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.

On a general theory for compressing process and aeroacoustics:linear analysis

Of the three mutually coupled fundamental processes （shearing, compressing, and thermal） in a general fluid motion, only the general formulation for the compress- ing process and a subprocess of it, the subject of aeroacous- tics, as well as their physical coupling with shearing and thermal processes, have so far not reached a consensus. This situation has caused difficulties for various in-depth complex multiprocess flow diagnosis, optimal configuration design, and flow/noise control. As the first step toward the desired formulation in fully nonlinear regime, this paper employs the operator factorization method to revisit the analytic linear theories of the fundamental processes and their decomposi- tion, especially the further splitting of compressing process into acoustic and entropy modes, developed in 1940s-1980s. The flow treated here is small disturbances of a compressible, viscous, and heat-conducting polytropic gas in an unbounded domain with arbitrary source of mass, external body force, and heat addition. Previous results are thereby revised and extended to a complete and unified theory. The theory pro- vides a necessary basis and valuable guidance for developing corresponding nonlinear theory by clarifying certain basic issues, such as the proper choice of characteristic variables of compressing process and the feature of their governing equations.

High Accuracy Finite Volume Method for Solving Nonlinear Aeroacoustics Problems on Unstructured Meshes

The paper presents a finite volume numerical method universally applicable for solving both linear and nonlinear aeroacoustics problems on arbitrary unstructured meshes. It is based on the vertexcentered multi-parameter scheme offering up to the 6th accuracy order achieved on the Cartesian meshes. An adaptive dissipation is added for the numerical treatment of possible discontinuities. The scheme properties are studied on a series of test cases, its efficiency is demonstrated at simulating the noise suppression in resonance-type liners.

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.

High-Order DG Schemes with Subcell Limiting Strategies for Simulations of Shocks,Vortices and Sound Waves in Materials Science Problems

Shock waves,characterized by abrupt changes in pressure,temperature,and density,play a significant role in various materials science processes involving fluids.These high-energy phenomena are utilized across multiple fields and applications to achieve unique material properties and facilitate advanced manufacturing techniques.Accurate simulations of these phenomena require numerical schemes that can represent shock waves without spurious oscillations and simultaneously capture acoustic waves for a wide range of wavelength scales.This work suggests a high-order discontinuous Galerkin(DG)method with a finite volume(FV)subcell limiting strategies to achieve better subcell resolution and lower numerical diffusion properties.By switching to the FV discretization on an embedded sub-cell grid,the method displays advantages with respect to both DG accuracy and FV shock-capturing ability.The FV scheme utilizes a class of high-fidelity schemes that are built upon the boundary variation diminishing(BVD)reconstruction paradigm.The method is therefore able to resolve discontinuities and multi-scale structures on the subcell level,while preserving the favorable properties of the high-order DG scheme.We have tested the present DG method up to the 6th-order accuracy for both smooth and discontinuous noise problems.

Studies on the Aeroacoustics of Turbomachinery

In this paper, the research work on aeroacoustics in turbomachinery done in the Thermoturbomachinery institute, Department of Power Machinery Engineering, Shanghai Jiao Tong University, for the past 15 years is summarized. It includes: aeroacoustic similarity law and correction ofnon--similarity) mechanism of acoustic control of separated flow, aerodynamic and aeroacoustic comprehensive optimum calculation, and the influence of inlet turbulence and inlet distortion.

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.

Design method for an acoustic cloak in flows by topology optimization

In this paper,a design method for an acoustic cloak in the presence of background mean flows is proposed by using topology optimization,which enables the associated fabrication of the cloaking design.The density-based topology optimization method is used to allocate the designated materials,thus providing the structure of the cloak.The optimization problem is efficiently solved with the gradient-based globally convergent method of moving asymptotes,which utilizes the derivative information from the finite element simulation studies of the linearized acoustic potential equation.This paper introduces the whole design method first then numerically demonstrates the corresponding performance,which shall constitute the main contribution of the present work.

