A Hybrid Solution for Transient Pipe Flow based on Method of Characteristics and Lax-Friedrichs Scheme

Method of Characteristics (MOC) has long been an excellent and widely established technique for analyzing transient flow, especially in a single pipeline with constant wave speed. But, this method has some limitations in terms of mesh sizing while studying multi-pipe systems or systems with different wave speeds. More specifically, it needs all pipes to satisfy the Courant number to be unity while the same time step should be chosen for all pipes. With this, one reach in each pipe remains, which does not satisfy the Courant requirement. As one possible remedy to this shortcoming, a hybrid numerical method based on MOC and a two-step variant of the Lax-Friedrichs method (MOC-LF) is suggested in the present study. This method is compared against the conventional MOC scheme, which adapts interpolation for the remaining length per pipe (MOC-MOC). In the approach, two significant effects of FSI in fluid-filled tubes, namely Poisson and junction coupling, are introduced. The computational simulations are carried out for a reservoir-pipe-valve system with instantaneous and gradual closure of the downstream valve. The results of proposed scheme and those of MOC with interpolation are in good agreement with solutions obtained by MOC with a very fine grid, which are taken as a reference. Detailed comparison of the computational methods in terms of error indicates that the proposed MOC-LF can be a good alternative for conventional MOC schemes.