François Garcin

Mistuning Phenomena on Bladed Disk: Industrial Methods and Applications

This paper deals with the methods used at Snecma (SAFRAN Group) to simulate the real mistuned dynamic behavior of bladed disks. Many applications of the method on industrial cases are also presented. The dissymmetry of a real bladed disk, which is mainly generated by small deviations in its geometry, leads to different dynamic impedances of the blades. These variations in impedance cause significant amplification of the forced response of the bladed disk. We propose to focus on industrial methods that can be used to simulate the mistuned dynamic behavior of a bladed disk in the case of small “frequency mistuning” or of greater “large mistuning”. This method, based on the modal synthesis technique (Benfield and Hruda family), can be classified as a “first generation method”. The first originality of this method consists in taking into account major mistuning, such as that induced by an FOD event. A second innovation concerns the use of these methods to assess in probabilistic terms the correction factor to apply to the maximum dynamic level measured during engine certification tests. The first part of the paper relates to the proposed method and its validation for large-scale “shape” mistuning on an academic test case. In the second section, we present a number of industrial applications: • An application of “geometric mistuning” is presented on a bladed disk after an FOD event. The same kind of analysis as previously described is carried out and the numerical results are commented. • The mistuning analyses are carried out on an industrial blisk with a local defect. The sensitivity of the response amplification factor is calculated versus the mistuning rate using a probabilistic approach. • Another application of the mistuning strategy is discussed: it concerns the use of the mistuning method to specify the correction factor of partial dynamic strain gauge measurements in order to assess the maximum level of a bladed disk. • The last application is dedicated to the simulation of intentional mistuning to increase the aeroelastic stability of an industrial test case.Copyright

Redesign of a High-Pressure Compressor Blade Accounting for Nonlinear Structural Interactions

Recent numerical developments dedicated to the simulation of rotor/stator interaction involving direct structural contacts have been integrated within the Snecma industrial environment. This paper presents the first attempt to benefit from these developments and account for structural blade/casing contacts at the design stage of a high-pressure compressor blade. The blade of interest underwent structural divergence after blade/abradable coating contact occurrences on a rig test. The design improvements were carried out in several steps with significant modifications of the blade stacking law while maintaining aerodynamic performance of the original blade design. After a brief presentation of the proposed design strategy, basic concepts associated with the design variations are recalled. The iterated profiles are then numerically investigated and compared with respect to key structural criteria such as: (1) their mass, (2) the residual stresses stemming from centrifugal stiffening, (3) the vibratory level under aerodynamic forced response and (4) the vibratory levels when unilateral contact occurs. Significant improvements of the final blade design are found: the need for an early integration of nonlinear structural interactions criteria in the design stage of modern aircraft engines components is highlighted.Copyright

Snecma’s Viewpoint on the Numerical and Experimental Simulation of Blade-Tip/Casing Unilateral Contacts

Aircraft engine manufacturers are developping a new generation of turbojet engines featuring a lower impact on the environment, increased performances as well as reduced gas consumption. The efficiency of an engine is mostly driven by the operating clearance between the rotating parts and the stator. Accordingly, modern designs focus on the minimization of these clearances. In this context, unavoidable rotor imbalances or mistuning stemming from manufacturing processes as well as distortions resulting from thermal expansion or assembly conditions may generate blade-tip/casing contacts that are now considered as non-accidental operating conditions. In order to minimize the consequences of such events, an abradable coating is sprayed along the inner surface of the casing and acts as a fuse when the blade and the casing are in contact. However, even when an abradable coating is used, significant structural damages and wear as well as blade failures have been witnessed experimentally. The understanding of the physical phenomena at play called, on one hand, for throrough experimental investigations of rotor/stator contacts on full-scale stages of compressors and underlined that blade failure is mainly due to vibratory fatigue although the abradable coating is worn. On the other hand, numerical simulations have been performed to better understand the blade dynamics: over the last decade Snecma and its academic partners jointly developed a code for the simulation of contacts between rotor and stator: COROS. This code allows for the simulation of contacts — with a Lagrange multiplier contact treatment procedure — between full 3D models of engine components and accounts for abradable coating material removal. In particular, the simulation of experimental set-ups with COROS highlighted the correlation between the blade vibratory response and the abradable material removal. Yet still an experimental code, this paper addresses the integration of COROS within the design process of aircraft engine blades at Snecma. The paper focuses on on-going research for the identification of critical parameters in the arising of interactions as early as the design stage of components. A particular attention is paid to the mechanical properties of the abradable coating for which both experimental and numerical investigations are detailed.© 2015 ASME

High-pressure compressor blade dynamics under aerodynamic and blade-tip unilateral contact forcings

Recent studies focused on the numerical prediction of structural instabilities that may arise in rotating components of an aircraft engine. These instabilities are commonly classified into two categories: those induced by aerodynamic phenomena (such as the pressure applied on the blade by the incoming air flow) and those related to structural phenomena (such as potential blade/casing contacts). Based on an existing numerical strategy for the analysis of rotor/stator interactions induced by unilateral contacts between rotating and static components, this paper aims at combining both types of instabilities and provides a qualitative analysis of structural interactions that may arise within the high-pressure compressor of an aircraft engine. The aerodynamic pressure on the blade is simplified as a sinusoidal external load whose frequency depends on the number of upstream guide vanes. Results are presented both in time and frequency domains. Detailed bifurcation diagrams and Poincare maps underline the fundamental differences in the nature of the witnessed interactions with and without aerodynamic loading on the blade.Copyright

Numerical Study of a Rotor/Stator Interaction Case Experimentally Simulated With an Industrial Compressor

Higher aircraft energy efficiency may be achieved by minimizing the clearance between the rotating blade tips and respective surrounding casing. A common technical solution consists in the implementation of an abradable liner which improves both the operational safety and the efficiency of modern turbomachines. Recently, unexpected abradable wear removal mechanisms were observed in experimental set-ups and during maintenance procedures. The present study introduces a numerical strategy capable to address this occurrence.After focusing on the analysis of the experimental results, the good agreement between experimental observations and numerical results is illustrated in terms of critical stress levels within the blade as well as final wear profiles of the abradable liner. New blade designs are also explored in order to assess the impact of blade design on the outbreak of the interaction phenomenon. The prevalence of three dominant parameters in the interaction onset is shown: (1) blade design, (2) abradable material mechanical properties and (3) the need for a global distortion of the casing to synchronize blade-tip/abradable coating contacts.Copyright