Daniel L. Sutliff

Low-Speed Fan Noise Reduction with Trailing EDGE Blowing

An experimental proof-of-concept test was conducted to demonstrate rotor-stator interaction tone noise reduction through rotor trailing edge blowing. The velocity deficit from the viscous wake of the rotor blades was reduced by injecting air into the wake from a trailing edge slot. Composite hollow rotor blades with internal flow passages were designed based on Computational Fluid Dynamics codes modeling the internal flow. The hollow blade with interior guide vanes creates flow channels through which externally supplied air flows from the root of the blade to the trailing edge. The impact of the rotor wake-stator interaction on the acoustics was also predicted analytically. The Active Noise Control Fan, located at the NASA Glenn Research Center, was used as the proof-of-concept test bed. In-duct mode and farfield directivity acoustic data were acquired at blowing rates (defined as mass flow supplied to trailing edge blowing system divided by fan mass flow) ranging from 0.5% to 2.0%. The first three blade passing frequency harmonics at fan rotational speeds of 1700 to 1900 rpm were analyzed. The acoustic tone mode power levels (PWL) in the inlet and exhaust were reduced 11.5&–0.1, 7.2&11.4, 11.8&19.1 PWL dB, respectively. The farfield tone power levels at the first three harmonics were reduced 5.4, 10.6, & 12.4 dB PWL. At selected conditions, two-component hotwire and stator vane unsteady surface pressures were acquired. These measurements show the modification of the rotor wake due to trailing edge blowing and its effect on the stator vane to illustrate the physics behind the noise reduction.

Rotating rake turbofan duct mode measurement system

An experimental measurement system was developed and implemented by the NASA Glenn Research Center in the 1990s to measure turbofan duct acoustic modes. The system is a continuously rotating radial microphone rake that is inserted into the duct. This rotating rake provides a complete map of the acoustic duct modes present in a ducted fan and has been used on a variety of test articles: from a low‐speed, concept test rig, to a full‐scale production turbofan engine. The rotating rake has been critical in developing and evaluating a number of noise reduction concepts as well as providing experimental databases for verification of several aero‐acoustic codes.

Turbofan Duct Mode Measurements Using a Continuously Rotating Microphone Rake

An experimental measurement system was developed and implemented at the NASA Glenn Research Center in the 1990s to measure fan duct acoustic modes. The system is a radial array of microphones inserted into the duct that continuously rotates about the duct centerline. This Rotating Rake provides a complete map of the acoustic duct modes at fan harmonics in a ducted fan. It has been used on a variety of test articles: from a low-speed, concept test fan rig, to a full-scale production turbofan engine. The Rotating Rake has been critical in developing and evaluating a number of noise reduction concepts as well as providing experimental databases for verification of several aero-acoustic codes.

Assessment of Soft Vane and Metal Foam Engine Noise Reduction Concepts

Two innovative fan-noise reduction concepts developed by NASA are presented - soft vanes and over-the-rotor metal foam liners. Design methodologies are described for each concept. Soft vanes are outlet guide vanes with internal, resonant chambers that communicate with the exterior aeroacoustic environment via a porous surface. They provide acoustic absorption via viscous losses generated by interaction of unsteady flows with the internal solid structure. Over-the-rotor metal foam liners installed at or near the fan rotor axial plane provide rotor noise absorption. Both concepts also provide pressure-release surfaces that potentially inhibit noise generation. Several configurations for both concepts are evaluated with a normal incidence tube, and the results are used to guide designs for implementation in two NASA fan rigs. For soft vanes, approximately 1 to 2 dB of broadband inlet and aft-radiated fan noise reduction is achieved. For over-the-rotor metal foam liners, up to 3 dB of fan noise reduction is measured in the low-speed fan rig, but minimal reduction is measured in the high-speed fan rig. These metal foam liner results are compared with a static engine test, in which inlet sound power level reductions up to 5 dB were measured. Brief plans for further development are also provided.

Baseline acoustic levels of the NASA active noise control fan rig

Extensive measurements of the spinning acoustic mode structure in the NASA 48-in. 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. Far-field 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 far-field 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 nonacoustic pressure. The Eversman radiation code accurately extrapolates the far-field levels and directivity pattern when all in-duct modes are included. (Author)

