Flutter susceptibility approximation via curve fitting and MAC-analysis in an automated optimization design process

Abstract

At present, several institutes of the German Aerospace Center (DLR) jointly work on the design of a counter rotating fan made of carbon fiber reinforced material. To generate blades with superior aerodynamic properties, a multidimensional optimization tool developed by the Institute of Propulsion Technology is used. Since aeroelastic problems often arise during late design stages and are inevitably very expensive to solve, the goal is to include aeroelastic estimations into this optimization tool which is used in the early design phase. Assuming that rotors generally show a relatively sinusoidal behavior of the damping-over-IBPA-diagram at operating points sufficiently far away from the surge line, not all IBPAs need to be investigated in order to determine the lowest aerodynamic damp- ing. An algorithm was implemented that predicts the IBPA with the lowest damping value from three already calculated IBPAs through which a sine curve is fitted. It was established that this algorithm is valid for damping-vs-IBPA-curves that possess a roughly sinusoidal shape. A further investigation of the possibilities of time reduction in the automated design process revealed that flow solutions and structural properties do not change significantly during one optimization run. Consequently, a method was found to measure the similarity of two different cases. For very similar cases it can be assumed that the aeroelastic properties do not change significantly. Therefore, just the IBPA with the predictably lowest aerodynamic damping value needs to be calculated. An introduction to the theory of these approaches is presented and examples arising during the design of the counter rotating fan are given