Parallel$-$disk viscometer$$A fluid whose viscosity is to be measured is placed in the gap of thickness B between the two disks of radius R$.$ One measures the torque Tz required to turn the upper disk at an angular velocity $,$ which helps calculate viscosity. Assume fully developed, steady, unidirectional flow.

$1.$What is the proper coordinate system to use for this problem& what are thee boundary conditions.

$2.$ simplify the Navier$-$Stokes equation for a small $$