A Noise Assessment Methodology for Highly-Integrated Propulsion Systems with Inlet Flow Distortion

Jeff Defoe
Advisor: Prof. Spakovszky
Reducing emissions, fuel burn, and noise are the main drivers for innovative aircraft design. Embedded propulsion systems, such as those used in hybrid wing-body aircraft, can offer fuel burn and noise reduction benefits but one of the major challenges in high-speed fan stages used in these embedded propulsion systems is inlet distortion noise, in particular multiple-pure-tone (MPT) noise. MPT noise consists of shaft-order tones due to shock interaction caused by small (+/- 0.2°) blade-to-blade stagger angle variations. The key challenge to MPT noise prediction in embedded propulsion systems is the inherent coupling of the acoustics and aerodynamics due to the presence of non-uniform flow. A new approach was developed to solve this problem based on a body force description of the fan blade row. The body force field not only represents the overall rotor characteristics, capable of capturing the distortion transfer effects, but for the first time is also used as the fan noise source. An unsteady perturbation force field captures the blade-to-blade flow variations that cause MPT noise.
The approach has been validated on NASA's Source Diagnostic Test fan and inlet, showing good agreement with experimental data for aerodynamic performance, acoustic source generation and far-field noise spectra. The approach was then employed with the objective of quantifying the effects of non-uniform flow on the generation and propagation of MPT noise. First-of-their-kind back-to-back coupled aero-acoustic computations were carried out, comparing the conventional inlet used in the validation case to a serpentine inlet. Both inlets delivered flow to the same NASA/GE R4 fan rotor at equal corrected mass flow rates. Although the source strength at the fan is increased by 45 dB in sound power level due to the non-uniform inflow, far-field noise for the serpentine inlet duct is increased on average by only 7 dB (3 dBA) overall sound pressure level in the forward arc. This is due to the redistribution of acoustic energy to frequencies below 11 times the shaft frequency and the apparent cut-off of tones at higher frequencies including blade-passing tones. The circumferential extent of the inlet swirl distortion at the fan was found to be 2 blade pitches, or 1/11th of the circumference, suggesting a relationship between the circumferential extent of the inlet distortion and the cut-off frequency perceived in the far field. The streamwise vortices associated with the inlet distortion locally alter the relative Mach number and create a region of evanescent wave behavior which is conjectured to be the cause of the changes in the far-field spectra.
In the final phase of the project, a parametric study of serpentine inlet designs is currently underway to quantify the effects of non-uniform flow on MPT noise generation and propagation. The results will be used in the formulation of a response surface model suitable for incorporation into NASA's ANOPP noise prediction framework. The understanding gained from the parametric study will also be useful in forming design guidelines for integrated propulsion systems.