P. Croaker

Fast Low-Storage Method for Evaluating Lighthill’s Volume Quadrupoles

An m-multipole particle-condensation method is proposed to spatially condense the volumetric quadrupole sources based on Lighthill’s acoustic analogy that are extracted from computational-fluid-dynamics data. The purpose of the method is to reduce both the amount of data that must be stored during the computational-fluid-dynamics analysis and the number of acoustic sources driving the subsequent acoustic-propagation analysis, while preserving the accuracy of the predicted sound pressure field. The method uses a particle approximation of the quadrupole source distribution and employs a Taylor series expansion of the harmonic Green’s function to spatially condense the underlying acoustic sources and preserve their multipole moments. Low-Mach-number flow around rigid-walled stationary bodies in a medium at rest at infinity is used to demonstrate the m-multipole particle-condensation method. The directivity of the sound pressure field due to the quadrupole sources is presented for the first four harmonics of ...

Acoustic scattering for 3D multi-directional periodic structures using the boundary element method

An efficient boundary element formulation is proposed to solve three-dimensional exterior acoustic scattering problems with multi-directional periodicity. The multi-directional periodic acoustic problem is represented as a multilevel block Toeplitz matrix. By exploiting the Toeplitz structure, the computational time and storage requirements to construct and to solve the linear system of equations arising from the boundary element formulation are significantly reduced. The generalized minimal residual method is implemented to solve the linear system of equations. To efficiently calculate the matrix-vector product in the iterative algorithm, the original matrix is embedded into a multilevel block circulant matrix. A multi-dimensional discrete Fourier transform is then employed to accelerate the matrix-vector product. The proposed approach is applicable to a periodic acoustic problem for any arbitrary shape of the structure in both full space and half space. Two case studies involving sonic crystal barriers are presented. In the first case study, a sonic crystal barrier comprising rigid cylindrical scatterers is modeled. To demonstrate the effectiveness of the proposed technique, periodicity in one, two, or three directions is examined. In the second case study, the acoustic performance of a sonic crystal barrier with locally resonant C-shaped scatterers is studied.

Experimental and numerical studies on the flow-induced vibration of propeller blades under nonuniform inflow

This article presents the experimental and numerical studies on the flow-induced vibration of propeller blades under periodic inflows. A total of two 7-bladed highly skewed model propellers of identical geometries but different elastic characteristics were operated in four-cycle and six-cycle inflows to study the blade vibratory strain response. A total of two kinds of wire mesh wake screens located 400 mm upstream of the propeller plane were used to generate four-cycle and six-cycle inflows. A laser Doppler velocimetry system located 100 mm downstream of the wake screen plane was used to measure the axial velocity distributions produced by the wake screens. Strain gauges were bonded onto the propeller blades in different positions. Data from strain gauges quantified vibratory strain amplitudes and excitation frequencies induced by the wake screens. The propellers were accelerated through the flexible propeller’s fundamental frequency to investigate the effect of resonance on vibratory strain response. The numerical work was conducted using large eddy simulation and moving mesh technique to predict the unsteady forces acting on the propeller blade when operating in a nonuniform inflow.

Aeroacoustic Scattering Using a Particle Accelerated Computational Fluid Dynamics/Boundary Element Technique

A particle accelerated computational fluid dynamics/boundary element method technique to predict the sound pressure field produced by low Mach number flow past a rigid body is presented. An incompressible computational fluid dynamics solver is used to calculate the transient hydrodynamic flowfield. A near-field formulation based on Lighthill’s analogy is coupled with a particle condensation technique to predict the incident acoustic field and its normal derivative on the body. The near-field formulation involves singular surface and volume integrals, which are regularized via singularity subtraction. A particle condensation technique is applied to accelerate the incident field computations and reduce the amount of data that must be stored during the computational fluid dynamics analysis. The incident field is then combined with a boundary element method model of the body, and the scattered sound pressure field is obtained by solving the Burton–Miller boundary integral equations. The accuracy and computati...

Effect of a serrated trailing edge on sound radiation from nearby quadrupoles

A periodic boundary element technique is implemented to study the noise reduction capability of a plate with a serrated trailing edge under quadrupole excitation. It is assumed for this purpose that the quadrupole source tensor is independent of the trailing edge configuration and that the effect of the trailing edge shape is to modify sound radiation from prescribed boundary layer sources. The flat plate is modelled as a continuous structure with a finite repetition of small spanwise segments. The matrix equation formulated by the periodic boundary element method for this 3D acoustic scattering problem is represented as a block Toeplitz matrix. The discrete Fourier transform is employed in an iterative algorithm to solve the block Toeplitz system. The noise reduction mechanism for a serrated trailing edge in the near field is investigated by comparing contour plots obtained from each component of the quadrupole for unserrated and serrated trailing edge plate models. The noise reduction due to the serrated trailing edge is also examined as a function of the source location.

