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April 11, 2006, 02:01 
Finite element method  number of elements

#1 
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Hello,
I am using 2 dimensional finite element method to determine the temperature on a cylindrical wall in which fluid expands from inlet to outlet. My solution is dependent on the number of grids (620). I understand that the accuracy of the solution depends on the number of grids, of course after a certain number the accuracy will not change. But when i change the grids by 2 or 3, the solution is absurd, % error is more than 1000 sometimes. How can I make my solution less sensitive on the number of grids? Regards Poornima 

April 11, 2006, 12:19 
Re: Finite element method  number of elements

#2 
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Your solution should be grid independent. So either the number of grid is to low for your problem or you have a bug in your program.
Angen 

April 12, 2006, 07:35 
Re: Finite element method  number of elements

#3 
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I agree that the solution should be grid independent. but i am able to find solution for 6X6 grid and 10x10 grid but not for 10X6 grid, what could be the problem in this case. i guess i need to do scaling so that the equations are not a strong functions of number of grids. but i am struggling with how to design the scaling factors. i need some help in this regard.
Poornima 

April 12, 2006, 12:08 
Re: Finite element method  number of elements

#4 
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Your Grid of 10x10 or 10x6 is really too small for any cfd problem, even in 2D. Please increase your grid to say 100x60 or 100x100 an then compare the solutions.


April 13, 2006, 08:25 
Re: Finite element method  number of elements

#5 
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Despite the very coarse grids, it seems the problem is not on the grid dimensions, but on its shape. You only get acceptable results in square grids, right? Maybe you should check your mesh generation and element matrices calculation.
Márcio 

April 14, 2006, 01:42 
Re: Finite element method  number of elements

#6 
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The geometry in context is an annular cylinder and hence the shape of the grid is cylindrical. I am only trying to discetrize the equations and solve, why should changing the number of grids result in an absurd solution. Surprisingly the problem disappears when I change the dimensions i.e the thickness and the length. Is it that aspect ratio is playing a significant role in the solution. If so, please suggest me how to overcome this problem.


April 14, 2006, 07:23 
Re: Finite element method  number of elements

#7 
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I suspect that the problem is because of the coupling between deltar and deltaz. because i am able to get solution for 100*6, 6*100, but not for 100*10, I am able to get solution for 10*10, 6*6 , 6*10, but not for 10*6. The ratio of D/L is known as aspect ratio, can any one suggest how to take this into consideration inorder to obtain solution to the problem.
regards Poornima 

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