Verification of the Inverse Cut-Off Effect in a Turbomachinery Stage Part 1-Numerical Results

Within the scope of the author’s previous papers, the general setup of an acoustic turbine test facility at the Graz University of Technology has been presented. In addition, its significance for the validation of acoustic prediction tools and the verification of noise reduction features related to the LPT (Low Pressure Turbine) has been highlighted. In contrast to the passive noise reduction feature presented last year, an innovative integrated acoustic absorber within the Turbine Exit Case (TEC), the purpose of the present paper is the investigation of a source noise reduction technology. In this case, the TEC strut count has been selectively chosen to achieve an ’Inverse Cut-off’ effect for the (last) blade TEC interaction. This inversely cut-off design comprises a comparably high strut count at an accordingly reduced chord length. Therefore, the existing TEC strut geometry has been modified to maintain its aerodynamic functionality while allowing for the inverse cut-off design. This acoustically designed TEC has then been numerically integrated into the existing rig environment to predict the achievable acoustic effect. These predictions are based on the MTU in-house Linearized Euler tool capable of simulating coupled multistage configurations. The results show a very promising noise reduction potential of the inversely cut-off TEC at the first Blade Passage Frequency (BPF) at approach power where the inverse cut-off is effective. In contrast, the take-off conditions are only slightly affected due to modified scattering effects. For an experimental verification, the inversely cut-off TEC predictions will be compared to upcoming rig measurements in a subsequent paper.