STUDY ON FLAP SIDE-EDGE NOISE BASED ON THE FLY-OVER MEASUREMENTS WITH A PLANAR MICROPHONE ARRAY

A large planar microphone array, which consists of 111 microphones, was successfully applied to measure a two dimensional mapping of the sound sources on landing aircraft. The focus was on the flap side edge noise source in this paper. The spectra, directivity and sound pressure level of flap side edge noise of 10 aircraft were presented in this paper. It is found that the spectrum of flap side edge noise is a broadband noise with some tones in some cases. Two different types of tone sources are found. It is proposed that one type of these tone sources is trailing edge semi baffled dipole source, and another is produced from the shedding of vortex from the wing cusp. The total sound pressure level of flap side edge broadband noise has no obvious directionality. However, the directivity of the tone noise in the flap side edge noise spectrum is obvious. It is demonstrated that the local flow field is the key to controlling the flap side edge noise.

Effect of cavity flow control on high-speed train pantograph and roof aerodynamic noise

The pantograph and its recess on the train roof are major aerodynamic noise sources on high-speed trains.Reducing this noise is particularly important because conventional noise barriers usually do not shield the pantograph.However,less attention has been paid to the pantograph recess compared with the pantograph.In this paper,the flow features and noise contribution of two types of noise reduction treatments rounded and chamfered edges are studied for a simplified high-speed train pantograph recess,which is represented as a rectangular cavity and numerically investigated at 1/10 scale.Improved delayed detached-eddy simulations are performed for the near-field turbulent flow simulation,and the Ffowcs Williams and Hawkings aeroacoustic analogy is used for far-field noise prediction.The highly unsteady flow over the cavity is significantly reduced by the cavity edge modifications,and consequently,the noise radiated from the cavity is reduced.Furthermore,effects of the rounded cavity edges on the flow and noise of the pantographs(one raised and one folded)are investigated by comparing the flow features and noise contributions from the cases with and without rounding of the cavity edges.Different train running directions are also considered.Flow analysis shows that the highly unsteady flow within the cavity is reduced by rounding the cavity edges and a slightly lower flow speed occurs around the upper parts of the raised pantograph,whereas the flow velocity in the cavity is slightly increased by the rounding.Higher pressure fluctuations occur on the folded pantograph and the lower parts of the raised pantograph,whereas weaker fluctuations are found on the panhead of the raised pantograph.This study shows that by rounding the cavity edges,a reduction in radiated noise at the side and the top receiver positions can be achieved.Noise reductions in the other directions can also be found.

OPTIMIZATION OF ACOUSTIC IMPEDANCE，GEOMETRIC STRUCTURE AND OPERATING CONDITION OF LINERS MOUNTED IN ENGINE DUCT

Acoustically absorptive treatment in aircraft engine nacelle is an essential part of the overall aircraft noise reduction effort. The investigation on the optimization of multi-liners plays an important role in noise reduction. Based upon the mode analysis method of sound propagation in a circular duct with multiple liners, a flexible tolerance method is used to optimize the acoustic parameters(impedance), geometric structure parameters(such as open area ratio, cavity depth and hole diameter) and operating condition parameters(such as blade passing frequency). The mathematical models for these kinds of optimization are presented here. The optimum values of the design variables are determined when the in-duct sound suppression approaches a maximum. It can be derived from the optimum results that the emphasis of the engineering optimization design of the perforated plate honey-comb structure should be placed on the optimum choice of the open area ratio and cavity depth. Some reference criteria for the engineering design of the multi-linings are also provided.

High-order schemes for predicting computational aeroacoustic propagation with adaptive mesh refinement

High-order schemes based on block-structured adaptive mesh refinement method are prepared to solve computational aeroacoustic （CAA） problems with an aim at improving computational efficiency. A number of numerical issues associated with high-order schemes on an adaptively refined mesh, such as stability and accuracy are addressed. Several CAA benchmark problems are used to demonstrate the feasibility and efficiency of the approach.

Simplified permeable surface correction for frequency-domain Ffowcs Williams and Hawkings integrals

A simplified surface correction formulation is proposed to diminish the far-field spurious sound generated by the quadrupole source term in Ffowcs Williams and Hawkings(FW-H)integrals.The proposed formulation utilizes the far-field asymptotics of the Green’s function to simplify the computation of its high-order derivatives,which circumvents the difficulties reported in the original frequency-domain surface correction formulation.The proposed formulation has been validated by investigating the benchmark case of sound generated by a convecting vortex.The results show that the proposed formulation successfully eliminates the spurious sound.The applications of the proposed formulation to flows with some special parameters are also discussed.

