Velocity profiles and unsteady pipe friction in transient flow

Transient conditions in closed conduits have traditionally been modeled as 1D flows with the implicit assumption that velocity profile and friction lo sses can be accurately predicted using equivalent to velocities. Although m ore complex fluid models have been suggested, there has been little direct experimental basis for selecting one model over another. This paper briefly reviews the significance of the 1D assumption and the historical approache s proposed for improving the numerical modeling of transient events. To add ress the critical need for better data, an experimental apparatus is descri bed, and preliminary measurements of velocity profiles during two transient events caused by valve operation are presented. The velocity profiles reco rded during these transient events clearly show regions of flow recirculati on, flow reversal, and an increased intensity of fluid turbulence. The expe rimental pressures are compared to a water hammer model using a conventiona l quasi-steady representation of head loss and one with an improved unstead y loss model, with the unsteady model demonstrating a superior ability to t rack the decay in pressure peak after the first cycle. However, a number of details of the experimental pressure response are still not accurately rep roduced by the unsteady friction model.