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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

NASA Investigation of Flow Direction Effects on Impedance Eduction for Acoustic Liners

In support of a collaboration with the International Forum for Aviation Research (IFAR), the NASA Langley Liner Physics Team has acquired a detailed dataset for use in the evaluation of flow direction effects on the acoustic liner impedance eduction process. Measurements are acquired with four acoustic liners designed to cover a range of sensitivities to source sound pressure level and tangential mean flow effects, specifically at source levels of 120 and 140 dB and centerline Mach numbers of 0.0, 0.1, 0.2, and 0.3. These measurements include detailed flow profile surveys upstream and downstream of the liner test window, as well as acoustic pressure data over the extent of the test window. The results clearly demonstrate the importance of the choice of Mach number used in the impedance eduction analysis. Average Mach number differences of less than 0.01 result in significant variation in the educed impedance spectra. These data will soon be supplied to the IFAR partners and will be made available for the general public within the next year.

acoustic↗
NASA NTRS1975

Duct wall impedance control as an advanced concept for acoustic impression

Models and tests on an acoustic duct liner system which has the property of controlled-variable acoustic impedance are described. This is achieved by a novel concept which uses the effect of steady air flow through a multi-layer, locally reacting, resonant-cavity absorber. The scope of this work was limited to a 'proof of concept.' The test of the concept was implemented by means of a small-scale, square-section flow duct facility designed specifically for acoustic measurements, with one side of the duct acoustically lined. The test liners were designed with the aid of previously established duct acoustic theory and a semi-empirical impedance model of the liner system. Over the limited range tested, the liner behaved primarily as predicted, exhibiting significant changes in resistance and reactance, thus providing the necessary concept validation.

Dean, P. D.↗
NASA NTRSDate not supplied

A Comparison of Impedance Eduction Test Rigs with Different Flow Profiles

The experimental characterization of acoustic liners applied for turbofan engines has been in the spotlight of the community for the last few decades. In general, such characterization is done by measurements of the liner acoustic impedance using different techniques in conditions as close as possible to those encountered in turbofan engines. Although a great amount of work has been published related to these techniques, few comparisons between different experimental setups using identical samples are available. The goal of the present study is to provide a comparison between educed acoustic impedances for two nominally identical liner samples in theUFSC Impedance Test Rig and the NASA Langley Research Center Grazing Flow ImpedanceTube (GFIT). Due to the geometrical differences between the test rigs, it is possible to consider the effect of different grazing flow profiles on the educed impedance. Impedance measurements between the two facilities show similar results in absence of grazing flow, while different ones when the grazing flow is present. Results are presented with both test rigs targeted to two different conditions: (i) same centerline Mach number and; (ii) same average Mach number.Both comparisons suggest a higher acoustic resistance obtained with the UFSC Impedance TestRig. A comparison using semi-empirical predictive models was also conducted. The results suggest that the main source for the observed difference is the grazing flow profile, represented by its boundary layer displacement thickness.

impedance↗
NASA NTRSDate not supplied

A Comparison of Impedance Eduction Test Rigs with Different Flow Profiles

The experimental characterization of acoustic liners applied for turbofan engines has been in the spotlight of the community for the last few decades. In general, such characterization is done by measurements of the liner acoustic impedance using different techniques in conditions as close as possible to those encountered in turbofan engines. Although a great amount of work has been published related to these techniques, few comparisons between different experimental setups using identical samples are available. The goal of the present study is to provide a comparison between educed acoustic impedances for two nominally identical liner samples in theUFSC Impedance Test Rig and the NASA Langley Research Center Grazing Flow ImpedanceTube (GFIT). Due to the geometrical differences between the test rigs, it is possible to consider the effect of different grazing flow profiles on the educed impedance. Impedance measurements between the two facilities show similar results in absence of grazing flow, while different ones when the grazing flow is present. Results are presented with both test rigs targeted to two different conditions: (i) same centerline Mach number and; (ii) same average Mach number.Both comparisons suggest a higher acoustic resistance obtained with the UFSC Impedance TestRig. A comparison using semi-empirical predictive models was also conducted. The results suggest that the main source for the observed difference is the grazing flow profile, represented by its boundary layer displacement thickness.

impedance↗
NASA NTRS2014

Sub-Surface Windscreen for Outdoor Measurement of Infrasound

A windscreen is configured for measuring outdoor infrasonic sound. The windscreen includes a container and a microphone. The container defines a chamber. The microphone is disposed in the chamber and can be operatively supported by the floor. The microphone is configured for detecting infrasonic sound. The container is advantageously formed from material that exhibits an acoustic impedance of between 0 and approximately 3150 times the acoustic impedance of air. A reflector plate may be disposed in the container. The reflector plate operatively can support the microphone and provides a doubling effect of infrasonic pressure at the microphone.

