Steady state versus time-accurate CFD in an automated airfoil section optimization of a counter rotating fan stage

Abstract

Automated CFD-based optimization procedures have become an essential part of modern aerodynamic compressor design. Although time-accurate CFD provides a higher physical accuracy, due to limited resources still mainly steady state CFD is used. With a constantly growing computing power the question arises, whether it is worth it increasing the computing effort per evaluation using more accurate CFD codes, in order to improve the optimization results. This work investigates how the results of an automated aerodynamic compressor optimization depend on the simulation procedure used to calculate the flow solutions during the optimization. Two configurations of a counter-rotating fan stage with different axial inter-blade spacing have been optimised using a Q3D approach for the midspan airfoil sections. The configurations were chosen, as to represent the two possibilities of low and high unsteady flow interaction between the blade rows. In each case two automated optimizations have been performed. One based on a steady simulation procedure (RANS), the other on a time-accurate (URANS). In addition, the configuration with low axial spacing has been optimized using a RANS procedure with a determinsitic stress model (RANS-DS). A dependency of the optimization results on the CFD method used has been observed for cases showing high unsteady interaction between the blade rows. The best optimization results were obtained using a time-accurate URANS CFD-solver. A comparison between RANS and RANS-DS showed an advantage of using RANS-DS.