Evaluation of Unsteady Friction in Transient Flow

Transient flow is generated by an instantaneous reduction in flow rate due to rapid valve closure or pump failure in a water network. Transient flow creates high and low pressure waves that it may be to damage pipelines. Comparison of experimental and numerical results in previous studies revealed that quasi-steady models with assumption of uniform and 1D velocity profile could not correctly predict the damping process of the pressure waves. So¡ the aims of this study are investigation of velocity profile at the downstream end section of pipeline to give a correct understanding of transient flow dynamics. Also¡ this paper evaluates the ability of the instantaneous acceleration–based (IAB) unsteady friction models to estimate pressure history in transient flow using the method of characteristics. To achieve this goal¡ a pipeline¡ made of PVC¡ with a nominal diameter of 63 mm in the two different lengths of 40 m and 80 m was used. In addition¡ a ball valve at the downstream end was installed to create transient flow. The data of velocity distribution and dynamic pressure oscillation during these transient events were collected by Ultrasonic Doppler Velocimetry and Pressure transducers. Results of experiments indicate existence of inflection points in the base flow velocity profile and the large velocity gradient near the pipe wall. Furthermore¡ the results show that two-coefficient models produce a better match with the experimental measurements. The terms Vt and Vx in unsteady friction models affect the phase shift and the damping of transient pressure waves¡ respectively. Moreover¡ the coefficients of acceleration terms vary from 0.0037 to 0.006 for kt and from 0.0325 to 0.052 for kx.