Session T05: Acoustics II: Hydro and Aero

Subconvective characteristics of wall-pressure fluctuations in low-Mach-number turbulent channel flow

Accurate prediction and modeling of flow-induced structural vibration and noise require a comprehensive understanding of turbulent wall-pressure fluctuations. In this study, compressible direct numerical simulations are conducted to investigate the spectral characteristics of wall-pressure fluctuations in turbulent channel flows at friction Reynolds number of 180 and bulk Mach numbers of 0.4, 0.2 and 0.1, with a focus on the subconvective wavenumber range. While acoustic peaks are barely visible in the one-dimensional streamwise wavenumber-frequency spectra, they are clearly observed in the two-dimensional wavenumber-frequency spectra at the zeroth spanwise wavenumber, with magnitudes several decades lower than the convective peak. At lower frequencies, the acoustic peaks correspond to propagating longitudinal and oblique waves that match the theoretical predictions of two-dimensional duct modes with a uniform mean flow. They decay with decreasing Mach number but remain distinctly identifiable even at the nearly incompressible Mach number of 0.1. At high frequencies, in contrast, no propagating waves are found, and the spectral level in the low-wavenumber range increases with decreasing Mach number.

  

Verification and Application of Time-domain Impedance Boundary Condition in CFD-CAA Method

In engineering applications, the surface structure plays a crucial role in influencing flow fields and noise generation. The CFD-CAA method commonly assumes smooth and purely reflective wall surfaces; however, experiments often involve surface treatments to explore noise control techniques. Consequently, there is a growing interest in incorporating impedance boundary conditions into CFD-CAA simulations. Since impedance boundary conditions are defined in the frequency domain, while CFD-CAA simulations operate in the time domain, direct implementation is not possible. To address this issue, several methods have been proposed to define time-domain impedance boundary conditions in simulations. The present study employed the wall softness model to examine a vortex whistle featuring an acoustically permeable surface. In the simulations, an impedance boundary condition representing the properties of melamine foam was defined over the surface of a cylindrical cavity. LES was used for CFD simulations, while FW-H analogy was employed for CAA simulations. The simulation results were validated against experimental data obtained from a vortex whistle with melamine foam. The findings revealed that the impedance of the melamine foam contributed to noise reduction at high frequencies. Additionally, at low airflow rates, the impedance boundary condition demonstrated the ability to enhance the signal-to-noise ratio for the low-frequency peak, which is advantageous in clinical applications.   

Nondivergent Behavior of Spin-Orbit Features in Acoustic Vortex Beams

We consider effects due to interplay of the orbital angular momentum and spin in the propagating acoustic vortex beams. It is shown that the relation beween longitudinal and transverse wave motion is strictly controlled by the continuity equation, resulting in observable features that do not diverge during wave propagation in a paraxial limit, while the overall wave's intensity profile expands due to diffraction. Similar non-divergent features reveal themselves in the sound generation from acoustic multipoles. We show that point-like acoustic dipoles, quadrupoles, etc. generate sound waves with inherent cylindrically-symmetric properties with embedded chiral topologies. Direct analogies are drawn between acoustic waves and effects previously identified in electromagnetic radiation [1,2].

References

1. Afanasev, Andrei, Jack J. Kingsley-Smith, Francisco J. Rodríguez-Fortuño, and Anatoly V. Zayats. "Nondiffractive three-dimensional polarization features of optical vortex beams." Advanced Photonics Nexus 2, no. 2 (2023): 026001-026001.

2. Vernon, Alex J., Andrew Kille, Francisco J. Rodríguez-Fortuño, and Andrei Afanasev. "Non-Diffracting Polarisation Features around Far-Field Zeros of Electromagnetic Radiation." arXiv preprint arXiv:2306.03278 (2023).   

Development of an OpenFOAM Solver for Hydroacoustic Simulations: An Application for Acoustic Fish Deterrence

The objective of this project is to develop a general-purpose solver for the acoustics analysis in a narrow channel of shallow water. The motivation is to understand a novel acoustic system designed to deter invasive fish species in rivers. Complexity arises from the channel's geometry which induces complex patterns of reflections.

The solver is developed using the OpenFOAM framework. This initial study deals only with acoustics transmission from the source, so no flow noise sources are included. The simulations consider the acoustic transmission from monochromatic sound waves. The superposition principle is used to investigate the effect of complex sound sources on the channel. As the acoustic solutions for various frequencies operate on different time scales, algorithms for temporal interpolation are investigated in order to apply superposition. This presentation will review the simulation results, discuss error minimization strategies, and evaluate sound pressure levels within the lock.   

