Numerical study on accuracy of frequency-domain modelling of transients

The integration of the governing equations of transients in the frequency domain has the appeal in that spatial discretization is not required, but the linearization of the equations is needed for the steady-friction term in turbulent flows. In this paper, to investigate the effects of such a linearization, a transient generated by a complete closure in a simple reservoir-pipe-valve system is considered, to exclude other linearization effects in the boundary conditions. A new approach is proposed to evaluate the linearized friction term, not only taking into account the flow dependency of the friction factor but also changing the operating point where the friction term is evaluated. By means of this analysis, significant improvements are gained in the frequency domain model performances for both elastic and viscoelastic pipes, in terms of their equivalence to the time domain models, which are not affected by the linearization error.