Final work : Numerical Analysis of the Mutual Aerodynamic Excitation between Probe and Rotor

Abstract

Within the development phase of new engine designs, experimental investigations are necessary to verify numerical results. As part of a multidisciplinary project in cooperation between the DLR and an industrial partner, experimental investigations of a two-stage compressor will be performed using a pressure probe. The presence of the probe alters the steady flow field and a mutual excitation between the downstream rotor and the upstream probe occurs.

The aim of this thesis was to predict the forced responses of the pressure probe and blades of a rotor, to assess the risk of collision and fatigue. For this analysis, the pressure probe was positioned between the variable inlet guide vane (VIGV) and the first rotor of the two-stage compressor. Critical resonance conditions were identified and a forced response analysis was conducted by use of steady state RANS computations and a time-linearisation around the steady flow field. Results show, that low engine order excitations of the rotor induced by the pressure probe are the essential source of high vibrational amplitudes. Furthermore, depending on the circumferential position of the probe, high engine order excitations of the VIGV were enhanced or reduced. A change in operating conditions indicated an increased forcing during choked flow as compared to stalled flow. However, operation during stalled flow was more critical due to an observed significant reduction in aerodynamic damping.

In conclusion, no risk of collision or fatigue is predicted for the rotor, while the forced response analysis of the probe indicated a risk of fatigue at a high rotational speed. The highly unsteady flow caused by the pressure probe was not entirely captured by the time-linearisation of the steady state RANS computation. Further non-linear investigations are needed, to allow a more comprehensive assessment of the flow characteristics induced by the pressure probe.