A Large Hemi-Anechoic Enclosure for Community-Compatible Aeroacoustic Testing of Aircraft Propulsion Systems

Transcription

1 A Large Hemi-Anechoic Enclosure for Community-Compatible Aeroacoustic Testing of Aircraft Propulsion Systems Beth A. Cooper a) Received 1993 May 14; revised 1993 November 20 A large hemi-anechoic sound-absorbing walls and acoustically hard floor noise control enclosure was erected around a complex of test stands at the NASA Lewis Research Center in Cleveland, Ohio. This new state-of-the-art Aeroacoustic Propulsion Laboratory provides an all-weather, semisecure test environment while minimizing noise levels in surrounding residential neighborhoods. The 39.6-m-diam 130-ft-diam geodesic dome houses the new nozzle aeroacoustic test rig, an ejector-powered Mach 0.3 free-jet facility for acoustical testing of supersonic aircraft exhaust nozzles and turbomachinery. A multiaxis, force-measuring powered lift facility stand for testing short-takeoff vertical-landing vehicles is also located in the dome. The design of the Aeroacoustic Propulsion Laboratory efficiently accommodates the research functions of the two separate rigs, while providing a specialized environment for measuring far-field sound-pressure levels from the nozzle aeroacoustic test rig. Sound-absorbing fiberglass wedges on the interior surface of the dome provide a hemi-anechoic environment. The Aeroacoustic Propulsion Laboratory is the first known geodesic dome structure to incorporate transmission-loss properties as well as interior absorption in a free-standing community-compatible, hemi-anechoic test facility. Primary subject classification: , Secondary subject classification: Introduction The noise control structure Fig. 1 was originally conceived to solve a community noise problem arising from open-air testing at the powered lift facility PLF. However, midway through the design process for a simple acoustical barrier, an additional aeroacoustic anechoic nozzle test facility was needed to augment the capacity of the Lewis Research Center s 9 15-ft low-speed anechoic wind tunnel. Because of the advantages of shared air supply services, control room, and noise control structure, the nozzle aeroacoustic test rig NATR was located along with the existing PLF in a geodesic dome-shaped structure that would provide transmission loss as well as a hemi-anechoic interior environment. Thus the designs of the Aeroacoustic Propulsion Laboratory APL and the NATR evolved simultaneously. the exit of the free-jet duct, scale-model nozzles can be tested in the free-jet airflow by means of a strut mount containing a force balance and a high-temperature combustor. The scale-model nozzles are supplied with highpressure air and gaseous hydrogen fuel, which when burned create the high-temperature, high-velocity exhaust gases that simulate in model scale the takeoff and climbout noise characteristic of supersonic aircraft. The geometry of the APL dome with respect to the existing PLF and the location and orientation of the NATR in the dome were based on an extensive tradeoff study to determine the most space-efficient and economical floor plan Figs. 3 a and 3 b. The NATR and PLF exhaust axes are oriented toward a 50 opening in the 39.6-m-diam 130-ftdiam structure, which was sized to accommodate the 2. Description of Facility The NATR, a 1.2-m-diam 4-ft-diam free jet Fig. 2 with a design Mach number of 0.3, is used to simulate forward flight during the aeroacoustic testing of scale-model aircraft exhaust nozzles and turbomachinery. 1 The required airflow to achieve simulated takeoff conditions is provided by an ejector system Fig. 2 in which the primary airstream is supplied by laboratory air services through a circular array of 25-mm-diam 1-in.-diam choked nozzles that feed into an annular bellmouth. The resulting low pressure just downstream of the bellmouth causes a pumping action that entrains the secondary airstream from ambient air. At a) Beth A. Cooper is an Aerospace Research Engineer in the Propeller and Acoustic Technology Branch of the Aeronautics Directorate at the National Aeronautics and Space Administration s Lewis Research Center, Cleveland, Ohio Ms. Cooper is a Registered Professional Engineer in the State of Ohio. Fig. 1 Aeroacoustic Propulsion Laboratory. Completed air intake enclosure not shown during this phase of construction Institute of Noise Control Engineering 1

