With unsteady flow simulations of industrial turbomachinery configurations  becoming more and more affordable there is a growing need for accurate inlet and outlet boundary conditions as numerical reflections alone can lead to incorrect trends in engine efficiency, noise and  aeroelastic analysis parameters.      
This is the first of two papers on the formulation of unsteady boundary conditions which have been implemented for both time-domain and frequency-domain solvers. Giles' original idea for steady solvers to formulate the boundary condition in terms of characteristics generalizes to both the time-domain and the frequency-domain solvers. The boundary condition drives the value of the incoming characteristics to ideal values that are computed using the modal decomposition of linearized 2D Euler flows. The latter could, in theory,  be replaced with a 3D modal decomposition. The authors, however, have  experienced robustness issues when the 3D method is applied to real turbomachinery configurations.

The present paper explains how to generalize 2D nonreflecting boundary conditions to real 3D annular domains by applying them in certain conical rotational surfaces. For a flow with zero radial component and an annular boundary that is perpendicular to the machine axis, these surfaces are the cylindrical streamsurfaces. For more general flows and geometries, however, there is no natural choice for the rotational surfaces. In this paper, two choices are discussed: the surfaces that are generated by the boundary normals and those that are defined by the circumferentially averaged meridional velocity.

The impact of the boundary condition on the  stability of both the time-domain and the harmonic-balance solvers is analyzed by studying the time evolution of certain energy integrals. For a model problem which consists of a small disturbance of an inviscid flow, the increase or decrease of this energy integral is shown to be directly related to the normal characteristic variables along the boundary. This shows that the actual boundary condition should be formulated as a control problem for the normal characteristics. Moreover, the application of the harmonic balance solver to a simple duct configuration with prescribed disturbances demonstrates that using the characteristics based on the meridional velocity may prevent the solver from converging.  In contrast, the 2D theory can be formulated  in a different surface without impairing the robustness of the overall approach.  
These findings are illustrated by simple test cases.

The impact of the choice of the rotational surface for the 2D theory is studied for a  duct segment and     
a low-pressure turbine configuration in the second paper. There it is shown that applying the 2D theory  
to the meridional velocity surfaces may be advantageous in that it leads to more accurate results.


On the Formulation of Nonreflecting Boundary Conditions for Turbomachinery Configurations: Part I \u2014  Theory and Implementation

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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