Coupling Strategies for Compr. Flow Algorithms in OpenFOAM

This proposal aims to carry out research on coupled finite volume algorithms for compressible fluid flows. The resulting approach is expected to be fast, robust and accurate at a time. The applicant has already established extensive know-how in the field of equation coupling of incompressible flows that shall be extended to compressible flows. In comparison to standard segregated approaches he showed that considerable gains, both in terms of robustness and convergence speed can be made. Since a 5x5xN system of equations arises from coupling the compressible flow variables for 3-D flows (N being the number of finite volume cells), a very fast linear multi- grid solver that scales linearly with the number of cells is necessary to overcome the disadvantage of having 25 times more entries in the matrix to be solved. Hence one part of the proposed project shall be the elaboration and the implementation of a suitable preconditioning and agglomeration strategy for such a solver. The second part of the project shall focus on the elaboration and implementation of coupled transition and turbulence models that are also expected to considerably improve the robustness and convergence speed compared to standard segregated approaches. As a development framework the open source CFD library OpenFOAM was selected to serve as a platform for collaborative work since it enriches and facilitates the exchange with other researchers.

OpenFOAM (Open Source Field Operation And Manipulation) is a free, open source Computational Fluid Dynamics (CFD) software package that is used by a rapidly increasing number of engineers and scientists across the world. It is highly customizable and builds a parallel and high performance foundation. So OpenFOAM allows for the solution of a very large range of fluid flow problems from single to multiphase flows, within continuous and particulate flows including flow problems with dynamics meshes and fluid-structure interaction. However for some important specific applications, as for turbomachinery flow investigations, it lacks robustness meaning that for many cases it fails to converge to a solution. This has meant that in the turbomachinery industry (power generation, pumps, turbines .) adoption of OpenFOAM has been quite slow. Therefore the aim of this project was to address this shortcoming through the implementation of a fully coupled CFD solver within the OpenFOAM framework that can be used for the solution of the range of flows that occur in turbo machinery applications. Fully coupled refers to the treatment of the velocity-pressure coupling that is at the core of the Navier-Stokes equations. In this work the coupling is accounted for in a fully implicit fashion and the system of algebraic equations resulting from the discretization of the momentum and continuity equations is resolved as one system. This is in contrast to the segregated approach that decouples the velocity and pressure equations and treats the coupling in a semi-implicit fashion. The result is a substantial improvement in the convergence rate and robustness and a decrease in the computational time as compared to the standard segregated solvers. The solver was tested for both incompressible and compressible flows with performance scaling nearly linearly with mesh size.