Aerodynamic and Basic Acoustic Optimization of a Counter Open Rotor with Experimental Verification

Abstract

The present study demonstrates an aerodynamic and acoustic optimization methodology of a contra-rotating open rotor (CROR). The main challenge of the design effort undertaken was to improve the overall propulsive efficiency at top of climb operating conditions while at the same time improving the acoustic characteristics at take-off or sideline conditions respectively. Apart from these global objectives, a number of constraints such as fulfilling thrust requirements at various flight regimes as well as basic mechanical criteria such as stress limitations to ensure a safe operation (in the present case for the test rig) needed to be respected and pose additional challenges for the design effort. However, both objectives were successfully met, which was confirmed by experiments. This was achieved by applying DLR’s multi-objective optimization procedure AutoOpti6, which is based on an evolutionary algorithm, in combination with the DLR’s 3D uRANS method TRACE8. Appropriate cost functions for aerodynamics and basic acoustics were applied already within the design phase. The acoustic evaluation was carried out with a coupled uRANS and an analytic far field prediction method based on an integral Ffowcs-Williams Hawkings (FW-H) approach (APSIM7). Both the aerodynamic and acoustic characteristics of several typical CROR configurations will be discussed by means of a coupled CFD/FW-H approach and corresponding experimental data taken in the WT104 wind tunnel of TsAGI by both TsAGI and DLR. The main driver of the acoustic improvement will be discussed, here mainly in terms of a reduction of the wake velocity deficit of the front rotor. This wake strength reduction led to a decrease of the sum of the interaction tones over the entire directivity, here an average acoustic benefit of 3.2 dB at sideline conditions for the optimized version was measured. Since all design efforts were undertaken neglecting the installation of the open rotor to the aircraft, the paper concludes with an assessment of the installation effect, here mainly on open rotor acoustic characteristics