A circular disk of diameter \(d\) and of negligible thickness is rotated at a constant angular speed, \(\omega\), in a cylindrical casing filled with a liquid of viscosity \(\mu\) and density \(ho\). The casing has an internal diameter \(D\), and there is a clearance \(y\) between the surfaces of disk and casing. Derive an expression for the torque required to maintain this speed if it depends only on the foregoing variables.