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Abstract
This paper discusses the existing approaches to unsteady friction and presents some new ideas on the modelling of unsteady friction in turbulent pipe flows. This paper aims to contribute to a physically-based unsteady friction model that captures the known physical characteristics of friction phenomena in transient turbulent pipe flows. For practical applicability, a new model should not require extensive computer memory from previously calculated flows, nor complex iterative procedures, since unsteady friction computations will be carried out in every calculation node at every time step of any transient scenario. A new formulation for the unsteady shear stress is proposed and validated against eight transient scenarios in four different systems, conveying water with steady state Reynolds numbers varying between 1940 and 1.5 million.