Zuckerwar, Allan J.↗
NASA NTRS1971

Acoustic grazing flow impedance using waveguide principles

A grazing flow apparatus was designed to measure the impedance of acoustic materials when installed in environments that subject the material to grazing airflow. The design of the apparatus and the data analysis technique is based on the solution of the convected wave equation in an infinite length waveguide.

Armstrong, D. L.↗
NASA NTRS1978

Impedance measurement using a two-microphone, random-excitation method

The feasibility of using a two-microphone, random-excitation technique for the measurement of acoustic impedance was studied. Equations were developed, including the effect of mean flow, which show that acoustic impedance is related to the pressure ratio and phase difference between two points in a duct carrying plane waves only. The impedances of a honeycomb ceramic specimen and a Helmholtz resonator were measured and compared with impedances obtained using the conventional standing-wave method. Agreement between the two methods was generally good. A sensitivity analysis was performed to pinpoint possible error sources and recommendations were made for future study. The two-microphone approach evaluated in this study appears to have some advantages over other impedance measuring techniques.

Seybert, A. F.↗
NASA NTRS2000

Experimental Impedance of Single Liner Elements with Bias Flow

An experimental investigation was conducted to generate a high quality database, from which the effects of a mean bias flow on the acoustic impedance of lumped-element single-degree-of-freedom liners was determined. Acoustic impedance measurements were made using the standard two-microphone method in the NASA Langley Normal Incidence Tube. Each liner consisted of a perforated sheet with a constant-area cavity. Liner resistance was shown to increase and to become less frequency and sound pressure level dependent as the bias flow was increased. The resistance was also consistently lower for a negative bias flow (suction) than for a positive bias flow (blowing) of equal magnitude. The slope of the liner reactance decreased with increased flow.

Follet, J. I.↗
NASA NTRS2014

Polyimide Aerogels and Porous Membranes for Ultrasonic Impedance Matching to Air

This work investigates acoustic impedance matching materials for coupling 200 kHz ultrasonic signals from air to materials with similar acoustic properties to that of water, flesh, rubber and plastics. Porous filter membranes as well as a new class of cross-linked polyimide aerogels are evaluated. The results indicate that a single impedance matching layer consisting of these new aerogel materials will recover nearly half of the loss in the incident-to-transmitted ultrasound intensity associated with an air/water, air/flesh or air/gelatin boundary. Furthermore, the experimental results are obtained where other uncertainties of the "real world" are present such that the observed impedance matching gains are representative of real-world applications. Performance of the matching layer devices is assessed using the idealized 3-layer model of infinite half spaces, yet the experiments conducted use a finite gelatin block as the destination medium.

acoustic impedance↗
DOE OSTI2024

3-D Geological Modeling for Numerical Flow Simulation Studies of Gas Hydrate Reservoirs at the Kuparuk State 7-11-12 Pad in the Prudhoe Bay Unit on the Alaska North Slope

