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NASA NTRS2015

Investigating the Feedback Path in a Jet-Surface Resonant Interaction

A resonant interaction between an 8:1 aspect ratio rectangular jet and flat-plates, placed parallel to the jet, is addressed in this study. For certain relative locations of the plates, the resonance takes place with accompanying audible tones. Even when the tone is not audible the sound pressure level spectra is often marked by conspicuous peaks. The frequencies of the spectral peaks, as functions of the streamwise length of the plate and its relative location to the jet as well as the jet Mach number, are explored in an effort of understand the flow mechanism. It is demonstrated that the tones are not due to a simple feedback between the plates trailing edge and the nozzle exit; the leading edge also comes into play in determining the frequency. An acoustic feedback path, involving diffraction from the leading edge, appears to explain the frequencies of some of the spectral peaks.

Jet Noise↗

NASA NTRS1973

Highly noise suppressed bypass 6 engine for STOL application

A TF-34 engine with an acoustically treated ground test nacelle was built and tested to determine the feasibility of suppressing fan and core engine noise to the stringent levels required for STOL or short-haul commercial aircraft. The design incorporates wall treatment for the fan and core plus three treated splitter rings in the inlet and two treated splitters in the aft fan duct. Maximum suppression of fan tone noise of 40-45 dB was obtained from both the inlet and aft fan treatment. At rated fan speed, overall noise was reduced by 21 PNdB to a value of 94 PNdB on a 500-foot sideline. The overall noise reduction value was limited by the jet noise floor. Thrust losses due to the acoustic treatment are also discussed.

Jones, W. L.↗

NASA NTRS1978

Evaluation of aero Commander propeller acoustic data: Static operations

Acoustic data are analyzed from a series of ground tests performed on an Aero Commander propeller-driven aircraft with an array of microphones flush-mounted on one side of the fuselage. The analyses were concerned with the propeller blade passage noise during static operation at several different engine speeds and included calculations of the magnitude and phase of the blade passage tones, the amplitude stability of the tones, and the spatial phase and coherence of the tones. The results indicate that the pressure field impinging on the fuselage represents primarily aerodynamic (near field) effects in the plane of the propeller at all frequencies. Forward and aft of the propeller plane aerodynamic effects still dominate the pressure field at frequencies below 200 Hz; but at higher frequencies, the pressure field is due to acoustic propagation from an equivalent center located about 0.15 to 0.30 blade diameters inboard from the propeller hub.

Piersol, A. G.↗

NASA NTRS1992

Acoustic interactions between an altitude test facility and jet engine plumes: Theory and experiments

The overall objective of the described effort was to develop an understanding of the physical mechanisms involved in the flow/acoustic interactions experienced in full-scale altitude engine test facilities. This is done by conducting subscale experiments and through development of a theoretical model. Model cold jet experiments with an axisymmetric convergent nozzle are performed in a test setup that stimulates a supersonic jet exhausting into a cylindrical diffuser. The measured data consist of detailed flow visualization data and acoustic spectra for a free and a ducted plume. It is shown that duct resonance is most likely responsible by theoretical calculations. Theoretical calculations also indicate that the higher discrete tones observed in the measurements are related to the screech phenomena. Limited experiments on the sensitivity of a free 2-D, C-D nozzle to externally imposed sound are also presented. It is shown that a 2-D, C-D nozzle with a cutback is less excitable than a 2-D C-D nozzle with no cutback. At a pressure ratio of 1.5 unsteady separation from the diverging walls of the nozzle is noticed. This separation switches from one wall to the opposite wall thus providing an unsteady deflection of the plume. It is shown that this phenomenon is related to the venting provided by the cutback section.

Ahuja, K. K.↗

NASA NTRS2004

A Hybrid Approach To Tandem Cylinder Noise

Aeolian tone generation from tandem cylinders is predicted using a hybrid approach. A standard computational fluid dynamics (CFD) code is used to compute the unsteady flow around the cylinders, and the acoustics are calculated using the acoustic analogy. The CFD code is nominally second order in space and time and includes several turbulence models, but the SST k - omega model is used for most of the calculations. Significant variation is observed between laminar and turbulent cases, and with changes in the turbulence model. A two-dimensional implementation of the Ffowcs Williams-Hawkings (FW-H) equation is used to predict the far-field noise.

