Does Fluent provide DNS just for laminar flow? Beside all turbulent models(such as DES,LES), I can not see any other option for turbulent flow. I would be appreciated if someone answer my question.

NO

Can you please explain more?

when doing dns you solve the laminar flow equations. when your flow is turbulent the grid should be fine enough to resolve all the turbulent scales but the equations are the same. so you should select the "laminar" option in the viscous panel.

Thanks for the answer. Do I have to deal with "komogolov length scale" in determining how fine the mesh should be?

exactly.