2 Fig. 2 Nozzle aeroacoustic test rig simulates forward flight for aeroacoustic testing of scale-model nozzles and turbomachinery. Ejector nozzles (upstream end) supply primary airstream for NATR. Fig. 4 Typical microphone array configuration employed during aeroacoustic testing of nozzles. exhaust plumes of both rigs as well as operational vehicles requiring access. The free jet is positioned such that, during scale-model nozzle testing, the model-nozzle exit plane is located close to the dome center, providing a clear line of sight to microphone arrays located in the NATR half of the dome; thus far-field sound-pressure levels can be measured in a 120 sector off the nozzle exhaust axis Fig. 3 b. Typically, during nozzle testing, far-field, high-frequency sound-pressure signals are measured with an array of pole microphones Fig. 4 at the nozzle centerline height positioned at 10 increments along a polar arc at a 15.2-m 50- ft radius. Microphones mounted inverted at 5 mm above the concrete ground plane, at equivalent radii and solid angles, are used to acquire low-frequency signals that are free of the ambiguity caused by ground reflections. A 32- channel computerized data acquisition and processing system 2 provides narrow-band and one-third-octave-band spectral analysis with compensation for shear layer effects, 3 microphone frequency response, and directivity and correction to sea level standard day conditions. This system Fig. 3 Interior of Aeroacoustic Propulsion Laboratory. (a) Nozzle aeroacoustic test rig (left) and powered lift facility (right). Photo taken during construction phase. (b) Floor plan of APL showing location and orientation of NATR and PLF facilities in dome footprint. Fig. 5 NASA Lewis Research Center and surrounding areas. Exhaust axes of NATR and PLF facilities are directed toward Cleveland Hopkins Airport. Affected residential communities are located approximately 1/2 mile away. 2

3 Fig. 6 Dome wall panels provide STC 55 noise attenuation performance. (a) Wall panel construction. (b) The multilayer sandwich panels fit in the 203-mm (8-in.) channels in the dome structural I beams. (c) Completed dome with interior aluminum skin partially installed. allows for next day turnaround of processed data, providing timely support for test program decision making. 3. Development of Noise Control Requirements Noise control requirements for the APL were the result of a history of complaints from neighboring residential communities Fig. 5 during a short-takeoff vertical-landing aircraft development program from 1987 to Although the exhaust axis of the PLF is directed toward Cleveland Hopkins Airport and away from residential areas, complaints received during approximately 30% of PLF test sessions over a 13-month period repeatedly forced PLF personnel to suspend operations. To restore testing productivity, a plan was developed for permanently reducing community noise levels during testing. A review of the literature on community and airport noise research suggested that, in order to minimize complaints from the nearby residential community, maximum noise levels and operational procedures during testing should result in an A-weighted day night average sound level no greater than 60 db in residential areas. 4 6 Day night average sound level, a time-integrated noiseevaluation quantity commonly used in community noise and airport noise studies, reflects a community s cumulative exposure to a variety of noise sources over a 24-h period and includes a 10-dB weighting for sounds occurring at night. Both in the residential area adjacent to NASA Lewis Research Center and nationally, a day night average sound level of 60 db is typical of noise ordinances for communities with published numerical noise ordinances. Additional weightings are often applied to predicted day night average sound levels in an attempt to account for demographic 3