Lockard, David P.↗

NASA NTRSDate not supplied

Acoustic Simulations of Two-stage Fans Behind a Supersonic Inlet

NASA created, validated, and applied a high fidelity simulation toolchain for noise prediction of a fan system for a near-term civilian supersonic aircraft engine. A high fidelity inlet and multistage fan design were acquired as the basis for physics based computational studies. The overall tone noise was computed, which was used to help improve empirical models of fan inlet noise for system analysis purposes. Additional insights were gained during the study of the fan installation details and these are shared in this paper. Both overall amplitudes and variations due to design details should be of interest to engineers tasked with designing quiet fan systems for supersonic commercial aircraft. The outlook for high-fidelity simulations of aft fan noise and installation effects will also be discussed.

fan↗

NASA NTRS1979

Tone Noise of Three Supersonic Helical Tip Speed Propellers in a Wind Tunnel

Three supersonic helical tip speed propellers were tested in the NASA Lewis 8- by 6-foot wind tunnel. This is a perforated-wall wind tunnel but it does not have acoustic damping material on its walls. The propellers were tested at tunnel through flow Mach numbers of 0.6, 0.7, 0.75, 0.8, and 0.85 with different rotational speeds and blade setting angles. The three propellers, which had approximately the same aerodynamic performance, incorporated different plan forms and different amounts of sweep and yielded different near field noise levels. The acoustically designed propeller had 45 deg of tip sweep and was significantly quieter at M = 0.8 cruise than the straight bladed propeller. The intermediate 30 deg tip sweep propeller, which was swept for aerodynamic purposes, exhibited noise that was between the other two propellers. Noise trends with varying helical tip Mach number and blade loading were also observed.

Dittmar, J. H.↗

NASA NTRS1989

Near-field acoustic characteristics of a single-rotor propfan

The near-field noise characteristics of the SR-7L, an eight-blade, single-rotor, wing-mounted, tractor propfan have been determined. It is found that the noise is dominated by discrete tones, usually at the first order (and occasionally at the second or third order) of the blade-passage frequency. The highest noise levels were noted at conditions of high tip helical speeds and high dynamic pressures.

Bartel, H. W.↗

NASA NTRSDate not supplied

Acoustic Simulations of Two-stage Fans Behind a Supersonic Inlet

fans↗

NASA NTRS1982

High-intensity sound in air saturated fibrous bulk porous materials

The interaction high-intensity sound with bulk porous materials in porous materials including Kevlar 29 is reported. The nonlinear behavior of the materials was described by dc flow resistivity tests. Then acoustic propagation and reflection were measured and small signal broadband measurements of phase speed and attenuation were carried out. High-intensity tests were made with 1, 2, and 3 kHz tone bursts to measure harmonic generation and extra attenuation of the fundamental. Small signal standing wave tests measured impedence between 0.1 and 3.5 kHz. High level tests with single cycle tone bursts at 1 to 4 kHz show that impedance increases with intensity. A theoretical analysis is presented for high-porosity, rigid-frame, isothermal materials. One dimensional equations of motion are derived and solved by perturbation. The experiments show that there is excess attenuation of the fundamental component and in some cases a close approach to saturation. A separate theoretical model, developed to explain the excess attenuation, yields predictions that are in good agreement with the measurements. Impedance and attenuation at high intensities are modeled.

Kuntz, H. L., II↗

NASA NTRS1990

Aeroacoustic effects of reduced aft tip speed at constant thrust for a model counterrotation turboprop at takeoff conditions

A model high-speed, advanced counterrotation propeller, F7/A7, was tested in the NASA Lewis Research Center's 9- by 15-foot anechoic wind tunnel at simulated takeoff and approach conditions of Mach 0.2. The propeller was operated in a baseline configuration with the forward and aft rotor blade setting angles (36.2deg and 35.4 deg) and forward and aft rotational speeds essentially equal. Two additional configurations were tested with the aft rotor at increased blade setting angles and the rotational speed reduced to achieve overall performance similar to that of the baseline configuration. The aft rotor blade angles were adjusted such that the thrust and power absorption for each rotor remained the same as for the baseline configuration. Acoustic data were taken with an axially translating microphone probe that was attached to the tunnel floor. Concurrent aerodynamic data were taken to define propeller operating conditions. The aft rotor fundamental tone was about 6 dB lower with the 36.2 deg and 38.4 deg blade setting angles, and about 9 dB lower with the 36.2 and 41.4 deg blade setting angles. Predicted noise reductions based on tip speed considerations were 5 and 9.5 dB, respectively, for the two altered blade setting angles.

Woodward, Richard P.↗

NASA NTRS2003

Improving Plating by Use of Intense Acoustic Beams

An improved method of selective plating of metals and possibly other materials involves the use of directed high-intensity acoustic beams. The beams, typically in the ultrasonic frequency range, can be generated by fixed-focus transducers (see figure) or by phased arrays of transducers excited, variously, by continuous waves, tone bursts, or single pulses. The nonlinear effects produced by these beams are used to alter plating processes in ways that are advantageous.

