the use of RMSD and NRMSD

Hi everyone

I have simulated the velocity in an object using fluid dynamics in transiet mode using different mesh-sizes. When the mesh-size is increased the accuracy of the simulation will increase. However, to a point there will be mesh independency, but the simulation variables (e.g. velocity of flow) will of course still vary across the different mesh-sizes but the difference in the variables will decrease with increasing mesh-size. I want to figure out which of the mesh-size will be good enough to choose and discuss the error comparing it with the other mesh-sizes (as increasing mesh-size requires large amount of time and memory on disk which I dont have enough of). I have 3 different mesh sizes respectively, mesh 1, mesh 2 and mesh 3 and for each mesh there will be 200 seconds/ time-step and for each time-step a velocity is calculated. I want to figure out how large the error will be when I choose mesh 1 instead of mesh 2 and mesh 3.
I was wondering how it was possible using RMSD and NRMSD and if you could recommend be some articles or literature?