Mixed-fidelity CFD Simulations for Aero-engines: A Fan-intake Interaction Study

Abstract

Engine system consists of complex components, where aerodynamics can be coupled. In these coupled problems, flow separation may exist and the multiscale turbulence needs to be finely resolved to obtain an accurate solution. A high-fidelity simulation in such scenarios, however, is still infeasible for industrial applications due to the limitation of current computational resources. To make it possible, a mixed-fidelity CFD method based on the Immersed Boundary Method (IBM) is proposed. In this hierarchical method, geometries can be replaced by forces, i.e. the standard IBM or eIBM, whereas turbulence can be resolved by the Large Eddy Simulation. The thesis proposed this method and applied it to an important issue for engine design: fan-intake interaction. The method was validated on a Darmstadt Transonic Rotor with a distortion generator to replicate the unsteady distortion at incidence, the NASA Rotor 67 with steady pressure distortion, and a triangular prism for turbulence statistics. The results indicated that this method can accurately simulate the performance map, separation transfer and total pressure distributions, compared to the experimental data and Direct Mesh Resolved (DMR) case. The method was then applied to reveal the mechanism of fan influence on intake distortion. It was shown that there are two aspects of such influence: the suction effect of a fan can accelerate the flow in the upstream and directly change its streamline curvature; on the other hand, the recirculating flows can also intensify the turbulence, indirectly increase the mixing process and finally alleviate the distortion. The main flow effect was further investigated in different parameters of fan type, location and distortion size. Results showed that a tip-loaded fan is more effective in suppressing intake separation; a nearer fan to the upstream has more significant reduction of distortion; a greater distortion can be suppressed more. These results demonstrate that a fan can be an essential component for intake distortion control. Further investigations of the results from RANS and LES interpreted the dominance of the influence via main flow or turbulence. It was found that stronger main flow acceleration by a fan can mitigate the inaccuracy of turbulence models. This indicates that for a short intake design, conventional turbulence models may be capable of predicting flow separation.