boundray layer
 

i want to simulate free convection in closed cavity: shall i create a boundray layer?or creating a boundary layers is only done in flow fluid(as water)only?if yes i must create a boundry layer plz tell me how to do that shortlythnx

you meant liquid (not fluid), because liquids or gas are fluids and obey to NS equations (newtonians fluids)
Anyway, you are not enforce to use BL, but it is recommended.
Check y+ & turbulence theory
http://aerojet.engr.ucdavis.edu/gamb...tm#mesh_blayer