mesh size: getting worse results with finer mesh

AliZuhaira · June 10, 2015, 06:08

Hello everybody,
I am using a model which is dealing with the turbulence and waves, the problem when I made the validation for the programme with the experiment data, it showed a good result; however, when i reduced the mesh size to get more accurate solution, the results getting far from the experiment data and the relative error between the experiment data and the numerical result increased and reached to 50% (when i used small mesh size)
why that happened????
any explain for that case??
Thank you in advance for any answer,

RodriguezFatz · June 11, 2015, 08:11

Possible reasons (among others):

Your fine mesh has lower quality for some reason. Thus, the solution is worse.
Your coarse mesh solution coincidentally equals the measured / correct values. Maybe because some errors cancel each other. When the grid becomes finer, numerical error decreases and the errors don't cancel each other any longer.
Your solution is not converged regarding grid size (not grid independent).

Anyway, after checking the first point, can you afford to run an even finer mesh?

June 10, 2015, 06:08	mesh size: getting worse results with finer mesh	#1
AliZuhaira New Member Ali Adel Join Date: Nov 2014 Posts: 6 Rep Power: 12	Hello everybody, I am using a model which is dealing with the turbulence and waves, the problem when I made the validation for the programme with the experiment data, it showed a good result; however, when i reduced the mesh size to get more accurate solution, the results getting far from the experiment data and the relative error between the experiment data and the numerical result increased and reached to 50% (when i used small mesh size) why that happened???? any explain for that case?? Thank you in advance for any answer,

June 11, 2015, 08:11		#2
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 27	Possible reasons (among others): Your fine mesh has lower quality for some reason. Thus, the solution is worse. Your coarse mesh solution coincidentally equals the measured / correct values. Maybe because some errors cancel each other. When the grid becomes finer, numerical error decreases and the errors don't cancel each other any longer. Your solution is not converged regarding grid size (not grid independent). Anyway, after checking the first point, can you afford to run an even finer mesh? __________________ The skeleton ran out of shampoo in the shower.

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