Hi there, I'm attempting to solve a relatively simple problem on steady state rotational couette flow in a rotational viscometer. There are two cylinders, one within the other, seperated by a layer of viscous fluid. The internal cylinder is stationary, the external one is rotating at a given constant angular velocity with a given torque. End effects are ignored. I have to find the general form of the tangential shear stress in the fluid as a function of the radius, r. I have set-up and simplified the tangential Navier Stokes equation with the given initial conditions and I am attempting to equate it to the tangential linear momentum equation, which simplifies to: Sum of the forces acting on the control volume = 0. The only force acting on the control volume is the applied torque. My question is: how do I use the torque which is applied to the outer cylinder? Is it simply: F*(internal Radius of the outer cylinder) = torque or F = torque/(internal Radius of the outer cylinder) Or do I have to do something more such as applying it over the entire surface of the outer cylinder?