# SNES Problem

Nils Erik Svangård nilserik at gmail.com
Mon Feb 27 08:07:51 CST 2006

```Hi all,

I have problems solving some CFD problems using SNES and my custom
back-euler. I have 7 equations that I want to solve.

First my FormFunction copies the values from the Vec that SNES uses to
the variables that my code use:

RO(L)=xx(1,L)
RU(L)=xx(2,L)
RV(L)=xx(3,L)
RW(L)=xx(4,L)
ET(L)=xx(5,L)
RQ(L)= xx(6,L)
REPS(L)=xx(7,L)

Here I also print the values of ET(2) for debugging:

write(6,*)"ET(2) = xx(5,2) : ",ET(2)," = ",xx(5,2)

Then I get the fluxes by calling custom functions

call AUXVR
call FLUX
call KESRC

Then I perform back-euler  save the new value to so that I can use it
in the next iterationi (TSF(L) is Time step function, which is set by
hand and is the same for all L):

ff(1,L) = RO(L)-OLD(1,L)-TSF(L)*DRO(L)
ff(2,L) = RU(L)-OLD(2,L)-TSF(L)*DRU(L)
ff(3,L) = RV(L)-OLD(3,L)-TSF(L)*DRV(L)
ff(4,L) = RW(L)-OLD(4,L)-TSF(L)*DRW(L)
ff(5,L) = ET(L)-OLD(5,L)-TSF(L)*DET(L)
ff(6,L) = RQ(L)-OLD(6,L)-TSF(L)*DRQ(L)
ff(7,L) = REPS(L)-OLD(7,L)-TSF(L)*DREPS(L)

And save the new value of RO-REPS to use in the next iteration of back-euler:

OLD(1,L)=RO(L)
OLD(2,L)=RU(L)
OLD(3,L)=RV(L)
OLD(4,L)=RW(L)
OLD(5,L)=ET(L)
OLD(6,L)=RQ(L)
OLD(7,L)=REPS(L)

Here I print the values of L=2 as above for debugging:

write(6,*)ff(5,2)," = ",ET(2),"-",OLD(5,2),"-",TSF(2),"*",DET(2)

The program exit abnormaly after 33 runs of the FormFunction. The
cause of this is that AUXVR tries to perform sqrt(ET(2)) when ET(2) is
negative, ET is the total energy and should never be negative. It
seems that all of a sudden the PETSc SNES solver decides to supply the
FormFunction with a negative ET value. I need help understanding why
and how to fix it (if it is fixable).

Here is the output of:
write(6,*)"ET(2) = xx(5,2) : ",ET(2)," = ",xx(5,2)
Which is in the beginning of my FormFunction.
And:
write(6,*)ff(5,2)," = ",ET(2),"-",OLD(5,2),"-",TSF(2),"*",DET(2)
Which is in the end of my Formfunction.

1. ET(2) = xx(5,2) :   253250.  =   253250.000000000
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
2. ET(2) = xx(5,2) :   253250.  =   253250.000010259
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
3. ET(2) = xx(5,2) :   253250.  =   253249.999999988
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4132419437260  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941324E+08
4. ET(2) = xx(5,2) :   253250.  =   253250.000048432
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
5. ET(2) = xx(5,2) :   253250.  =   253249.999999872
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4177283437784  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941773E+08
6. ET(2) = xx(5,2) :   253250.  =   253250.000071113
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
7. ET(2) = xx(5,2) :   253250.  =   253249.999980115
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4453347441010  =
253250. -  253250.000000000 -  1.000000E-07 * -2.944533E+08
8. ET(2) = xx(5,2) :   253250.  =   253250.000037298
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4082723436679  =
253250. -  253250.000000000 -  1.000000E-07 * -2.940827E+08
9. ET(2) = xx(5,2) :   253250.  =   253249.999942706
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4082723436679  =
253250. -  253250.000000000 -  1.000000E-07 * -2.940827E+08
10. ET(2) = xx(5,2) :   253250.  =   253249.999948703
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
11. ET(2) = xx(5,2) :   253250.  =   253250.000007808
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.7283811474088  =
253250. -  253250.000000000 -  1.000000E-07 * -2.972838E+08
12. ET(2) = xx(5,2) :   253250.  =   253250.000018146
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
13. ET(2) = xx(5,2) :   253250.  =   253250.000001865
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   28.3742819315846  =
253250. -  253250.000000000 -  1.000000E-07 * -2.837428E+08
14. ET(2) = xx(5,2) :   253250.  =   253249.999997296
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
15. ET(2) = xx(5,2) :   253250.  =   253250.000000049
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   30.6548963582361  =
253250. -  253250.000000000 -  1.000000E-07 * -3.065490E+08
16. ET(2) = xx(5,2) :   253250.  =   253250.000022870
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
17. ET(2) = xx(5,2) :   253250.  =   253249.999999716
