Recent advances of computational aeroacoustics

Computational aeroacoustics （CAA） is an interdiscipline of aeroacoustics and computational fluid dynamics （CFD） for the investigation of sound generation and propagation from various aeroacoustics problems. In this review, the foundation and research scope of CAA are introduced firstly. A review of the early advances and applications of CAA is then briefly surveyed, focusing on two key issues, namely, high order finite difference scheme and non-reflecting boundary condition. Furthermore, the advances of CAA during the past five years are highlighted. Finally, the future prospective of CAA is briefly discussed.

ACOUSTIC PROPAGATION IN SHEARED MEAN FLOW USING COMPUTATIONAL AEROACOUSTICS

Acoustic propagation problems in the sheared mean flow are numerically investigated using different acoustic propagation equations , including linearized Euler equations ( LEE ) and acoustic perturbation equations ( APE ) .The resulted acoustic pressure is compared for the cases of uniform mean flow and sheared mean flow using both APE and LEE.Numerical results show that interactions between acoustics and mean flow should be properly considered to better understand noise propagation problems , and the suitable option of the different acoustic equations is indicated by the present comparisons.Moreover , the ability of APE to predict acoustic propagation is validated.APE can replace LEE when the 3-D flow-induced noise problem is solved , thus computational cost can decrease.

Aircraft noise and its nearfield propagation computations

Noise generated by civil transport aircraft during take-off and approach-to-land phases of operation is an environmental problem. The aircraft noise problem is firstly reviewed in this article. The review is followed by a description and assessment of a number of sound propagation methods suitable for applications with a background mean flow field pertinent to aircraft noise. Of the three main areas of the noise problem, i.e. generation, propagation, and ra- diation, propagation provides a vital link between near-field noise generation and far-field radiation. Its accurate assessment ensures the overall validity of a prediction model. Of the various classes of propagation equations, linearised Euler equations are often casted in either time domain or frequency domain. The equations are often solved numerically by computational aeroacoustics techniques, bur are subject to the onset of Kelvin-Helmholtz （K-H） instability modes which may ruin the solutions. Other forms of linearised equations, e.g. acoustic perturbation equations have been proposed, with differing degrees of success.

Numerical simulation on the NACA0018 airfoil self-noise generation

Airfoil self-noise is a common phenomenon for many engineering applications. Aiming to study the underlying mechanism of airfoil self-noise at low Mach number and moderate Reynolds number flow, a numerical investigation is presented on noise generation by flow past NACA0018 airfoil. Based on a high-order accurate numerical method, both the near-field hydrodynamics and the far-field acoustics are computed simultaneously by performing direct numerical simulation. The mean flow properties agree well with the experimental measurements. The characteristics of aerodynamic noise are investigated at various angles of attack. The obtained results show that inclining the airfoil could enlarge turbulent intensity and produce larger scale of vortices. The sound radiation is mainly towards the upper and lower directions of the airfoil surface. At higher angle of attack, the tonal noise tends to disappear and the noise spectrum displays broad-band features.

COMPACT FINITE VOLUME SCHEMES AND THEIR APPLICATIONS

A class of Compact Finite Volume Schemes (CFVS) with small support stencils are developed based on a new reconstruction method for cell face variables. The accumulative errors of these schemes for a scalar wave equation are analyzed and compared. The established compact schemes are proved suitable for steady and unsteady flow simulations by several numerical experiments in this paper.

Numerical Modelling of Aerodynamic Noise in Compressible Flows

The solution of the AeroAcoustics (CAA) problems by means of the Direct Numerical Simulation (DNS) or even the Large Eddy Simulation (LES) for a large computational domain is very time consuming and cannot be applied widely for engineering applications. In this paper the in-house CFD and CAA codes are presented. The in-house CFD code is based on the LES approach whereas the CAA code is an acoustic postprocessor solving the non-linearized Euler equations for fluctuating (acoustic) variables. These codes are used to solve the aerodynamically generated sound field by a flow over a rectangular cavity with inlet Mach number 0.53.

Numerical Investigation of Sound Generation due to Laminar Flow Past Elliptic Cylinders

Numerical investigation of sound generation due to unsteady laminar flow past elliptic cylinders has been carried out using direct numerical simulation(DNS)approach at a free-stream Mach number of 0.2.Effects of aspect ratio(0.6≤AR≤1.0)and Reynolds number(100≤Re≤160)on the characteris-tics of radiated sound fields are analyzed.Two-dimensional compressible fluid flow equations are solved on a refined grid using high resolution dispersion relation pre-serving(DRP)schemes.Using present DNS data,equivalent noise sources as given by various acoustic analogies are evaluated.Amplitudes and frequencies associated with these noise sources are further related to characteristics of disturbance pressure fields.Disturbance pressure fields are intensified with increase in Reynolds number and aspect ratio.Thus,radiated sound power increases with increase in Reynolds number and aspect ratio.Among various cases studied here,minimum and max-imum values of radiated sound power are found at Re=120&AR=0.6 and Re=160&AR=1.0,respectively.Directivity patterns show that the generated sound fields are dominated by the lift dipole for all cases.Next,proper orthogonal decomposition(POD)technique has been implemented for decomposing distur-bance pressure fields.The POD modes associated with the lift and the drag dipoles have been identified.POD analyses also clearly display that the radiated sound fields are dominated by the lift dipole only.Further,acoustic and hydrodynamic modes obtained using Doak’s decomposition method have confirmed the patterns of radiated sound field intensities.

Sound Propagation Properties of the Discrete-Velocity Boltzmann Equation

As the numerical resolution is increased and the discretisation error decreases,the lattice Boltzmann method tends towards the discrete-velocity Boltzmann equation(DVBE).An expression for the propagation properties of plane sound waves is found for this equation.This expression is compared to similar ones from the NavierStokes and Burnett models,and is found to be closest to the latter.The anisotropy of sound propagation with the DVBE is examined using a two-dimensional velocity set.It is found that both the anisotropy and the deviation between the models is negligible if the Knudsen number is less than 1 by at least an order of magnitude.