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Old   May 1, 2012, 17:36
Default convergence criteria
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ramila.h
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I am trying to write a convergence criteria for a code ans I really need your help to do so
it's like (tempnew-tempold) less than 1e-6
do k=1,mstep

do i=0,n
do j=0,n
temp(0,0)=0.0
temp(i,j)=th(i,j)
diff=temp(i,j)-temp1(i,j)
enddo
enddo
temp1=temp
if (diff.lt.temp1e-6)then
stop
endif
enddo

Last edited by afnene; May 1, 2012 at 17:40. Reason: error
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Old   May 1, 2012, 21:48
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Chris DeGroot
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Your logic doesn't make sense.

do k=1,mstep
do i=0,n
do j=0,n
temp(0,0)=0.0 <-- Why is this inside the loop; doesn't depend on i, j, or k
temp(i,j)=th(i,j) <-- What is "th"? It is never used
diff=temp(i,j)-temp1(i,j) <-- You calculate "diff" inside the loop and do nothing with it; it's not an array so it will be erased each time the loop is incremented
enddo
enddo
temp1=temp <-- What is this supposed to do?
if (diff.lt.temp1e-6)then <-- "diff" is meaningless here since it is only that value for i=j=n
stop
endif
enddo

To check time convergence you could do something like:

maxdiff=0.0
do i=0,n
do j=0,n
diff=abs(tempnew(i,j)-tempold(i,j))
if (diff>maxdiff) then
maxdiff=diff
end if
end do
end do
if (maxdiff<1.0e-6) then
stop
end if
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Old   May 2, 2012, 14:40
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ramila.h
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Thank you so much for your precious help
I am just a beginner in CFD computation, The first step I did was to try how convergence can be written in a code
Please tell me how to read the old a new value of temperature inside iterrations?
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Old   May 2, 2012, 14:47
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Chris DeGroot
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Quote:
Originally Posted by afnene View Post
Thank you so much for your precious help
I am just a beginner in CFD computation, The first step I did was to try how convergence can be written in a code
Please tell me how to read the old a new value of temperature inside iterrations?
I'm not sure I totally understand your question, but I will try my best to answer. I think you are asking about how to access an old value of temperature inside your loop. If you have a temperature array T(n,n) that you are working with, you should also define an array to store your old temperature, for example TO(n,n). Before updating T with your new solution you should transfer it to TO so you still have access to it when you go to check convergence. See below for an outline of what you should do.

1. Initialize T and TO
2. According to your numerical model find a new temperature field T
3. Check convergence between T and TO (see my previous post)
3a. If converged, exit
3b. If not converged, transfer T to TO and go to step 2
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Old   May 2, 2012, 15:05
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Thanks again,
I will do what you explained and let you know if it works for me
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Old   May 2, 2012, 17:36
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Well, It sounds working,but I really want that you check that with me and of course if you don't mind
the subroutine which compute temperature is as following:


subroutine tcalcu(f,temp,temp1,n,m)
real f(0:8,0:n,0:m),temp(0:n,0:m),temp1(0:n,0:m)
temp1=temp
do j=0,m
do i=0,n
sum=0.0
do k=0,8
sum=sum+f(k,i,j)
end do
temp(i,j)=sum
enddo
enddo

I called 'temp1' the new temperature value and insert it inside the subroutine and 'temp' as an old value......does it make sense?
return
end
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Old   May 6, 2012, 22:52
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Your subroutine seems to be doing what you intend, but I can't really say if it is "correct" without knowing a bit more about what sort of algorithm you are using. So you are filling in temp1 with the existing temp array, i.e. as an old value. Then you update temp as the sum of all the local elements of f. If that is what you intend, then you are good.
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Old   May 7, 2012, 18:21
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It worked fine, I checked that by printing old and new values of temperature.

Thanks again Cdegroot, you were very helpful.

Afnene
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