Engineering topics

Hersh, A. S.

Publications and source records attributed to Hersh, A. S..

At least 19 records

NASA NTRS1999

Hybrid Active/Passive Jet Engine Noise Suppression System

A novel adaptive segmented liner concept has been developed that employs active control elements to modify the in-duct sound field to enhance the tone-suppressing performance of passive liner elements. This could potentially allow engine designs that inherently produce more tone noise but less broadband noise, or could allow passive liner designs to more optimally address high frequency broadband noise. A proof-of-concept validation program was undertaken, consisting of the development of an adaptive segmented liner that would maximize attenuation of two radial modes in a circular or annular duct. The liner consisted of a leading active segment with dual annuli of axially spaced active Helmholtz resonators, followed by an optimized passive liner and then an array of sensing microphones. Three successively complex versions of the adaptive liner were constructed and their performances tested relative to the performance of optimized uniform passive and segmented passive liners. The salient results of the tests were: The adaptive segmented liner performed well in a high flow speed model fan inlet environment, was successfully scaled to a high sound frequency and successfully attenuated three radial modes using sensor and active resonator arrays that were designed for a two mode, lower frequency environment.

Parente, C. A.↗

NASA NTRS1983

Effect of flow on the acoustic performance of extended reaction lined ducts

A model is developed for the effects of uniform and boundary-layer mean flow on the attenuation and propagation of harmonically excited sound waves in an extended reaction lined cylindrical duct. A duct geometry consisting of an annular outer region of bulk material surrounding an inner cylinder of air is utilized. A numerical solution is obtained for the coupled wave equations governing the motion of the sound in both the inner and annular regions. It is found that the numerically predicted attenuation and propagations constants are in excellent agreement with measured values using Kevlar as the liner material for plane-wave mode (O,O) excitation over a wide range of mean flows and sound frequency. The boundary-layer effects are determined to be unimportant, at least for plane-wave sound. In addition, numerical studies indicate small differences between the use of either the radial velocity or the radial displacement boundary conditions.

Hersh, A. S.↗

NASA NTRS1981

Analytical and experimental investigation of the propagation and attenuation of sound in extended reaction lined ducts

Results are presented of an analytical and experimental study of the attenuation and propagation of harmonically excited sound waves in an extended reaction lined cylindrical duct. The duct geometry considered consisted of an annular outer region of bulk material surrounding an inner cylinder of air. The coupled wave equations governing the motion of the sound in both the inner and annular regions were solved numerically. The numerically predicted attenuation and propagation constants were in excellent agreement with measured values using Kevlar as the liner material for plane-wave mode (0,0) excitation over the frequency from 100 to 7,000 Hz. Although the numerical model was verified using Kevlar, it can be used with any fibrous constructed bulk liner. The results of this study demonstrate that a good start has been made on the numerical modelling of the acoustic performance of extended reaction liners.

Hersh, A. S.↗

NASA NTRS1980

Acoustic behavior of fibrous bulk materials

A semiempirical model of the acoustic behavior of fibrously constructed bulk materials of Hersh and Walker (1979) is generalized to take into account the filtration or removal of particles by fibrous mats and heat conduction between the material fibers and the surrounding fluid. Equations governing the propagation and attenuation of sound waves in a fibrous material are derived on the basis of a solution of the Navier Stokes equation for momentum conservation and a one-dimensional model of heat transfer between the sound field and the fibers. A comparison of the two propagation constants and material impedance specified by the model and experimental measurements for Kevlar 29 indicates the accuracy of the model over a wide range of sound frequencies, material porosities and specimen thickness. It is also found that heat transfer effects are relatively unimportant, while the attenuation constants and material characteristic impedance are a function of fiber orientation relative to sound field propagation direction.

Hersh, A. S.↗

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 NTRS1979

Effects of refraction by means flow velocity gradients on the standing wave pattern in three-dimensional, rectangular waveguides

The influence of a mean vortical flow on the connection between the standing wave pattern in a rectangular three dimensional waveguide and the corresponding duct axial impedance was determined analytically. The solution was derived using a perturbation scheme valid for low mean flow Mach numbers and plane wave sound frequencies. The results show that deviations of the standing wave pattern due to refraction by the mean flow gradients are small.

