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This paper describes the development and validation steps of a characteristics-based explicit along with a novel fully implicit mixing plane implementation for turbomachinery applications. The framework is an unstructured 3D RANS in-house modified solver, based on open-source libraries. Particular attention was paid to mass-conservation, accurate variables interpolation, and algorithm stability in order to improve robustness and convergence. By introducing a specific interface, allowing the use of algebraic multigrid solvers together with multiprocessor computation, a speed up of the numerical solution procedure was achieved. The validation of both mixing plane algorithms is carried out on an industrial radial compressor and a cold air 1.5 stages axial turbine.

This paper describes the development and validation steps of a characteristics-based explicit as well as a novel fully implicit mixing plane implementation for turbomachinery applications. The framework is an unstructured 3D RANS in-house modified solver, based on open-source libraries. Particular attention was paid to mass-conservation, accurate variables interpolation and algorithm stability in order to improve robustness and convergence. By introducing a specific interface, allowing the use of algebraic multigrid solvers together with multiprocessor computation, a speed up of the numerical solution procedure was achieved. The validation of both mixing plane algorithms is carried out on an industrial radial compressor and a cold air 1.5 stages axial turbine.

The CFD assisted design of modern single- and multi-stage turbomachines is usually performed with the mixing plane approach in order to assess the components matching. While the mixing-plane state-of-the-art is based on a boundary-condition based approach, hereafter called explicit, the authors presented last year a novel, fully implicit method, which shows considerable advantages compared to the explicit one. In the present paper the quality and advantages of the novel approach compared to the state-of-the-art will be shown through a variety of detailed examples. The main issues discussed are the built-in ability to reduce incoming disturbances and to manage backflow at the interface due to the implicit formulation. With selected cases it is shown that no special care has to be taken to avoid reflections at the interface also for inviscid transonic or fully-supersonic cases. Moreover, detailed results for a high-pressure centrifugal compressor are presented, showing that the proposed approach is able to capture both, the global behavior as well as local flow-features over the complete speed-line, while the explicit approach partially fails on the same test case.

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