Search NASA⌕ Search

SEARCH · Search NASA

Results for “Duct acoustics”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Non-linear wave propagation in acoustically lined circular ducts

An analysis is presented of the nonlinear effects of the gas motion as well as of the acoustic lining material on the transmission and attenuation of sound in a circular duct with a uniform cross-section and no mean flow. The acoustic material is characterized by an empirical, nonlinear impedance in which the instantaneous resistance is a nonlinear function of both the frequency and the acoustic velocity. The results show that there exist frequency bandwidths around the resonant frequencies in which the nonlinearity decreases the attenuation rate, and outside which the nonlinearity increases the attenuation rate, in qualitative agreement with experimental observations. Moreover, the effect of the gas nonlinearity increases with increasing sound frequency, whereas the effect of the material nonlinearity decreases with increasing sound frequency.

Nayfeh, A. H.↗
NASA NTRS1973

Propagation of waves of acoustic frequencies in curved ducts

The propagation of waves of acoustic frequencies in curved ducts is studied for the first four modes. The analysis makes use of Bessel functions to construct curves of wave number in the duct versus imposed wave number. The results apply to ducts of arbitrary width and arbitrary radii of curvature. The characteristics of motion in a bend are compared with propagation of waves in a straight duct, and important differences in the behavior of waves are noted.

Rostafinski, W.↗
NASA NTRS1974

Analysis of propagation of waves of acoustic frequencies in curved ducts

The propagation of waves of acoustic frequencies in curved ducts of rectangular cross section is studied for the first four modes. The analysis makes use of Bessel functions of the order (n + 1/2) to construct curves of wavenumber in the duct versus imposed wavenumber and to determine the profile of vibrational velocities. A wide range of duct widths and unrestricted radii of curvature have been considered. The characteristics of motion in a bend are compared with propagation of waves in a straight duct, and important differences in the behavior of waves are noted.

Rostafinski, W.↗
NASA NTRS1973

Acoustic propagation in ducts with varying cross sections and sheared mean flow.

An analysis is presented of the wave propagation and attenuation in two-dimensional ducts with slowly varying cross sections which carry sheared mean flow. A uniformly valid asymptotic expansion of the acoustic waves is obtained in terms of the maximum slope of the wall by using the method of multiple scales. The result is a first-order differential equation describing the amplitude variation with the axial distance. This equation is used to evaluate the effects of variations of the duct cross section and growing boundary layers on the different acoustic modes.

Nayfeh, A. H.↗
NASA NTRS1971

On propagation of long waves in curved ducts

Long acoustic wave propagation in curved ducts and junctions between straight and curved ducts utilizing Bessel functions for steady and decaying fields of motion

Rostafinski, W.↗
NASA NTRS1976

Design of optimum acoustic treatment for rectangular ducts with flow

A design optimization technique for acoustic treatment in rectangular ducts with uniform mean flow is presented. The technique is based on the acoustic wave solution in terms of series of characteristic duct modes. The analysis allows multiple axial treatment sections along the length of the duct and requires a known modal characterization of the sound source. Conditions of acoustic pressure and acoustic velocity continuity are used to match modal solutions at planes of impedance discontinuity in the duct. Experimental techniques for obtaining this modal characterization are presented. Using duct modes measured at the source plane, the optimization technique is exercised to design an optimized single element liner in a case without mean flow, and optimized single and dual element liners in cases with mean flow. The validity of the program for predicting noise suppression is demonstrated by comparing analytical predictions with measured data for several (non-optimum) cases. Application to treatment design in turbomachinery exhaust ducts is considered.

Motsinger, R. E.↗
NASA NTRS1976

Aero-acoustic tests of duct-burning turbofan exhaust nozzles

The acoustic and aerodynamic characteristics of several exhaust systems suitable for duct burning turbofan engines are evaluated. Scale models representing unsuppressed coannular exhaust systems are examined statically under varying exhaust conditions. Ejectors with both hardwall and acoustically treated inserts are investigated.

