14th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
The higher and higher bypass ratio of modern aircraft turbofan engines is associated with reduced fan rotation speed, exhaust jet speed and relative nacelle length. Therefore, the fan stage becomes a major source of noise. In this context, the understanding and prediction of secondary noise sources, such as the tip-leakage vortex of the rotor blades, are crucial needs. However, few models to predict tip-clearance noise exist in the literature. The present study is aimed at improving the understanding of the unsteady aerodynamic mechanisms responsible for the generation of tip-clearance noise. A numerical investigation of a tip-leakage flow configuration studied experimentally at École Centrale de Lyon in an open-jet facility is performed. The configuration consists of a stationary airfoil held vertically at non-zero angle of attack between two horizontal plates, with a gap between the tip of the airfoil and the lower plate. The Mach number is 0.2 and the chord-based Reynolds number is 933,000. A computational approach accounting for wind-tunnel installation effects is described. A Reynolds-Averaged Navier-Stokes (RANS) simulation of the whole configuration including the nozzle is first carried out, in order to reproduce the nozzle-jet flow deflection caused by the airfoil lift. To save computational cost, a Large Eddy Simulation (LES) is then performed in a smaller domain using outputs of the RANS simulation as an inlet boundary condition. Simulations are validated by comparing the computed results to the measurements. In order to track the position and diameter of the tip-leakage vortex, flow data are extracted in planes almost perpendicular to its trajectory. The tip-leakage vortex wandering/unsteadiness and its interaction with the airfoil are of primary interest, as a possible source of sound. Power spectral densities of the airfoil surface pressure and tip-leakage vortex velocities, extracted from the LES, are compared to the measurements. Acoustic wave propagation is resolved within the core of the jet and through the mixing layers. The acoustic far-field pressure is computed using the Ffowcs-Williams & Hawking acoustic analogy and also compared to measurements.