MATCH-MODE METHOD AND ITS NUMERICAL SIMULATION FOR ROTOR-STATOR INTERACTION SOURCE MODES AND PROPAGATION MODES IN AN ANNULAR DUCT WITH MULTI-TREATMENTS

The suppressing design of the engine nacelle in an aircraft can benefit from the development of the prediction system for the sound fields in engine ducts which includes the prediction of the source generation and that of sound propagation in ducts. First, the acoustic match mode principle between the source modes of rotor stator interaction noise and the propagation modes is presented in this paper. Second, by utilizing this principle, the theoretical prediction method for rotor stator interaction noise generation and its propagation and attenuation in an annular duct with multi treatments is developed. That means that the prediction of sound propagation and attenuation in the segmented ducts might no longer completely depend on the in duct mode measurements, and the investigation on the sound propagation and attenuation in ducts can be accomplished not only by acoustic mode measurement, but also by making use of the source prediction to determine the source modes excited by rotor stator interaction. The effects of fan speed, blade/vane numbers, axial spacing between rotor and stator on the in duct sound attenuation and generated sound power level before and after ducts (also including the sound power level of blade passing frequency and its harmonics at the inlet of ducts) have been numerically calculated by using this prediction method. The reliability of this prediction method is verified by reasonable agreement between the predicted results with measured results in references. By analyzing the results of calculating cases, some reference criteria are provided for the engineering design of the suppressing engine nacelle.

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.

Flow aeroacoustic damping using coupled mechanical electrical impedance in lined pipeline

We report a new noise-damping concept which utilizes a coupled mechanical-electrical acoustic impedance to attenuate an aeroacoustic wave propagating in a moving gas confined by a cylindrical pipeline. An electrical damper is incorporated to the mechanical impedance, either through the piezoelectric, electrostatic, or electro-magnetic principles. Our numerical study shows the advantage of the proposed methodology on wave attenuation. With the development of the micro-electro-mechanical system and material engineering, the proposed configuration may be promising for noise reduction.

WEAKLY NON-LINEAR ANALYSIS ON IDENTIFICATION OF VORTEX,SOUND & HEAT AND THEIR INTERACTIONS IN SHEAR FLOW

The identification of vortex,vortex,sound and heat motions and the interactions among them are discussed by means of velocity vector split and perturbation method in this paper.Especially the shear.flow is considered.All the obtained weakly non-linear equarions have clear physics concept. Basing on the analysis.the interaction between first order sound and vortex.and the creation of the secnd order vortex are studied and some.experiment phenomena of airfoil.flow control by sound are explained.

Fast prediction of aerodynamic noise induced by the flow around a cylinder based on deep neural network

Accurate and fast prediction of aerodynamic noise has always been a research hotspot in fluid mechanics and aeroacoustics.The conventional prediction methods based on numerical simulation often demand huge computational resources,which are difficult to balance between accuracy and efficiency.Here,we present a data-driven deep neural network(DNN)method to realize fast aerodynamic noise prediction while maintaining accuracy.The proposed deep learning method can predict the spatial distributions of aerodynamic noise information under different working conditions.Based on the large eddy simulation turbulence model and the Ffowcs Williams-Hawkings acoustic analogy theory,a dataset composed of 1216samples is established.With reference to the deep learning method,a DNN framework is proposed to map the relationship between spatial coordinates,inlet velocity and overall sound pressure level.The root-mean-square-errors of prediction are below 0.82 dB in the test dataset,and the directivity of aerodynamic noise predicted by the DNN framework are basically consistent with the numerical simulation.This work paves a novel way for fast prediction of aerodynamic noise with high accuracy and has application potential in acoustic field prediction.

Electrodynamics of Inhomogeneous (Laminated, Angular) Structures

The consistent physic-mathematical model of propagation of an electromagnetic wave in a heterogeneous medium is constructed using the generalized wave equation and the Dirichlet theorem. Twelve conditions at the interfaces of adjacent media are obtained and justified without using a surface charge and surface current in explicit form. The conditions are fulfilled automatically in each section of counting schemes for calculations. A consistent physicomathematical model of interaction of nonstationary electric and thermal fields in a layered medium with allowance or mass transfer is constructed. The model is based on the methods of thermodynamics and on the equations of an electromagnetic field and is formulated without explicit separation of the charge carriers and the charge of an electric double layer. The influence of a slowly moving medium on the electromagnetic wave propagation is considered. The calculation results show the absence of the influence of the medium’s motion on the phase shift of waves, which is consistent with experimental data.