Hersh, A. S.↗

NASA NTRS1979

Acoustic behavior of a fibrous bulk material

A semiempirical model is presented describing the acoustic behavior of Kevlar 29, a bulk absorbing material. The model is based on an approximate solution to the one-dimensional equations representing conservation of fluctuating mass, momentum and energy. By treating the material as a momentum sink, theoretical expressions of the material complex propagation constants and characteristic impedance were derived in terms of a single constant. Evaluating the constant at a single frequency for a particular specimen, excellent agreement between prediction and measurement was achieved for a large range of sound frequencies and material porosities and thicknesses. Results show that Kevlar 29 absorbs sound efficiently even at low frequencies. This is explained in terms of a frequency dependent material phase speed.

Hersh, A. S.↗

NASA NTRS1978

Effect of grazing flow on the acoustic impedance of Helmholtz resonators consisting of single and clustered orifices

A semiempirical fluid mechanical model is presented which predicts impedance of a Helmholtz resonator consisting of a single cavity-backed orifice as a function of grazing flow speed, boundary-layer thickness, incident sound amplitude and frequency, and resonator geometry. The incident and cavity sound fields are connected in terms of an orifice discharge coefficient. The effect of multiple orifices was studied experimentally. Interaction between orifices is important only for orifices aligned parallel to the grazing flow. Resistance was virtually independent of both orifice relative spacing and number. Reactance was found to be quite dependent upon orifice spacing but insensitive to the number of orifices.

Hersh, A. S.↗

NASA NTRS1978

Effects of grazing flow on the steady-state flow resistance and acoustic impedance of thin porous-faced liners

The effects of grazing flow on the steady state flow resistance and acoustic impedance of seven Feltmetal and three Rigimesh thin porous faced liners were studied. The steady-state flow resistance of the ten specimens was measured using standard fluid mechanical experimental techniques. The acoustic impedance was measured using the two microphone method. The principal findings of the study are that the effects of grazing flow were measured and found to be small; small differences were measured between steady-state and acoustic resistance, and a semi-empirical model was derived that correlated the steady-state resistance data of the seven Feltmetal liners and the face sheet reactance of both the Feltmetal and Rigimesh liners.

Hersh, A. S.↗

NASA NTRS1977

Effect of grazing flow on the acoustic impedance of interacting cavity-backed orifices

The two-microphone method was used to investigate the impedance of interacting cavity-backed orifices in the presence of a grazing flow; the investigation has relevance for the control of turbomachinery noise generated within jet engines. The number (varied from one to 16), the diameter, and the spacing of the orifices were the chief parameters studied in the experimental program. It was found that interactions between adjacent orifices, while increasing reactance, do not significantly alter resistance. In addition, the grazing flow appears to reduce the rate of increase of the reactance.

Hersh, A. S.↗

NASA NTRS1977

Fluid mechanical model of the Helmholtz resonator

A semi-empirical fluid mechanical model of the acoustic behavior of Helmholtz resonators is presented which predicts impedance as a function of the amplitude and frequency of the incident sound pressure field and resonator geometry. The model assumes that the particle velocity approaches the orifice in a spherical manner. The incident and cavity sound fields are connected by solving the governing oscillating mass and momentum conservation equations. The model is in agreement with the Rayleigh slug-mass model at low values of incident sound pressure level. At high values, resistance is predicted to be independent of frequency, proportional to the square root of the amplitude of the incident sound pressure field, and virtually independent of resonator geometry. Reactance is predicted to depend in a very complicated way upon resonator geometry, incident sound pressure level, and frequency. Nondimensional parameters are defined that divide resonator impedance into three categories corresponding to low, moderately low, and intense incident sound pressure amplitudes. The two-microphone method was used to measure the impedance of a variety of resonators. The data were used to refine and verify the model.

Hersh, A. S.↗

NASA NTRS1976

Semiempirical airframe noise prediction model and evaluation with flight data

A semiempirical maximum overall sound pressure level (OASPL) airframe noise model was derived. Noise radiated from aircraft wings was modeled on the trailing edge diffractes quadrupole sound theory. The acoustic dipole sound theory was used to model noise from the landing gear. The model was correlated with maximum OASPL flyover noise measurements obtained for three jet aircraft. One third octave band sound pressure level flyover data was correlated and interpreted.