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.5326051451209  =
253250. -  253250.000000000 -  1.000000E-07 * -2.953260E+08
18. ET(2) = xx(5,2) :   253250.  =   253250.000074328
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4165187437643  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941652E+08
19. ET(2) = xx(5,2) :   253250.  =   253250.000049809
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4085059436707  =
253250. -  253250.000000000 -  1.000000E-07 * -2.940851E+08
20. ET(2) = xx(5,2) :   253250.  =   253250.000019253
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
21. ET(2) = xx(5,2) :   253250.  =   253250.000000189
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.8851075492403  =
253250. -  253250.000000000 -  1.000000E-07 * -2.988511E+08
22. ET(2) = xx(5,2) :   253250.  =   253249.999997897
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4085059436707  =
253250. -  253250.000000000 -  1.000000E-07 * -2.940851E+08
23. ET(2) = xx(5,2) :   253250.  =   253249.999975142
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.3961539435263  =
253250. -  253250.000000000 -  1.000000E-07 * -2.939615E+08
24. ET(2) = xx(5,2) :   253250.  =   253250.000018324
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4085059436707  =
253250. -  253250.000000000 -  1.000000E-07 * -2.940851E+08
25. ET(2) = xx(5,2) :   253250.  =   253249.999987073
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4165187437643  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941652E+08
26. ET(2) = xx(5,2) :   253250.  =   253249.999980982
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4165187437643  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941652E+08
27. ET(2) = xx(5,2) :   253250.  =   253249.999994645
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
28. ET(2) = xx(5,2) :   253250.  =   253249.999962364
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4165187437643  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941652E+08
29. ET(2) = xx(5,2) :   253250.  =   253249.999999921
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
30. ET(2) = xx(5,2) :   253250.  =   253250.000000058
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   28.4019971319085  =
253250. -  253250.000000000 -  1.000000E-07 * -2.840200E+08
31. ET(2) = xx(5,2) :   253250.  =   253250.000040348
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4085059436707  =
253250. -  253250.000000000 -  1.000000E-07 * -2.940851E+08
32. ET(2) = xx(5,2) :   253250.  =   253250.000005245
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
33. ET(2) = xx(5,2) :   253250.  =   253250.000005245
ff(5,2) = ET(2)-OLD(5,2)-TSF(2)*DET(2) :   29.4126563437192  =
253250. -  253250.000000000 -  1.000000E-07 * -2.941266E+08
34. ET(2) = xx(5,2) :  -848141.  =  -848141.388090847
--------------------------------------------------------------------------
Petsc Release Version 2.3.0, Patch 32, April, 26, 2005
See docs/faq.html for hints about trouble shooting.
See docs/index.html for manual pages.
-----------------------------------------------------------------------
./SNESG3D2 on a linux-gnu named sethnx004.vac.com by yy26539 Mon Feb
27 14:21:13 2006
Configure run at Thu Oct 13 08:23:56 2005
Configure options --with-cc=gcc --with-fc="f77 -N109"
-----------------------------------------------------------------------
[0]PETSC ERROR: Caught signal number 8 FPE: Floating Point
Exception,probably divide by zero
[0]PETSC ERROR: Try option -start_in_debugger or -on_error_attach_debugger
[0]PETSC ERROR: likely location of problem given in stack below
[0]PETSC ERROR: --------------- Stack Frames ---------------
[0]PETSC ERROR: Note: The EXACT line numbers in the stack are not available,
[0]PETSC ERROR:       INSTEAD the line number of the start of the function
[0]PETSC ERROR:       is given.
[0]PETSC ERROR: [0] SNES user function line 0 unknownunknown
[0]PETSC ERROR: [0] SNESComputeFunction line 788 src/snes/interface/snes.c
[0]PETSC ERROR: [0] SNESLineSearchCubic line 514 src/snes/impls/ls/ls.c
[0]PETSC ERROR: --------------------------------------------
[0]PETSC ERROR: User provided function() line 0 in unknown directory
unknown file
[0]PETSC ERROR:  !

Suggestions and tips are very welcome!
/nisse

--
Nils-Erik Svang�rd
MSN: schweingaard at hotmail.com
Skype: schweingaard

```