Parthiv N. Shah

High-Resolution Continuous Scan Acoustical Holography Applied to High-Speed Jet Noise

Acoustical holography is a technique that has been used extensively to characterize the surface velocities and acoustic pressures of coherently vibrating structures such as engines and gearboxes. For aeroacoustic noise sources such as jets with multiple distributed, partially correlated source mechanisms, scan-based techniques using reference and response transducers and singular value decomposition (SVD) have been applied to acoustical holography to decompose these noise sources into partial fields. Partial fields can reconstruct an overall sound field and also provide a near-field representation of the source that may aid in understanding the physics of jet noise. This paper presents model-scale experimental evidence that advanced signal processing techniques can enable the generation of high resolution acoustical holograms of the hydrodynamic and acoustic near-fields of high-speed jets with a reasonable number of microphones. Two key innovations discussed in the paper are: (1) the construction of partial fields using continuously moving response (i.e., “hologram”) microphones and spatially fixed reference microphones with Chebyshev-spaced sampling points to achieve sufficient averaging, and (2) the computation of transfer functions using a method of canonical coherences. This method uses a tensorial formulation of coherence analysis to filter off all signal components at or below the noise floor. The current experimental evidence is to be used in the development of a full-scale acoustical holography system to be demonstrated on a military jet aircraft.

A Novel Turbomachinery Air-Brake Concept for Quiet Aircraft

A novel air-brake concept for next-generation, low-noise civil aircraft is introduced. Deployment of such devices in clean airframe configuration can potentially reduce aircraft source noise and noise propagation to the ground. The generation of swirling outflow from a duct, such as an aircraft engine, is demonstrated to have high drag and low noise. The simplest configuration is a ram pressure-driven duct with stationary swirl vanes, a so-called swirl tube. A detailed aerodynamic design is performed using first principles based modeling and high-fidelity numerical simulations. The swirl-drag-noise relationship is guantified through scale-model aerodynamic and aeroacoustic wind tunnel tests. The maximum measured stable flow drag coefficient is 0.83 at exit swirl angles close to 50 deg. The acoustic signature, extrapolated to full-scale, is found to be well below the background noise of a well-populated area. Vortex breakdown is found to be the aerodynamically and acoustically limiting phenomenon, generating a white-noise signature that is about 15 dB louder than a stable swirling flow.

Engine Air-Brakes for Quiet Air Transport

A key operational aspect of future aircraft may be a slow and steep, “clean-airframe” approach trajectory, both to reduce source noise and to increase propagation attenuation on approach. In previous work, we have demonstrated that a ram air driven device that provides swirling outflow from a duct, a so-called swirl tube, offers the potential for quiet drag to enable such trajectories. The idea is that a steady streamwise vortex at the duct outlet is relatively quiet and is supported by a radial pressure gradient that results in pressure drag. The noise-drag relationship of the swirl tube may be contrasted with conventional high-drag devices such as flaps, slats, and landing gear, that are suggested to have a strong correlation between drag and noise. The present paper provides a quantification of the benefits and a description of the challenges associated with implementation and integration of such devices into an airframe/propulsion system. A comparison between the ram air driven swirl tube and an upstream fan stage driven swirl tube, a so-called engine air-brake, suggests that the increased mass flow ingested by a pumping fan stage upstream of a set of ducted swirl vanes increases effective drag capability. A full-scale ram air driven swirl tube is found to have a noise signature well below other sources on a typical airframe/propulsion system. Fan stage pumping is expected to increase the engine air-brake noise to a level that must be quantified in future experiments and compared to the existing sources on the aircraft to assess system level feasibility. Simple scaling law estimates suggest that such devices may still maintain noise signatures below other airframe/propulsion system sources when integrated into ultra-high bypass ratio turbofans with low approach idle pressure ratios. We envision deployable swirl vanes located in the bypass or mixer section of such engines that create effective drag coefficients of about 3 based on throughflow area- enough to steepen a conventional three degree approach glideslope to about six degrees, with a 6 dB potential overall noise reduction. This device concept may be compatible with a thrust reverser system, whereby the deployed vanes can completely close on landing to serve as blocker doors.

Validation of Methods to Predict Vibration of a Panel in the Near Field of a Hot Supersonic Rocket Plume

This paper describes the measurement and analysis of surface fluctuating pressure level (FPL) data and vibration data from a plume impingement aero-acoustic and vibration (PIAAV) test to validate NASA’s physics-based modeling methods for prediction of panel vibration in the near field of a hot supersonic rocket plume. For this test – reported more fully in a companion paper by Osterholt & Knox at 26 th Aerospace Testing Seminar, 2011 the flexible panel was located 2.4 nozzle diameters from the plume centerline and 4.3 nozzle diameters downstream from the nozzle exit. The FPL loading is analyzed in terms of its auto spectrum, its cross spectrum, its spatial correlation parameters and its statistical properties. The panel vibration data is used to estimate the in-situ damping under plume FPL loading conditions and to validate both finite element analysis (FEA) and statistical energy analysis (SEA) methods for prediction of panel response. An assessment is also made of the effects of non-linearity in the panel elasticity.

