Session E03: Acoustics: Aeroacoustics (3:10pm - 3:55pm CST)

Aerofoil wall pressure fluctuations in the presence of a serrated trailing edge

A cambered Joukowski aerofoil with 12\% thickness is studied in a wall-resolved large-eddy simulation at $Re_{\infty}=500000$, $M=0.2$ and $6^{\circ}$ incidence. The main objective is to investigate the behaviour of the pressure fluctuations on the aerofoil surface and its impact on the broadband noise reduction while using a serrated trailing edge geometry. A sawtooth serrated trailing edge with aspect ratio 1.0 is compared to a reference baseline trailing edge. The present investigation delivers an in-depth spectral analysis and shows how both the magnitude and the phase of the surface pressure fluctuations play a role in the noise reduction mechanism. Furthermore, the study also shows how the wall pressure fluctuations information lying upstream of the trailing edge has a significant impact in the far-field sound prediction and how this information can be useful to elaborate a serrated trailing edge wall pressure fluctuations model.   

Aerodynamic noise generation in boundary layer flows.

Based on the linearized Euler equation and the normal-mode approach, the Pridmore-Brown equation for acoustics of parallel shear flows is solved analytically for a boundary layer flow with an exponential velocity profile. The general solution to this equation is given in terms of the confluent Heun function (CHF). Together with the boundary conditions of vanishing disturbances at infinity, and zero wall-normal velocity, the boundary value problem is converted to an algebraic eigenvalue problem. The eigenvalues obtained by the CHF-based solution does not involve any spurious numerical modes, which are always difficult to distinguish from physical modes. In the temporal stability analysis, the first three acoustic modes are computed as functions of Mach numbers and wavenumbers, where the first mode is always the most unstable one of all, and the present critical Mach number of acoustic instability is close to 2.2. The unstable modes reveal a mechanism of resonance, where waves spatially grow towards to wall. Defining an acoustic boundary layer thickness, which essentially quantifies how far eigenfunctions reach into the area far from the boundary layer, we find that sound is especially for large Mach numbers beyond 2 and large wave numbers are widely audible.   

Extraction of Large-Scale Coherent Structures Associated with Broadband Shock-Associated Noise in Supersonic Jets

Broadband shock-associated noise (BBSAN) is analyzed with a large-eddy simulation (LES) of two off-design supersonic jets. The simulation results are validated with experiments. An acoustic analogy based on the decomposition of the Navier-Stokes equations is used to isolate the source of BBSAN. The BBSAN source term is the scalar product of anisotropic velocities and mean pressure gradients. The source term was validated in a previous work using a statistical method. A deterministic approach is adopted in the present work, which uses an LES database to obtain the BBSAN spectra. Proper orthogonal decomposition (POD) is applied to the scalar source field to extract the large-scale coherent structures associated with BBSAN. The eigenvalue spectra show low-rank behavior at the dominant BBSAN frequencies. Directivity plots of the BBSAN intensity are generated from different POD modes at the peak frequency. The leading modes of the source term show strong amplification in the observer direction. The key features of the BBSAN spectra are preserved when only a small fraction of POD modes is used. Noise source contour plots calculated with these POD modes reveal that most noise sources are distributed at the shock turbulence interactions and within the potential core.   

Modelling the trailing edge noise using optimisation techniques within a rapid distortion theory framework

The sound radiated by a jet flow interacting with a flat plate has received much recent attention in Aero-acoustics research owing to its canonical representation of jet installation effects. Rapid-distortion theory (RDT) uses linear theory to determine both this sound and the hydrodynamic field that generates it. When the plate is positioned parallel to the level curves of the streamwise mean flow , the theory shows that the unsteady flow is related to an upstream convected quantity (the input) that is an arbitrary function of its arguments. This quantity is then related to a measureable turbulence correlation function ($R_{22}$) to determine the far field radiated sound (the response). \\ We extend this model by including anti-correlation effects in the function form of $R_{22}$ . There are several parameters within the model which need to be selected in order to find the optimum acoustic spectrum across acoustic Mach number and far-field angle. Here, we discuss various global optimisation techniques used to obtain the parameters in acoustic spectrum formula. This was achieved by either optimising $R_{22}$ or optimising the acoustic model. We discuss these approaches and show how they yield very accurate sound predictions across the Mach number and observation angle regime.   

