Scott M. Bartram

Performance Analysis of a Cost-Effective Electret Condenser Microphone Directional Array

Microphone directional array technology continues to be a critical part of the overall instrumentation suite for experimental aeroacoustics. Unfortunately, high sensor cost remains one of the limiting factors in the construction of very high-density arrays (i.e., arrays containing several hundred channels or more) which could be used to implement advanced beamforming algorithms. In an effort to reduce the implementation cost of such arrays, the authors have undertaken a systematic performance analysis of a prototype 35-microphone array populated with commercial electret condenser microphones. An ensemble of microphones coupling commercially available electret cartridges with passive signal conditioning circuitry was fabricated for use with the Langley Large Aperture Directional Array (LADA). A performance analysis consisting of three phases was then performed: (1) characterize the acoustic response of the microphones via laboratory testing and calibration, (2) evaluate the beamforming capability of the electret-based LADA using a series of independently controlled point sources in an anechoic environment, and (3) demonstrate the utility of an electret-based directional array in a real-world application, in this case a cold flow jet operating at high subsonic velocities. The results of the investigation revealed a microphone frequency response suitable for directional array use over a range of 250 Hz - 40 kHz, a successful beamforming evaluation using the electret-populated LADA to measure simple point sources at frequencies up to 20 kHz, and a successful demonstration using the array to measure noise generated by the cold flow jet. This paper presents an overview of the tests conducted along with sample data obtained from those tests.

Development of a Microphone Phased Array Capability for the Langley 14- by 22-Foot Subsonic Tunnel

A new aeroacoustic measurement capability has been developed for use in open-jet testing in the NASA Langley 14- by 22-Foot Subsonic Tunnel (14x22 tunnel). A suite of instruments has been developed to characterize noise source strengths, locations, and directivity for both semi-span and full-span test articles in the facility. The primary instrument of the suite is a fully traversable microphone phased array for identification of noise source locations and strengths on models. The array can be mounted in the ceiling or on either side of the facility test section to accommodate various test article configurations. Complementing the phased array is an ensemble of streamwise traversing microphones that can be placed around the test section at defined locations to conduct noise source directivity studies along both flyover and sideline axes. A customized data acquisition system has been developed for the instrumentation suite that allows for command and control of all aspects of the array and microphone hardware, and is coupled with a comprehensive data reduction system to generate information in near real time. This information includes such items as time histories and spectral data for individual microphones and groups of microphones, contour presentations of noise source locations and strengths, and hemispherical directivity data. The data acquisition system integrates with the 14x22 tunnel data system to allow real time capture of facility parameters during acquisition of microphone data. The design of the phased array system has been vetted via a theoretical performance analysis based on conventional monopole beamforming and DAMAS deconvolution. The performance analysis provides the ability to compute figures of merit for the array as well as characterize factors such as beamwidths, sidelobe levels, and source discrimination for the types of noise sources anticipated in the 14x22 tunnel. The full paper will summarize in detail the design of the instrumentation suite, the construction of the hardware system, and the results of the performance analysis. Although the instrumentation suite is designed to characterize noise for a variety of test articles in the 14x22 tunnel, this paper will concentrate on description of the instruments for two specific test campaigns in the facility, namely a full-span NASA Hybrid Wing Body (HWB) model entry and a semi-span Gulfstream aircraft model entry, tested in the facility in the winter of 2012 and spring of 2013, respectively.

Application of MEMS Microphone Array Technology to Airframe Noise Measurements

NASA Langley Research Center, Hampton, Virginia, 23681Current generation microphone directional array instrumentation is capable ofextracting accurate noise source location and directivity data on a variety of aircraftcomponents, resulting in significant gains in test productivity. However, with this gain inproductivity has come the desire to install larger and more complex arrays in a variety ofground test facilities, creating new challenges for the designers of array systems. Toovercome these challenges, a research study was initiated to identify and develop hardwareand fabrication technologies which could be used to construct an array system exhibitingacceptable measurement performance but at much lower cost and with much simplerinstallation requirements. This paper describes an effort to fabricate a 128-sensor arrayusing commercially available Micro-Electro-Mechanical System (MEMS) microphones. TheMEMS array was used to acquire noise data for an isolated 26%-scale high-fidelityBoeing 777 landing gear in the Virginia Polytechnic Institute and State University StabilityTunnel across a range of Mach numbers. The overall performance of the array wasexcellent, and major noise sources were successfully identified from the measurements.

