William C. Horne

High-Speed Jet Noise Reduction Using Microjets on a Jet Engine

by a jet engine. Experiments were conducted at the NASA Ames Research Center using a General Electric YJ97-GE-3 turbofan jet engine that was equipped with a 317.5 mm converging nozzle. The engine was operated at conditions that resulted in jets with fully expanded Mach numbers of 0.9 and 1.3. The microjets were generated using up to 48 evenly spaced micro-nozzles that had exit diameters of 1.2 and 2.4 mm. The operating pressure of the microjets was varied from 7.9 to 42.4 bar. Various microjet configurations were used resulting in a total mass flux of the microjets that ranged from 0.5 to 2.3 % of the primary mass flux for the subsonic jet and from 0.3 to 1.0 % of the primary mass flux for the supersonic jet. Through the various configurations it was found that reductions of up to 2 dB in the OASPL could be obtained for both the subsonic and supersonic jets. The reductions for the subsonic jet were seen at all frequencies while they were seen primarily at the higher frequencies for the supersonic jet. A reduction of about 2 dB in the shock noise of the supersonic jet was also observed.

Concepts for reducing the self-noise of in-flow acoustic sensors and arrays

Several recent studies have demonstrated the feasibility of aeroacoustic measurements in closed-section wind tunnels using m-flow phased microphone arrays. Such applications subject the sensors to boundary-layer noise that can limit the acoustic response of the array. This paper presents a simple estimate of the effects cf boundary-layer self-noise on measurement accuracy, which include effects of sensor count, individual sensor signal-to-noise ratio, and data block count. This estimate compares favorably with the results of a Monte Carlo simulation of three different arrays with 35, 70, and 140 elements in a typical array test configuration. The effect of sensor self-noise on spatial pattern dynamic range was also evaluated. Two methods for directly reducing in-flow sensor self noise, recessed microphone placement, and laminar flow control on the array fairing, were evaluated experimentally and found to be effective in reducing self-noise by 5 to 15 dB, which result in levels. comparable to isolated microphones with quiet forebodies. Further development of these noise-reducing concepts is needed for practical application to large arrays.

AMELIA CESTOL Test: Acoustic Characteristics of Circulation Control Wing with Leading-and Trailing-Edge Slot Blowing

The AMELIA Cruise-Efficient Short Take-off and Landing (CESTOL) configuration concept was developed to meet future requirements of reduced field length, noise, and fuel burn by researchers at Cal Poly, San Luis Obispo and Georgia Tech Research Institute under sponsorship by the NASA Fundamental Aeronautics Program (FAP), Subsonic Fixed Wing Project. The novel configuration includes leading- and trailing-edge circulation control wing (CCW), over-wing podded turbine propulsion simulation (TPS). Extensive aerodynamic measurements of forces, surfaces pressures, and wing surface skin friction measurements were recently measured over a wide range of test conditions in the Arnold Engineering Development Center(AEDC) National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Ft Wind Tunnel. Acoustic measurements of the model were also acquired for each configuration with 7 fixed microphones on a line under the left wing, and with a 48-element, 40-inch diameter phased microphone array under the right wing. This presentation will discuss acoustic characteristics of the CCW system for a variety of tunnel speeds (0 to 120 kts), model configurations (leading edge(LE) and/or trailing-edge(TE) slot blowing, and orientations (incidence and yaw) based on acoustic measurements acquired concurrently with the aerodynamic measurements. The flow coefficient, Cmu= mVSLOT/qSW varied from 0 to 0.88 at 40 kts, and from 0 to 0.15 at 120 kts. Here m is the slot mass flow rate, VSLOT is the slot exit velocity, q is dynamic pressure, and SW is wing surface area. Directivities at selected 1/3 octave bands will be compared with comparable measurements of a 2-D wing at GTRI, as will as microphone array near-field measurements of the right wing at maximum flow rate. The presentation will include discussion of acoustic sensor calibrations as well as characterization of the wind tunnel background noise environment.

Advanced Background Subtraction Applied to Aeroacoustic Wind Tunnel Testing

An advanced form of background subtraction is presented and applied to aeroacoustic wind tunnel data. A variant of this method has seen use in other fields such as climatology and medical imaging. The technique, based on an eigenvalue decomposition of the background noise cross-spectral matrix, is robust against situations where isolated background auto-spectral levels are measured to be higher than levels of combined source and background signals. It also provides an alternate estimate of the cross-spectrum, which previously might have poor definition for low signal-to-noise ratio measurements. Simulated results indicate similar performance to conventional background subtraction when the subtracted spectra are weaker than the true contaminating background levels. Superior performance is observed when the subtracted spectra are stronger than the true contaminating background levels. Experimental results show limited success in recovering signal behavior for data where conventional background subtraction fails. They also demonstrate the new subtraction techniques ability to maintain a proper coherence relationship in the modified cross-spectral matrix. Beam-forming and de-convolution results indicate the method can successfully separate sources. Results also show a reduced need for the use of diagonal removal in phased array processing, at least for the limited data sets considered.

