Session U11: Acoustics: Aero & Hydro

Models of coalescing Mach waves based on experiment and simulation

Turbulent structures in supersonic jet flows generate acoustic Mach waves, which have the potential to intersect and coalesce. The coalescence process has been proposed as a significant contributor to acoustic waveform steepening in the near field of a jet and consequently to the noise referred to as crackle [Baars et al., AIAA (2013); Fiévet et al., AIAA (2016)]. Simulations of intersecting waveforms using the Khokhlov-Zabolotskaya-Kuznetzov nonlinear parabolic wave equation demonstrate that coalescence does increase steepening, dependent on intersection angle, waveform duration, and geometrical spreading [Willis et al., AIAA (2022)]. To observe this experimentally, a spark source is used to generate high-amplitude waveforms inside an enclosure with side-by-side holes, thus producing intersecting waveforms. Schlieren images of the coalescing waves are analyzed using Proper Orthogonal Decomposition (POD) in translating coordinates to identify modal behavior. By isolating coalescence events in schlieren images of Mach waves emitted from a lab-scale Mach 3 jet flow, modal behavior can be compared between the simulation, simplified experiment, and actual turbulence-generated waveforms.   

Modeling the nonlinear aeroacoustic response of a harmonically forced side branch aperture under turbulent grazing flow

Hydrodynamic modes in the turbulent mixing layer over a cavity can constructively interact with the acoustic modes of that cavity and lead to aeroacoustic instabilities. The resulting limit cycles can cause undesired structural vibrations or noise pollution in many industrial applications. To further the predictive understanding of this phenomenon, we propose two physics-based models which describe the nonlinear aeroacoustic response of a side branch aperture under harmonic forcing with variable acoustic pressure forcing amplitude p_a. One model is based on Howe's classic vortex sheet formulation, and the other on an assumed vertical velocity profile in the side branch aperture. These models are validated against experimental data. Particle image velocimetry (PIV) was performed to quantify the turbulent and coherent fluctuations of the shear layer under increasing p_a. The specific acoustic impedance Z of the aperture was acquired over a range of frequencies f for different bulk flow velocities U and acoustic pressure forcing amplitudes p_a. We show that, once the handful of parameters in the two models for Z have been calibrated using experimental data at a given condition, it is possible to make robust analytical predictions of this impedance over a broad range of f, U and p_a. In particular, the models allow prediction of a necessary condition for instability, implied by negative values of the acoustic resistance Re(Z). Furthermore, we demonstrate that the models are able to describe the nonlinear saturation of the aeroacoustic response caused by alteration of the mean flow at large forcing amplitudes, which was recently reported in literature. This effect stabilizes the coupling between the side branch opening and the acoustic field in the cavity, and its quantitative description may be of value for control of aeroacoustic instabilities.   

A study of the interaction and coupling of wavepackets in supersonic twin jets using plane-marching PSE

Supersonic twin jets are a common configuration encountered in the propulsion systems of high-speed flight vehicles and rocket launchers. The interaction between the fluctuating pressure fields of the jets leads to their coupling and results in more complex physical mechanisms than those of a single jet. In this work, we apply plane-marching Parabolized Stability Equations to model the large-scale turbulent structures, often referred to as wavepackets, responsible for the dominant part of the noise in twin-jet configurations. By analyzing several frequencies for different jet spacings, the following conclusions are reached:

First, the jet-jet interaction significantly modifies the dynamics of the wavepackets, altering their amplification and convection speed. For the non-toroidal modes, the coupling favors flapping motions of the jet plumes over the helical motions that dominate in single jets. Second, the Mach-wave noise radiation associated with the wavepackets is also altered beyond the linear superposition of emission by two incoherent jets. An overall reduction of the acoustic emission is found for most modes and azimuthal angles, while the pressure radiated in the direction perpendicular to the jet-containing plane is increased up to 50% for some oscillation modes.   

Optimization of microjet control for supersonic impinging jet

Supersonic impinging jets tend to experience resonance and generate a highly unsteady flowfield. In Short Take-Off and Vertical Landing (STOVL) applications, these highly unsteady jets produce loud noise and cause severe lift-loss. Therefore, a control method to reduce the jet resonance is required to improve the safety and efficiency of STOVL aircraft. Microjet injection into a round impinging jet is studied in this work as a method for reducing impinging jet resonance where the effectiveness of microjet control varies with impingement distance. In the present study, optimization of microjet control on a supersonic round jet (Mach 1.5) is performed by using a Genetic Algorithm to minimize jet noise at selected impingement distances. Sixteen internal microjets in the diverging section of the supersonic nozzle and sixteen external microjets at the lip of the nozzle are installed to reduce jet resonance, and an automated experimental optimization procedure is implemented to find the best actuator locations to reduce jet noise. The effectiveness of the optimization is examined with the help of schlieren flow visualization and acoustics measurements.   

Array Processing of Porous Trailing Edge Aeroacoustics

This presentation concerns characterization of the sound generated by a vortex ring passing near the edge of a non-compact porous plate, using a microphone array.  These measurements were performed to test theoretical predictions of how source sound power, waveform, and acoustic directivity all change with plate porosity.  The measurements were performed in the ARL Penn State anechoic chamber, for four plate porosities, and for vortex ring convection speeds ranging from 39 m/s to 86 m/s.  Vortex ring motion, including convection speed, was estimated from high-speed Schlieren imaging.  A circular 12 microphone array, centered on the edge, measured the far-field radiated sound, synchronously with the vortex ring motion video capture.  Using principal component analysis, dimensional reduction of the measured microphone signals was used to estimate the aeroacoustic source waveform, sound power scaling and directivity.