Infrasound Measurements of Tornadoes and Other Severe Storm Events at Close Range

Recent experimental evidence suggests that, during tornadogenesis and through the life a tornado, acoustic waves at frequencies below human hearing (termed infrasound) are produced. To date, data required to identify the fluid mechanism responsible for this infrasound has been limited, gathered by large fixed installations. To expand the number of samples and enable close-range measurements, which would mitigate the measurement uncertainty associated with long distance atmospheric propagation, the design and deployment of a mobile Ground-based Local Infrasound Data Acquisition (GLINDA) system was completed at Oklahoma State University. GLINDA has been deployed alongside Oklahoma-based media storm chasers since May 2020 and has already returned data over multiple severe weather events, including tornadic measurements acquired with GLINDA on 22 May 2020 in Lakin, Kansas. This presentation will cover system design, measurement, processing and integration considerations, deployment of the mobile infrasound package, and preliminary results from a selection of severe weather events.   

Comparison between microscopic and macroscopic models of porous materials in aerodynamic sound generated from a flow past a cylinder

Covering rigid bodies in high-speed flows with porous materials is one of the promising methods for reduction of aeroacoustic noise. In order to optimize the shape and porosity of the porous materials, numerical methods that can accurately predict the aeroacoustic sound generated in a flow involving rigid bodies and porous materials are expected. We study the aeroacoustic sound generated from a flow past a cylinder which is covered by porous materials. Two models are compared: (i) the microscopic model in which the porous materials are modeled by collection of small cylinders and (ii) the macroscopic model in which continuous modeling like Darcy's law are employed. The compressible Navier-Stokes equations are solved numerically with high-precision methods to obtain the sound pressure directly. In both models the acoustic power is reduced to $0.1\%$ of that of a bare cylinder for optimized parameters. The results obtained by the two models are linked by the theory of Carman-Kozney, showing that the macroscopic model can be used for prediction of the aeroacoustic sound.   

The importance of the shock-cell structure in the A1 and A2 jet screeching modes

This work focuses on exploring jet screech closure mechanisms of the axisymmetric modes in shock-containing jets. The analysis is based on different types of waves supported by the jet medium (Mancinelli et al. 2019), and the interaction between the Kelvin-Helmholtz mode and the shock-cell structure (Tam $\&$ Tanna 1982, Shen $\&$ Tam 2002). Analysis of the convective terms in the Navier-Stokes equations expanded around a streamwise-oscillatory mean flow shows that new forcing terms arise in particular wavenumbers by the interaction between instability waves and shocks, creating new energy transfer paths for the generation of upstream-travelling waves that can close resonance. Predictions using locally parallel spatial stability analysis and the wavenumber spectrum of the shock-cell structure, educed from experiments, suggest that the A1 mode resonance is closed by the peak wavenumber of the shock-cell spectrum interacting with the Kelvin-Helmholtz mode, and the A2 mode is closed by a secondary peak, which arises from the spatial variation of the shock-cell wavenumber. Results are in good agreement with experiments in the region of dominance of each mode, and an analysis of the dominance of each mode is performed.   

Direct numerical simulations of aerofoil noise due to flow separation and stall

Aerofoil flow separation and stall is a significant source of self-noise for many engineering applications, particularly when operating at high angles of attack in unsteady inflow condition. Despite this, the noise generation mechanisms for stalled flows remains relatively unexplored. The aim of this work is to provide an improved understanding of both the dipole and quadrupole noise sources utilising direct numerical simulations. A NACA0012 aerofoil with a large spanwise domain size (one chord length) is considered for a Reynolds number of 50,000 and Mach number of 0.4. The far-field sound is calculated with a Ffowcs Williams and Hawkings solver at three angles of attack ($\alpha$): pre-stall ($\alpha=5^\circ$), near-stall ($10^\circ$), and full-stall ($15^\circ$). The radiated noise is significantly increased at low frequencies for the full-stall case. It is found that the full-stall case produces a more in-phase source distribution at most frequencies, resulting in more efficient radiation. The frequency filtered flow field is used to identify the flow structures responsible for the noise generation. At low frequency coherent vortices appearing in the separated shear layer dominate, while at high frequency vortices shed from the trailing-edge play a prominent role.   

