Hugo E. Camargo

Calibration and Demonstration of the New Virginia Tech Anechoic Wind Tunnel

A unique new removable anechoic system and new acoustic treatment for the Virginia Tech Stability Wind Tunnel is described. The new system consists of a Kevlar-walled acoustic test section flanked by two anechoic chambers. In its new configuration the facility is closed aerodynamically and open acoustically, allowing far-field acoustic measurements with a flow quality comparable to that of a hardwalled wind tunnel. An extensive program of experiments has been conducted to examine the performance of this new hardware under a range of conditions, both to examine the effects of acoustic treatment on overall test-section noise levels and to ascertain the aerodynamic characteristics of the new test section. Noise levels in the test section of the anechoic facility are down by as much as 25 dB compared to the original hard-walled configuration. Lift interference corrections (for a baseline NACA 0012 airfoil) are less than half those expected in an open-jet wind tunnel. Acoustic measurements of airfoil trailing edge noise using a microphone phased array are compared to past experiments conducted on similar airfoils in an open-jet facility.

Development and Testing of a Novel Acoustic Wind Tunnel Concept

*† ‡ § A new anechoic wind tunnel test section concept was developed and tested. The new concept involves the construction and installation of a test section with walls formed largely from tensioned Kevlar cloth embedded in an anechoic chamber. This acoustically open design has the potential to eliminate the need for a jet catcher and reduce interference effects. The concept was tested in the Virginia Tech Stability Wind Tunnel. The existing wind tunnel test section was modified to incorporate prototype acoustic treatment, two large Kevlar side-walls and surrounding acoustic enclosures. In-flow microphone measurements over the whole speed range of the facility showed that the acoustic treatment was effective in reducing noise levels by 8 to 15 dB. A NACA0012 airfoil model was placed inside the modified test section to perform aerodynamic and aeroacoustic measurements for proof-ofconcept. Tests with and without an airfoil model showed the Kevlar side-walls can quietly and stably contain the flow. Furthermore, they significantly reduce lift interference. Phased array measurements of the trailing edge noise produced by the airfoil demonstrate the acoustically transparency of the Kevlar walls and the practicality of making noise measurements through them. I. INTRODUCTION This paper describes the preliminary development and testing of a new anechoic wind tunnel test section design. The design uses large areas of ballistic Kevlar cloth to provide a stable flow boundary. Sound generated in the flow can propagate through the Kevlar cloth into a surrounding anechoic chamber, where detailed noise measurements can be made. The specific application of this work is to the acoustic upgrade of the Virginia Tech Stability Wind Tunnel. This facility can generate flows of up to 80 m/s through its 1.83 m x1.83 m square test section. The use of Kevlar walls in the test section of this facility offers the possibility of providing anechoic capability without the complication and noise generated by a jet catcher. This approach also holds the promise of controlling lift interference, which can be a large factor in open-jet wind tunnels. The objective of this work is a wind tunnel design that, in the case of the Stability Wind Tunnel, will allow the testing of the aerodynamics and aeroacoustics of large scale lifting configurations (particularly airfoils) at realistic Reynolds numbers. In this paper we describe the design of the prototype acoustic test section, including acoustic treatment, acoustic enclosures and the Kevlar cloth walls. We also describe the development of a 63-microphone phased array system used to make measurements through these acoustic windows. We report on the aerodynamic behavior of the Kevlar walls over the full speed range of the facility, and the effects of the windows and treatment on in-flow noise levels. We also report on the aerodynamic interference generated by the cloth walls observed through mean pressure distributions measured on a large NACA 0012 airfoil as a function of angle of attack and flow speed. Finally we demonstrate that trailing-edge noise measurements of the same model can be made through the Kevlar walls with the phased array system. The results show a significant reduction in the background noise level of the flow. More importantly, they also show that large Kevlar panels can be used to quietly and stably contain the flow eliminating

Novel Kevlar-Walled Wind Tunnel for Aeroacoustic Testing of a Landing Gear

which allowed aeroacoustic measurements to be carried out in the far field and in an environment with significantly less reflections. The model was a very faithful replica of the full-scale landing gear, designed to address the issues associated with low-fidelity models. A 63-element microphone phased array was used to locate the noise-source components of the landing gear from different streamwise positions, both in the near and far fields. The same landing-gear model was previously tested in the original hard-walled configuration of the tunnel with the same phased array mounted on the wall of the test section (i.e., near-field position). The new anechoic configuration of the Virginia Polytechnic Institute and State University wind tunnel offered a unique opportunity to directly compare data collected in hard-walled and semi-anechoic test sections, using the same landing-gear model and phased-array instrumentation. Through these tests, some of the limitations associated with testing in hard-walled wind tunnels were addressed. Nomenclature Cj = components of the steering vector d = distance from the array to the model f = frequency ffull-scale = full-scale frequency fmeasured = measured frequency k = wave number M = flow Mach number rj = distance traveled by an acoustic ray from the grid point with coordinates xn to array microphone j Rj = distance between the grid point with coordinates xn and array microphone j xn = coordinates of the grid point to which the array is being steered � = wavelength

