On the Fast Prediction of Open Rotor Tonal Interaction Noise

Abstract

This paper discusses the noise prediction of Contra Rotating Open Rotors (CROR) in the context of a multi-disciplinary rotor design. Propeller noise is believed to be one of the dominant sources of noise on CROR engines in all flight conditions that require high thrust. It is therefore important to consider acoustics as early as possible in the design process. At Snecma, blade design is tackled as a multi-disciplinary approach that involves among others; mechanics, dynamics, aerodynamics, acoustics and engine integration constraints. In this context, accurate prediction methods based on unsteady Computational Fluid Dynamics (CFD) cannot be used to predict tonal noise radiation because of the prohibitive time required for convergence. This paper focuses on the prediction of far field tonal interaction noise using the fast prediction platform Sandra, which complies with the timescale of a rotor design project. Sandra is composed of an aerodynamic module and an acoustic module. As a first approximation, the aerodynamic module uses either a lifting-line method or a lifting-surface method to compute the pressure fluctuations on the blades. A map of the flow velocity, which contains the velocity gradients of the wake and of the tip vortex behind the front rotor, is extracted from a Reynolds Averaged Navier-Stokes (RANS) CFD simulation downstream of the front rotor. A quasi-stationary approach, which uses the space and time symmetry of the isolated rotors CFD, allows the flow field to be decomposed along the azimuthal angle. The pressure fluctuations on the rear blade are finally computed for each position. The acoustic propagation is then performed in the time domain using Farassat’s formulation 1A of the Ffowcs Williams and Hawkings equation. It is shown that the physics of the interaction between the front and the rear rotor is well captured. Direct comparisons with the far field noise computed from a uRANS solution, and with experimental data, show very good agreement of the position of the angles of maximum noise radiation for harmonics of order 2 or lower. It is also shown that the relative noise radiation between various rotor geometries is reasonably well captured, which is a requirement to provide a fast and relevant method for multi-disciplinary propeller blade design.