Abstract
This numerical study aims at predicting the reflective behavior of different conventional inlet and outlet far-field boundary conditions as well as available non-reflecting boundary conditions (NRBC) implemented in the commercial CFD solver ANSYS CFX. An isolated rotor model of an axial turbine stage with prescribed blade displacement is applied as test case to consider a representative application case, while at the same time provoke an unsteady flow field featuring pronounced flow perturbations in the far-field. Since the reflective behavior of the implemented boundary conditions was found inadequate in the given application case, a zonal treatment of the inlet and outlet far-field, based on a modification of the governing Navier-Stokes equations, is investigated. The applied approach has proven its capability to suppress spurious reflections reliably, while at the same time ensures a preservation of the reference flow conditions within the required domain extensions. The results of a case study considering calculation domains of different spatial extent and different treatments of their respective far-fields suggest variations in the steady flow aerodynamics to be of moderate influence on the predicted aerodynamic damping, while spurious reflections were found to falsify the unsteady aerodynamics considerably.
Highlights
Turbomachinery CFD calculations are generally performed on truncated domains, either by considering an isolated blade row, an arbitrary multi-row configuration, or the entire machine
Given the fact that evaluating the harmonic blade surface pressure is the primary objective in most aeromechanical investigations, the reflective behavior of boundary conditions is assessed in representative manner by evaluating the associated perturbations in the static pressure field
An isolated rotor model of an axial turbine stage with prescribed blade displacement was applied as a representative test case to investigate the reflective behavior of different conventional inlet and outlet far-field boundary conditions as well as available nonreflecting boundary conditions (NRBC) implemented in the commercial CFD solver ANSYS CFX. Their individual reflection characteristics were assessed by extracting the induced reflected acoustic wave from the resultant harmonic flow field and quantified by means of corresponding reflection coefficients, proving all conventional boundary conditions to be of inadequate reflective behavior for the simulation of unsteady flow featuring pronounced flow perturbations in the far-field, while the provided non-reflecting boundary conditions (NRBC) were found to be limited to reflection coefficients of undesirable magnitude
Summary
Turbomachinery CFD calculations are generally performed on truncated domains, either by considering an isolated blade row, an arbitrary multi-row configuration, or the entire machine. Imposing specific steady-state reference flow conditions while avoiding spurious reflections from entering the computational domain of interest, the zonal treatment applied to the inlet and outlet far-field can be interpreted as a type of gradual, spatially distributed non-reflecting boundary condition.
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