Friction factor for a rotating disk, a thin circular disk of radius R is immersed in a large body of fluid with density p and viscosity µ. If a torque Tz is required to make the disk rotate at an angular velocity 12, then a friction factor f may be defined analogously to Eq. 6.1-1 as follows, Tz/R = AKf where reasonable definitions for K and A are K = ?1 p(OR)2 and A = 2(pR2). An appropriate choice for the Reynolds number for the system is Re = R2Op/µ. For laminar flow an exact boundary layer development gives Tz = 0.616ppR4vµO3/p for laminar flow for turbulent flow, an approximate boundary layer treatment based on the 1/7 power velocity distribution leads to T = 0.073pO2R5 5vµ/R2Op express these results as relations between f and Re.