4 TABLE 1 Installed noise reduction of Aeroacoustic Propulsion Laboratory dome structure. Octave midband frequency, Hz Noise reduction, db a a a Received levels were within 3 db of background noise level. 10 increments in directivity along a circular arc around an F110 engine at intermediate power. Wave-divergence and minimum-attenuation atmospheric absorption effects 9 were assumed over the distance between the source and the nearest residential neighborhood at each angle. Corresponding neighborhood-dependent day night average sound levels were then computed for typical operational procedures e.g., the number and duration of individual nozzle tests per test session as well as the temporal and spectral quality of the noise. Source noise attenuation requirements to achieve these day night average sound levels were computed on a one-third-octave-band soundpressure level basis at each increment of 10 directivity angle around the engine. Fig. 7 Noise-attenuating air intake enclosure. (a) Exterior view of APL showing enclosure. (b) Interior view of enclosure showing noise-attenuating louvers (taken prior to construction of self-noise-attenuating ejector enclosure). factors that may make a particular community more or less receptive to the noise source. Demographic information and existing contours of day night average sound level for the affected neighborhoods were obtained and used for this purpose. 7,8 The community-compatibility plan involved adopting some immediate changes in operational procedures effectively lowering day night average sound levels as well as developing a long-term solution for containing the noise in a manner that would not compromise research objectives. In order to ensure unrestricted testing of sound sources that generate high sound-pressure levels, the noise control structure was designed to accommodate noise generated by an F110 aircraft engine. Received spectra in a number of neighboring communities surrounding the source were predicted using one-third-octave-band far-field soundpressure levels provided by the engine manufacturer at 4. Design of APL Structure to Meet Community Noise Goals Each APL dome wall panel was designed to provide a noise reduction equal to the maximum of the calculated noise reduction required over the range of directivity angles encompassed by the residential neighborhoods. A sound transmission class STC requirement of 55 was determined 10 such that noise reduction requirements would be met at all one-third octave bands below 20 khz. The customdesigned multilayer sandwich panels Figs. 6 a 6 c were tested per ASTM E by an independent acoustical laboratory prior to the dome construction. Each sandwich consists of two aluminum panels of different thicknesses exterior, 1.8 mm or 0.07 in.; interior, 4.8 mm or 0.19 in. separated by a 152-mm 6-in. airspace containing 51 mm 2 in. of thermal insulating wool fiberglass. The exterior surface of the dome is covered by a thin aluminum skin under which the individual sandwich panels are enclosed and acoustically sealed with a silicone sealant in the 203-mm-deep 8-in.-deep channels of the dome structural beams Figs. 6 b and 6 c. Secondary air for the ejector-powered free jet is entrained from ambient outdoor air through a noiseattenuating, low-pressure-drop air intake enclosure Figs. 7 a and 7 b. The enclosure is designed to provide the required airflow area as well as a reduction of forwardquadrant noise generated by the annulus of ejector nozzles. Outdoor air entrained by the ejector flows into the bellmouth through a wall of double-stacked, noise-attenuating louvers Fig. 7 b, each of which consists of a cascade of parallel airfoil-shaped splitters filled with sound-absorbing 4

5 Fig. 8 Anechoic wedge treatment covers interior of dome wall surface. (a) Wedge construction. (b) Photo taken during wedge installation showing wedges mounted on dome wall surface by means of a track system. Wedges are offset from interior surface of dome by 51 mm (2 in.). (c) Hardware cloth screen covers entire interior (wedged) surface of dome to prevent damage to wedges. material. The remaining walls of the air intake enclosure are designed to acoustically and visually match the construction of the dome. Noise reduction requirements for the air intake enclosure were specified so that the ejector nozzle noise attenuated by the louvers would be reduced to the same level in the community as the test nozzle noise attenuated by the dome wall panels. Noise levels measured during initial NATR checkout tests in the Spring of 1992 indicated that the dome wall panels provided the noise reduction shown in Table 1. Received sound levels in the nearest residential neighborhoods were not detectable above the background noise level during these or subsequent tests with a nozzle sound source; in addition, no noise complaints have been received since the completion of dome construction. 5. Hemi-Anechoic Interior Environment of Dome The fiberglass wedge treatment on the entire interior wall surface of the dome provides a hemi-anechoic soundabsorbing walls and hard floor surface interior environment for the accurate acoustical measurements required to meet research program goals. The 0.61-m 24-in. depth of the acoustical treatment Figs. 8 a 8 c is installed on a track system with a 51-mm 2-in. airspace Fig. 8 b between the wedge base and the interior surface of the dome wall. The wedges are fully encased in fiberglass cloth to prevent erosion and to protect the wedge treatment from the elements as well as from high velocity air flows. Wedges are held in their frames with mm 1/2 1-in. hardware cloth on all sloping edges of the wedge peaks. A hardware cloth screen Fig. 8 c installed over the entire interior surface of the wedges protects against physical damage and prevents birds from nesting in the wedge peaks. Results of the manufacturer s impedance tube tests performed on the wedge material indicated that the normalincidence sound absorption coefficient was at least 0.99 above 125 Hz when tested in accordance with ASTM C384-90a. 12 5

