Consider a two$-$dimensional square enclosure of side L$.$ The enclosure is filled with an incompressible fluid of viscosity $\backslash$mu and density$\backslash$rho $.$ The top side of the enclosure is covered with an infinite horizontal wall moving with a constant velocity U$.$ The other sides are fixed in place. Due to the motion of the top wall a flow field is established within the enclosure. Using appropriate dimensionless variables, write the simplified momentum equation for the flow field in dimensionless form showing that the Reynolds number $($Re$=\backslash$rho UL$/\backslash$mu $)$ is the only dimensionless group affecting the flow.