hello to all
 

in your opinion the mesh size of a problem is difference with that problem with for example 0.01 scale?
i think this is a joke but i am confused about it?
for example you suppose that you have two problems :
1-flow around a sphere with 1 meter diameter
2- same problem with .01 meter diameter
so,what is the sale of minimum mesh size for each other? is the ratio 0.01 or about it?
i applogize everyone due to my bad english and i wish i could get my goal.

Unfortunatly there is no blanket rule for mesh sizing. You will need to already predict some characteristics about the flow and then decide where you want higher resolution and therefore more cells. For the topics you mentioned, you probably want a refined wake behind the sphere to capture the unsteady nature of the flow. Mesh sizing is usually just a compromise between accuracy vs time to solve and computational resources.

Stu