A moving piston boundary condition including gap flow in OpenFOAM

摘要

Fuel injection as well as digital switching strategies in fluid power applications are not only famous representatives of a large field of technology but also a main reason for the increasing interest in wave propagation effects in research. While there is a huge number of works dealing with the pressure drop of different hydraulic components in the steady state, many issues still remain unresolved in the transient regime, even in the case of laminar fluid flow. A better understanding of these processes would be a great benefit as it would lead to a higher accuracy of predicted system responses. In order to reach a higher degree of precision, the highly sophisticated computational fluid dynamics (CFD) codes are a wide-spread tool. These codes solve the famous Navier-Stokes equations in all three dimensions of space and therefore result in the full resolution of the pressure field as well as of the velocity field. A very awkward topic of performing a CFD simulation is the choice of the boundary condition, which should correspond to a physical one. At the latest when measurements for validation are carried out, the boundary condition of the experimental setup should match the one used in the simulation. Especially the use of a volumetric flow rate boundary condition is fraught with problems. Using a moving piston, a definite volumetric flow rate could be forced on a boundary. In an experimental setup only the measurement of the position of the piston would be necessary to use it in the simulation. This measurement has no backlash on the system, which is therefore well separated. In this work a moving piston boundary condition including gap flow is implemented and used in OpenFOAM. For this reason moving walls have to be used and the mesh has to change during the simulation. Results of simulations done with this moving piston boundary condition are compared with simulations done with an ordinary volumetric boundary condition.