6 Fig. 10 Safety-mandated fan provides quiet continuous ventilation during nozzle testing. (a) Fan housing mounted above (exterior to) dome surface. (b) Interior view of fan opening, treated with anechoic wedges to prevent reflections from top of dome. Fig. 9 Noise-attenuating enclosure. (a) Enclosure surrounds ejector system to contain and shield microphones from ejector self-noise. (b) Wedged roll-away, plug-style doors provide access to interior of enclosure for NATR systems maintenance. Potentially reflective surfaces on internal dome structures were covered or shielded with a variety of soundabsorbing materials to ensure a high-quality acoustical environment. In particular, the exterior surfaces of the NATR were covered with sound-absorbing blankets; a freestanding, fiberglass-faced wall prevents reflected sound from the PLF Fig. 3 a ; facility instrumentation such as pole microphone stands were wrapped with soundabsorbing material. Facility lighting and video cameras were selected for low frontal area and were recessed in the wedged interior walls to be acoustically unobtrusive. In addition, electrical conduit and junction boxes were installed behind the wedges, the access to which is provided by custom-wedged doors. Evaluation of the interior acoustical environment will be included in a series of facility qualification procedures to be performed pending completion in early 1994 of all internal facility systems installations and checkouts. To further maintain the research-quality interior acoustical environment, facility self-noise levels were minimized by requiring safety and operational systems to meet strict 6

7 noise criteria for direct and reflected sound, specifically 20 db below the predicted one-third-octave-band soundpressure levels for a typical noise-suppressor nozzle. The NATR itself is by design a low self-noise system: aftquadrant self-noise generated by the annulus of ejector nozzles is attenuated as it travels downstream through the NATR by sound-absorbing treatment in the walls of the diffuser and plenum sections. The microphone arrays are shielded from direct, radiated, aft-quadrant ejector selfnoise by a sealed noise-attenuating STC 54 enclosure that surrounds the ejector portion of the NATR Figs. 9 a and 9 b. To prevent reflections, the exterior surfaces of this enclosure are covered with wedges. A fan producing a volume flow rate of 18.9 m 3 /s or ft 3 /min at the top of the dome Figs. 10 a and 10 b provides the continuous exhaust mandated for safety reasons while the NATR facility is burning gaseous hydrogen fuel and also meets the 20-dB noise-reduction criterion. The acoustical integrity of the facility was of primary importance during the process of new equipment installations and facility modifications; each action was considered with regard to its impact on the research quality of the acoustical environment. Further facility upgrades and modifications to accommodate new test programs on both the PLF and the NATR will again be accomplished in an acoustically responsible manner. 6. Summary The all-weather, semisecure geodesic dome structure of the Aeroacoustic Propulsion Laboratory at the NASA Lewis Research Center achieved the goal of reducing noise levels in adjacent residential communities while providing a research-quality hemi-anechoic interior environment for the acoustical testing of supersonic aircraft exhaust nozzles. During initial checkout testing, the dome performed as expected; community noise levels were significantly reduced and complaints have been eliminated since the facility was completed. 7. References 1 M. Long-Davis and B. Cooper, LeRC NATR free-jet development, presented at the NASA/Industry HSR Nozzle Symposium D. J. Hall and J. Bridges, A sophisticated, multi-channel data acquisition and processing system for high frequency noise research, NASA Contractor Report CR R. Schlinker and R. Amiet, Refraction and scattering of sound by a shear layer, NASA Contractor Report CR Information on levels of environmental noise requisite to protect public health and welfare with an adequate margin of safety, Environmental Protection Agency Report No. PB /4 EPA-550/ L. L. Beranek, Noise and Vibration Control Institute of Noise Control Engineering, Washington, DC, C. M. Harris, Handbook of Acoustical Measurements and Noise Control, 3rd ed. McGraw-Hill, New York, FAR 150 airport noise compatibility program, City of Cleveland Department of Port Control, Cleveland, OH FAR Part 150 noise exposure map project, City of Cleveland Department of Port Control, Cleveland, OH Standard values of atmospheric absorption as a function of temperature and humidity, ARP 866A, Society of Automotive Engineers, Warrendale, PA Classification for rating sound insulation, ASTM E , American Society for Testing and Materials, Philadelphia, PA Standard method for laboratory measurement of airborne sound transmission loss of building partitions, ASTM E-90-90, American Society for Testing and Materials, Philadelphia, PA Standard test method for impedance and absorption of acoustical materials by the impedance tube method, ASTM C a, American Society for Testing and Materials, Philadelphia, PA