Noise Reduction of a Model-Scale Landing Gear Measured in the Virginia Tech Aeroacoustic Wind Tunnel

The effectiveness of various fairings for landing gear noise reduction was measured in the Virginia Tech (VT) Stability Wind Tunnel. This wind tunnel was recently upgraded to an aeroacoustic facility, which allowed acoustic measurements to be carried out in the far-field, out of the flow, and in a low reverberant environment. The model was a very faithful replica of the full-scale landing gear, designed to address the issues associated with low-fidelity models. A 63-element microphone phased array was used to locate the noise source components of the landing gear in its baseline and streamlined configurations, and to measure the noise reduction potential of the fairings. Measurements were carried out from two far-field positions on the flyover path of the landing gear. Through a comparison between the noise levels of the landing gear with and without fairing, the noise reduction potential of each fairing could be estimated. The results from these experiments also showed that if phased-array measurements of the landing gear noise are carried out in the near-field, the noise reduction potential of the fairings could be largely overestimated.

Aeroacoustic Study of a 26%-Scale, High-Fidelity, Boeing 777 Main Landing Gear in a Semi-Anechoic-Wind-Tunnel Test Section

Experiments were conducted on a 26%-scale high fidelity Boeing 777 main landing gear in the Virginia Tech (VT) Stability Wind Tunnel. This wind-tunnel was recently upgraded to a semi-anechoic facility, which allowed aeroacoustic measurements to be carried out in the far-field and in an environment with significantly less reflections. The model was a very faithful replica of the full-scale landing gear, designed to address the issues associated with low-fidelity models. A 63-element microphone phased array was used to locate the noise source components of the landing gear on the flyover path, both in the near- and far-field. The same landing gear model was previously tested in the original hard-walled configuration of the VT tunnel with the same phased array mounted on the wall of the test section, i.e. near-field position. The new anechoic configuration of the VT wind tunnel offered a unique opportunity to directly compare, using the same gear model and phased array instrumentation, data collected in hard-walled and semi-anechoic test sections. Through these tests some of the limitations associated with hard-walled wind tunnels were discussed. The tests also allowed the noise source components of the landing gear on the flyover path to be identified. It was shown that noise from the landing gear on the flyover path cannot be characterized by only taking phased array measurement straight under the gear.

Noise Source Identification on a Continuous Mining Machine

Noise Induced Hearing Loss is the most common occupational disease in the U.S. and of paramount importance in the mining industry. According to data for 2006 from the Mine Safety and Health Administration (MSHA), Continuous Miner operators accounted for 30.2% of underground mining equipment operators with noise doses exceeding the Permissible Exposure Limit (PEL). This figure becomes more significant considering that 49% of the 2006 national underground coal production was extracted using continuous mining methods. Thus, there is a clear need to reduce the sound radiated by Continuous Mining Machines. The first step towards efficient noise control of a Continuous Mining Machine requires identification of the various noise sources under controlled operating conditions. To this end, a 42-microphone phased array was used in conjunction with 4 reference microphones to sample the acoustic field of a machine in the Hemi-anechoic chamber of the Pittsburgh Research Laboratory. These data were processed using a frequency-domain beamforming algorithm to obtain acoustic maps of 5 sides of the machine. The focus of the test was on the conveyor noise since previous studies showed that operation of the conveyor is the most important contributor to the sound radiated by the machine. From the acoustic maps, the following potential areas for noise control were identified, and included: chain-tail-roller interaction, chain flight tip-side board interaction, and chain-upper deck interaction.© 2008 ASME

Application of Phased Array Technology for Identification of Low Frequency Noise Sources

