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.