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. However, unexpected abradable wear removal mechanisms were recently observed in experimental set-ups as well as during maintenance procedures. Based on a numerical strategy previously developed, the present study introduces a numerical-experimental comparison of such occurrence.Attention is first paid to the review and analysis of existing experimental results. Good agreement with numerical predictions is then illustrated in terms of critical stress levels within the blade as well as final wear profiles of the abradable liner. Numerical results suggest an alteration of the abradable mechanical properties in order to explain the outbreak of a divergent interaction. New blade designs are also explored in this respect and it is found that the interaction phenomenon is highly sensitive to (1) the blade geometry, (2) the abradable material properties and (3) the distortion of the casing.

International audienceIn rotating machinery, notably in modern high efficiency compressors, a critical requirement for optimal performance consists in minimizing radial clearances between the rotating bladed disk and the casing. This solution significantly increases the risks of contact between rotating bladed disk and casing and may lead in specific conditions to catastrophic behavior (component failure, etc.). The physical phenomena and mechanisms involved in blade-casing contact interaction situations are still misunderstood. In order to highlight these mechanisms, specific experiments have been performed on an experimental multi-stage compressor of a turbojet with dedicated dynamic and thermal instrumentations. For all configurations tested, major damages are noticed: blades had cracks and the abradable coating of the casing was heavily machined. Results show that the blade failure refers to fatigue limit with first natural mode excitation of the blade. The paper is focused on the analysis of the successive stages of blade dynamic response before the failure. It is shown that this response is influenced by the variations of the blade-casing contact conditions. These conditions are linked to the thermomechanical behavior and wear of coating, illustrated by high thermal levels and non uniform wear profile. Coupling between thermomechanics, wear and dynamic has to be considered to highlight the transient mechanisms leading to the cases of blade failure

a b s t r a c tThis contribution addresses a confrontation between the experimental simulation of a rotor/stator interaction case initiated by structural contacts with numerical predictions made with an in-house numerical strategy. Contrary to previous studies carried out within the low-pressure compressor of an aircraft engine, this interaction is found to be nondivergent: high amplitudes of vibration are experimentally observed and numerically predicted over a short period of time. An in-depth analysis of experimental data first allows for a precise characterization of the interaction as a rubbing event involving the first torsional mode of a single blade. Numerical results are in good agreement with experimental observations: the critical angular speed, the wear patterns on the casing as well as the blade dynamics are accurately predicted. Through out the article, the in-house numerical strategy is also confronted to another numerical strategy that may be found in the literature for the simulation of rubbing events: key differences are underlined with respect to the prediction of non-linear interaction phenomena.

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 com pressor blade. The blade of interest underwent structural divergence after blade!abrad able coating contact occurrences on a rig test. The design improvements were carried out in several steps with significant modifications o f the blade stacking law while main taining 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 stem ming 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. 1 In tro d u c tio nThe existence of several nonlinear interfaces within aircraft engines and turbines, in general-such as the blade-tip/casing or the shaft/bearing interfaces-are propitious to the occurrence of a large variety of rotor/stator interactions that may threaten the engine structural integrity. As a rule, modem designs feature tighter operating clearances in order to increase the engine effi ciency. Consequently, structural contacts are now accepted as nor mal operating conditions and subsequent interactions, such as the nabbing of a single blade on the abradable coating [1,2], modal interaction [3,4], or backward and forward whirl motions [5], have been widely investigated over the last years. The growing understanding of these interactions allows designers to anticipate of blades robust to structural contacts, in particular blades in order to mitigate events such as blade failure or blade loss. A numerical strategy dedicated to the analysis of blade/casing and blade/abradable coating interactions previously introduced [6,7] has been implemented in the industrial environment of Snecma. This paper focuses on the first attempt to benefit from this strategy as early as the blade design stage.The blade of interest belongs to the high-pressure compressor of a modem aircraft engine. At this stage of the engine, an abrad able coating is deposited along the casing circumference in order to avoid damages when contacts occur. However, a structural divergence, defined as an abnormal increase of the amplitude of vibration of the blade possibly leading to blade failure for a given rotational frequency, was witnessed on a rig test after abradable coating/blade contacts. Such phenomenon was previously wi...

This article deals with the numerical investigation of the unilateral contact-induced dynamics of a turbomachine blade rotating within a perfectly rigid yet distorted casing. This investigation is motivated by unelucidated vibratory behaviours observed experimentally. The simulations are based on an in-house time-marching strategy incorporating Lagrange multipliers for the unilateral contact treatment, as well as centrifugal stiffening and abradable coating removal. Significant extensions are proposed through the implementation of (1) aerodynamic loading on the blade and, (2) post-processing techniques involving the empirical mode decomposition which provides fruitful insights on important transient phenomena. A thorough bifurcation analysis with and without aerodynamic loading highlights the existence of flip bifurcations with period-doubling and period-halving sequences over a broad angular speed range. Numerical simulations with external aerodynamic loading yields quasi-periodic and likely to be chaotic motions that could not be observed under vacuum. The proposed numerical investigations underline the key role of the aerodynamic loading in the blade dynamics and suggest that unexplained experimental vibratory behaviours are related to the vacuum conditions of the experiment.