International audienc

The following numerical investigations are performed in the frame of a research project that aims at a better understanding of the flow unsteadiness that develops in a multistage high-speed axial compressor. First, the paper presents a new version of the 3.5 stages high-speed axial compressor CREATE (Compresseur de Recherche pour l’Etude des effets Aérodynamiques et TEchnologiques), which has been designed by Snecma and is based at the LMFA (Laboratory for Fluid Mechanics and Acoustics) on a 2MW test rig. This paper is based on numerical results obtained with 3D steady and unsteady RANS computations using the CREATE configuration. The unsteady RANS simulations are carried out over the whole spatial and temporal periodicity of the compressor. The main numerical setup has been fixed according to the state of the art. Second, the effect of three different time discretizations on the flow field in CREATE is discussed. The global performance of the compressor is not significantly affected. However the change in the time discretization impacts the structure of the flow at specific locations. The main focus of this study lies on the transport of flow structures and the analysis of their interactions. A double modal decomposition method, which highlights the specific contribution of the interactions on the overall flow field, is applied for the study of the highly complex and unsteady flow field. It allows identifying which interactions are more sensitive to the change in the time discretization.

In this work, unsteady numerical results and high-frequency measurements are investigated from nominal to loaded operating points with the objective to contribute to the understanding of pre-stall rotating disturbances. A 3.5 stages high speed axial multistage compressor is investigated on a 2 MW test rig in the laboratory of fluid mechanics (LMFA) at Ecole Centrale de Lyon, France. The compressor has been built by Snecma, and is representative of modern high-pressure rear blocks of a modern aircraft engine. The unsteady numerical results predict a rotating disturbance in the tip flow field of the rotor 2 at the loaded operating point. It causes a frequency which is not a multiple of the periodicity of the compressor, and is rotating at about 72% of the shaft speed. The mesh independency of this disturbance is ensured. The analysis of the circumferential and axial propagation of the disturbance reveals a rotating instability like phenomenon. Most characteristic is the very important periodic oscillation of the tip leakage vortex trajectory, leading to a modulation of the leakage flow in the neighboring tip gap. The influence of the neighboring blade rows is investigated by filtering their unsteady contribution by means of mixing planes up and/or downstream of the rotor 2. In either case, the rotating disturbance is found to be still present. There are no traces of this rotating disturbance in the high-frequency measurements investigated at a near surge operating point. A spike like stall inception and almost instantaneous surge inception is identified. The mis-prediction of the tip region flow field in the rotors 2 and 3 is believed to cause the mis-prediction of the pre-stall disturbance.