A non-reflective boundary condition for prediction of acoustic tones in turbomachinery using computational fluid dynamics for non-uniform flows in 3-D

A new Non-Reflective Boundary Condition (NRBC) is developed for computational fluid dynamics (CFD) prediction of acoustic noise at discrete frequencies in ducts containing nonuniform mean flow. The method is demonstrated for downstream and upstream acoustic propagation in a three-dimensional axisymmetric duct and compared with existing boundary treatments. Extension of the method to acoustics in ducts of non-uniform geometries is discussed. The target of this work is the prediction of acoustic tones in turbomachinery, with a particular focus on low order (low spatial curvature) modes, which are well captured by both the CFD and the eigenvalue calculation and for which the effect of turbulent viscosity can be considered negligible. The method is demonstrated using a commercial time-unsteady Reynolds averaged Navier Stokes (RANS) CFD solver to predict propagation of an acoustic signal in uniform and non-uniform mean flow.