A phased-array system comprised of custom-made and commercially available microelectromechanical system (MEMS) silicon microphones and custom ancillary hardware has been developed for use in aeroacoustic testing in hard-walled and acoustically treated wind tunnels. Recent advances in the areas of multi-channel signal processing and beam forming have driven the construction of phased arrays containing ever-greater numbers of microphones. Traditional obstacles to this trend have been posed by (1) the high costs of conventional condenser microphones, associated cabling, and support electronics and (2) the difficulty of mounting conventional microphones in the precise locations required for high-density arrays. The present development overcomes these obstacles. One of the hallmarks of the new system is a series of fabricated platforms on which multiple microphones can be mounted. These mounting platforms, consisting of flexible polyimide circuit-board material (see left side of figure), include all the necessary microphone power and signal interconnects. A single bus line connects all microphones to a common power supply, while the signal lines terminate in one or more data buses on the sides of the circuit board. To minimize cross talk between array channels, ground lines are interposed as shields between all the data bus signal lines. The MEMS microphones are electrically connected to the boards via solder pads that are built into the printed wiring. These flexible circuit boards share many characteristics with their traditional rigid counterparts, but can be manufactured much thinner, as small as 0.1 millimeter, and much lighter with boards weighing as much as 75 percent less than traditional rigid ones. For a typical hard-walled wind-tunnel installation, the flexible printed-circuit board is bonded to the tunnel wall and covered with a face sheet that contains precise cutouts for the microphones. Once the face sheet is mounted, a smooth surface is established over the entire array due to the flush mounting of all microphones (see right side of figure). The face sheet is made from a continuous glass-woven-fabric base impregnated with an epoxy resin binder. This material offers a combination of high mechanical strength and low dielectric loss, making it suitable for withstanding the harsh test section environment present in many wind tunnels, while at the same time protecting the underlying polyimide board. Customized signal-conditioning hardware consisting of line drivers and antialiasing filters are coupled with the array. The line drivers are constructed using low-supply-current, high-gain-bandwidth operational amplifiers designed to transmit the microphone signals several dozen feet from the array to external acquisition hardware. The anti-alias filters consist of individual Chebyshev low-pass filters (one for each microphone channel) housed on small printed-circuit boards mounted on one or more motherboards. The mother/daughter board design results in a modular system, which is easy to debug and service and which enables the filter characteristics to be changed by swapping daughter boards with ones containing different filter parameters. The filter outputs are passed to commercially- available acquisition hardware to digitize and store the conditioned microphone signals. Wind-tunnel testing of the new MEMS microphone polyimide mounting system shows that the array performance is comparable to that of traditional arrays, but with significantly less cost of construction.

Thermal sensitive paints (TSPs) are used for global nonintrusive detection of boundary layer transition in flow over the surface of wind tunnel research models. Since the transition is a transient process, the TSP should have a fast response characteristic. A low paint thermal conductivity is required for fast response. A thin-film thermal conductivity meter (TFTCM) was designed and built to measure thermal conductivity of the TSPs, which are typically between 50 and 150 microm thick. In this paper, the design and operating features of the TFTCM are described. Measurement of the thermal conductivity with this TFTCM of three standard thin-film low conductivity specimens, Kapton, Teflon, and Borofloat glass, showed good agreement with the manufacturer quoted values, thus validating the instrument and the procedure. Consistently repeatable values for thermal conductivity (k=0.41 +/- 0.02 W/m K) were also obtained for the TSP specimen (TSB-B, 75 microm) tested.