Velocity Fields of Axisymmetric Hydrogen-Air Counterflow Diffusion Flames from LDV, PIV, and Numerical Computation

Laminar fuel-air counterflow diffusion flames (CFDFs) were studied using axisymmetric convergent-nozzle and straight-tube opposed jet burners (OJBs). The subject diagnostics were used to probe a systematic set of H m a a i r CFDFs over wide ranges of fuel input (22 to 100% Ha), and input axial strain rate (130 to 1700 Us) just upstream of the airside edge, for both plug-flow and parabolic input velocity profiles. Laser Doppler Velocimetry (LDV) was applied along the centerline of seeded air flows from a convergent nozzle OJB (7.2 mm i.d.), and Particle Imaging Velocimetry (PIV) was applied on the entire airside of both nozzle and tube OJBs (7 and 5 mm i.d.1 to characterize global velocity structure. Data are compared to numerical results from a one-dimensional (1-D) CFDF code based on a stream function solution for a potential flow input boundary condition. Axial strain rate inputs at the airside edge of nozzle-OJB flows, using LDV and PIV, were *Research Scientist (member AIAA) , NASA Langley Research Center, Hampton, VA 23681 =Laser Applications Specialist, NASA Langley ?Research Technician, Lockheed Engineering and Sciences IAssociate Professor (member AIAA), Dept. Mechanical and Aerospace Engineering and Engineering Mechanics Copyright

Application of particle image velocimetry to Mach 6 flows

This paper describes particle image velocimetry measurements obtained in a Mach 6 flow field. The measurements were performed in the Langley Mach 6 High Reynolds Number Tunnel facility. A wedge model oriented at an angle-of-attack of -15 degree(s) was used to generate an oblique 22.7 degree(s) bow shock. Using 1.0-micrometer aluminum oxide powder as the seed material, PIV photographs in the vicinity of the bow shock region were taken on the centerline of the model at a location 110 mm from the leading edge. Using the two- dimensional velocity fields obtained from analysis of the photographs, normal and tangential components of velocity with respect to the shock angle were obtained. These velocity components were then used to infer the aerodynamic particle sizes present in the tunnel. Results indicated that the 1.0 micrometer seed material introduced into the tunnel had an aerodynamic size of approximately 1.0 - 2.0 micrometers. Differences were noted between the measured and predicted normal component of velocity downstream of the shock after full particle relaxation had occurred. Using qualitative flow visualization, it was determined that due to cavity flow along the test section walls, reflected shocks in the vicinity of the PIV measurements could account for these differences.

Flow-Field Investigation of Gear-Flap Interaction on a Gulfstream Aircraft Model

Off-surface flow measurements of a high-fidelity 18% scale Gulfstream aircraft model in landing configuration with the main landing gear deployed are presented. Particle Image Velocimetry (PIV) and Laser Velocimetry (LV) were used to measure instantaneous velocities in the immediate vicinity of the main landing gear and its wake and near the inboard tip of the flap. These measurements were made during the third entry of a series of tests conducted in the NASA Langley Research Center (LaRC) 14- by 22-Foot Subsonic Tunnel (14 x 22) to obtain a comprehensive set of aeroacoustic measurements consisting of both aerodynamic and acoustic data. The majority of the off-body measurements were obtained at a freestream Mach number of 0.2, angle of attack of 3 degrees, and flap deflection angle of 39 degrees with the landing gear on. A limited amount of data was acquired with the landing gear off. LV was used to measure the velocity field in two planes upstream of the landing gear and to measure two velocity profiles in the landing gear wake. Stereo and 2-D PIV were used to measure the velocity field over a region extending from upstream of the landing gear to downstream of the flap trailing edge. Using a special traverse system installed under the tunnel floor, the velocity field was measured at 92 locations to obtain a comprehensive picture of the pertinent flow features and characteristics. The results clearly show distinct structures in the wake that can be associated with specific components on the landing gear and give insight into how the wake is entrained by the vortex at the inboard tip of the flap.