Acoustic scattering for rotational and translational symmetric structures in nonuniform potential flow

© Copyright 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. An efficient approach is proposed to predict acoustic scattering with nonuniform potential flow effects for structures with rotational and translational symmetries. The convected wave equation is transformed to Helmholtz and Laplace equations using a time transformation. The boundary-element method is used to formulate scattering by rotationally symmetric structures as two separate block circulant matrix equations and, similarly, as two separate block Toeplitz matrix equations for structures with translational symmetry. Discrete Fourier transform is employed to solve the block circulant systems. The block Toeplitz systems are solved using the generalized minimal residual method along with the discrete Fourier transform. Solving the convected wave equation using structured matrices significantly reduces computational time and storage requirements. To demonstrate the application of the formulation, two exterior acoustic case studies are considered. The first case study examines acoustic scattering from a sphere submerged in potential flow under monopole source excitation. Directivity plots obtained using the proposed technique are compared with analytical results. The second case study examines flow-induced noise generated by a rigid cylinder immersed in low-Mach-number flow, with the effect of mean flow on the scattered acoustic field taken into account using nonuniform potential flow. The fluctuating flowfield is obtained using an incompressible computational fluid dynamics solver. Acoustic sources based on Lighthills analogy are extracted from the flowfield data using a high-order reconstruction scheme. Results from the hybrid computational fluid dynamics-boundaryelement method technique are presented for turbulent flow past the cylinder, with Reynolds number based on cylinder diameter of ReD = 46;000 and Mach numberM = 0.21. The aeroacoustic results are compared with data from literature.

Aeroacoustic analysis of a cylinder in low mach number flow using a periodic CFD-BEM technique

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.The flow-induced noise generated by a rigid cylinder immersed in low Mach number flow is predicted using a hybrid computational fluid dynamics (CFD)-boundary element method (BEM) technique. The fluctuating flow field is obtained using an incompressible CFD solver. A high-order reconstruction scheme is used to extract acoustic sources based on Lighthills acoustic analogy from the flow field data. The convected wave equation is transformed to Helmholtz and Laplace equations, with the effect of the mean flow on the scattered acoustic field taken into account using non-uniform potential flow. A periodic BEM technique is used to formulate the acoustic problem as two separate block Toeplitz systems. Solving the aeroacoustic problem using block Toeplitz systems significantly reduces computational time and storage requirements. The generalized minimal residual method is then employed along with the discrete Fourier transform to solve the block Toeplitz systems. The results from the hybrid CFD-BEM technique are presented for turbulent flow past a circular cylinder, with Reynolds number based on the cylinder diameter of ReD= 46000 and Mach number M=0.21. The aeroacoustic results are compared with experimental data from literature.

Dynamic strain measurements of marine propellers under non-uniform inflow

An experimental investigation was conducted to determine the dynamic strain characteristics of marine propellers under non-uniform inflow. Two 7-bladed highly skewed model propellers of identical geometries, but different elastic characteristics were tested at various rotational speeds and free stream velocities in the water tunnel. Two kinds of wire mesh wake screens located 400mm upstream of the propeller plane were used to generate four-cycle and six-cycle inflows. A laser doppler velocimetry (LDV) system located 100mm downstream of the wake screen plane was used to measure the axial velocity distributions produced by the wake screens. Strain gauges were bonded onto the propeller blades in different positions. A customized underwater data acquisition system which can record data off-line was used to record the dynamic strain. The results show that the frequency properties of the blade dynamic strain are determined by the harmonics of the inflow and that the stiffness of the propeller has an essential effect on the dynamic strain amplitudes.

Structural and acoustic responses of a fluid loaded shell due to propeller forces

The low frequency structural and acoustic responses of a fluid loaded shell to propeller induced fluid pressures are investigated. The propeller operates in the non-uniform wake field and produces fluctuating pressures on the blades of the propeller. This in turn generates acoustic waves and a near field that excites the surface of the shell. The resulting incident pressure is scattered and diffracted by the shell surface, and also excites structural vibration. A potential flow panel code is coupled with the Ffowcs-Williams and Hawkings acoustic analogy to predict the fluctuating propeller forces, blade pressures and the resulting incident field on the surface of the fluid loaded shell due to operation of the propeller in a non-uniform inflow. The propeller induced incident pressure field is then combined with a coupled three-dimensional finite element/boundary element model of the submerged shell to predict the vibro-acoustic and scattered field responses.

A computational flow-induced noise and time-reversal technique for analysing aeroacoustic sources

A simulation technique to analyse flow-induced noise problems that combines computational fluid dynamics (CFD), the boundary element method (BEM) and an aeroacoustic time-reversal (TR) source localisation method is presented. Hydrodynamic data are obtained from a high-fidelity CFD simulation of flow past a body and aeroacoustic sources are extracted based on Lighthills acoustic analogy. The incident pressure field on the body due to the aeroacoustic sources is combined with a BEM representation of the body to obtain the spectrum of the direct, scattered and total acoustic pressure fields at far-field microphone locations. The microphone data are then used as input for the time-reversal simulations which are implemented by numerically solving two-dimensional linearized Euler equations. Decomposing the far-field pressure enables the TR simulation of the direct, scattered and total acoustic fields to be performed separately which yields the location and nature of the corresponding aeroacoustic sources. To demonstrate the hybrid CFD-BEM-TR technique, the sound generated by a cylinder in low Mach number cross-flow is considered. The nature of the aeroacoustic sources at the vortex shedding frequency and its second harmonic for the direct, scattered and total fields are identified.