Direct and adjoint analysis of turbomachinery aeroacoustics

With ever more stringent efficiency, emission and noise regulations, much interest and efforts are presently directed towards the analysis and eventual performance optimization of turbomachinery. In these applications the flow domain geometries typically consist of identical airfoils arranged in annular rows, with alternating stationary and rotating components making up the working sections of the device. With the large numbers of blades, traditional modal, non-modal and sensitivity analyses focusing on isolated single passages are incapable of modelling the large-scale synchronization effects in the unsteady flow field and an alternative approach must be utilized. In this work we present a novel computational framework for the direct and adjoint analysis of unsteady turbomachinery flows. By modelling an idealized compressor geometry, we investigate the roles played by self-excited aerodynamic instabilities and the effects of aerodynamic mistuning though an n-periodic mean flow stability analysis. We then augment this by applying a high-order time-domain nonlinear-adjoint sliding plane approach for identifying flow sensitivities in the rotor-stator interaction problem - a configuration where mean flow analysis is no longer suitable.   

The Dynamics of Azimuthal Fourier Modes in Rectangular Jets

Rectangular propulsion jets exhibit airframe-integration and thrust-vectoring advantages over the more commonly studied circular jets but have not been analyzed as extensively. This work seeks to leverage techniques developed for axisymmetric jets to analyze three rectangular jets with aspect ratios (AR) 1, 4 and 8. Large-Eddy Simulation databases are employed for each, together with a benchmark circular jet to provide a reference. Key variations from axisymmetric jets include: a decrease in potential core length with an increase in the AR, axis switching (in AR=4), and a peak azimuthal asymmetry of 2dB in the noise field (in AR=8). To utilize the azimuthal Fourier analysis usually adopted for axisymmetric jets, the acoustic dynamics of the rectangular jets are projected on a cylindrical coordinate system. All rectangular jets show good convergence of the dynamics on to the leading azimuthal modes indicating the possibility of simplifying their modelling by considering only a few modes similar to circular jets. Additionally, two mechanisms unique to the high AR rectangular jets, namely, preferential flapping in the minor axis direction and a coupling of the axisymmetric and second azimuthal modes are identified as causes of the asymmetry in nearfield RMS acoustic intensity.   

Nozzle effects on global instabilities of supersonic jets

We conduct global linear stability analyses of supersonic jets, with the aim of assessing the effect of the nozzle lip on the instability mechanism. The inclusion of the nozzle within the computational domain gives rise to absolute instability via a coupling between the downstream-traveling Kelvin-Helmholtz mode and the upstream-traveling acoustic wave in the vicinity of the nozzle lip. The features of the eigenmodes are discussed by applying the momentum potential theory of Doak (1989). The effect of varying the thickness of the nozzle is investigated and found to have a marginal influence on the instability mechanism. The sensitivity to the exit Mach number and the nozzle pressure ratio are explored.   

