Prediction of flow noise from in-duct spoilers using computational fluid dynamics

This paper presents a method for the prediction of flow noise from in-duct spoilers using Computational Fluid Dynamics (CFD). Previous work conducted by Mak [Mak CM. A prediction method for aerodynamic sound produced by multiple elements in air ducts. J Sound Vib 2005;287:395–403] and Mak et al. [Mak CM, Wu J, Ye C, Yang J. Flow noise from spoilers in ducts. J Acoust Soc Am 2009;125:3756–65] provides a method for such a prediction based on experimental data. In this work, the advanced Large-eddy Simulation turbulence model (LES) is used to compute the mean pressure drop across spoilers and the fluctuating drag forces acting on them. The predictions of the numerical simulation agree well with those based on experimental data and the actual, measured levels. With the aid of CFD simulation, engineers are now able to use the prediction method developed by the authors to predict the flow noise from in-duct spoilers using the LES turbulence model.     

A study of the aerodynamic and acoustic performance of an indoor unit of a DC-inverter split air-conditioner

The aerodynamic and acoustic performance of an indoor unit of a DC-inverter split air-conditioner is simulated by computational fluid dynamics (CFD) and computational aerodynamic acoustics (CAA). Numerical predictions of the flow rate (FR) and band sound pressure level (BSPL) are verified by experimental data and used to create an improved design. The parametric influence upon the values of FR and BSPL for the unit is analyzed for various cases of geometrical parameters of the indoor unit and the cross-flow fan. Two improved designs are developed that can increase the FR for the existing prototype of indoor unit while the BSPL remains lower.     

基于面元法的水平轴风力机数值模拟

使用基于速度面元法的势流数值模拟方法,以NREL PhaseⅥ为例进行了叶片气动载荷和风轮近尾流场的数值模拟。将势流数值模拟、叶素动量理论和计算流体力学CFD方法的计算结果与实验数据进行了对比分析。结果表明使用速度面元法计算风轮绕流场具有较高的计算精度和求解效率,为大规模风力机群的流场计算和出力预报提供支撑。     

Aeroacoustics of the swinging corrugated tube: voice of the Dragon

When one swings a short corrugated pipe segment around one's head, it produces a musically interesting whistling sound. As a musical toy it is called a "Hummer" and as a musical instrument, the "Voice of the Dragon." The fluid dynamics aspects of the instrument are addressed, corresponding to the sound generation mechanism. Velocity profile measurements reveal that the turbulent velocity profile developed in a corrugated pipe differs notably from the one of a smooth pipe. This velocity profile appears to have a crucial effect both on the non-dimensional whistling frequency (Strouhal number) and on the amplitude of the pressure fluctuations. Using a numerical model based on incompressible flow simulations and vortex sound theory, excellent predictions of the whistling Strouhal numbers are achieved. The model does not provide an accurate prediction of the amplitude. In the second part of the paper the sound radiation from a Hummer is discussed. The acoustic measurements obtained in a semi-anechoic chamber are compared with a theoretical radiation model. Globally the instrument behaves as a rotating (Leslie) horn. The effects of Doppler shift, wall reflections, bending of the tube, non-constant rotational speed on the observed frequency, and amplitude are discussed.     

Development of a prediction method for flow-generated noise produced by duct elements in ventilation systems

Flow-generated noise generated on the quiet side of the primary attenuators of a ventilation system is the result of interaction between air flow and duct discontinuities. It is of engineering importance to predict the flow-generated noise caused by air duct elements in ventilation systems at the design stage. However, all prediction methods are based upon an isolated in-duct element that is very different from a real ventilation system. Until recently, Mak and Yang have produced a prediction method for flow-generated noise produced by the interaction of two elements in air ducts. In this paper, an attempt has been made to modify their equations so that their predictive equations can possibly be used to predict noise produced by “real” duct discontinuities. By comparing their predictive values with the experimental results of Oldham and Ukpoho, their validity can be proved. The modified Mak-Yang predictive equations, therefore, provide a basis for permitting a more accurate prediction of flow-generated noise produced by various configurations of two in-duct elements and duct dimensions.     

