Aeroacoustic Scaling of Turbomachinery Blade Rows

An understanding of sound propagation through components of modern turbofan engines is indispensable for the further development of technologies to reduce noise from aircraft engines. Analytical models are used to predict noise emissions in early design phases of aircraft engines and turbomachinery components, e.g. axial compressors or turbines. However, only few experimental investigations have been carried out to validate the model predictions. Since most turbomachinery test rigs lack the acoustically optimized boundary conditions necessary for aeroacoustic measurements, the blading must be scaled in order to generate representative validation data. Based on a dimensional analysis, a scaling approach is derived to yield a finite number of dimensionless quantities. The hypothesis states that these characteristic numbers need to be maintained when scaling to establish aeroacoustic similarity between a reference and its scaled geometry. The approach and the hypothesis are validated for an axial compressor stator vane row and a 1.5-stage axial turbine configuration for various operating conditions. Numerical validation of aerodynamic and sound transmission similarity is determined with the agreement of steady RANS and unsteady harmonic balance simulations results for the original and scaled configurations. To reproduce real compressor conditions, a unique experiment based on an aeroacoustic wind tunnel is developed and rotor-stator-interaction modes are excited by means of a speaker array by applying a mode synthesis. Extensive aerodynamic and acoustic measurements using the acoustically optimized test rig with the scaled compressor vane row were conducted to experimentally validate the approach. Subsequently, steady and unsteady simulations were carried out. An analytical sound transmission model by Smith, as well as a broadband noise prediction model by Posson, were additionally applied. The numerical and experimental findings showed a good agreement for frequencies below 4.5 kHz regarding reflection and transmission across the scaled stator vane row. Differences occur when comparing the results with the model by Smith and reveal discrepancies due to the two-dimensional model assumptions. A similar stator vane self-noise was established. Overall, the results validate the scaling approach numerically, experimentally, and analytically. They confirm that it can be utilized for real aircraft engine applications and deliver a broad validation data set for analytical model validation.