Spatio-temporal measurement of underwater ultrasound field using non-contact technique

We successfully measured the spatio-temporal ultrasound acoustic field in water using Background-oriented schlieren (BOS) technique. Other the ability to obtain the 3D-pressure field easily and quickly, BOS needs only a background image and a camera to measure the pressure field without disturbing the flow field. To discuss the measurement accuracy of BOS, we compared its results with the measurement performed using a hydrophone, which is a contact measurement technique. BOS demonstrated good agreement with the hydrophone and theoretical values at a given wavelength and period. The spatio-temporal acoustic field measured by BOS showed that the reflected ultrasound from the hydrophone changed the acoustic field. Especially, BOS could visualize the influence of the shape and size of hydrophone sensor on the propagation of reflected wave and acoustic field. This BOS technique can also measure the acoustic field of other multiphase flow.   

Design considerations for water-flow wall-pressure fluctuation measurements using pinholes in a turbulent channel flow

Flush-mounted-transducer measurements of turbulent wall-pressure fluctuations in water flows provide poor spatial resolution because no transducers simultaneously provide the requisite size and sensitivity. However, successful air flow measurements have been made with microphones behind pinholes. This pinhole measurement method is imperfect in water flows because of the detrimental effects of trapped air bubbles, pinhole-opening flow dynamics, Helmholtz resonance(s), and residual spatial averaging by the pinhole opening. In this presentation, all four limitations are illustrated and addressed via pressure fluctuation spectra collected in a turbulent channel flow (half height = 3.5 mm, 14:1 width-to-height aspect ratio) with 5.5-mm-diameter pressure transducers mounted behind 0.5 mm, 0.75 mm, 1.0 mm, and 2.0 mm diameter pinholes at frequencies from 10 and 20 kHz at nominal average water flow speeds from less than 2 to more than 6 m/s. The effects of air bubbles are suppressed by using deaerated water (30% dissolved oxygen). The effects of pinhole-opening flow dynamics and spatial averaging are reduced by decreasing the opening size. Unfortunately, the importance of the mounting geometry's Helmholtz resonance becomes more prominent as the opening size decreases. In this study, the 0.5 mm pinhole showed the greatest potential for successful turbulent wall-bounded water-flow applications while providing more than a factor of 10 improvement is spatial resolution.   

Multi-Domain Approach to CFD Prediction of Underwater Radiated Noise from Marine Propellers

Underwater Radiated Noise (URN) from shipping has increasingly become a topic of concern, as its harmful effects on marine ecosystems become better understood. The shed vorticity in the wake of a marine propeller is broadly responsible for driving sound-generating phenomena across a wide range of operating conditions where cavitation is either present or absent. Accounting for the unsteady pressure in the propeller wake due to the shed vorticity is therefore crucial for simulation of acoustic emissions. The vorticity is itself dependant on both the motion of the propeller and the propeller inflow. The propeller inflow is, in turn, determined by the motion of the vessel and the development of its wake. As a result, prediction of the propeller noise by CFD necessitates prediction of flow features across a wide range of spatial and temporal scales. We present a multi-domain approach to URN prediction that relies on three solutions of increasing fidelity at decreasing physical length scales and discuss the relevant features of each of the corresponding computational domains. At the largest scale, a steady-state Reynolds-averaged simulation of a bare hull is used to establish relevant boundary conditions, while at the smallest scale, a Delayed Detached-Eddy Simulation (DDES) is used to resolve the fluctuating pressure field in the propeller wake. We applied the multi-domain methodology to a model-scale vessel and compared the numerical fluctuating pressure results to experimental measurements.   

Inertio-viscous hydrodynamic interactions between particles in oscillatory flow

 

Particles suspended in oscillatory flows and acoustic fields are known to exhibit complex behavior, including collective dynamics due to interactions. Here, we study how particles in a uniform oscillatory flow interact with each other by generating oscillatory disturbances, and how the inertia of the flow is rectified into time-averaged particle motion. We present a comprehensive theory of the dynamics of a pair of spherical particles. Utilizing multipole expansions around each particle, we apply the method of reflections to calculate oscillatory hydrodynamic interactions between the particles. We then use these interacting oscillatory flows in an analytic framework to evaluate time-averaged secondary hydrodynamic force on the particles. The forces produce either attraction or repulsion of the particles depending on their orientation relative to the flow, their relative separation distance, density contrast relative to the fluid, and the Stokes layer thickness. The theory is found to be in good agreement with previous direct numerical simulations.