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.
In the present contribution the results of two three-dimensional viscous flutter analyses for a turbine cascade, Standard Configuration no. 11, are presented. The steady state and transient flow simulations were performed using the commercially available CFD solver ANSYS CFX 13.0 and a modified version of the CFD solver TBLOCK developed by Denton which is widely used in turbomachinery industry. The flutter analyses are performed under two different flow conditions. A subsonic, attached flow case and an off-design transonic case with a separated flow region near the trailing edge and a normal shock which are both located on the suction side. For each flutter analysis, the aeroelastic solution is computed for a large number of interblade phase angles. The results of ANSYS CFX and TBLOCK are compared to one another as well as to other CFD codes and experimental data. To reduce computing time, a phase-shifted boundary condition was implemented in TBLOCK. First results are shown in comparison to ANSYS CFX and its new implemented Fourier transformation method. The results of TBLOCK and ANSYS CFX agree well with experimental results. First results applying the phase-shifted boundary condition show that this method is suitable for calculating the aerodynamic damping with less numerical effort.
During the aeromechanical design process of turbomachinery blading, one of the main goals is to improve the blade loading which may lead to a higher risk of flutter. To avoid flutter induced blade failure during operation, the final blade design has to fulfill certain aero mechanical requirements. These refer to the permitted static and dynamic stress levels as well as the aeroelastic stability constraint of flutter for the whole operating range. In this contribution, an efficient workflow for three-dimensional viscous flutter stability analyses will be presented using the three-dimensional viscous flow solver TBLOCK and the open-source software package CalculiX for FE modal analyses. For this purpose, the workflow is applied to the first compressor rotor of a state of the art gas turbine. The flutter analysis is performed for several operating points to predict an accurate flutter envelope for the whole operating range of the investigated compressor stage. To reduce the numerical effort, only the first two mode shapes are considered with respect to different shaft speeds. In addition, phase-shifted boundary conditions are applied to all flutter calculations using the traveling wave mode domain taking all possible inter-blade phase angles into account. The results of the flutter analysis show no indications for flutter within the projected operating range of the rotor and for the considered mode shapes. In conclusion, the described workflow is able to determine the critical flutter stability boundaries of the investigated compressor rotor with reasonable numerical effort.
Usually, in a turbine an uneven number of blades are selected for vane and blade rows to reduce the level of interaction forces. To consider all unsteady flow phenomena within a turbine the computation of the full annulus is required causing considerable computational cost. Transient blade row methods using few passages reduce the numerical effort significantly. Nevertheless, those approaches provide accurate results. This contribution presents three different unsteady approaches to compare the accuracy and the computational effort, using a full annulus unsteady CFD simulation as a reference. The first approach modifies the blade-to-blade ratio whereas the second method scales the circumferential flow pattern to reach spatial and temporal periodicity. Third approach is based on time-inclining method to overcome unequal blade pitches with less numerical effort. All unsteady CFD simulations are carried out for the transonic test turbine VKI BRITE EURAM using the commercial CFD solver ANSYS CFX 14.5. The resulting unsteady pressure disturbances and blade forces of the different transient blade row methods are compared to each other as well as to experimental data. Finally, the accuracy and the computational costs are discussed in more detail.
Transient blade loading caused by the interaction of different blade rows is one limiting factor of turbo machinery blading lifetime especially when it leads to forced response. In order to reduce the calculation effort associated with 3D computational fluid dynamics (CFD), modern transient blade row methods such as the time transformation (TT) method are used. This paper investigates the capability of the TT method to predict these phenomena by the example of the last two stages of a shrouded 4-stage air-turbine. Numerical simulations are performed using the commercial flow solver ANSYS CFX. In a detailed time step and grid influence study the sufficient resolution of all predominant flow features is ensured. The results of the TT method are then compared to a reference (REF) calculation using direct periodic treatment with focus on the force signals, frequency spectra and the distribution of the first harmonic amplitude of pressure signal along the blade surface.
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