Implementation and validation of a consistent averaging method for the mixing plane interface in OpenFOAM

Authors

Stefan Fraas - Institute of Fluid Mechanics and Hydraulik Machinery (IHS) - University of Stuttgart
Jonas Wack - Institute of Fluid Mechanics and Hydraulik Machinery (IHS) - University of Stuttgart
Alexander Tismer - Institute of Fluid Mechanics and Hydraulik Machinery (IHS) - University of Stuttgart
Stefan Riedelbauch - Institute of Fluid Mechanics and Hydraulik Machinery (IHS) - University of Stuttgart

Abstract

Due to the interaction between rotating and non-rotating components in a turbomachine, a simplified model at the interface between these components is required for a steady state simulation. One such simplified model is the mixing plane, which provides the flow variables averaged over the circumference resulting in a mixing-out of the wake. With the existingimplementation of an area or mass flow averaging in OpenFOAM it is not possible to reach the conservation of mass flow as well as the conservation of momentum and angular momentum. Therefore, the implementation of a consistent averaging method for incompressible fluids is presented, which provides the conservation of the mass flow as well as the conservation ofmomentum and angular momentum.Two test cases were used to validate the implementation of the consistent averaging method. Both are based on the tutorial test case for the mixing plane in OpenFOAM, which consists of a bulb turbine. The conserved quantities have to be obtained for any type of interface, such as a radial, axial or conical interface. Therefore, an axial interface was used in one test case and a conical interface in the other. The deviation of mass flow, momentum and angular momentum across the interface was determined and compared with the deviation when using the existing area averaging method for averaging the pressure and the velocity components. In addition to the determination of the deviation directly across the interface, faceZones downstream and upstream of the interface were defined. This made it possible to verify the correct numerical treatment of the boundary condition.For each test case two meshes of different refinement were used to exclude a discretization error. The result of the validation is that, in contrast to an area or mass flow averaging, the newly implemented consistent averaging method allows the conservation of the mass flow as well as the conservation of the momentum and angular momentum.In addition, a new user defined method of creating the ribbon patch, which is used for averaging the flow quantities, is presented. In the existing implementation, the next ribbon coordinate is driven by the largest jump either upstream or downstream of the mesh discretization. This leads to a poorribbon patch discretization when using tetrahedral meshes or in case of a conical interface. These problems are solved with the new implemented user defined method of creating the ribbon patch.