Infrasonic windscreens, designed for service at frequencies below 20Hz, were fabricated from a variety of materials having a low acoustic impedance, and tested against four specifications (the first three in a small wind tunnel): (1) wind-generated noise reduction (“insertion loss”) at a free-stream wind speed of 9.3m∕s, (2) transmission of low-frequency sound from a known source (subwoofer), (3) spectrum of sound generated from trailing vortices (aeolian tones), and (4) water absorption (to determine suitability for all-weather service). The operating principle is based on the high penetrating capability of infrasound through solid barriers. Windscreen materials included three woods (pine, cedar, and balsa), closed-cell polyurethane foam, and Space Shuttle tile material. The windscreen inside diameter ranged from 0.0254to0.1016m (1to4in.), and wall thickness from 0.003175to0.01905m (18to34in.). A windscreen made of closed-cell polyurethane foam revealed a wind noise reduction of 10–20dB from 0.7to25Hz, transmission coefficient near unity from 10to20Hz, and spectral peaks beyond 20Hz due to vortex-generated sound. Following a description of past methods, the principle of operation, and the experimental method, experimental data are presented for a variety of windscreens.

A compact windscreen has been conceived for a microphone of a type used outdoors to detect atmospheric infrasound from a variety of natural and manmade sources. Wind at the microphone site contaminates received infrasonic signals (defined here as sounds having frequencies <20 Hz), because a microphone cannot distinguish between infrasonic pressures (which propagate at the speed of sound) and convective pressure fluctuations generated by wind turbulence. Hence, success in measurement of outdoor infrasound depends on effective screening of the microphone from the wind. The present compact windscreen is based on a principle: that infrasound at sufficiently large wavelength can penetrate any barrier of practical thickness. Thus, a windscreen having solid, non-porous walls can block convected pressure fluctuations from the wind while transmitting infrasonic acoustic waves. The transmission coefficient depends strongly upon the ratio between the acoustic impedance of the windscreen and that of air. Several materials have been found to have impedance ratios that render them suitable for use in constructing walls that have practical thicknesses and are capable of high transmission of infrasound. These materials (with their impedance ratios in parentheses) are polyurethane foam (222), space shuttle tile material (332), balsa (323), cedar (3,151), and pine (4,713).

An improved and compact windscreen was conceived for a microphone of a type used outdoors to detect atmospheric infrasound from a variety of natural and artificial sources. This cylindrically shaped compact windscreen, made of closed-cell polyurethane foam, had dimensions of 0.0762 m i.d. ×0.2286 m height ×0.0127 m wall (3×9×0.5 in.). The low acoustic impedance of the foam showed a transmission coefficient near unity. The same closed-cell polyurethane foam was used to fabricate a spherical windscreen of 0.254 m (10 in.) diameter with 0.0127-m (0.5 in.) wall thickness. In this paper the effectiveness of cylindrical and spherical windscreens is evaluated using a low-speed wind tunnel facility that enables controlled and repeatable experiments. Wind noise levels are characterized for both windscreens as a function of frequency and mean wind velocity.

Current generation microphone directional array instrumentation is capable of extracting accurate noise source location and directivity data on a variety of aircraft components, resulting in significant gains in test productivity. However, with this gain in productivity has come the desire to install larger and more complex arrays in a variety of ground test facilities, creating new challenges for the designers of array systems. To overcome these challenges, a research study was initiated to identify and develop hardware and fabrication technologies which could be used to construct an array system exhibiting acceptable measurement performance but at much lower cost and with much simpler installation requirements. This paper describes an effort to fabricate a 128-sensor array using commercially available Micro-Electro-Mechanical System (MEMS) microphones. The MEMS array was used to acquire noise data for an isolated 26%-scale high-fidelity Boeing 777 landing gear in the Virginia Polytechnic Institute and State University Stability Tunnel across a range of Mach numbers. The overall performance of the array was excellent, and major noise sources were successfully identified from the measurements.

Surface-mount microphones based on MEMS (micro-electromechanical system) technologies have recently become viable as component-level engineering solutions for acoustic measurements. In addition, advances in microelectronics, flexible circuitry, and array processing have motivated the design of a high-speed, low-cost acoustic array system for aeroacoustic measurements. A variety of microphones are available in the market today. Each type of microphone has its benefits and drawbacks. For example, standard condenser microphones have excellent sensitivity, stability, and high frequency response, but tend to be unwieldy, expensive, and require relatively high operational voltages. Electret microphones are small and fairly inexpensive but their performance deteriorates if exposed to moderately elevated temperatures. MEMS microphones combine the best features of the electret and condenser microphones while occupying a volume of less than 20 cubic millimeters. This paper details the custom-made 128-channel MEMS-based acoustic array for wind tunnel applications as well as the electrical, mechanical, and acoustic properties of the MEMS microphones utilized here.

