The decay of swirl in a pipe

Abstract The way in which swirling flow in a pipe decays has been computed by solving an approximation to the Navier-Stokes equations. The approximation was performed using an order-of-magnitude analysis and a turbulent viscosity that is computed as part of the solution. At a sufficient distance downstream of a swirl-inducing fitting, the flow is predicted to rotate approximately as a solid body: experimental data confirm this prediction. Swirl decay rates have been computed and found to be very nearly proportional to pipe friction factor: this result is also in good agreement with experiment and implies that swirl is extremely persistent in smooth pipes at high Reynolds numbers.