Hersh, A. S.↗

NASA NTRS1976

Sound propagation in choked ducts

The linearized equations describing the propagation of sound in variable area ducts containing flow are shown to be singular when the duct mean flow is sonic. The singularity is removed when previously ignored nonlinear terms are retained. The results of a numerical study, for the case of plane waves propagating in a one-dimensional converging-diverging duct, show that the sound field is adequately described by the linearized equations only when the axial mean flow Mach number at the duct throat M sub th 0.6. For M sub th 0.6, the numerical results showed that acoustic energy flux was not conserved. An attempt was made to extend the study to include the nonlinear behavior of the sound field. Meaningful results were not obtained due, primarily, to numerical difficulties.

Hersh, A. S.↗

NASA NTRS1976

Semi-empirical airframe noise prediction model

A semi-empirical maximum overall sound pressure level (OASPL) airframe noise model was derived. The noise radiated from aircraft wings and flaps was modeled by using the trailing-edge diffracted quadrupole sound theory derived by Ffowcs Williams and Hall. The noise radiated from the landing gear was modeled by using the acoustic dipole sound theory derived by Curle. The model was successfully correlated with maximum OASPL flyover noise measurements obtained at the NASA Dryden Flight Research Center for three jet aircraft - the Lockheed JetStar, the Convair 990, and the Boeing 747 aircraft.

Hersh, A. S.↗

NASA NTRS1976

Fluid mechanical model of the acoustic impedance of small orifices

A fluid mechanical model of the acoustic behavior of small orifices is presented which predicts orifice resistance and reactance as a function of incident sound pressure level, frequency, and orifice geometry. Agreement between predicted and measured values is excellent. The model shows the following: (1) The acoustic flow in immediate neighborhood of the orifice can be modeled as a locally spherical flow. Within this near field, the flow is, to a first approximation, unsteady and incompressible. (2) At very low sound pressure levels, the orifice viscous resistance is directly related to the effect of boundary-layer displacement along the walls containing the orifice, and the orifice reactance is directly related to the inertia of the oscillating flow in the neighborhood of the orifice. (3) For large values of the incident acoustic pressure, the impedance is dominated by nonlinear jet-like effects. (4) For low values of the pressure, the resistance and reactance are roughly equal.

Hersh, A. S.↗

NASA NTRS1975

The effect of grazing flow on the steady state resistance of square-edged orifices

A simple inviscid interaction model has been developed which uses thin airfoil theory to account for pressure forces exerted at the interface between the orifice and grazing flow together with a one-dimensional discharge coefficient concept. The effect of grazing flow boundary layer thickness was also included in the model. Resistance measurements for a wide range of grazing flow speeds and orifice flow rates collapse into a single curve when plotted in terms of effective discharge coefficient versus orifice-to-grazing velocity ratio. The correlation curves for inflow and outflow are different. Data for clustered orifices collapse in the same way as the single orifice. The effect of boundary layer thickness is compared with model predictions. The effect of orifice length-diameter ratio is shown to be significant for inflow but negligible for outflow.

Rogers, T.↗

NASA NTRS1975

Fluid mechanical model of the acoustic impedance of small orifices

A fluid mechanical model of the acoustic behavior of small orifices is presented which predicts orifice impedance as a function of incident sound pressure level, frequency, and orifice geometry. Agreement between predicted and measured values (in both water and air) of orifice impedance is excellent. The model shows that (1) the acoustic flow in the immediate neighborhood of the orifice can be modelled as a locally spherical flow, (2) within this near field, the flow is, to a first approximation, unsteady and incompressible, and (3) at very low sound pressure levels, the orifice viscous resistance is directly related to the effect of boundary-layer displacement along the walls containing the orifice, and the orifice reactance is directly related to the inertia of the oscillating flow in the orifice neighborhood.-

Hersh, A. S.↗

NASA NTRS1974

Investigation of acoustic effects of leading-edge serrations on airfoils

This paper presents and interprets a series of extensive studies of the application of leading-edge serrations as a device for reducing the vortex noise radiated from stationary and rotating airfoils in low Reynolds number flow. In these studies, a variety of serrations were attached at selected locations near the leading edge of stationary and rotating airfoils. The noise levels of the airfoils were reduced considerably with the serrations attached. An explanation of the aeroacoustic flow mechanisms involved is given.

Hersh, A. S.↗