Oeftering, Richard C.↗

NASA NTRS1993

Ground characterization for JAPE

Above-ground propagation modelling at the JAPE (Joint Acoustic Propagation Experiment) site requires a reasonably accurate model for the acoustical properties of the ground. Various models for the JAPE site are offered based on theoretical fits to short range data and to longer range data obtained with random noise and pure tones respectively from a loudspeaker under approximately quiescent isothermal conditions.

Attenborough, Keith↗

NASA NTRS1981

A survey of the acoustical field of a rectangular wall jet

The acoustic field of a rectangular wall jet was surveyed for Reynolds numbers between 2000 and 9200. The study includes measurements of amplitude directivity, acoustic power spectra, and flow visualization. The effects of jet speed, wall length and trailing edge angle were examined. The flow visualization survey suggests an important role of discrete vortex convection in wall tone generation.

Horne, W. C.↗

NASA NTRS1992

Acoustic characteristics and dynamic structural loading of an ASTOVL aircraft in hover

Measurements of surface dynamic loading and freestream acoustics were made for an ASTOVL model in hover, to quantify the effects of elevated temperature on the acoustic field and surface loading. Data were acquired for a many combinations of operating parameters: model height above the ground plane, nozzle pressure ratio, and jet exit stagnation temperature. For many conditions, strong tones were observed, with amplitudes up to 150 dB. The frequencies of the strongest tones were well predicted by a model assuming feedback between the nozzle exit and the ground plane. The model also accounts for many of the variations in frequency observed with changes in model height, nozzle pressure ratio, and jet temperature. Broadband sound pressure levels up to 170 dB were also recorded. The maximum levels occurred at approximately 3 equivalent jet diameters above the ground plane. For the majority of the cases, the increase in noise due to temperature was less than expected based on free jet correlations.

Mitchell, L. K.↗

NASA NTRS1981

Sources, control, and effects of noise from aircraft propellers and rotors

Source noise predictions are compared with measurements for conventional low-speed propellers, for new high speed propellers (propfans), and for a helicopter. Results from a light aircraft demonstration program are described, indicating that about 5-dB reduction of flyover noise can be obtained without significant performance penalty. Sidewall design studies are described for interior noise control in light general aviation aircraft and in large transports using propfan propulsion. The weight of the added acoustic treatment is estimated and tradeoffs between weight and noise reduction are discussed. A laboratory study of passenger response to combined broadband and tonal propeller like noise is described. Subject discomfort ratings of combined tone broadband noises are compared with ratings of broadband (boundary layer) noise alone, and the relative importance of the propeller tones is examined.

Mixson, J. S.↗

NASA NTRS1990

The prediction of STOVL noise - Current semiempirical methods and comparisons with jet noise data

The prediction of conventional or STOVL turbojet propulsion system-using aircraft noise is presently undertaken by means of a method incorporating empirical models for jet-mixing noise, engine core noise, and broadband shock noise. The free-jet noise is coupled with a novel empirical equation for ground-interaction noise generated by a vertically impinging jet, and supplemented with the out-of-ground-effect free-jet acoustic directivity pattern of a Harrier-type vectoring nozzle installation. This acoustic-prediction method yielded reasonable agreement with measured far-field Harrier noise during hover in and out of ground effect. Unlike small-scale studies of jet impingement on a hard surface, no tones were found in the present Harrier nozzle spectra.

Soderman, Paul T.↗

DOE OSTI2022

Predicting Local Material Thickness from Steady-State Ultrasonic Wavefield Measurements Using a Convolutional Neural Network

Acoustic steady-state excitation spatial spectroscopy (ASSESS) is a full-field, ultrasonic non-destructive evaluation (NDE) technique used to locate and characterize defects in plate-like structures. ASSESS generates a steady-state, single-tone ultrasonic excitation in a structure and a scanning laser Doppler Vibrometer (LDV) measures the resulting full-field surface velocity response. Traditional processing techniques for ASSESS data rely on wavenumber domain analysis. This paper presents the alternative use of a convolutional neural network (CNN), trained using simulated ASSESS data, to predict the local plate thickness at every pixel in the wavefield measurement directly. The defect detection accuracy of CNN-based thickness predictions are shown to improve for defects of greater size, and for defects with higher thickness reductions. The CNN demonstrates the ability to predict thickness accurately in regions where Lamb wave dispersion relations are complex or unknown, such as near the boundaries of a test specimen, so long as the CNN is trained on data that accounts for these regions. The CNN also shows generalizability to ASSESS experimental data, despite an entirely simulated training dataset.

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