Noise generation by a vortex ring near porous edges: Experiment

The quiet flight of large owl species has been attributed to their porous trailing edge (TE) plumage. Theoretical and computational efforts by Jaworski and Peake 2013 and Cavalieri et al. 2016, respectively, predicted how the TE sound power scaling law and acoustic directivity change as a function of edge porosity. These predictions have proved difficult to validate in wind tunnels because of background noise. The current study addresses this issue by performing measurements in the ARL Penn State anechoic chamber, and by abstracting the TE noise problem to the convection of a vortex ring past the edge of a non-compact plate. In this manner, the only sound produced is due to the vortex ring/edge interaction. Experiments were performed for a series of plates, each with a different porosity, the control case being a rigid, impermeable plate. The vortex rings, produced by a shock tube, exited from a 6 mm nozzle. Vortex ring motion and size were estimated from Schleiren imaging of the vortex ring motion, captured at 25.1 kHz. Ring speed ranged from 20 m/s to 70 m/s, while the ring radius was 4.5 mm. Twelve microphones, arranged in a circle centered on the plate edge, were used to measure farfield sound pressure and directivity. These measurements were used to estimate the exponent in the \textit{sound power} \textasciitilde $U^{\mathrm{n}}$ scaling law. Observed changes in both $n$ and farfield sound directivity with porosity were compared to theoretical predictions.   

Noise generation by a vortex ring near porous edges: Theory

The noise generated by a porous edge is investigated analytically using a vortex ring source in a quiescent fluid. Use of a vortex ring on a rectilinear path near an edge permits the investigation of scaling behaviors for the radiated acoustic power that are analogous to those derived for turbulence edge-noise. A solution methodology based on Green’s functions solves for the time-dependent scattered acoustic field. In the highly-porous limit, the acoustic power is shown to scale with the inverse fifth power of the minimum distance between the vortex path and the edge, and on the sixth-power of the acoustic power on vortex ring speed. The vortex-ring configuration in a still fluid furnishes a more direct theoretical comparison with experimental validation efforts, where corruption of the weak porous-edge noise signal by secondary noise sources in aeroacoustic wind tunnels is avoided.   

Numerical investigation of coupling between twin supersonic rectangular jets

In this work we perform large-eddy simulations (LES) using Voronoi-CharLES from Cascade Technologies to study coupling between cold over-expanded supersonic jets, issuing from two closely placed rectangular nozzles with an aspect ratio of 2. The far-field sound generated from these jets is obtained by projecting the near-field LES data using the Ffowcs Williams-Hawkings method and the effects of jet-to-jet interactions are studied. Noise characteristics predicted by LES compare favorably with the experiments conducted at the University of Cincinnati, including screech. The LES captures out-of-phase flapping motions of the two jets with respect to each other, constructing 3-d coupled oscillations. At the screech frequency and its harmonics, the LES detects strong coherence between the two jets with the phase lag suggesting anti-symmetric coupling, consistent with the out-of-phase jet flapping. By applying spectral proper orthogonal decomposition analysis to near-field pressure and velocity fluctuations, coupling modes at such frequencies are further identified, and mechanisms behind the twin-jet coupling and its impact on the screech noise generation are discussed.   

Sources of Broadband Fan Noise in the NASA/GE Source Diagnostic Test Fan

As a result of continuous progress in reducing the jet exhaust noise issued from commercial, high bypass ratio turbofan engines, it is now known that broadband fan noise (BBFN) is the dominant source of noise at take-off. Broadband fan noise contains a wide range of frequencies and is associated with turbulent flow and its interaction with a solid boundary, with emphasis on rotor self-noise and rotor-stator interaction. Rotor self-noise is caused by trailing edge scattering of hydrodynamic turbulent pressure fluctuations into sound while rotor-stator interactions involves the rotor-shed turbulent wake impinging upon the downstream stator. Existing semi-empirical noise models for efficiently predicting BBFN are limited because of the challenges associated with modeling the on-blade and interstage turbulence. In this talk we present a large-eddy simulation of the NASA/GE Source Diagnostic Test fan whose goal is to directly simulate the turbulent sources of noise and provide data for assessing and improving the existing noise models. The fan was extensively studied at the NASA Glenn Research Center and we compare predicted flow data to measured flow data.   

