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NASA NTRS2015

Acoustic Radiation from High-Speed Turbulent Boundary Layers in a Tunnel-Like Environment

Direct numerical simulation of acoustic radiation from a turbulent boundary layer in a cylindrical domain will be conducted under the flow conditions corresponding to those at the nozzle exit of the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT) operated under noisy-flow conditions with a total pressure p(sub t) of 225 kPa and a total temperature of T(sub t) equal to 430 K. Simulations of acoustic radiation from a turbulent boundary layer over a flat surface are used as a reference configuration to illustrate the effects of the cylindrical enclosure. A detailed analysis of acoustic freestream disturbances in the cylindrical domain will be reported in the final paper along with a discussion pertaining to the significance of the flat-plate acoustic simulations and guidelines concerning the modeling of the effects of an axisymmetric tunnel wall on the noise field.

Duan, Lian↗

NASA NTRS1979

An experimental investigation of acoustic radiation from a source inside a large turbulent free jet

An experiment has been conducted in NASA Langley's free-jet anechoic facility to assess the effect of a free-jet shear layer on sound radiation from an acoustic source placed inside the jet. The nozzle diameter was 1.22 m and operated at an exit velocity of 35 m/s. Two different types of sources were used; one was a compact source with a nearly uniform radiation pattern, the other was a noncompact source with highly directional radiation pattern. The acoustic source was driven by pure tone and 1/3-octave band white noise. Acoustic measurements were made both inside the jet flow in the potential core region and outside the jet in the acoustic farfield. The effects of mean flow convection and mean shear refraction on sound radiation were determined. The experimental results compared favorably with analytical predictions. The effect of free-jet turbulence on sound scattering was found to be insignificant.

Ozkul, A.↗

NASA NTRS1977

Far-field multimodal acoustic radiation directivity

Approximate equations for the far field acoustic radiation patterns in the forward quadrant from a flanged circular duct were compared with exact equations for both single and multimodal excitations. The single mode comparison showed good agreement between the exact and approximate equations for the principal lobes of higher radial order modes. For lower and especially for zero radial order modes, there was some error in the angular location and decibel level of principal lobe peak pressure obtained from the approximate equation. Some agreement of sidelobes was also observed although the approximate equation was not intended to simulate the sidelobes. The multimodal approximate summation equations consisting only of a simple function of directivity angle and an acoustic power biasing function were checked against the exact equations for several distributions of modal power and showed excellent agreement with exact equations for all cases. Although many modes contribute to the final level and shape of the directivity curve, the major contributions appear to come from the higher radial order modes.

Saule, A. V.↗

NASA NTRS1995

A numerical solution method for acoustic radiation from axisymmetric bodies

A new and very efficient numerical method for solving equations of the Helmholtz type is specialized for problems having axisymmetric geometry. It is then demonstrated by application to the classical problem of acoustic radiation from a vibrating piston set in a stationary infinite plane. The method utilizes 'Green's Function Discretization', to obtain an accurate resolution of the waves using only 2-3 points per wave. Locally valid free space Green's functions, used in the discretization step, are obtained by quadrature. Results are computed for a range of grid spacing/piston radius ratios at a frequency parameter, omega R/c(sub 0), of 2 pi. In this case, the minimum required grid resolution appears to be fixed by the need to resolve a step boundary condition at the piston edge rather than by the length scale imposed by the wave length of the acoustic radiation. It is also demonstrated that a local near-field radiation boundary procedure allows the domain to be truncated very near the radiating source with little effect on the solution.

Caruthers, John E.↗

NASA NTRS2004

A Spectral Analysis Approach for Acoustic Radiation from Composite Panels

A method is developed to predict the vibration response of a composite panel and the resulting far-field acoustic radiation due to acoustic excitation. The acoustic excitation is assumed to consist of obliquely incident plane waves. The panel is modeled by a finite element analysis and the radiated field is predicted using Rayleigh's integral. The approach can easily include other effects such as shape memory alloy (SMA) ber reinforcement, large detection thermal postbuckling, and non-symmetric SMA distribution or lamination. Transmission loss predictions for the case of an aluminum panel excited by a harmonic acoustic pressure are shown to compare very well with a classical analysis. Results for a composite panel with and without shape memory alloy reinforcement are also presented. The preliminary results demonstrate that the transmission loss can be significantly increased with shape memory alloy reinforcement. The mechanisms for further transmission loss improvement are identified and discussed.

