Acoustic Mode Measurements in the Inlet of a Model Turbofan Using a Continuously Rotating Rake

Comprehensive measurements of the spinning acoustic mode structure in the inlet of the Advanced Ducted Propeller (ADP) have been completed. These measurements were taken using a unique and previously untried method which was first proposed by T.G. Sofrin. A continuously rotating microphone system was employed. The ADP model was designed and built by Pratt & Whitney and tested in the NASA Lewis 9- by 15-foot Anechoic Wind Tunnel. Three inlet configurations were tested with cut-on and cutoff stator vane sets. The cutoff stator was designed to suppress all modes at the blade passing frequency. Rotating rake measurements indicate that several extraneous circumferential modes were active. The mode orders suggest that their source was an interaction between the rotor and small interruptions in the casing tip treatment. The cut-on stator produced the expected circumferential modes plus higher levels of the unexpected modes seen with the cutoff stator

Acoustic evaluation of the Helmholtz resonator treatment in the NASA Lewis 8- by 6-foot supersonic wind tunnel

The acoustic consequences of sealing the Helmholtz resonators of the NASA Lewis 8- by 6-Foot Supersonic Wind Tunnel (8x6 SWT) were experimentally evaluated. This resonator sealing was proposed in order to avoid entrapment of hydrogen during tests of advanced hydrogen-fueled engines. The resonators were designed to absorb energy in the 4- to 20-Hz range; thus, this investigation is primarily concerned with infrasound. Limited internal and external noise measurements were made at tunnel Mach numbers ranging from 0.5 to 2.0. Although the resonators were part of the acoustic treatment installed because of a community noise problem their sealing did not seem to indicate a reoccurrence of the problem would result. Two factors were key to this conclusion: (1) A large bulk treatment muffler downstream of the resonators was able to make up for much of the attenuation originally provided by the resonators, and (2) there was no noise source in the tunnel test section. The previous community noise problem occurred when a large ramjet was tested in an open-loop tunnel configuration. If a propulsion system producing high noise levels at frequencies of less than 10 Hz were tested, the conclusion on community noise would have to be reevaluated

Unsteady blade pressure measurements on a model counterrotation propeller

In an exploratory effort an advanced counterrotation propeller instrumented with blade-mounted pressure transducers was tested in the NASA Lewis 9- by 15-Foot Anechoic Wind Tunnel at a simulated takeoff and landing speed of Mach 0.20. The propeller's aft diameter was reduced to investigate possible noise reductions resulting from reduced blade row interaction with the tip vortex. The propeller was tested at three blade row spacings at fixed blade setting angles, at the maximum blade row spacing at higher blade setting angles and at propeller axis angles attack to the flow up to + or - 16 deg. A limited number of unsteady blade surface pressure measurements were made on both rotors of the model counterrotation propeller. Emphasis was placed on determining the effects of rotor-rotor interactions on the blade surface pressures. A unique method of processing the pressure signals was developed that enables even weak interaction waveforms and spectra to be separated from the total signal. The interaction on the aft rotor was many times stronger than that on the forward rotor. The fundamental rotor interaction tone exhibited complicated behavior but generally increased with rotational speed and blade setting angle and decreased with rotor spacing. With the propeller axis at an angle to the flow, the phase response of the aft rotor appeared to be significantly affected by the presence of the forward rotor

Advanced turboprop wing installation effects measured by unsteady blade pressure and noise

A single rotation model propeller (SR-7A) was tested at simulated takeoff/approach conditions (Mach 0.2), in the NASA Lewis 9- by 15-Ft Anechoic Wind Tunnel. Both unsteady blade surface pressures and noise measurements were made for a tractor configuration with propeller/straight wing and propeller alone configurations. The angle between the wing chord and propeller axis (droop angle) was varied along with the wing angle of attack to determine the effects on noise and unsteady loading. A method was developed that uses unsteady blade pressure measurements to provide a quantitative indication of propeller inflow conditions, at least for a uniform (across the propeller disk) inflow angle. The wing installation caused a nearly uniform upwash at the propeller inlet as evidenced by the domination of the pressure spectra by the first shaft order. This inflow angle increased at a rate of almost 150 percent of that of the wing angle-of-attack for a propeller-wing spacing of 0.54 wing chords at a constant droop angle. The flyover noise, as measured by the maximum blade passing frequency level, correlates closely with the propeller inflow angle (approx. 0.6 dB per degree of inflow angle) for all droop angles and wing angles of attack tested, including the propeller alone data. Large changes in the unsteady pressure responses on the suction surface of the blade were observed as the advance ratio was varied. The presence of a leading edge vortex may explain this behavior since changes in the location of this vortex would change with loading (advance ratio)

Far-field noise and internal modes from a ducted propeller at simulated aircraft takeoff conditions

