Assessment of the Extended 1D Unsteady Friction Model Applied to Transient Pipe Flows

This study evaluates the extended one-dimensional (1D) unsteady friction model developed by Guerrero, Lambert, and Chin (GLC model) for transient pipe-flow applications. The model incorporates an empirical term to represent the turbulence response, derived from direct numerical simulation data. However, the GLC model was formulated for single flow-change scenarios (e.g., linear accelerations or decelerations). In practice, velocity variations are typically nonlinear due to the operation of flow devices such as pumps and valves. Moreover, water-hammer events often involve alternating accelerations and decelerations at intermediate pipe sections, caused by wave reflections at boundaries. Therefore, it is essential to examine the applicability of the GLC model under more complex flow conditions, including nonlinear velocity variations and combinations of accelerations and decelerations, before applying it to wall shear stress (WSS) or water-hammer analysis of full pipeline systems. To this end, the present work validates the GLC model against computational fluid dynamics simulations performed with the Launder–Sharma turbulence model. The assessment covers a range of unsteady flow cases, including different initial and final Reynolds numbers, ramp rates, nonlinear accelerations, decelerations, and their combinations. The results show that the GLC model provides more accurate predictions of WSS than the Vardy–Brown model for long-term transients, where turbulence contributions cannot be neglected. The model therefore has strong potential for application in the design of emerging cardiac-cycle-inspired unsteady flow transport strategies in long transmission pipelines aimed at energy savings, where accurate prediction of WSS waveforms is required. Nonetheless, further refinement of the model is needed, particularly with respect to the turbulent response term.