Turner, Travis L.↗

NASA NTRS1990

Acoustic radiation from lifting airfoils in compressible subsonic flow

The far field acoustic radiation from a lifting airfoil in a three-dimensional gust is studied. The acoustic pressure is calculated using the Kirchhoff method, instead of using the classical acoustic analogy approach due to Lighthill. The pressure on the Kirchhoff surface is calculated using an existing numerical solution of the unsteady flow field. The far field acoustic pressure is calculated in terms of these values using Kirchhoff's formula. The method is validated against existing semi-analytical results for a flat plate. The method is then used to study the problem of an airfoil in a harmonic three-dimensional gust, for a wide range of Mach numbers. The effect of variation of the airfoil thickness and angle of attack on the acoustic far field is studied. The changes in the mechanism of sound generation and propagation due to the presence of steady loading and nonuniform mean flow are also studied.

Atassi, Hafiz M.↗

NASA NTRS1990

Acoustic radiation from lifting airfoils in compressible subsonic flow

Atassi, Hafiz M.↗

NASA NTRS1971

Directional acoustic radiation from a supersonic jet generated by shear layer instability

Directional acoustic radiation from supersonic jet, discussing generation mechanism theory based on shear layer instability close to nozzle

Tam, C. K. W.↗

NASA NTRS1981

Acoustic radiation force on a rigid sphere in a resonance chamber

The acoustic radiation force on a rigid sphere has been measured in a resonance chamber for a range of pressures, positions, sizes, and for various gases. In the low to medium intensity region of less than 150 dB, the measured force is consistent with King's theory (1934) when analyzed in terms of the fundamental pressure. However, in the high intensity region of greater than 150 dB, the measured force starts to deviate systematically from King's calculations.

Leung, E.↗

NASA NTRS1996

Stabilization and Low-Frequency Oscillation of Capillary Bridges with Modulated Acoustic Radiation Pressure

In the work reported here it is demonstrated that acoustic radiation pressure may be used in simulated low gravity to produce stable bridges significantly beyond the Rayleigh limit with S as large as 3.6. The bridge (PDMS mixed with a dense liquid) has the same density as the surrounding water bath containing an ultrasonic standing wave. Modulation was first used to excite specific bridge modes. In the most recent work reported here the shape of the bridge is optically sensed and the ultrasonic drive is electronically adjusted such that the radiation stress distribution dynamically quenches the most unstable mode. This active control simulates passive stabilization suggested for low gravity. Feedback increases the mode frequency in the naturally stable region since the effective stiffness of the mode is increased.

Marston, Philip L.↗

NASA NTRS1979

Directivity of acoustic radiation from sources

The radiation properties of acoustic monopoles and dipoles are described. The directivity of radiation from these sources in a free field and in the presence of an absorptive surface is described. The kinematic effects on source radiation due to translation and rotation are discussed. Experimental measurements of sound from an acoustic monopole in motion and the characteristics of helicopter rotor and propeller noise are reviewed. An introduction is provided to several essential concepts required by noise control engineers making measurements of noise from moving sources in the proximity of the ground.

Lansing, D. L.↗

NASA NTRS1979

Directivity of acoustic radiation from sources

The radiation properties of acoustic monopoles and dipoles are described, as well as the directivity of radiation from these sources in a free field and in the presence of an absorptive surface. The kinematic effects on source radiation due to translation and rotation are discussed. Experimental measurements of sound from an acoustic monopole in motion and the characteristics of helicopter rotor and propeller noise are reviewed. Several essential concepts required by noise control engineers making measurements of noise from moving sources in the proximity of the ground are introduced.