The ducted propeller offers structural and acoustic benefits typical of conventional turbofan engines while retaining much of the aeroacoustic benefits of the unducted propeller. A model Advanced Ducted Propeller (ADP) was tested in the NASA Lewis Low-Speed Anechoic Wind Tunnel at a simulated takeoff velocity of Mach 0.2. The ADP model was designed and manufactured by the Pratt and Whitney Division of United Technologies. The 16-blade rotor ADP was tested with 22- and 40-vane stators to achieve cut-on and cut-off criterion with respect to propagation of the fundamental rotor-stator interaction tone. Additional test parameters included three inlet lengths, three nozzle sizes, two spinner configurations, and two rotor rub strip configurations. The model was tested over a range of rotor blade setting angles and propeller axis angles-of-attack. Acoustic data were taken with a sideline translating microphone probe and with a unique inlet microphone probe which identified inlet rotating acoustic modes. The beneficial acoustic effects of cut-off were clearly demonstrated. A 5 dB fundamental tone reduction was associated with the long inlet and 40-vane sector, which may relate to inlet duct geometry. The fundamental tone level was essentially unaffected by propeller axis angle-of-attack at rotor speeds of at least 96 percent design

Coupling of Low Speed Fan Stator Vane Unsteady Pressures to Duct Modes: Measured versus Predicted

Uniform-flow annular-duct Green's functions are the essential elements of the classical acoustic analogy approach to the problem of computing the noise generated by rotor-stator interaction inside the fan duct. This paper investigates the accuracy of this class of Green's functions for predicting the duct noise levels when measured stator vane unsteady surface pressures are used as input to the theoretical formulation. The accuracy of the method is evaluated by comparing the predicted and measured acoustic power levels for the NASA 48 inch low speed Active Noise Control Fan. The unsteady surface pressures are measured,by an array of microphones imbedded in the suction and pressure sides of a single vane, while the duct mode levels are measured using a rotating rake system installed in the inlet and exhaust sections of the fan duct. The predicted levels are computed using properly weighted integrals of measured surface pressure distribution. The data-theory comparisons are generally quite good particularly when the mode cut-off criterion is carefully interpreted. This suggests that, at least for low speed fans, the uniform-flow annular-duct Green's function theory can be reliably used for prediction of duct mode levels if the cascade surface pressure distribution is accurately known

Azimuthal Directivity of Fan Tones Containing Multiple Modes

The directivity of fan tone noise is generally measured and plotted in the sideline or flyover plane and it is assumed that this curve is the same for all azimuthal angles. When two or more circumferential (m-order) modes of the same tone are present in the fan duct, an interference pattern develops in the azimuthal direction both in the duct and in the farfield. In this investigation two m-order modes of similar power were generated in a large low speed fan. Farfield measurements and a finite element propagation code both show substantial variations in the azimuthal direction. Induct mode measurement were made and used as input to the code. Although these tests may represent a worst case scenario, the validity of the current practice of assuming axisymmetry should be questioned

Low-noise fan exit guide vanes

Low-noise fan exit guide vanes are disclosed. According to the present invention a fan exit guide vane has an outer shell substantially shaped as an airfoil and defining an interior cavity. A porous portion of the outer shell allows communication between the fluctuations in the air passing over the guide vane and the interior cavity. At least one acoustically resonant chamber is located within the interior cavity. The resonant chamber is in communication with the porous portion of the outer perimeter. The resonant chamber is configured to reduce the noise generated at a predetermined frequency. In various preferred embodiments, there is a plurality of acoustically resonant chambers located within the interior cavity. The resonant chambers can be separated by one or more partitions within the interior cavity. In these embodiments, the resonant chambers can be configured to reduce the noise generated over a range of predetermined frequencies

Active Noise Control of Low Speed Fan Rotor-Stator Modes

This report describes the Active Noise Cancellation System designed by General Electric and tested in the NASA Lewis Research Center's 48 inch Active Noise Control Fan. The goal of this study was to assess the feasibility of using wall mounted secondary acoustic sources and sensors within the duct of a high bypass turbofan aircraft engine for active noise cancellation of fan tones. The control system is based on a modal control approach. A known acoustic mode propagating in the fan duct is cancelled using an array of flush-mounted compact sound sources. Controller inputs are signals from a shaft encoder and a microphone array which senses the residual acoustic mode in the duct. The canceling modal signal is generated by a modal controller. The key results are that the (6,0) mode was completely eliminated at 920 Hz and substantially reduced elsewhere. The total tone power was reduced 9.4 dB. Farfield 2BPF SPL reductions of 13 dB were obtained. The (4,0) and (4,1) modes were reduced simultaneously yielding a 15 dB modal PWL decrease. Global attenuation of PWL was obtained using an actuator and sensor system totally contained within the duct

Baseline acoustic levels of the NASA Active Noise Control Fan rig

Extensive measurements of the spinning acoustic mode structure in the NASA 48 inch Active Noise Control Fan (ANCF) test rig have been taken. A continuously rotating microphone rake system with a least-squares data reduction technique was employed to measure these modes in the inlet and exhaust. Farfield directivity patterns in an anechoic environment were also measured at matched corrected rotor speeds. Several vane counts and spacings were tested over a range of rotor speeds. The Eversman finite element radiation code was run with the measured in-duct modes as input and the computed farfield results were compared to the experimentally measured directivity pattern. The experimental data show that inlet spinning mode measurements can be made very accurately. Exhaust mode measurements may have wake interference, but the least-squares reduction does a good job of rejecting the non-acoustic pressure. The Eversman radiation code accurately extrapolates the farfield levels and directivity pattern when all in-duct modes are included