pipe may be The flow through a closed, circular-sectioned metered by measuring the speed of rotation of a propeller having its axis along the pipe centre-line. Derive a relation between the volume flow rate and the rotational speed of the propeller, in terms of the diameters of the pipe and the pro- peller and of the density and viscosity of the fluid. A propeller of 75 mm diameter, installed in a 150 mm pipe carrying water at 42.5 L-s, was found to rotate at 130 rad s (20.7 rev/s). If a geometrically similar propeller of 375 mm diameter rotates at 64.5 rad s (10.9 rev/s) in air flow through a pipe of 750 mm diameter, estimate the volume flow rate of the air. The density of the air is 1.28 kg m-3 and its dynamic vis- cosity 1.93 10-5 Pas. The dynamic viscosity of water is 1.145 10- Pa.s. . .