Boundary Condition Study for the Juncture Flow Experiment in the NASA Langley 14x22-Foot Subsonic Wind Tunnel

Because future wind tunnel tests associated with the NASA Juncture Flow project are being designed for the purpose of CFD validation, considerable effort is going into the characterization of the wind tunnel boundary conditions, particularly at inflow. This is important not only because wind tunnel flowfield nonuniformities can play a role in integrated testing uncertainties, but also because the better the boundary conditions are known, the better CFD can accurately represent the experiment. This paper describes recent investigative wind tunnel tests involving two methods to measure and characterize the oncoming flow in the NASA Langley 14by 22-Foot Subsonic Tunnel. The features of each method, as well as some of their pros and cons, are highlighted. Boundary conditions and modeling tactics currently used by CFD for empty-tunnel simulations are also described, and some results using three different CFD codes are shown. Preliminary CFD parametric studies associated with the Juncture Flow model are summarized, to determine sensitivities of the flow near the wing-body juncture region of the model to a variety of modeling decisions.

Comparison of Stereo-PIV and Plenoptic-PIV Measurements on the Wake of a Cylinder in NASA Ground Test Facilities.

A series of comparison experiments have been performed using a single-camera plenoptic PIV measurement system to ascertain the systems performance capabilities in terms of suitability for use in NASA ground test facilities. A proof-of-concept demonstration was performed in the Langley Advanced Measurements and Data Systems Branch 13-inch (33- cm) Subsonic Tunnel to examine the wake of a series of cylinders at a Reynolds number of 2500. Accompanying the plenoptic-PIV measurements were an ensemble of complementary stereo-PIV measurements. The stereo-PIV measurements were used as a truth measurement to assess the ability of the plenoptic-PIV system to capture relevant 3D/3C flow field features in the cylinder wake. Six individual tests were conducted as part of the test campaign using three different cylinder diameters mounted in two orientations in the tunnel test section. This work presents a comparison of measurements with the cylinders mounted horizontally (generating a 2D flow field in the x-y plane). Results show that in general the plenoptic-PIV measurements match those produced by the stereo-PIV system. However, discrepancies were observed in extracted pro les of the fuctuating velocity components. It is speculated that spatial smoothing of the vector fields in the stereo-PIV system could account for the observed differences. Nevertheless, the plenoptic-PIV system performed extremely well at capturing the flow field features of interest and can be considered a viable alternative to traditional PIV systems in smaller NASA ground test facilities with limited optical access.

Development of MEMS microphone array technology for aeroacoustic testing

A new approach to aeroacoustic microphone array design and implementation is described and demonstrated. Using commercially available, low‐cost MEMS microphones exhibiting a suitable low‐frequency response, a series of 128‐channel arrays were constructed on flexible Kapton circuit boards which were bonded to rigid aluminum backplates. Cover panels with precision cutouts for the microphones were bonded on top of the Kapton circuit boards to create a smooth surface providing flush‐mounting for all microphones. Connections for the microphones were created by extending strips of Kapton containing power and signal busses to the rear of the backplates. All channels were powered from a common 3 V power source, and all signals were conditioned using custom‐manufactured filtering and line‐driving hardware. The conditioned signals were digitized and processed in near real‐time using both commercially available and customized data acquisition and analysis hardware. This new type of array construction addresses two c...

Calibration of electret microphone cartridges for application in a multichannel acoustic array

Electret condenser microphone cartridges, having the advantages of small size, low cost, few lead wires, low output impedance, and acoustic specifications comparable to those of air condenser microphones, are well suited for application in multichannel acoustic arrays. Amplitude and phase calibration of electret cartridges by two methods is described. In the first, a test cartridge, reference microphone, and driver are inserted into a coupler, whereby the calibration takes place by a comparison method. In the second method, the calibration is performed in a commercial acoustic calibrator (B&K type 4226), which was modified by the manufacturer to permit the phase calibration. Further, a special adapter was fabricated to seat the cartridge at the proper position in the calibrator coupler. Amplitude and phase calibrations of 80 electret microphones were performed. The calibrated microphone cartridges were flush‐mounted in an 80‐channel array to measure dynamic surface pressures. The array was installed in a ...