Energy-Based Evaluation of 1D Unsteady Friction Models for Classic Laminar Water Hammer with Comparison to CFD

Abstract

Under the umbrella of one-dimensional (1D) transient flow analysis, unsteady friction models are popular tools to better represent the decay in pressure signal associated with high wall shear stresses. There are generally two families of unsteady friction models, namely those based on instantaneous acceleration (IAB) and those based on convolution (CB). This paper considers a classic laminar water hammer and compares two unsteady friction models with both experimental results and those obtained using a high-resolution computation fluid dynamics (CFD) model, with CFD having a more native representation of frictional losses. Because dissipation is fundamentally an energetic phenomenon, an integrated total energy (ITE) approach is used here as a lens to view and compare results. The capabilities and limitations of the ITE approach are discussed. More specifically, the ITE method is used not only to quantify dissipation but also to spatially resolve when, where, and how energy is dissipated in each model. Most notably, CFD and CB models are found to differ significantly for positive flows, with CFD results exhibiting little difference between positive and zero flows.