Accurate reservoir evaluation requires reliable three-dimensional (3-D) geological models. Here, this study conducted 3-D geological modeling for numerical flow simulation of the B1 sand gas hydrate reservoir at the Kuparuk State 7-11-12 pad, Prudhoe Bay Unit, Alaska North Slope. The model integrates well logs, core, and seismic data to address spatial heterogeneity in geological structures and reservoir properties. Two modeling types were performed: structural framework modeling and petrophysical property modeling. For structural framework modeling, seismic data and well log markers were used to reproduce subsurface structures characterized by a normal fault system. A volume-based modeling algorithm and stair-stepping grid were applied. The resulting 3-D model comprised 2,640,000 grid cells across 264 layers, including seven fault grids. For petrophysical property modeling, total porosity was initially modeled using sequential Gaussian simulation with collocated cokriging. To reproduce the upward coarsening of the B1 sand, upscaled log-derived total porosity and a three-dimensional (3-D) trend depicting total porosity variation were used as primary and secondary data, respectively. Gas hydrate saturation distribution was modeled similarly, with secondary data from estimated porosity distribution and seismic-derived acoustic impedance map enhancing accuracy. Results indicate higher gas hydrate saturation in the upper part of the B1 sand and areas with higher acoustic impedance. Intrinsic permeability was modeled from the total porosity and clay-bound water volume, and effective permeability was derived from the gas hydrate saturation and intrinsic permeability distributions based on the “Tokyo model”. Effective permeability distributions were influenced by the total porosity, gas hydrate saturation, and intrinsic permeability. Within the same layer, higher gas hydrate saturation leads to decreased effective permeability. In total, 100 sets of multiple scenarios were prepared, providing input data for dynamic flow simulations to evaluate the effects of lateral heterogeneity in reservoir properties and the hydraulic characteristics of faults on production behavior for preassessment before the long-term production test.

58 GEOSCIENCES↗
NASA NTRS1999

Advanced Turbofan Duct Liner Concepts

The Advanced Subsonic Technology Noise Reduction Program goal is to reduce aircraft noise by 10 EPNdB by the year 2000 relative, to 1992 technology. The improvement goal for nacelle attenuation is 25% relative to 1992 technology by 1997 and 50% by 2000. The Advanced Turbofan Duct Liner Concepts Task work by Boeing presented in this document was in support of these goals. The basis for the technical approach was a Boeing study conducted in 1993-94 under NASA/FAA contract NAS1-19349, Task 6, investigating broadband acoustic liner concepts. As a result of this work, it was recommended that linear double layer, linear and perforate triple layer, parallel element, and bulk absorber liners be further investigated to improve nacelle attenuations. NASA LaRC also suggested that "adaptive" liner concepts that would allow "in-situ" acoustic impedance control also be considered. As a result, bias flow and high-temperature liner concepts were also added to the investigation. The major conclusion from the above studies is that improvements in nacelle liner average acoustic impedance characteristics alone will not result in 25% increased nacelle noise reduction relative to 1992 technology. Nacelle design advancements currently being developed by Boeing are expected to add 20-40% more acoustic lining to hardwall regions in current inlets, which is predicted to result in and additional 40-80% attenuation improvement. Similar advancements are expected to allow 10-30% more acoustic lining in current fan ducts with 10-30% more attenuation expected. In addition, Boeing is currently developing a scarf inlet concept which is expected to give an additional 40-80% attenuation improvement for equivalent lining areas.

Bielak, Gerald W.↗
NASA NTRS1983

Turbofan aft duct suppressor study

Suppressions due to acoustic treatment in the annular exhaust duct of a model fan were theoretically predicted and compared with measured suppressions. The predictions are based on the modal analysis of sound propagation in a straight annular flow duct with segmented treatment. Modal distributions of the fan noise source (fan-stator interaction only) were measured using in-duct modal probes. The flow profiles were also measured in the vicinity of the modal probes. The acoustic impedance of the single degree of freedom treatment was measured in the presence of grazing flow. The measured values of mode distribution of the fan noise source, the flow velocity profile and the acoustic impedance of the treatment in the duct were used as input to the prediction program. The predicted suppressions, under the assumption of uniform flow in the duct, compared well with the suppressions measured in the duct for all test conditions. The interaction modes generated by the rotor-stator interaction spanned a cut-off ratio range from nearly 1 to 7.

Syed, A. A.↗
NASA NTRS1979

Effect of grazing flow on the nonlinear acoustic behavior of helmholtz resonators

A semi empirical fluid mechanical model is derived of the acoustic behavior of thin walled single orifice Helmholtz resonators in a grazing flow environment. The model assumes that the flow field incident to a resonator orifice consists of a spherical sound particle velocity field superimposed upon a mean grazing flow. The incident and cavity sound fields are connected in terms of an orifice discharge coefficient whose values are determined experimentally using the two microphone method. With regard to its application to rocket motor interiors, the most important finding of this study is that the acoustic impedance of Helmholtz resonators is affected by grazing flow when the product of the amplitude of the sound pressure incident to the resonator orifice and the rocket motor interior mean grazing flow speed are less than 0.5. For values greater than 0.5, the acoustic impedance is independent of the grazing flow.

