Session ZC05: Aeroacoustics II

Mechanism of noise suppression due to subcavity in turbulent supersonic cavity flow

The perturbation of the shear layer at the leading edge of a cavity flow due to an upstream traveling acoustic wave (generated at the trailing edge) causes self-sustained flow oscillations and generates noise. Typically, the noise reduction in a turbulent supersonic cavity flow is achieved by disrupting the feedback loop of disturbances. One of the passive control techniques for noise suppression involves construction of a subcavity at the leading edge of the cavity. In this work, we investigate the mechanism of this passive control technique through Large Eddy Simulation (LES) of the flow over a cavity at Mach 1.71 and length (L) to depth (D) ratio of 2. A preliminary investigation by placing a subcavity of length (l) =  0.2L reveals that it suppresses the perturbation of the shear layer from upstream-propagating acoustic waves. This is achieved through the interaction between the reflected acoustic wave from the subcavity and the upstream-propagating acoustic wave. From LES simulation, the wavelet coherence of the probed pressure signals at upstream and downstream locations about the wave-interaction point reveals a destructive interference between the two waves and supports above hypothesis. Moreover, the same subcavity, when placed at the trailing edge, resulted in a constructive interference. The present study will be extended to more length ratios (l/L), and the effect of acoustic waves interaction on the attenuation of shear layer oscillations will be closely examined.   

Microfiber coating as a noise-reducing device for aerial vehicles

A microfiber coating has been studied as a noise-reducing device for a propeller blade in small aerial vehicles. It is a hair-like structure attached to the trailing edge of a blade. To determine the optimal microfiber locations for a blade-noise reduction, the microfiber coating is placed symmetrically at different spanwise locations on each blade of the 15-inch diameter rotor. The Reynolds numbers based on the velocity and chord at the 75% span station is 7.4 × 10⁴. Sound level around the rotating blade is measured by using a sound-level meter in an anechoic chamber. When the coating is placed along the trailing edge of the outer portion of the rotor, the microfiber coating leads to noise reductions of up to 2.2 dBA. The location near the wingtip corresponded with larger noise reduction than the location close to the rotation center. The current status of this study as well as our future studies will be presented.    

The roles of the evolution of vortices on the aerodynamic noise of flapping wings

The effects of the evolution of vortices on the noise generated by a low-aspect-ratio flapping wing during hovering flight are investigated by solving the three-dimensional incompressible Navier-Stokes (N-S) equations. A simplified model based on the Ffowcs Williams-Hawkings (FW-H) acoustic analogy is developed here for investigating the aerodynamic noise of flapping wings, where the time derivative of the surface pressure is considered as the main contributor of far-field sound. A rigid rectangular wing with an aspect ratio of 1.5 undergoing both pitching and flapping motions at a Reynolds number (Re) of 1000 and a Mach number (M) of 0.04 is chosen for this study. By incorporating a Green's function, the time derivative of the surface pressure is found to be determined by the time derivatives of the divergence of the convection, the centrifugal acceleration and Coriolis acceleration terms in a non-inertial reference frame, with the first being the dominant source which is mainly within the vortical structure. Further, the Green's function is found to determine the time derivative of the surface pressure, which is related to the movement of the vortices. A scaling analysis is conducted on the time derivative of the surface pressure forces, revealing a scaling relationship with the cube of the flapping frequency f_o.   

Trailing-edge bluntness vortex shedding noise attenuation using perforated flat plates

The reduction of trailing-edge (TE) bluntness noise from a flat plate with a 3 mm thickness is investigated experimentally using perforations along the chord, as inspired by the porous trailing edges of owl wings. Edge porosity has been shown to reduce flow noise in experimental and theoretical works, where in the latter a single dimensionless parameter is shown to control the scaling of noise generation due to a vortical or turbulence source. This insight is applied to the analysis of the broadband noise produced by a boundary layer over a porous plate. An open jet wind tunnel gathers acoustic and flow measurements of flat plates with blunt TEs and different perforation designs. Sounds maps are generated using microphone array data and hot-wire testing investigates the specific flow behavior. Periodic vortex shedding occurs at the TE due to edge bluntness and creates tonal sound peaks that decrease in magnitude with increasing number of perforations. The presence of perforations also drastically increase roughness-induced noise, and the mechanisms behind this noise and associated vortex shedding at the TE are investigated by interrogating the velocity field with hot-wire probes.   