Aerodynamic and Aeroacoustic Analysis of a Passive-Adaptive Slat for a Wind Turbine Airfoil

This study presents results of numerical aerodynamic and aeroacoustic simulations on a reference airfoil DU 91-W2-250 combined with a passive-adaptive slat. The leading-edge slat was designed by German aerospace centre (DLR) as a part of the common German research project ''Smart Blades II''. An extended numerical aerodynamic analysis based on computational fluid dynamics (CFD) was performed in OpenFOAM. The studies were conducted for two slat configurations: open and closed conditions. Large Eddy Simulations (LES) in combination with Wall Adapting Local Eddy-viscosity (WALE) turbulence model were performed for various angles of attack for two different free stream velocities. Besides a detailed flow analysis, the physical flow mechanisms responsible for noise generation were identified. In the computation of far-field slat noise, unsteady flow data from LES models were extracted at a permeable surface in the near-field of the profile and used as input for FfowcsWilliams-Hawkings (FW-H) equations. The farfield noise spectra were characterised by narrow-band peaks, broadband noise and a single broad tone. The results were compared with experimental data, and quantitative agreement was obtained in terms of mean pressure distribution and the far-field noise spectra.   

Momentum transferred to the environment by a Helmholtz resonator

The momentum transferred from a Helmholtz resonator driven at high amplitudes is investigated experimentally and numerically for frequencies around resonance and higher. As visualized, the outward flow consists of a vortex-ring discharged every positive cycle. Momentum transfer in the outward cycle is estimated experimentally from the velocity and size of the vortex. Numerically, the momentum transfer is estimated from a Lumped Element Model (LEM) of the flow in the neck of the resonator. Results from the LEM are in good agreement with the experimental measurements. Under most conditions considered, the motion of air through the neck is dominated by the aerodynamic damping accounted for by the Bernoulli effect, and driven by the sound pressure within the resonator. A dimensionless scaling derived from the LEM, suggests that momentum transfer in the outward cycle scales as the ratio of the Reynolds and the Strouhal numbers. Such scaling is in good agreement with the experimental and numerical results, collapsing graphically into a common curve.   

Small Unmanned Aircraft Systems (sUAS) rotor acoustic noise at unsteady thrusts

There is immense engineering interest to alleviate the acoustic noise of drones to pave the path of mass drone deployment for applications like Urban Air Mobility. Plentiful acoustic studies of rotor-propelled drones have been done in the past few years (Tinney et al., 2018; Kloet et al., 2017). However, most of them relied on an assumption of hovering or static thrust output. In this study, sound fields produced by a controlled small drone rotor under unsteady thrusts are recorded by a microphone array. Acoustic time-frequency analysis tools like wavelet transform and empirical mode decomposition are used (Stephenson et al., 2014). The acoustic signatures are studied with temporal rotor aerodynamics. In future work, 2D time-resolved PIV data on the rotor blade may be also incorporated.   

Not Participating

The interaction between acoustic waves and particles suspended in a fluid are relevant to many present-day applications, for example propagation of sound through fog and dust, acoustic waves interaction with aluminum particles in rocket engines, or the injection of water particle to reduce jet noise. The focus will be on how the concentration and physical properties of the suspended particles influence the attenuation and dispersion of sound propagating through the surrounding fluid. High-fidelity numerical simulations will be performed utilizing the CHEM code which is a chemically-reacting Navier-Stokes finite-volume solver for generalized grids. Both the Eulerian and Lagrangian particle simulation approaches will be evaluated for their ability to model the interaction between rigid particles and acoustic waves in conjunction with the implemented fourth order low dissipation skew symmetric flux scheme. How sound-particle interactions are affected by the interface boundaries present in overset meshes will also be investigated.   

