ErRoR StUdY
Can somebody advice me how to do an error study in a grid convergance study !!!

Re: grid convergence
The way I do a grid convergence study is to look at the parameters in which I'm interested, in the region I'm studying, for at least three different mesh densities. For example, at an inlet where I want to know velocity and/or velocity fluctuation, I look at the individual values at grid nodes and also the mean across the inlet region for each of the mesh densities.
The error can be as simple as solution_1solution_2 or you can look at the percent difference from one solution to the next: (solution_1solution_2/solution_1)*100. You can then plot the error. If the curve flattens, i.e. the error remains constant (and low, like nearly 0), then you can qualitatively conclude grid convergence. As far as I know, this is really all you need to demonstrate whether or not you have grid convergence. 
Re: grid convergence
thanks for repy, do you mean by this
solution_1solution_2 the NORM !!! & under which crateria you obtain the results.... 
Re: grid convergence
as in a vector norm? no, this is just an absolute value.
the criteria for obtaining the results? not sure what you mean by that. the method of taking the absolute value of the difference between two results from different iterations (grid convergence iterations in this case, of which there only a few) is very common in numerical analysis. 
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