
  Coefficient based has shown to be more robust and not mesh dependent. So go for it.
- February 20, 2019 at 1:13 pm
  
  tayyaba.bano
  Subscriber
  
  Thank you very much for your reply.
  
  Is it possible to run the existing case with these changes? or should I have to run the case from beginning which is quite hard (:
  
  as I got the error almost at the middle of solution.
  
  Regards
  
  Tayyaba
- February 20, 2019 at 2:21 pm
  
  DrAmine
  Ansys Employee
  
  Best to run from scratch as usual.
- February 4, 2020 at 4:14 pm
  
  utkug
  Subscriber
  
  Hello Abenhadj,
  
  I have a two way FSI related issue and have been reading the discussions and came across your comment. Whenever I try to use implicit update, fluent says that implicit update values will be overwritten by system coupling settings. I was wondering if there is a way to get around that or is it actually updating the dynamic mesh implicitly?
  
  Check the documentation. In Transient run do not reduce the URF to smaller value (best to be one not lower then 0.. You need to monitor the force on the FSI walls in Fluent to monitor its evolution of the number of steps.
  
  You have in Fluent the Solution Stabilization: Dynamic Mesh>System Coupling boundary (Solver Options).
  
  And for general Dynamic Mesh Motion: Implicit Update