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 challenges which currently limit the widespread use of large channel-count arrays for aeroacoustic applications, namely by providing a lower cost-per-channel solution and by providing a simpler method for mounting microphones in wind tunnels. The MEMS arrays have been extensively tested in anechoic and hard-walled facilities, and their performance has been found comparable to that of condenser microphone arrays.

This report describes the NASA Langley 16-Ft. Transonic Tunnel Pressure Sensitive Paint (PSP) System and presents results of a test conducted June 22-23, 2000 in the tunnel to validate the PSP system. The PSP system provides global surface pressure measurements on wind tunnel models. The system was developed and installed by PSP Team personnel of the Instrumentation Systems Development Branch and the Advanced Measurement and Diagnostics Branch. A discussion of the results of the validation test follows a description of the system and a description of the test.

This paper presents the calibration results and uncertainty analysis of a high-precision reference pressure measurement system currently used in wind tunnels at the NASA Langley Research Center (LaRC). Sensors, calibration standards, and measurement instruments are subject to errors due to aging, drift with time, environment effects, transportation, the mathematical model, the calibration experimental design and other factors. Errors occur at every link in the chain of measurements and data reduction from the sensor to the final computed results. At each link of the chain, bias and precision uncertainties must be separately estimated for facility use, and are combined to produce overall calibration and prediction confidence intervals for the instrument, typically at a 95 percent confidence level. The uncertainty analysis and calibration experimental designs used herein, based on techniques developed at LaRC, employ replicated experimental designs for efficiency, separate estimation of bias and precision uncertainties, and detection of significant parameter drift with time. Final results, including calibration confidence intervals and prediction intervals given as functions of the applied inputs, not as a fixed percentage of the full-scale value are presented. System uncertainties are propagated beginning with the initial reference pressure standard, to the calibrated instrument as a working standard in the facility. Among the several parameters that can affect the overall results are operating temperature, atmospheric pressure, humidity, and facility vibration. Effects of factors such as initial zeroing and temperature are investigated. The effects of the identified parameters on system performance and accuracy are discussed.

Temperature sensitive paints (TSP s) have recently been used to detect small temperature differences on aerodynamic model surfaces. These types of applications impose stringent performance requirements on a paint system. The TSP s must operate over a broad temperature range, must be physically robust (cannot chip or peel), must be polishable to at least the smoothness of the model surface, and must have sufficient sensitivity to detect small temperature differences. TSP coatings based on the use of metal complexes in polymer binders were developed at NASA Langley Research Center which meet most of the requirements for detection of small temperature differences under severe environmental conditions.

Measurements of pressures on walls of wind tunnels and on surfaces of models in wind tunnels made with help of electronically scanned pressure-measurement (ESP) system. Voltage-to-frequency converter circuit, designed to convert 0- to 5-Vdc analog output voltage from high-line-pressure, low-differential-pressure standard to required frequency range. Enables selection of wider variety of high-accuracy pressure standards to enhance accuracy of measurement of ESP instrumentation while requiring little modification of manufacturer's system and no modification of operating software of system. Useful primarily in wind-tunnel instrumentation and readily adaptable to commercial instruments currently in use.

An automated pressure data acquisition system for testing of pressure sensitive phosphorescent paints was designed, assembled, and tested. The purpose of the calibration system is the evaluation and selection of pressure sensitive paint chemistries that could be used to obtain global aerodynamic pressure distribution measurements. The test apparatus and setup used for pressure sensitive paint characterizations is described. The pressure calibrations, thermal sensitivity effects, and photodegradation properties are discussed.