    

Revisiting the frozen gust assumption through the aeroacoustic scattering of wavepackets by a semi-infinite plate

One of the main sources of aerodynamic noise is the interaction of boundary-layer turbulence with a wing trailing edge. The aeroacoustic theory used to predict this trailing-edge noise is often based upon the `frozen gust' assumption, which posits that boundary-layer vorticity is unaffected by the flow field local to the trailing edge. This classical theory has been successful for straight edges, but the agreement between predictive theory and computational or experimental results is often poor for serrated or other geometrical variations to the trailing edge. Several computational studies have shown that trailing-edge serrations affect the vorticity field around the edge, suggesting that the frozen gust assumption might not be valid there. In the current research we develop an analytical model to predict the noise scattered by a traveling wavepacket passing near the edge of a semi-infinite flat plate. The solution is derived in the time domain for a wavepacket of either constant or spatially varying wavenumber, which allows us to examine the effect of relaxing the frozen gust assumption on the prediction of trailing-edge noise. Our results shed light on the role that spatial variations in the vorticity field local to the trailing edge have on the far-field scattered noise.   

Aeroacoustic Simulations of a Landing Gear Benchmark Problem using Wall-Modeled Large-Eddy Simulations

The capability of predicting airframe noise using the wall-modeled large-eddy simulation (WMLES) with an immersed boundary Cartesian approach in the Launch, Ascent, and Vehicle Aerodynamics (LAVA) framework is analyzed. WMLESs of the flow past the LAGOON nose landing gear are conducted using Cartesian octree meshes with different resolutions to facilitate the grid sensitivity analysis of the near-field and far-field numerical noise predictions. With the effects of the tripping devices in the experiments modeled, the near-field mean and unsteady acoustic results from the high resolution WMLESs show good agreement with the experimental results. The spectra of far-field nosie radiated with the use of Ffowcs Williams-Hawkings acoustic analogy also have reasonable comparison with the experimental data in the low and medium frequency ranges.   

Not Participating

Ships are the largest source underwater radiated noise, which threatens marine ecosystems. Propellers are typically the dominant source of the radiated noise. Thus, accurate predictive tools for noise emissions from marine propellers are of primary importance for the development of effective mitigation strategies. The numerical prediction of both cavitation- and vortex-induced noise requires detailed simulation of the unsteady turbulent structures in the wake, including the fluctuating component of pressure. Simultaneously, the solution needs to account for the interaction between the hull, the wake, and the propeller. Balancing the range of scales in simulations presents a formidable challenge. We present a methodology that uses one-way-coupled solution to increase fidelity of the simulation and to decrease the domain size, beginning with Reynolds-averaged simulation of the flow around an entire ship, and concluding with a detached eddy simulation of the propeller wake. Our methodology is sufficient to resolve the shed vortices at acoustically-relevant time scales using only the geometry and the operating conditions as inputs, while maintaining computational feasibility. The methodology is validated against fluctuating pressure measurements performed on a model-scale vessel   

Sounds of a drop impact on a liquid pool

We investigate the impact of a drop into a deep liquid pool, with focus on bubble entrapment and sound emission during the crater collapse¹. This process is related to the underwater sound generated by rain. Herein we study sound emission from such bubbles, when the drop and pool are of same and different immiscible liquids. The frame rates as high as 5 million fps are required to accurately capture the rapid shape evolution. The intricate crater shapes, which include a cascade of dimples, induce interesting scenarios like double bubbles pinch-offs. We synchronize the phase of the bubble oscillation, recorded by high-speed camera, to the acoustic signal from hydrophone and confirm that the onset of sound generation is at the instant of neck pinch-off. The tiny bubble from the initial drop contact with the liquid pool will also be induced to oscillate by the oscillation of the large bubble from the air cylinder pinch-off. The dynamic process of bubble oscillation is analyzed from the audio signal and we find good agreement with the Minnaert frequency when using the fluid properties of the drop. This applies reasonably well even for very small bubbles where the frequency ∼ 100 kHz.

¹Pumphrey H. C. & Elmore P. A., J. Fluid Mech. 220, 539 (1990).

    

Generation of sound waves by nonlinearly evolving ring-mode coherent structures on a turbulent subsonic circular jet: a comparative study of two mechanisms

Coherent structures (CS) are present on a subsonic turbulent jet and are known to constitute an important source of jet noise. With these structures being treated as wavepackets of instability modes supported by the mean flow, two acoustic radiation mechanisms have been identified. The first, referred to as generalised Mach-wave radiation (GMWR), is associated with the fact that a CS undergoing axial amplification and attenuation consists of supersonic components in its spectral tail, which radiate to the far field as sound waves. On the other hand, the nonlinear interaction of the CS generates a temporally and spatially modulated mean-flow distortion, which emits low-frequency sound waves. This second mechanism is referred to as envelope radiation (ER). We investigates, in a common mathematical setting, these two radiation processes for nonlinearly evolving CS of ring modes, which are described by strongly nonlinear critical-layer theory. Nonlinear effect is found to induce jittering, which enhances the GMWR significantly but suppresses ER slightly. The two mechanisms are both viable for CS of moderate amplitude, with GMWR and ER being dominant in the near axis and sideline regions respectively. The acoustic field due to the GMWR is in qualitative agreement with measurements.