Optimization of Geometric Robustness of Aero Structures in Early Design Phases

Licentiate thesis, 2012

Every manufactured product is different. This variation can be detrimental to quality and product functionality. In an aerospace context, this variation may even result in serious threats to the safety and reliability of aircraft. However, it is not the variation itself that is harmful, but the effects it has on functionality. This is an important distinction to make. Reducing sources of variation is often associated with tightening tolerances and increasing cost. Instead, it is preferable to eliminate the effects of this variation by making designs more robust. This idea is at the core of robust design methodology. The research presented in this thesis aims at identifying the role of robust design in general, and geometry assurance in particular, in early phases of aerospace component design. Methods for evaluating the effects of geometric variation on the functionality of aero engine components are put forth. A simulation tool for performing multidisciplinary analyses is developed. By connecting geometry assurance tools with computational fluid dynamics and finite element analysis software, the aerodynamic, thermal, and structural effects of geometric variation can be evaluated. In addition, optimization procedures for surpassing this variation are investigated. The validity of computer simulations as a tool comes from their ability to accurately predict reality. Validating a simulation model can be done by estimating all its potential uncertainties and errors. Geometric variation is only one source of uncertainty amongst many others. By evaluating geometric against the framework of uncertainty quantification, this thesis addresses the relative importance of geometry assurance against other product development activities. The case studies presented in this thesis show that simulation results are heavily affected by geometric variation in parts and assemblies. One result showed an 8% thermal stress increase in a critical area due to geometric variation. This emphasizes the fact that the effects of geometric variation cannot be neglected in early design phases. The results of this thesis also showcase automated simulation platforms as a powerful tool for performing robustness analysis. Its advantage lies in that it speeds up the design iteration loop, which simplifies experiential design significantly. Combined with the theoretical framework from uncertainty quantification, the platform may also be used to optimize the product development process, in addition to optimizing the products themselves. By balancing the level of detail in the all phases of each simulation activity, an optimal allocation a lot of resources and engineering time can be obtained.