OATAO is an open access repository that collects the work of some Toulouse researchers and makes it freely available over the web where possible. This is an author's version published in : http://oatao.univ-toulouse.fr/19782Official URL : https://dx.

Blade/casing rubbing interactions in aircraft engines: Numerical benchmark and design guidelines based on NASA rotor 37. Journal of Sound and Vibration, Elsevier, 2019, 460, pp.Abstract In order to improve the efficiency of aircraft engines, the reduction of clearances between blade tips and their surrounding casing is one avenue manufacturers consider to lower aerodynamic losses. This reduction increases the risk of blade tip/casing contact interactions under nominal operating conditions. Designers need tools to accurately predict subsequent nonlinear vibrations. Engineers and researchers have developed a variety of sophisticated numerical models to predict blades' responses. These models are related to distinct frameworks (time/frequency domain) and various solution algorithms (explicit/implicit time integration schemes, penalty/Lagrange multiplier contact treatment...) which calls for comparative analyses. However, published results are often limited for the sake of confidentiality thus preventing any detailed confrontation. While qualitative understanding can be gained from simplified academic models, full scale models are needed to predict complex interactions in a realistic manner. In this context, this paper proposes a benchmark featuring detailed simulations and analyses of a full 3D finite element model based on the open NASA rotor 37 compressor blade to facilitate reproducibility and collaboration across the research community. NASA rotor 37, a compressor stage widely used as a test case in aerodynamic simulations and validations, has the advantage of presenting a realistic blade geometry. The geometry of the blade is built from publicly available reports. The paper provides details on the geometry, the numerical model and the results to allow an easy use of this model across the fields of structural dynamics. Two contact scenarios are investigated: one with direct contact against the casing, and one with abradable material deposited on the casing to mitigate contact severity through wear. The nonlinear vibration response of the blade is simulated in the time domain. It is evidenced that the addition of the abradable material decreases the amplitude of vibration for most of the angular speeds investigated. However, new interactions appear for some angular speeds. The obtained results are consistent with previous simulations on industrial geometries. Based on works showing improved aerodynamic performances when the blade is tilted, a total of seven geometries are investigated: the reference blade, with a straight vertical stacking line similar to the original rotor 37, two forward-leaned blades, two backward-swept blades and two full forward chordwise swept blades. The sweep and lean variations are shown to have a dramatic impact on the vibration response: the backward sweep results in an increased blade's robustness to contact events and the full forward chordwise sweep in a reduced robustness, while the forward lean leads to a robustness similar to the reference blade. RésuméLa réduction des jeux au...

This article investigates the use of a recently developed fibrous core material to increase vibration damping in sandwich beams. The entangled cross-linked fibre (ECF) material is made of short carbon fibres cross-linked with epoxy resin. Dry friction between fibres provides energy dissipation when the material is deformed. Previous measurements on the material are post-processed to provide a simplified viscoelastic description of the material, for an easier interpretation of subsequent structural testings. Two sandwich beams are compared with reference honeycomb beams: a sandwich beam with an ECF core, and a hybrid beam with a honeycomb core and an ECF insert. Steady-state tests are performed on both types of beams to obtain their frequency responses for different excitation levels, and the corresponding apparent loss factors are computed. The beam with a full ECF core shows an apparent loss factor more than ten times higher than the reference honeycomb beam. The hybrid sandwich beam provides an apparent loss factor four times higher than the reference honeycomb beam. All beams exhibit nonlinear softening responses consistent with a dry friction phenomenon in the material: the resonance frequencies decrease with increasing excitation amplitude, and damping increases then decreases again at very high amplitudes while remaining largely superior to that of the honeycomb beams. Transient impact testings are also presented for a qualitative comparison of the ECF and reference beams, and the ECF beams lead to shorter decay times compared to the reference beams.

In modern aircraft engines, reduced operating clearances between rotating blade tips and the surrounding casing increase the risk of blade/casing structural contacts, which may lead to high blade vibration levels. Therefore, structural contacts must now be accounted for as early as in the engine design stage. As the vibrations resulting from contact are intrinsically nonlinear, direct optimization of blade shapes based on vibration simulation is not realistic in an industrial context. A recent study on a blade featuring significantly lower vibration levels following contact event identified a potential criterion to estimate a blade sensitivity to contact interactions. This criterion is based on the notion of dynamic clearance, a quantity describing the evolution of the blade/casing clearance as the blade vibrates along one of its free-vibration modes. This paper presents an optimization procedure, which minimizes the dynamic clearance as a first step toward the integration of structural criteria in blade design. A dedicated blade geometry parameterization is introduced to allow for an efficient optimization of the blade shape. The optimization procedure is applied to the three-dimensional (3D) properties of two different blades. In both cases, initial and optimized blades are compared by means of an in-house numerical tool dedicated to the simulation of structural contact events with a surrounding casing. The simulations focus on rubbing phenomena, involving the vibration of a single blade. Simulation results show a significant reduction of vibration levels following contact interactions for the optimized blades. Critical speeds related to the mode on which the dynamic clearance is computed are successfully eliminated by the blade shape optimization. For the investigated blade geometries, backward sweep and backward lean angles are associated with reduced contact interactions compared to forward sweep and forward lean angles.

This paper presents a novel trade-off chart to support the design of multilayer acoustic packages. In this multi-objective problem, a designer has to specify a combination of layers from a set of available acoustic materials and thicknesses. Material types may include porous, mass-weighted, facing, among others. The combination must meet requirements in terms of sound absorption, sound transmission loss, cluttering, mass, etc. While predictions and analyses can be made on predetermined multilayer acoustic packages using the transfer matrix method, statistical energy analysis, finite elements methods or modal analysis, comparing a large number of possible combinations is cumbersome. On the other hand, optimization methods can be used to identify optimal thicknesses or material properties for a given layer combination, but the obtained solution may not be industrially relevant since, in general, only a limited set of acoustic materials and layer thicknesses exist commercially. In this paper, a new design methodology is proposed, which takes into account only the feasible combinations and provides guidelines for compromises between different performance parameters. The three-step methodology is demonstrated through a case study inspired by the automotive industry. First, relevant categories of layer configurations are defined, and following these patterns, all possible combinations of materials from a given inventory are calculated and stored in a database. Then, for selected performance parameters, the Pareto set of "better combinations" is identified. Finally, the "better solutions" are displayed on a trade-off chart through utility functions that allow weighing the different performance parameters. The tool developed for doing so is applied to the case study, and two example situations are presented. For each situation, the trade-off chart provides several suitable solutions, which are discussed. The use of this new tool effectively induces gains of time at the early stage of design, when it is most crucial.