Author Topic: HOw to retrieve the 43th diagonal?  (Read 247 times)

NN

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HOw to retrieve the 43th diagonal?
« on: June 19, 2017, 02:20:31 pm »
When trying do run the simulation I get the following error:

         PCG has a matrix with 43th diagonal element <= 0.
       
It could maybe be an elevation problem but how can I retrieve this 43th diagonal?

Björn Kaiser

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Re: HOw to retrieve the 43th diagonal?
« Reply #1 on: June 19, 2017, 02:28:58 pm »
The diagonal in the matrix must not be <= 0. There are several possible reasons for negative entries along the principal diagonal of the matrix.
1.   Strong contrasts in material properties of adjacent elements. Try to refine the mesh horizontally/vertically.
2.   Topological errors of the mesh as triggered inconsistent input data (e.g. overlapping elements/element edges, wrong numeration of the nodes).

NN

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Re: HOw to retrieve the 43th diagonal?
« Reply #2 on: June 19, 2017, 02:35:37 pm »
I tried to correct the elevations in the 3D Layer configuration but it doesn't work.

I initially had 4 layers and then removed 3 of them, just having two slices with about 50 m vertical distance between them. In this configuration I'm 100% sure that I don't have any overlapping in elevations.

The problem remains.
My question is if there's a way to manually correct this problem in the 43th diagonal?

Björn Kaiser

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Re: HOw to retrieve the 43th diagonal?
« Reply #3 on: June 19, 2017, 04:18:12 pm »
You cannot manually correct the entry. The only way is to change the settings as suggested above.

NN

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Re: HOw to retrieve the 43th diagonal?
« Reply #4 on: June 20, 2017, 01:06:09 pm »
Just a short reply that could be useful to other users: the problem was in the data regionalization method (conductivity): the kriging created negative values of hydraulic conductivity; by setting "logarithmic true" it doesn't create any mode negative values.

Björn Kaiser

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Re: HOw to retrieve the 43th diagonal?
« Reply #5 on: June 21, 2017, 01:45:43 pm »
Great, thanks.

razipv

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Re: HOw to retrieve the 43th diagonal?
« Reply #6 on: November 12, 2019, 07:00:56 am »
What is the reason for SAMG: Row(s) with negative diagonal found! ERROR
How can i rectify that?
         

Peter Schätzl

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Re: HOw to retrieve the 43th diagonal?
« Reply #7 on: November 25, 2019, 12:27:35 pm »
The most likely reason is an instability somewhere in the model. Check for oszillations in the process variables and/or large error values (by adding the error norm history to the User Data distributions).
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