Semi-analytical tip vortex model for fast prediction of contrarotating open rotor noise

This paper presents an improved Reynolds-averaged Navier–Stokes (RANS)-informed model for the fast prediction of the interaction noise from contrarotating open rotors, which accounts for the effects of the front rotor tip vortices. A new semi-analytical three-dimensional model of the tip vortex flow, calibrated using data extracted from a RANS mixing plane simulation, is coupled with an existing RANS-informed model of the rotor viscous wake and potential flow perturbations. This provides a more general and accurate procedure for the reconstruction of the flow velocity perturbations at the rear rotor inlet. These perturbations are used to compute the unsteady blade loads and the interaction noise tones by means of analytical models from literature. The method is tested on the Open Rotor UDF F7/A7 and validated against unsteady RANS (URANS)-based results. It is found that the tip vortex model significantly improves the reconstruction of the velocity perturbations. The noise emissions computed analytically from reconstructed and URANS velocity perturbations agree remarkably better when the contribution of the tip vortices is modeled properly. A comparison with the results of the Ffowcs-Williams and Hawkings acoustic analogy shows some inaccuracies that are mainly ascribed to inherent limitations of the adopted analytical blade response and noise propagation models.