Challenges and opportunities of thermomechanical FEM simulations of high fidelity aero-engine models by implicit time-integration

摘要

Due to the demand of more and more detailed FEM simulation results of whole aero-engine models, the number of DOF of such models increases dramatically up to a few millions. Although the available computational power for the solution of such highly nonlinear, dynamic boundary value problems also increased during the last years, the overall computational time of these high-fidelity models for the implicit simulation of a few seconds of the running engine is in the order of weeks even on high-performance clusters with thousands of CPU cores. On the other hand, during the development of such models one is faced with a bunch of challenges, which do not occur in simplified models. Since in high-fidelity models even the bearings are not idealized or simplified the mesh of the surrounding parts needs to be fine enough to allow a kinematically correct rotation of the bearing. It will be shown that the necessary mesh density does not only depend on the geometric properties but also on the used contact algorithm. The used contact algorithm also has a big influence onto the time-step which is used by the implicit time-integration algorithm. Especially so-called Mortar contact formulations showed very promising results also with respect to the accuracy of the contact stresses since Mortar algorithms take into account the element shape functions. Of big importance is also the choice of the correct Finite Element formulation in combination with a proper anti-Hourglassing algorithm. Excessive tests have been performed with different formulations and some results will be presented. Also the used time-integration algorithm itself has to be chosen carefully when dealing with fast rotating structures since the classical Newmark time-integration scheme becomes instable in certain situations. To avoid these instability issues a so-called Newmark-3-Point-Euler-Backward time-integration scheme has been implemented. Finally, strategies for the reduction of the number of degrees of freedom have been investigated, which do not decrease the accuracy of the computational results of the model. Here, the substitution of solid elements by shell elements in areas where this is applicable is a very promising approach.