CFD Based Modeling of Wave Propagation in Liquid Transmission Line Junctions

Wave propagation effects are of increasing importance in simulation models for fluid power applications due to rising demands for both the accuracy of predicted system responses as well as the range of interesting operating frequencies. Well documented examples can be found in technologies such as fuel injection for internal combustion engines or hydraulically driven punching machines. The distribution system for the pressurized fluid is predominantly made up of pipes and hoses as well as cylindrical bores in the housings of various components. These cylindrical geometries lend themselves conveniently for modeling as a network of elements with a one or two-dimensional spatial representation. The junctions between these elements, for instance a sharp elbow bend or a T-junction between three pipelines is very often idealized as a lossless Kirchhoff-type node. The present paper shows a comparison of transmission line modeling based on the well known frequency-dependent friction theory for transient laminar flow of a weakly compressible viscous fluid in transmission lines with a CFD model resolving the local effects occurring at a junction or intersection of cylindrical elements. Results are given for a 90 degree elbow under step excitation with the material parameters of a typical ISO 46 mineral oil based hydraulic fluid. These results can be used in order to assess the importance of including the usually neglected local effects at the junctions. In further work they will form the basis for reduced order dynamic models of transmission line junctions.