Hersh, A. S.↗
NASA NTRS1975

Attenuation of sound in ducts with acoustic treatment: A generalized approximate equation

A generalized approximate equation for duct lining sound attenuation is presented. The specification of two parameters, the maximum possible attenuation and the optimum wall acoustic impedance is shown to completely determine the sound attenuation for any acoustic mode at any selected wall impedance. The equation is based on the nearly circular shape of the constant attenuation contours in the wall acoustic impedance plane. For impedances far from the optimum, the equation reduces to Morse's approximate expression. The equation can be used for initial acoustic liner design. Not least important is the illustrative nature of the solutions which provide an understanding of the duct propagation problem usually obscured in the exact calculations. Sample calculations using the approximate attenuation equation show that the peak and the bandwidth of the sound attenuation spectrum can be represented by quite simple functions of the ratio of actual wall acoustic resistance to optimum resistance.

Rice, E. J.↗
NASA NTRS1975

Attenuation of sound in ducts with acoustic treatment - A generalized approximate equation

A generalized approximate equation for duct lining sound attenuation is presented. The specification of two parameters, the maximum possible attenuation and the optimum wall acoustic impedance is shown to completely determine the sound attenuation for any acoustic mode at any selected wall impedance. The equation is based on the nearly circular shape of the constant attenuation contours in the wall acoustic impedance plane. For impedances far from the optimum, the equation reduces to Morse's approximate expression. The equation can be used for initial acoustic liner design. Not least important is the illustrative nature of the solutions which provide an understanding of the duct propagation problem usually obscured in the exact calculations. Sample calculations using the approximate attenuation equation show that the peak and the bandwidth of the sound attenuation spectrum can be represented by quite simple functions of the ratio of actual wall acoustic resistance to optimum resistance.

Rice, E. J.↗
NASA NTRS2015

Optimization of Microphone Locations for Acoustic Liner Impedance Eduction

Two impedance eduction methods are explored for use with data acquired in the NASA Langley Grazing Flow Impedance Tube. The first is an indirect method based on the convected Helmholtz equation, and the second is a direct method based on the Kumaresan and Tufts algorithm. Synthesized no-flow data, with random jitter to represent measurement error, are used to evaluate a number of possible microphone locations. Statistical approaches are used to evaluate the suitability of each set of microphone locations. Given the computational resources required, small sample statistics are employed for the indirect method. Since the direct method is much less computationally intensive, a Monte Carlo approach is employed to gather its statistics. A comparison of results achieved with full and reduced sets of microphone locations is used to determine which sets of microphone locations are acceptable. For the indirect method, each array that includes microphones in all three regions (upstream and downstream hard wall sections, and liner test section) provides acceptable results, even when as few as eight microphones are employed. The best arrays employ microphones well away from the leading and trailing edges of the liner. The direct method is constrained to use microphones opposite the liner. Although a number of arrays are acceptable, the optimum set employs 14 microphones positioned well away from the leading and trailing edges of the liner. The selected sets of microphone locations are also evaluated with data measured for ceramic tubular and perforate-over-honeycomb liners at three flow conditions (Mach 0.0, 0.3, and 0.5). They compare favorably with results attained using all 53 microphone locations. Although different optimum microphone locations are selected for the two impedance eduction methods, there is significant overlap. Thus, the union of these two microphone arrays is preferred, as it supports usage of both methods. This array contains 3 microphones in the upstream hard wall section, 14 microphones opposite the liner, and 3 microphones in the downstream hard wall section.

Jones, M. G.↗
NASA NTRS1983

Acoustic Ground-Impedance Meter

Helmoltz resonator used in compact, portable meter measures acoustic impedance of ground or other surfaces. Earth's surface is subject of increasing acoustical investigations because of its importance in aircraft noise prediction and measurment. Meter offers several advantages. Is compact and portable and set up at any test site, irrespective of landscape features, weather or other environmental condition.

Zuckerwar, A. J.↗