Session E07: Acoustics: Aeroacoustics

Supersonic Jet Noise Reduction using Micro Vortex Generators

A novel technology for jet noise reduction using micro vortex generators is developed and tested

on a laboratory scale model of a supersonic faceted nozzle. Nozzle experiments were conducted at UC Acoustic Test Facility along with NRL simulations using LES, for conditions relevant to tactical aircraft take off conditions in the overexpanded regime. The noise reduction technologies involve introducing streamwise vortices in supersonic nozzle flow to enhance the shear layer mixing and thus affecting the jet’s coherent structures. The vortex generators employed are fin type vortex generators that have the added benefit of altering the shock cell structures to

alter the broad band shock associated Noise. A set of design parameters affecting the level of noise reduction was systematically explored. The high-pressure gradient across the mvgs induces the formation of two counter rotating vortices that caused upwash between each pair and downwash between two neighboring pairs. Nozzle lip separation was delayed due to momentum transfer from the nozzle flow to the boundary layer. Far acoustic field measurement showed substantial reduction of the shock associated noise and mixing noise. The impact of different parameters of the MVG design will be shown.   

Screech feedback reinforcement by coupling of twin rectangular jets

We use high-fidelity LES data to investigate aeroacoustic coupling in twin supersonic rectangular jets with an aspect ratio of 2. The LES data was validated against the experiments conducted at University of Cincinnati. Twin geometry allows two different screech feedback loops. For each jet, one loop involves with upstream disturbances originating from itself (self-excitation), which can be internal or external modes. The other feedback loop is completed by disturbances arriving from the other jet (cross-excitation). These paths are extracted from the leading SPOD modes at the fundamental screech frequency after a wavenumber decomposition in the streamwise direction. Assuming the maximum receptivity at the nozzle exit, the cross-correlation analysis identifies points of return where upstream disturbances originating from such a point satisfy the resonance condition. The current analysis suggests that, while both internal and external modes play a role in the self-excitation feedback loop, some of the identified points of return for the cross-excitation are synchronized with the internal mode self-excitation. Such synchronization of the self- and cross-excitation modes disappears at non-resonating frequencies.   

Internal Acoustic Mode in Rectangular Jet Screech

Experimentally validated high-fidelity LES of rectangular under-expanded jets are used to study the feedback involved in screech generation. The leading SPOD modes at the screech fundamental frequency are extracted from pressure and transverse velocity fluctuations in the jet minor-axis plane, showing both the coherent structures inside the jet plume and the acoustic waves outside the jet. Spatial cross correlation with the SPOD data shows that upstream-travelling external acoustic waves and the internal acoustic mode coexist at the resonant and neighboring frequencies. However, the spatial amplification relative to nozzle exit reference for the internal mode is much larger at the resonant frequency. On the other hand, the spatial amplitude of the external mode is similar at resonant and non-resonant frequencies. The cross-correlation analysis is also applied to two other screech cases at different NPR values and similar results are obtained. The local mode shape of the internal mode agrees well with the theoretical model by Tam & Norum (1992).   

Reducing jet noise using resolvent-based sensitivities

To increase community acceptance of forthcoming supersonic civilian jet transports, we develop a resolvent-based technique to reduce the take-off radiated noise from their low-bypass-ratio turbofan engines with near-sonic exit velocities.  Linear resolvents are shown to be sensitive to the jet exhaust properties and to the exhaust nozzle design.  A method to directly connect changes in the resolvent to changes in the exhaust properties and the nozzle design and is developed and demonstrated on dual stream near-sonic jets that have been tested experimentally.  Both cold and heated jets are considered.  The presentation will include the details of the resolvent-based method, its application to RANS-predicted jet mean flows, noise reduction estimates compared to experimental measurements, and conclude with integration details into a nozzle design optimization process.   

Aeroacoustics of porous trailing edges

This presentation concerns the sound generated by a vortex ring passing near the edge of a non-compact porous plate. Theoretical predictions of the radiated sound level and directivity are compared against measurements performed in the ARL Penn State anechoic chamber for a series of plates, each with a different porosity. A shock tube produces the rectilinear motion of the vortex rings, whose speeds range from 39 m/s to 86 m/s, as estimated from high-speed Schlieren imaging. A ring of twelve microphones centered at the plate edge measures the far-field sound and its directivity in synchrony with video capture. These measurements are used to estimate a scaling law for the radiated acoustic power $\Pi \sim U^n L^m$ on the ring speed $U$ and minimum ring-to-edge distance $L$. Predicted variations in these exponents $n$ and $m$, in far-field sound directivity, and in source waveforms on changes in porosity compare favorably to the experimental results.   

