CFD and 1D approaches for the unsteady friction analysis of low Reynolds number turbulent flows

The aim of this paper is to investigate the complex nature of the transient energy dissipation by using a computational fluid dynamics (CFD) model with a high spatial and temporal resolution together with one-dimensional (1D) models incorporating different unsteady friction (UF) formulations. The analysis focuses on a transient event in a single pipe system with a smooth-wall turbulent flow (with an initial Reynolds number equal to 7,638) generated by an instantaneous valve closure. For the considered flow condition, the numerical experiments point out the importance of the used UF model with regard to the wall shear stress simulation. In such a context, it is shown that the convolution-based UF models better describe the pressure signal than the instantaneous acceleration-based ones because they take into account a set of previous time steps. This is due to the fact that, similarly to the laminar regime, to simulate the characteristics of low turbulent transients the flow time history plays a crucial role.