A computational investigation of acoustic characteristics of model ducted counter-rotating fan COBRA-1 with ultra-high bypass ratio BPR = 20, developed in CIAM in the framework of European Project COBRA (Innovative Counter rOtating fan system for high Bypass Ratio Aircraft engine), is presented in the paper. Calculations of acoustic characteristics of the fan were performed at five operational conditions. For three of them the sums of rotation frequencies of rotors corresponded to those expected for approach conditions, and they differed from each other by relations between frequencies of rotors, and for other two the sums of rotation frequencies corresponded to cutback and sideline operational conditions. Numerical investigations were conducted using the method of 3D numerical calculation of interaction between fan rows implemented in 3DAS (3 Dimensional Acoustics Solver) CIAM in-house solver. The data were compared with the results of the experiments conducted in the CIAM test rig C-3A. The results of comparison show satisfactory qualitative and, in some positions of microphones, quantitative agreement between the results of the simulation and the experiment. Both results demonstrate strong influence of relation between rotation frequencies of rotors on acoustic characteristics and significant dominance of noise radiated from the nozzle over the noise radiated from the inlet.

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