Kozlowski, H.↗
NASA NTRS1972

Acoustic attenuation analysis program for ducts with mean flow

A computerized acoustic attenuation prediction procedure has been developed to evaluate acoustically lined ducts for various geometric and environmental parameters. The analysis procedure is based on solutions to the acoustic wave equation, assuming uniform airflow on a duct cross section, combined with appropriate mathematical lining impedance models. The impedance models included in the analysis procedure are representative of either perforated sheet or porous polyimide impregnated fiberglass facing sheet coupled with a cellular backing space. Advantages and limitations of the analysis procedure are reviewed.

Kunze, R. K., Jr.↗
NASA NTRS1979

On the attenuation of sound by three-dimensionally segmented acoustic liners in a rectangular duct

Axial segmentation of acoustically absorbing liners in rectangular, circular or annual duct configurations is a very useful concept for obtaining higher noise attenuation with respect to the bandwidth of absorption as well as the maximum attenuation. As a consequence, advanced liner concepts are proposed which induce a modal energy transfer in both cross-sectional directions to further reduce the noise radiated from turbofan engines. However, these advanced liner concepts require three-dimensional geometries which are difficult to treat theoretically. A very simple three-dimensional problem is investigated analytically. The results show a strong dependence on the positioning of the liner for some incident source modes while the effect of three-dimensional segmentation appears to be negligible over the frequency range considered.

Koch, W.↗
NASA NTRS1974

Acoustic theory of axisymmetric multisectioned ducts

Equations are developed for the acoustic field in a duct system which is made up of a number of connected circular and annular ducts. These equations are suitable for finding the acoustic field inside of and radiated from an aircraft turbofan engine. Acoustic modes are used as generalized coordinates in order to develop a set of matrix equations for the acoustic field. Equations for these modes are given for circular and annular ducts with uniform flow. Modal source equations are derived for point acoustic sources. General equations for mode transmission and reflection are developed and detailed equations are derived for ducts with multiple sections of acoustic treatment and for ducts with circumferential splitter rings. The general theory is applied to the special case of a uniform area circular duct with multisection liners and it is shown that the mode reflection effects are proportional to differences of the acoustic admittances of adjacent liners. A numerical example is given which shows that multisection liners may provide greater noise suppression than uniform liners.

Zorumski, W. E.↗
NASA NTRS1993

Measured acoustic characteristics of ducted supersonic jets at different model scales

A large-scale (about a 25x enlargement) model of the Georgia Tech Research Institute (GTRI) hardware was installed and tested in the Propulsion Systems Laboratory of the NASA Lewis Research Center. Acoustic measurements made in these two facilities are compared and the similarity in acoustic behavior over the scale range under consideration is highlighted. The study provide the acoustic data over a relatively large-scale range which may be used to demonstrate the validity of scaling methods employed in the investigation of this phenomena.

Jones, R. R., III↗
NASA NTRS1975

The propagation of plane waves and higher order acoustic modes in multisectioned ducts

A theoretical and experimental study of acoustic propagation in an anechoically terminated multisectioned duct was performed. A unique source array consisting of two concentric rings of sources, providing phase and amplitude control in the radial as well as circumferential direction, was developed to generate plane waves and both spinning and non-spinning higher order modes. Measurement of attenuation and radial mode shapes were taken with various finite length liners inserted between the hard wall sections of a duct with an anechoic termination. A search technique was developed to find the complex eigenvalues for a liner under the assumption of a locally reacting boundary condition. The experimental results were compared with a theoretical analysis which includes the modal transmission and reflection at the interface of each duct section, and this indicated that the local reaction boundary condition is valid for three liner configurations.

Wyerman, B. R.↗
NASA NTRS1973

Acoustic propagation in ducts with varying cross sections

The method of multiple scales is used to derive the equations that describe the spatial and temporal variation of the amplitudes and phases of a wave packet propagating in slowly varying hard-walled or lined ducts. The analysis is carried out for rectangular as well as circular ducts. These equations are statements of the conservation of energy. For large admittance or high-frequency modes, an approximate expression is obtained for the attenuation. This expression shows that all possible acoustic modes are attenuating. The results also show that decreasing the cross sectional area can lead to elimination of some of the acoustic modes.

