I am solving flow in driven cavity(taking incompressible novier-stokes equation)how can i find out reynolds number(I have mue ,rho values & i have taken square cavity).

Reynold's Number is defined as:

Re = (\rho) *(Velocity Scale)*(length Scale)/ (\mu)

1. Determine your velocity scale. It could be the speed of your driving force?

2. Determine your length scale. size of box?

Usually other researchers will have already defined a Reynolds number. Its good to take their definition so you can compare results.

see: Fluid mechanics in a driven cavity

P.N. Shankar and M.D. Deshpande

Annual Review in Fluid Mechanics

V.32 pp93-136

this should give you too much info.