Coupling Modes and Standing Waves in Supersonic Twin Rectangular Jets

The existence of standing waves (SWs) in the irrotational hydrodynamic field (IHF) of screeching underexpanded circular single and closely-spaced (C-S) twin jets has been reported in the literature as has coupling of C-S twin circular and large aspect ratio rectangular jets. SWs in the IHF are generated by the interaction of upstream travelling acoustic waves and downstream convecting hydrodynamic waves generated by the convection of large-scale structures in the jet's shear layer. A new experimental facility, using C-S supersonic twin rectangular jets with a design Mach number of 1.5 and an aspect ratio of 2, has recently been commissioned at the Gas Dynamics and Turbulence Laboratory at OSU. SWs and jet coupling have been investigated from overexpanded (M$_{\mathrm{j\thinspace }}$of 1.15) to underexpanded (M$_{\mathrm{j\thinspace }}$of 1.85) jets using 6 near-field microphones located along the common major axis and both minor axes of the nozzles and schlieren imaging. The preliminary results show out-of-phase coupling along the minor axis in overexpanded regime transitioning to in-phase coupling in underexpanded regime. In both coupling cases, weak coupling exists at low M$_{\mathrm{j}}$s but transitions to stronger coupling as M$_{\mathrm{j}}$ is increased. Well-defined SWs are observed in both regimes in the range of M$_{\mathrm{j}}$s at which both coupling strength and screech amplitude are increasing. More detailed work is underway.   

Screech Generation in a Rectangular Jet

Screech emission from a rectangular jet with an aspect ratio of 4:1 is studied, using large-eddy simulations based on Cascade Technologies' solver, CharLES. LES results agree very well with experimental data from Florida State University (Dr. Ranjan Kumar, private communication) including mean flow, near and far field sound. LES data show the jet is flapping in the minor-axis plane. Screech tones are emitted from an effective sound source approximately between the 4th and the 5th shock cells. The fundamental tone propagates strongly upstream and less so downstream, and the first harmonic travels almost perpendicular to the jet. SPOD analysis reveals clear signature of upstream- and downstream- travelling wave packets. The upstream wave packet contains both an internal mode and upstream beamed sound. Varying NPR changes the strength of the wave packets, and its relation to the screech amplitude and frequency is examined. While evidence supports external sound waves being the key link in screech closure, the role of the internal mode is also examined by quantifying its amplitude and phase at the nozzle exit. Lastly, a comparative study on the effect of initial shear layer turbulence on screech is conducted by numerically tripping the boundary layer with geometric perturbations.   

Numerical Investigation of the Effect of Trailing Edge Deformations on Noise from Jets Exhausting Over Flat Plates

The design of aircraft propulsion configurations must digress from the typical configurations that are utilized on the majority of aircraft in order to consider the effects of environmental issues as well as the noise that is generated from the engines. One unconventional approach under consideration involves rectangular jets near flat surfaces that are parallel to the jet axis. This type of configuration makes an attempt to muffle the noise that propagates to the ground, but previous experimental work showed that the noise generated by this configuration was actually increased due to the effect that the plate trailing edge exerts on the flow. We conduct large eddy simulations to determine if wall deformations at the plate trailing edge could reduce the jet noise. A high aspect ratio rectangular nozzle is placed over a flat surface featuring sinusoidal deformations at the leading edge, with different wavenumbers and amplitudes. Our previous numerical simulations, which targeted configurations with small deformation amplitudes and high wavenumbers, showed that the trailing edge deformations only had a small effect on the noise. Here, we consider new configurations in an attempt to determine if a more significant reduction of the jet noise is possible.   

Analysis of a Coupled LES-Synthetic Turbulence Modeling Approach for Jet Noise Prediction

Turbulence modeling for jet noise remains a challenge due to the wide range of temporal and spatial scales present in the flow. Smaller turbulent scales generate higher-frequency jet noise content that resides in a critical range for human hearing. Resolving the sound generated by these smaller turbulent scales, while necessary for predicting jet noise spectra, requires an excessive amount of computational resources for either LES or DNS. With the goal of reducing the cost needed to predict higher-frequency jet noise content, we employ a Coupled LES-Synthetic Turbulence (CLST) method, where-in large-scale flow structures are resolved via very large eddy simulations (VLES) and the smaller-scale flow structures are modeled via a Synthetic Eddy Method (SEM). Sweeping is achieved by convecting the synthesized turbulent field with the large-scale resolved turbulent flow field. Acoustic sources are generated by combining large- and small-scale fluctuations in source terms for the linearized Euler equations (LEE). Initial near-field noise results demonstrate the feasibility of a CLST approach for jet noise prediction.