Noise Induced Hearing Loss (NIHL) is the most common occupational disease in the U.S. with devastating consequences particularly in the mining industry. A study conducted by the National Institute for Occupational Safety and Health (NIOSH) revealed that 90% of coal miners have hearing impairment by age 50, compared to only 10% of those not exposed to occupational noise. According to the Mine Safety and Health Administration (MSHA), Continuous Mining Machine (CM) operators account for 30% of workers exposed to noise doses exceeding the Permissible Exposure Level (PEL). This number becomes more dramatic considering that 49% of the total national underground coal production is extracted using these machines. In this context, NIOSH is conducting research to identify and control dominant noise sources in CMs. Previous noise source identification was performed using a Brüel &Kjær (B&K) 1.92-m diameter, 42-microphone phased array. These measurements revealed that the impacts from the conveyor chain onto the tail roller, and the impacts from the conveyor chain onto the upper deck are the dominant noise sources at the tail-section of the CM. The objectives of the work presented in this paper were: 1) To rank the noise radiated by the different sections of the conveyor, and 2) to determine the effect of a urethane-coated tail roller on the noise radiated by the tail-section. This test was conducted using an Acoustical and Vibrations Engineering Consultants (AVEC) 3.5-m diameter, 121-microphone phased array. The results from this new test show that a urethane-coated tail roller yields reductions in the tail-section of 2 to 8 dB in Sound Pressure Level in the frequency range of 1 kHz to 5 kHz. However, integration of the acoustic maps shows that the front-section and mid-section of the conveyor also contain dominant noise sources. Therefore, a urethane-coated tail roller in combination with a chain with urethane-coated flights that reduces the noise sources in the front and mid sections of the conveyor is required to yield a significant noise reduction on the CM operators overall exposure. These results show the applicability of phased array technology for low frequency noise source identification.

Engineered noise controls for miner safety and environmental responsibility

Abstract Noise-induced hearing loss (NIHL) continues to be one of the most prevalent diseases in the mining industry. Several factors contribute to the noise overexposure of miners, including confined workplace conditions, the utilization of heavy-duty mining equipment, and the proximity of machine operators to that equipment. In this context, the National Institute for Occupational Safety and Health (NIOSH) conducts research aimed at reducing the incidence of NIHL by developing noise controls for mining equipment, thereby lessening the level of exposure encountered by miners. To accomplish this task, experimental and computational state-of-the-art engineering tools are used to identify dominant noise sources and to explore different noise control approaches. This chapter provides an overview of these engineering tools, and presents three case studies where noise controls were developed and implemented into a longwall shearer, a roof bolter, and a continuous mining machine.

Development of noise controls for longwall shearer cutting drums

Noise-induced hearing loss is the second most pervasive disease in the mining industry. The exposure of miners to noise levels above the permissible exposure level results in hearing loss of approximately 80% of coal miners by retirement age. In addition, between 2002 and 2011, approximately 48% of longwall shearer operators were overexposed in coal mines in the United States. Previous research identified the two rotating cutting drums used by the longwall shearer to extract coal as the most significant sound-radiating components. In this context, the National Institute for Occupational Safety and Health conducted research to develop noise controls for longwall mining systems. To this end, structural and acoustic numerical models of a single cutting drum were developed to assess its dynamic and acoustic response, respectively. Once validated, these models were used to explore various noise control concepts including force isolation, varying structural damping and varying component stiffness. Upon multiple simulations, it was determined that structural modifications to increase the stiffness of the outer vane plates were the most practical and durable approach to reduce the sound radiated by the cutting drums. Furthermore, these modifications did not adversely affect the cutting performance, nor the loading ability of the drums. As a result, these structural modifications were implemented into an actual set of drums for evaluation purposes. Results from the underground evaluation, when the modified cutting drums were used under normal operation conditions, showed noise reduction across the entire frequency spectrum with an overall noise reduction of 3 dB in the sound pressure level at the operator location, confirming the validity of the developed noise controls.

Sound Radiation Analysis of a Longwall Cutting Drum

Operators of longwall mining systems experience sound levels of 93–105 dB(A) and receive noise exposures that place them at risk of noise-induced hearing loss. To address the problem, the National Institute for Occupational Safety and Health (NIOSH*) Office of Mine Safety and Health Research (OMSHR) has conducted research to develop engineering noise controls for longwall systems. In previous field surveys, the sound radiated by the cutting drums was identified as a major hazard, especially considering their close proximity to the operators. Cutting drums are complex structures consisting of curved metal pieces welded together, and NIOSH has used modeling and simulation to characterize the acoustic properties of this structure. Based on a finite element (FE) model of the drum, the boundary element method (BEM) was used to predict the sound radiated from the vibrating drum due to an excitation force applied to one of the cutting bits. Simulations were used to examine the following with respect to the radiated sound power: (1) the ramifications of adding the welds to the model rather than assuming direct attachment between the metal components; (2) the effect of weld stiffness; (3) the relative contributions of the vanes and the cylindrical part of the drum; and (4) the sensitivity to the direction of the applied force. Parametric studies have shown that including the weld in the finite element model has a significant effect on the predicted sound power level, while varying the weld Young’s modulus by 20% does not radically change the sound radiation. Panel contribution analysis indicates that the vanes contribute much more to the total sound power level, as compared to the cylindrical part of the drum. Consequently, it is expected that damping treatments would be most effective at controlling noise radiation if applied to the vanes rather than to the cylindrical portion. Finally, case study results show that the sound power levels are most sensitive to the tangential and bending forces above 500 Hz. For frequencies below 500 Hz, the sound power level is most sensitive to axial and bending forces.© 2013 ASME