Investigation of Inlet Distortion on the Flutter Stability of a Highly Loaded Transonic Fan Rotor

Abstract

The fundamental mechanisms of blade flutter in modern aircraft engines are very complex. Flutter is a self-excited aeroelastic instability phenomenon which can finally cause material fatigue and, in the worst case, leads to blade failure within a very short time. The risk of flutter has to be considered during the design process and it is necessary to avoid that safety risk. The aeroelastic stability has to be ensured over the whole operating range especially near operating limits or typical flutter boundaries, like at stall or choke conditions. Topic of this paper are inlet distortions, which can have an additional influence on the flutter stability of the fan and the first compressor stages of jet engines. For this purpose a sinusoidal steady total pressure inlet distortion was defined. The influence of this inlet distortion on the flow field and the flutter stability of a highly loaded transonic fan rotor (NASA rotor 67) is investigated. The static deflection of the manufactured blade was considered using an accurate mesh morphing algorithm to update the fan performance characteristic considering the deformed blade structure. The fan rotor interacts with the upstream distorted flow which leads to different blade loading between the adjacent blades. A decoupled flutter stability analysis using the three-dimensional viscous flow solver TBLOCK and the open-source software package CalculiX for pre-stressed modal analyses is carried out. The flutter stability analyses with TBLOCK are performed using the so-called energy method which was introduced by Carta. In order to predict the flutter stability under clean inflow conditions, two different formulations, the Influence Coefficient Method (ICM) and Traveling Wave Mode (TWM) formulation, are taken into account, whereas both formulations are compared to each other. The influence coefficients were directly calculated from the TWM formulation to determine the required number of passages for the ICM. It can be seen that the stability curves obtained with the ICM are in a good agreement to the TWM-method. The use of ICM reduces substantially the number of unsteady CFD calculations because of the fact that only one unsteady CFD calculation is needed to reconstruct the stability curve for each eigenmode and operating point. The effect of inlet distortion on flutter stability is investigated applying the TWM formulation only. Indeed, it was established that such flow disturbances have also for specific blades, considering the operating point, eigenmode and nodal diameter a destabilising impact on their aeroelastic behavior and can cause flutter, which is mostly determined by the time-averaged stability parameter. Just in the same manner a positive effect was observed for certain blades in the blade row.

Investigation of the Flutter Stability of a Transonic Fan with Inlet Distortion

“…The minimum damping occurs at ND = 0. This flutter stability behavior is consistent with that by Iseni (Iseni et al, 2016).…”

“…8, the deformation of tip region indicates that the first two modes are bending modes and the third mode is a torsion mode. Both the natural frequencies and mode shapes are consistent with that by Iseni (Iseni et al, 2016).…”

“…Iseni (Iseni et al, 2016) calculated the flutter boundary of NASA Rotor 67 at different operating conditions (peak efficiency, near stall, near chock), their analysis was subject to a sinusoidal total pressure distortion. Their results indicated no risk of flutter for all first three modes of Rotor 67.…”

Effect of Intake Acoustic Reflection on Blade Vibration Characteristics

“…This suggests that separation is the primary driver of blade flutter, while the shock wave enhances the aeroelastic stability of the blades. Similar findings were reported in studies by Iseni [12] and Purushothaman et al [13]. However, these conflicting conclusions were highlighted by Dong [14], who Aerospace 2024, 11, 358 2 of 17 indicated that the difference in the stabilizing or destabilizing effects of shock wave and flow separation is related to changes in the blade throat area, with the influencing mechanisms remaining unclear.…”