Nayfeh, A. H.↗
NASA NTRS1983

Experimental investigation of geometry and flow effects on acoustic radiation from duct inlets

An experimental study of the effects of geometry, flow, frequency and source structure on the radiation and reflection of sound from duct inlets has been conducted. This work extends a previous study by comparing two inlet shapes typical of static and flight testing and by considering higher order radial modes. Measurements of the internal and radiated acoustic fields are presented in terms of modal reflection coefficients and absolute radiated pressure directivity. Results indicate that geometry effects associated with these practical inlet shapes become less important for a fixed cutoff ratio as the mode order and frequency are increased. Also, static inflow, while reducing the reflected wave amplitude, is shown to have little effect on radiation directivity regardless of inlet geometry.

Silcox, R. J.↗
NASA NTRS1995

Internal Acoustics Measurements of a Full Scale Advanced Ducted Propulsor Demonstrator

Acoustics measurements of a Pratt & Whitney full-scale ADP (Advanced Ducted Propulsor), an ultrahigh by-pass ratio engine, were conducted in the NASA Ames 40- by 80-Foot Wind Tunnel. This paper presents data from measurements taken from sensors on a fan exit guide vane in the ADP. Data from two sensors, one at mid-span and the other at the tip of the fan exit guide vane, are presented. At the blade passage frequency (BPF), the levels observed at the various engine and wind speeds were higher at the mid-span sensor than the tip sensor. The coherence between these internal sensors and external microphones were calculated and plotted as a function of angle (angles ranged from 5 degrees to 160 degrees) relative to the ADP longitudinal axis. At the highest engine and wind speeds, the coherence between the tip sensor and the external microphones was observed to decrease at higher multiples of the BPF. These results suggest that the rotor-stator interaction tones are stronger in the mid-span region than at the tip.

Santa Maria, O. L.↗
NASA NTRS1976

Aerodynamic and acoustic tests of duct-burning turbofan exhaust nozzles

The static aerodynamic and acoustic characteristics of duct-burning turbofan (DBTF) exhaust nozzles are established. Scale models, having a total area equivalent to a 0.127 m diameter convergent nozzle, simulating unsuppressed coannular nozzles and mechanically suppressed nozzles with and without ejectors (hardwall and acoustically treated) were tested in a quiescent environment. The ratio of fan to primary area was varied from 0.75 to 1.2. Far field acoustic data, perceived noise levels, and thrust measurements were obtained for 417 test conditions. Pressure ratios were varied from 1.3 to 4.1 in the fan stream and from 1.53 to 2.5 in the primary stream. Total temperature varied from 395 to 1090 K in both streams. Jet noise reductions relative to synthesized prediction from 8 PNdB (with the unsuppressed coannular nozzle) to 15 PNdB (with a mechanically suppressed configuration) were observed at conditions typical of engines being considered under the Advanced Supersonic Technology program. The inherent suppression characteristic of the unsuppressed coannular nozzle is related to the rapid mixing in the jet wake caused by the velocity profiles associated with the DBTF. Since this can be achieved without a mechanical suppressor, significant reductions in aircraft weight or noise footprint can be realized.

Kozlowski, H.↗
NASA NTRS1981

Acoustic power dissipation on radiation through duct terminations - Experiments

An acoustic impulse technique is used to study the acoustic transmission characteristics of ducts, nozzles, orifices, and perforated plates. High intensity pulses were generated by discharging a capacitor across a spark gap for both heated and unheated flows, with and without simulated flights. The effect of a termination geometry on power absorption for high acoustic intensities was observed, showing the existence of power absorption at low frequencies for the duct, nozzles, orifice plates, and perforated plates at a no flow condition; the amount of power absorption was controlled primarily by the open area at the exit. Power conservation with flow was similar to that for no flow: Power absorption initially increased for the duct and conical nozzles and then decreased with increasing Mach number. Power absorption results for the suppressor nozzle were similar to those for the conical nozzle, and the nonlinear effect was found to be insignificant in the presence of flow. A power imbalance was noted for heated jets, with a greater power loss at low frequencies than for unheated jets. Low frequency power absorption increased with the simulated flight, and a small amount of low frequency power absorption was found even for low intensity sound. A typical schlieren photograph of vortex formation (Whiffen and Ahuja, 1981) helped to confirm that sound energy can be converted into vortical energy and may account for low frequency power losses.

Salikuddin, M.↗