Aeroacoustics of Swirling Exhaust Flows in High Bypass Ratio Turbofan Nozzles for Drag Management Applications

The aeroacoustics of swirling exhaust flows in high bypass ratio turbofan nozzles are assessed for aircraft approach drag management applications. Aerodynamic designs of swirl vane hardware were tested in an anechoic jet test facility to quantify their thrust reduction (or equivalent drag generation) capability and noise. Simple vaned-disk (visk) hardware using a row of periodically spaced swirl vanes was used to study flow interactions and noise generation mechanisms associated with swirl generated in the bypass duct. The swirling bypass exhaust stream creates suction on the core stream, which elevates the core flow and decreases the bypass ratio by up to 40 percent for the geometries tested. Swirling the bypass exhaust enhances mixing, and phased array measurements suggest that the apparent noise source moves upstream and radially inward toward the inner shear layer, and radiates preferentially in the direction of swirl. The presence of a pylon with a deflected trailing edge and asymmetric swirl vanes is found to produce a coherent swirling outflow with slightly greater noise generation than its idealized counterpart. The relationship between gross thrust reduction and noise generation is quantified and applied to a simple steep approach noise simulation at fixed aircraft speed. Results suggest a noise reduction potential of up to 3.1 dB, tonecorrected peak perceived noise level (PNLT) and 1.8 dB effective perceived noise level (EPNL) for a single-aisle, twin engine aircraft.

A High-Resolution, Continuous-Scan Acoustic Measurement Method for Turbofan Engine Applications

Detailed mapping of the sound field produced by a modern turbofan engine, with its multitude of overlapping noise sources, often requires a large number of microphones to properly resolve the directivity patterns of the constituent tonal and broadband components. This is especially true at high frequencies where the acoustic wavelength is short, or when shielding, scattering, and reflection of the sound field may be present due to installation effects. This paper presents a novel method for measuring the harmonic and broadband content of complex noncompact noise sources using continuously moving (referred to here as continuous-scan) microphones in conjunction with a state-of-the-art phase-referencing technique. Because the microphones are moving through the sound field produced by the noise sources, they effectively provide infinite spatial resolution of the sound directivity over the scan path. In this method, harmonic (i.e., shaft-coherent) content at the integer multiples of the instantaneous shaft rotational frequency is first extracted from the time signal using a tachometer signal and the Vold-Kalman filter. The residual broadband signal is then filtered in the time domain in fractional octave bands. The broadband spectra of the signals from the moving microphones are then computed at arbitrary positions along their scan paths using weighted averages (based on Chebyshev polynomial zero-crossings) and the assumption of a complex envelope that varies slowly over a spatial scale whose lower bound is set by the acoustic wavenumber. A benefit of this method is that the decomposition of the total measured sound field into a stochastic superposition of components preserves a meaningful phase definition for each “partial field” associated with a given shaft order. This preservation of phase data enables the forward or backward projection of each of these partial fields using acoustical holography. The benefits of the continuous-scan method are demonstrated using acoustic data acquired for a 22-inch scale-model fan stage run at the NASA Glenn Research Center’s 9-foot by 15-foot wind tunnel. Two key outcomes of the work include (1) significant improvement in the spatial resolution of the measured sound field and (2) reduction in the overall data acquisition time. Additionally, the methods described here lead to new opportunities for noise source diagnostics and visualization.Copyright

On the computation of farfield cross-spectra and coherences from reduced parameter models of high speed jet noise

Reduced parameter models of jet noise mechanisms serve as computational vehicles to estimate sound pressure and directivies at arbitrary locations in the farfield. The customary formulations have been successful at predicting autospectra and directivities, but the calculation of crossspectra and coherences has not been attempted. The authors will present a procedure for calculating crossspectra and coherences from the simple source model, i.e., an equivalent monopole density over a volume enclosing the jet noise sources. It will be shown that realistic coherences and crossspectra are only well defined when several mutually incoherent noise sources are being considered, and both convergence and spatial aliasing phenomena will be defined and investigated.

Direct Spectral Estimation Method for Continuous Scan Beamforming

The paper presents an initial methodology for the direct estimation of the spatio-spectral distribution of an acoustic source from microphone measurements that comprise fixed and continuously scanning sensors. The non-stationarity introduced by the sensor motion is quantified by means of the Wigner-Ville spectrum. Its strongest effect is on the correlations of the sensor signals. Suppression of the non-stationarity in the signal processing involves division of the signals into blocks and application of a frequency-dependent window within each block. The direct estimation approach entails the inversion of an integral that connects the source distribution to the measured coherence of the acoustic field. A Bayesian-estimation approach is developed that allows for efficient inversion of the integral and performs similarly to the much costlier conjugate gradient method. The methodology is applied to acoustic fields emitted by impinging jets approximating a point source and an overexpanded supersonic jet. The measurement setup comprises one continuously scanning microphone and a number of fixed microphones, all arranged on a linear array. Comparisons are made between array configurations with fixed microphones only and with the scanning microphone, all having the same sensor count. The noise source maps with the scanning microphone have improved spatial resolution and suppressed sidelobes. The ability of the continuous scan paradigm to provide high-definition noise source maps with low sensor count is demonstrated.

FULL-SCALE TURBOFAN DEMONSTRATION OF A DEPLOYABLE ENGINE AIR-BRAKE FOR DRAG MANAGEMENT APPLICATIONS

United States. National Aeronautics and Space Administration. Small Business Innovation Program. Phase II (contract NNX13CC78C)

Measurement and propagation of supersonic aeroacoustic noise sources using continuous scanning measurement technologies and the fast multipole boundary element method

ATA Engineering has developed a technique that uses a continuous scanning robot to take high-resolution measurements of supersonic jet plumes. The jet noise was modeled using a reduced-order model and propagated to far field microphone locations in the free-field. It is shown that the pressure at these microphones was successfully reconstructed across a range of frequencies. The capability to make predictions when scattering surfaces are present is also demonstrated using the fast multipole boundary element method in VA One. This work was originally designed for supersonic jets, but it can also be used for static firing tests of launch vehicle engines. The measured data could then be used for analytic predictions of the liftoff environment.