Locating and Quantifying Broadband Fan Sources Using In-Duct Microphones

Abstract In-duct beamforming techniques have been developed for locating broadband noise sources on a low-speed fan and quantifying the acoustic power in the inlet and aft fan ducts. The NASA Glenn Research Center’s Advanced Noise Control Fan was used as a test bed. Several of the blades were modified to provide a broadband source to evaluate the efficacy of the in-duct beamforming technique. Phased arrays consisting of rings and line arrays of microphones were employed. For the imaging, the data were mathematically resampled in the frame of reference of the rotating fan. For both the imaging and power measurement steps, array steering vectors were computed using annular duct modal expansions, selected subsets of the cross spectral matrix elements were used, and the DAMAS and CLEAN-SC deconvolution algorithms were applied. Introduction Significant reduction in aircraft noise is required to meet ongoing noise regulation in the USA and Europe. Since the turbofan engine is a large contributor to aircraft noise, any overall reduction in aircraft noise must include engine noise reduction. In order to efficiently achieve noise reduction, detailed understanding of the physics of noise source generation is required. The NASA focus area—Fundamental Aeronautics, Subsonic: Fixed Wing Program—emphasizes developing technologies for diagnostics of noise for subsonic aircraft. The work described here is intended to advance the state of the art for imaging broadband fan noise of turbofan engines using microphones that are flush mounted in the outer wall of the bypass flow path of the nacelle. The recordings are post-processed to form images of broadband noise sources on the rotating blades and to measure the modal amplitudes and the sound power in the inlet and aft fan ducts. The blade acoustic images and mode maps are intended to help clarify the mechanism for fan broadband noise generation and to identify any problem areas on a given set of blades. The sound power measurement may be a convenient and more-precise substitute for far field microphones.

Attenuation of FJ44 Turbofan Engine Noise With a Foam-Metal Liner Installed Over-the-Rotor

Abstract A Williams International FJ44-3A 3000-lb thrust class turbofan engine was used as a demonstrator for a Foam-Metal Liner (FML) installed in close proximity to the fan. Two FML designs were tested and compared to the hardwall baseline. Traditional single degree-of-freedom liner designs were also evaluated to provide a comparison. Farfield acoustic levels and limited engine performance results are presented in this paper. The results show that the FML achieved up to 5 dB Acoustic Power Level (PWL) overall attenuation in the forward quadrant, equivalent to the traditional liner design. An earlier report presented the test set-up and conditions. Nomenclature FML ςFoam-Metal Liner OTR ςOver-the-Rotor HW1 original hardwall flow path HW2 modified hardwall flow path A1 Target FML design (80 ppi/in.; 8% density) A1t FML w/forward 1/2-section taped A2 Target FML design (40 ppi/in.; 8% density) SDOF1 Single degree-of-freedom liner (designed for BPF at100% N1c) o SDOF2 Single degree-of-freedom liner (designed for BPF at 75% N1c) PWL Acoustic Power Level N1c Corrected Fan Speed BPF Blade Passing Frequency acoustic impedance

Foam-Metal Liner Attenuation of Low-Speed Fan Noise

A foam-metal liner for attenuation of fan noise was developed for and tested on a low speed fan. This type of liner represents a significant advance over traditional liners due to the possibility for placement in close proximity to the rotor. An advantage of placing treatment in this region is the modification of the acoustic near field, thereby inhibiting noise generation mechanisms. This can result in higher attenuation levels than can be achieved by liners located in the nacelle inlet. In addition, foam-metal liners could potentially replace the fan rub-strip and containment components, ultimately reducing engine components and thus weight, which can result in a systematic increase in noise reduction and engine performance. Foam-metal liners have the potential to reduce fan noise by 4 dB based on this study.

ACTIVE RESONATORS FOR CONTROL OF MULTIPLE SPINNING MODES IN AN AXIAL FLOW FAN INLET

A research program was undertaken to explore the potential and practicality of developing active control Helmholtz resonators to attenuate excessive turbofan engine noise. The active Helmholtz resonator provides increased acoustic bandwidth by adding a controlled velocity via a secondary sound source. Plane wave tests demonstrated that the concept could be used both the increase a resonator’s absorption bandwidth and induct insertion loss bandwidth. For effective absorption of sound in a two dimensional duct environment, the active resonator concept was combined with a unidirectional radiation technique, which was extended to provide simultaneous suppression of multiple azimuthal, multiple radial and multiple blade-pass harmonic sound fields. The program culminated in a demonstration in the ANCF at NASA Glenn Research Center

Low-Speed Fan Noise Attenuation from a Foam-Metal Liner

A foam-metal liner for attenuation of fan noise was developed for and tested on a low-speed fan. This type of liner represents a significant advance over traditional liners, due to the possibility of placement in close proximity to the rotor. An advantage of placing treatment in this region is that the acoustic near field is modified, thereby inhibiting the noise-generation mechanism. This can result in higher attenuation levels than could be achieved by liners located in the nacelle inlet. In addition, foam-metal liners could potentially replace the fan rub strip and containment components, ultimately reducing engine components and thus weight, which can result in a systematic increase in noise reduction and engine performance. Foam-metal liners have the potential to reduce fan noise by 4 dB based on this study.