Phased acoustic array measurements of a 5.75% hybrid wing body aircraft

Detailed acoustic measurements of the noise from the leading-edge Krueger flap of a 5.75% hybrid wing body aircraft model were acquired with a traversing phased microphone array in the Arnold Engineering Development Complex NFAC 40- by 80-foot wind tunnel. The spatial resolution of the array was sufficient to distinguish between individual support brackets over the full-scale frequency range of 100 to 2875 Hz. For conditions representative of landing and take-off configuration, the noise from the brackets dominated other sources near the leading edge. Inclusion of flight-like brackets for select conditions highlights the importance of including the correct number of leading-edge high-lift device brackets with sufficient scale and fidelity. These measurements support the development of a Krueger noise model which includes cove and bracket noise.

Phased Acoustic Array Measurements of a 5.75% Hybrid Wing body Aircraft (Invited)

Detailed acoustic measurements of the noise from the leading-edge Krueger flap of a 5.75% Hybrid Wing Body (HWB) aircraft model were recently acquired with a traversing phased microphone array in the AEDC NFAC 40by 80-Foot Wind Tunnel at NASA Ames Research Center. The spatial resolution of the array was sufficient to distinguish between individual support brackets over the full-scale frequency range of 100 to 2875 Hz. For conditions representative of landing and take-off configuration, the noise from the brackets dominated other sources near the leading edge. Inclusion of flight-like brackets for select conditions highlights the importance of including the correct number of leading-edge highlift device brackets with sufficient scale and fidelity. These measurements will support the development of new predictive models.

Initial Assessment of Acoustic Source Visibility with a 24-Element Microphone Array in the Arnold Engineering Development Center 80- by 120-Foot Wind Tunnel at NASA Ames Research Center

Measurements of background noise were recently obtained with a 24-element phased microphone array in the test section of the Arnold Engineering Development Center 80by120-Foot Wind Tunnel at speeds of 50 to 100 knots (27.5 to 51.4 m/s). The array was mounted in an aerodynamic fairing positioned with array center 1.2m from the floor and 16 m from the tunnel centerline, The array plate was mounted flush with the fairing surface as well as recessed 1⁄2 in. (1.27 cm) behind a porous Kevlar screen. Wind-off speaker measurements were also acquired every 15 on a 10 m semicircular arc to assess directional resolution of the array with various processing algorithms, and to estimate minimum detectable source strengths for future wind tunnel aeroacoustic studies. The dominant background noise of the facility is from the six drive fans downstream of the test section and first set of turning vanes. Directional array response and processing methods such as background-noise cross-spectral-matrix subtraction suggest that sources 10-15 dB weaker than the background can be detected.

Measurement of Approach Noise Footprints of a C-17 STOL Aircraft During Conventional and Noise-Mitigating Approach Trajectories

Several conventional and noise-mitigating landing approach patterns were recently successfully demonstrated with a C-17 STOL aircraft, as part of an ongoing study of the integration of ESTOL-class aircraft into the national airspace system. During the demonstration, ground measurements of approach noise were obtained with an array of ground measurement stations at the NASA Dryden Research Flight Center. The measurements confirm projected trends of reduced noise impact to the greater airport vicinity for noise-mitigating approaches relative to a conventional 3 ̊ glide-slope approach.

Subspace-based background subtraction applied to aeroacoustic wind tunnel testing

A subspace-based form of background subtraction is presented and applied to aeroacoustic wind tunnel data. A variant of this method has seen use in other fields such as climatology and medical imaging. The technique is based on an eigenvalue decomposition of the background noise cross-spectral matrix. Simulated results indicate similar performance to conventional background subtraction when the subtracted spectra are weaker than the true contaminating background levels. Superior performance is observed when the subtracted spectra are stronger than the true contaminating background levels, and when background data do not match between measurements. Experimental results show limited success in recovering signal behavior for data in which conventional background subtraction fails. The results also demonstrate the subspace subtraction technique’s ability to maintain a physical coherence relationship in the modified cross-spectral matrix. Deconvolution results from microphone phased array data indicate that array integration methods are largely insensitive to subtraction type, and that background subtraction with appropriate background data is an effective alternative to diagonal removal.

Measurements of unsteady pressure fluctuations in the near-field of a solid rocket motor plume

Near-plume fluctuating pressures were measured during five static burns of a two-stage solid rocket motor. An array of 11 water-cooled dynamic pressure sensors was used for the near-field survey, and a condenser microphone was used to monitor the far-field acoustic fluctuations. During the initial high-thrust phase of the burn, the plume was nearly ideally expanded, while in the following low-thrust phase, it was highly over-expanded and showed the presence of clear shock patterns. This paper presents time histories and spectra measured for the two thrust conditions. Spectra from very close to the plume show high levels of low-frequency fluctuations which are known to produce significant vibro-acoustic response of the spacecraft structures. The far-field microphone signal was dominated by mixing noise with little evidence of contribution from shock-associated noise, even for the over-expanded condition. The work was performed in support of an effort to improve predictions of the acoustic environment of a manned spacecraft, such as NASAs Orion Crew Vehicle, during pad abort scenarios.