Lansing, D. L.↗

NASA NTRS1983

Spinning mode acoustic radiation from the flight inlet

A mathematical model was developed for spinning mode acoustic radiation from a thick wall duct without flow. This model is based on a series of experiments (with and without flow). A nearly pure azimuthal spinning mode was isolated and then reflection coefficients and far field pressure (amplitude and phase) were measured. In our model the governing boundary value problem for the Helmholtz equation is first converted into an integral equation for the unknown acoustic pressure over a disk, S1, near the mouth of the duct and over the exterior surface, S2, of the duct. Assuming a pure azimuthal mode excitation, the azimuthal dependence is integrated out which yields an integral equation over the generator C1 of S1 and the generator C2 of S2. The sound pressure on C1 was approximated by a truncated modal expansion of the interior acoustic pressure. Piecewise linear spline approximation on C2 was used.

Moss, W. F.↗

NASA NTRS1973

Acoustic radiation patterns for a source in a hard-walled unflanged circular duct

Acoustic radiation patterns are measured over a 320 deg arc for a point source in a finite length, hard walled, unflanged circular duct. The measured results are compared with computed results which are based on the Wiener-Hopf solution for radiation from a semi-infinite unflanged duct. Measurements and computations are presented for frequencies slightly below and slightly above each of the first four higher order radial mode cutoff frequencies. It is found that the computed and measured patterns show better agreement below the mode cut-off frequencies than above and that the agreement is better at lower frequencies that at higher frequencies. The computed radiation patterns do not show fine lobes which are caused by diffraction from the back end of the duct.

Holm, R. G.↗

NASA NTRS1977

Far-field multimodal acoustic radiation directivity

A comparison is made between approximate equations for far-field acoustic radiation patterns and exact equations for single and multimodal excitations in order to determine the validity range for the approximate approach. It is found that for single-mode cases: (1) the gross behavior of the primary lobes is adequately described by the approximate equations, (2) some error is found for lower and zero radial order modes, and (3) some agreement is yielded between exact and approximate sidelobes, although the approximate equation was not intended to simulate sidelobes. Multimodal approximate equations are compared to exact equations for various distributions of modal power; for all cases excellent agreement is found. For multimodal patterns it is noted that many modes influence the final level and shape of the directivity curve, although the major contributions are from the higher radial order modes.

Saule, A. V.↗

NASA NTRS1973

Response of panels to turbulence-induced, surface-pressure fluctuations and resulting acoustic radiation to the flow field.

The response of finite rectangular panels, excited by turbulent boundary layers and the resulting acoustical radiation into the flow field is studied. The method of normal modes and the technique of spectral analysis are used to compute the power spectra of the panel amplitude and strain response. Computed results are compared with the corresponding response measurements for panels exposed to attached and separated flows at supersonic speeds and attached flow at subsonic speeds. The thin plate and the convected wave equations are coupled through the boundary condition on the panel to give a formula for the radiation pressure power spectra. A formula for the acoustical damping ratio of the panel is obtained through the principle of virtual work.-

Chyu, W. J.↗

NASA NTRS1997

Manipulating Liquids with Acoustic Radiation Pressure

At the NASA Lewis Research Center, high-intensity ultrasound is being used to create acoustic radiation pressure (ARP) on objects in liquids. It is also being used to create liquid currents or jets called acoustic streaming. NASA's interest in ARP includes remote-control agitation of liquid systems in space, such as in liquid space experiments and liquid propellant tanks. It can be used to eject or deploy droplets for droplet physics or droplet combustion experiments. It can also be used to manipulate bubbles, drops, and surfaces suspended in liquid experiments and propellant systems.

Oeftering, Richard C.↗

NASA NTRS1986

Measuring Acoustic-Radiation Stresses in Materials

System measures nonlinearity parameters of materials. Uses static strain generated by acoustic wave propagating in material. Since static strain is effectively "dc" component of waveform distortion, problems associated with phase-cancellation artifacts disappear. Further, sign of nonlinearity parameter obtained by simple inspection of measured signal polarity. These features make this system very amenable to use in field. System expected to become standard for acoustic-radiation-stress measurements for solids and liquids and for characterization of material properties related to strength and residual or applied stresses. Also expected to become standard for transducer calibration.

Cantrell, John H.↗