Experimental study of noise generated by porous trailing-edges on owl-inspired aircraft

A modular porous edge is designed for a radio-controlled aircraft and is tested to assess its effectiveness as a passive noise control strategy on flying vehicles. The porous edge draws inspiration from one of the plumage adaptations of owl, the tattered fringe along the back edge of the wing, which is thought to mitigate turbulence sound scattered by the trailing edge. The testing platform is designed to measure surface pressure fluctuations from interchangeable porous and non-porous surfaces, where the wing self-noise is measured by a microphone on the fuselage, as would be perceived by an owl in flight. Analysis of the effects of wing porosity on the local turbulent flow over the wing and on the radiated aerodynamic noise will be presented together in comparison with existing theoretical predictions for porous edge noise reduction.   

Analytical Green's function for the acoustic scattering by a flat plate with serrated edges

Serrations have attracted considerable attention in recent years owing to its capability to reduce trailing-edge and leading-edge noise in applications such as wind turbines and various rotating fans. Amiet's approach of modelling turbulence as gusts is widely used in recent analytical attempts. The approach of using Green's function, apart from an earlier attempt of Howe, is still rare, but it is very useful because it allows a direct comparison with experiments using laser-induced monopoles and offers the extensibility of including more realistic effects such as non-zero angle of attack. In this paper, we develop an analytical Green’s function for the serrated edge scattering problem using the Wiener-Hopf technique. A closed-form analytical Green’s function is obtained for sawtooth serrations and compared with the canonical Green’s function for straight edges. The analytical Green's function is verified using the finite element method. Both noise reduction spectra and directivity patterns are studied as a function of source position. Physical mechanism of sound reduction is discussed. This analytical Green’s function can be used to developed more accurate trailing-edge and leading-edge noise models and used as a benchmark solution for various numerical simulations.   

Sound generation by a rotor in a thick axisymmetric turbulent boundary layer

The sound generation by a rotor immersed in a thick turbulent boundary layer at the tail end of an axisymmetric body of revolution is investigated using large-eddy simulation and the Ffowcs Williams-Hawkings equation. Two rotor advance ratios corresponding to nominally zero-thrust and thrusting conditions are considered. The computed sound pressure spectra exhibit reasonable agreement with the experimental measurements at Virginia Tech (Hickling et al., AIAA-2019-2571) over a wide range of frequencies. The spectra show broadband noise with haystacking peaks caused by multiple blades cutting through the same turbulence structure. Two-point and space-time velocity correlations are examined to elucidate the evolution of the turbulence structures in the adverse-pressure-gradient boundary layer and their interaction with rotor blades. The strength of blade sectional unsteady-loading dipoles increases with the local chord length, turbulence intensity and rotational speed, and is largest near the location of maximum chord length. Significant blade-to-blade coherence is found and related to the boundary-layer structures and haystacking acoustic peaks.   

Using a direct-adjoint framework for the analysis of turbomachinery aeroacoustics

New efficiency and emission standards in contemporary aero-engine design are increasingly introducing the need for the understanding of fundamental flow processes and their optimisation. Here, a newly developed high-order, time-domain, direct-adjoint framework for the analysis and optimisation of turbomachinery aeroacoustics will be presented, and two classes of representative problems considered. First, self-excited instabilities and the resulting acoustic fields on an isolated compressor bladerow at off-design conditions are characterised by means of a direct-adjoint meanflow global stability analysis, with wavemaker regions calculated to identify self-reinforcing feedback mechanisms that perpetuate these features. An extension to multiple identical blade passages is then demonstrated by reinterpreting the periodic meanflow within each passage as a block-circulant system. A Bloch-wave decomposition of this system yields independent and computationally tractable subproblems, by means of which new flow structures are identified. To conclude, a time-domain, unsteady adjoint sliding plane treatment is introduced, and its utility highlighted, by considering the unsteady rotor-stator interaction problem from the adjoint sensitivity perspective.   

A physical model to predict indirect noise in multicomponent non-isentropic nozzle flows

Aircraft engine manufacturers are striving to make cleaner and less noisy engines. In aero-engine combustors, incomplete mixing and air cooling give rise to inhomogeneities in temperature and composition. These flow inhomogeneities accelerate downstream of the combustor in the nozzle guide vane and generate indirect noise. In this work, a quasi-one-dimensional model is mathematically developed and analyzed numerically to predict the indirect noise produced in non-isentropic nozzles. The physics-based model is compared with existing non-isentropic models, which make use of semi-empirical parameters. The results compare favorably with experimental data of the literature. It is found that friction significantly affects the gain/phase of the reflected and transmitted waves for compact and non-compact nozzles in the subsonic-choked regime. Furthermore, the effect of friction on thermoacoustics is studied, highlighting the sensitivity of thermoacoustic oscillations to the dissipation in the nozzle. The study opens new possibilities for the accurate prediction of indirect noise from nozzles.