Numerical simulation of the interaction between finite-amplitude acoustic fields and flow phenomena in open ducts

Finite-amplitude standing acoustic waves in narrow, open ducts are associated with phenomena such as shock formation, acoustic streaming and generation and shedding of vortices at the open end. Typically, these events are all observed when acoustic particle velocities increase inside the duct, and their appearance also depends strongly on the geometry of the duct. The competing effects of nonlinearity, thermoviscous attenuation and radiation at the open end must all be taken into account to adequately model acoustic propagation inside the duct, and, further, we find it is critical to reproduce correctly the behavior of the Stokes boundary layer. We use a full-wave finite volume axisymmetric code which allows for the coupling of the acoustic field with the aforementioned flow phenomena to identify the values of different parameters leading to generation of vorticity at the open end of the duct, and in particular the propagation of streets of vortices along the Stokes boundary layer inside the duct.   

Resonance Response Due To Shear Layer Vorticity in Cavity Exposed To High-Speed Freestream

A cavity exposed to a high-speed free stream will exhibit a resonance interaction between the shear layer over the cavity and acoustic waves in the cavity, developing discrete tones as predicted by the phenomenological model developed by Rossiter [1964] and refined by Heller [1971]. In this study, we experimentally observe the interaction between acoustic waves and vorticity within the cavity with a shadowgraph system and optically clear sidewalls, as well as discrete pressure transducers in the front and back walls. We demonstrate how the statistical distribution of flow physics events in the cavity produces some of the observed acoustic tones. Hitherto, all research has been conducted with cavities exposed to a continuous free stream; we will demonstrate how the response of the cavity aeroacoustic resonance is affected by a sliding door from fully closed to fully open in O(100) convective units.   

Aeroacoustic Source Separation using RPCA of Microphone Array Signals

We present an application of Robust Principal Component Analysis (RPCA) to acoustic measurements, where the aim is to accurately distinguish between two acoustic sources in noisy signals. Here, we analyze microphone array data from an individual vortex ring (VR) convecting past a semi-infinite half-plane in an anechoic chamber. VR generation is impulsive in character and acts as a second source, producing a weak shock wave. Acoustic pressure measurements using a circular array of 12 microphones centered on the VR/half-plane source are sampled synchronously with high speed Schlieren imaging of VR motion. In this application, RPCA is used to decompose microphone array signals into low-rank and sparse components in an attempt to separate the two sources. Temporal alignment of each acoustic source is performed by steering the array, which encourages higher representation of the desired source's signal energy in the low-rank portion of the RPCA decomposition. RPCA-estimated acoustic source waveforms are then used to estimate sound source parameters for the VR/half-plane interaction.   

Bayesian optimization of the acoustic response of short holes

The acoustic response of short circular holes with bias flow passing through them is relevant to many industrial applications, such as acoustic liners and perforated plates. Recent theoretical and numerical studies suggest that this acoustic response is sensitive to small geometry modifications. Additionally, flows through holes are often used to generate mean flow cooling.

In the current study, we consider a hole flow with a temperature gradient, and numerically optimize the hole geometry to maximise its acoustic absorption using Bayesian optimisation. The two-step numerical approach of Guzmán-Iñigo et al. (2022) is used, in which the incompressible Reynolds-Averaged Navier-Stokes (RANS) and energy equation are firstly solved to compute the mean flow through the hole in the presence of temperature gradient. The second step involves solving the flow perturbations superimposed on the mean hole bias flow via the linearised Navier-Stokes equations (LNSE). These two steps (including meshing) are repeated for every geometry optimization iteration.

The geometrical modification proposed uses rounded fillets on both the upstream and downstream edges of the hole, with the fillet radii as the optimisation parameters. The effect on a hole whose length and diameter have similar values, and which initially generates acoustic energy via “whistling” is demonstrated. The output of this study offers a route for designing short holes whose acoustic damping properties are maximized.   

Dynamic gas flow measurements using self-sustained acoustic resonance tracking

The National Institute of Standards and Technology is developing an acoustic, dynamic gas-flow standard. The standard consists of a large, unthermostated pressure vessel with known volume that we use as a gas source and as an acoustic resonator. We determine the mass flow dm/dt exiting the vessel by tracking the time-dependent pressure P(t) and the resonance frequency f_N(t) of an acoustic mode of the gas remaining in the vessel. We use P(t), f_N(t), and known values of the speed of sound w(T,P) to determine a mode-weighted average temperature <T>_φ of the gas in the vessel. As gas exits the vessel, the average temperature (and therefore f_N(t)) changes rapidly due to flow work. To track f_N(t), we sustain the gas oscillations using positive feedback; f_N responds to changes in <T>_φ within a time of order 1/f_N. We have validated this technique with calibrated critical flow venturis using radial modes of a spherical vessel (1.85 m³) and using longitudinal modes of a cylindrical vessel (0.3 m³) in flows ranging from 0.24 g/s to 12.4 g/s with an uncertainty of 0.51% (95 % confidence level). We have also measured leaks as small as 5 μg/s. Details of the acoustic tracking technique and requirements of the vessel/resonator will be discussed.