A computational modeling approach of the jet-like acoustic streaming and heat generation induced by low frequency high power ultrasonic horn reactors

High power ultrasound reactors have gained a lot of interest in the food industry given the effects that can arise from ultrasonic-induced cavitation in liquid foods. However, most of the new food processing developments have been based on empirical approaches. Thus, there is a need for mathematical models which help to understand, optimize, and scale up ultrasonic reactors. In this work, a computational fluid dynamics (CFD) model was developed to predict the acoustic streaming and induced heat generated by an ultrasonic horn reactor. In the model it is assumed that the horn tip is a fluid inlet, where a turbulent jet flow is injected into the vessel. The hydrodynamic momentum rate of the incoming jet is assumed to be equal to the total acoustic momentum rate emitted by the acoustic power source. CFD velocity predictions show excellent agreement with the experimental data for power densities higher than W(0)/V ≥ 25kWm(-3). This model successfully describes hydrodynamic fields (streaming) generated by low-frequency-high-power ultrasound.     

Scattering and absorption of sound at flow duct expansions

The scattering of plane acoustic waves at area expansions in flow ducts is analysed using the vortex sheet model where the flow at the expansion is modelled as a jet, with a vortex sheet emanating from the edge. Of particular interest is the influence of the flow field on acoustic scattering and absorption.First, it is demonstrated that the stability properties of the shear layer can be simulated by modifying the edge condition within the vortex sheet model. To this end the accuracy for the region where the shear layer is changing from unstable to stable is improved by introducing a gradually relaxed Kutta edge condition with empirically derived coefficients. For low Strouhal numbers the vortex sheet model applies and for higher Strouhal numbers the two models converge.Second, it is demonstrated that the acoustic transmission through the jet expansion region can be determined by neglecting the scattering there. Incorporating this assumption, the vortex sheet model reproduces well the experimental results on transmission and absorption for an area expansion. This result supports the assumption that the main part of the scattering occurs at the area expansion at least for the low-frequency range. Furthermore, the influence of the flow field is particularly strong for small Strouhal numbers.     

高速动能弹温度场数值模拟

随着兵器发射技术和空气动力学技术的发展,动能弹的发射初速和飞行状态正从超声速向高超声速发展,由此产生了气动热问题.准确预测动能弹温度场是其气动力和热防护设计的关键技术.采用CFD预测温度场的方法,包括平衡流流动控制方程及差分格式,构造平衡流通量Jacob矩阵,在差分格式矢通量分裂过程中嵌入平衡流真实气体模型模拟温度场,获得平衡流气体状态方程.对典型高速动能弹热环境进行验证,考察方法的合理性.对设计的一种新型高超声速动能弹温度场进行数值模拟,为其气动设计及热防护提供了较可靠的数据.     

Prediction of flow-generated noise produced by acoustic and aerodynamic interactions of multiple in-duct elements

Flow-generated noise problem caused by in-duct elements is due to the complicated acoustic and turbulent interactions of multiple in-duct flow noise sources. The approach of partially coherent sound fields used previously by Mak and Yang [C.M. Mak, J. Yang, Flow-generated noise radiated by the interaction of two strip spoilers in a low speed flow ducts, Acta Acust united with Acustica 88 (2002) 861–868] and Mak [C.M. Mak, A prediction method for aerodynamic sound produced by multiple elements in air ducts, J Sound Vib 287 (2005) 395–403] is adopted to formulate the sound powers produced by interactions of multiple elements at frequencies below and above the cut-on frequency of the lowest transverse duct mode. The study indicates that the level and spectral distribution of the additional acoustic energy produced by the interactions of multiple elements can be predicted based on the measured data with respect to the interactions. The proposed method can form a basis of a generalized prediction method for flow-generated noise produced by multiple elements. The application of the proposed method is supported by two engineering examples.     

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 1216 samples 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.     

Effect of cutter geometric configuration on aerodynamic noise generation in face milling cutters

Aerodynamic noise spectrum of rotary face milling cutters consists of a broad range of high frequencies and discrete tones. This paper aims to develop a method to calculate the aerodynamic noise generation and propagation by rotary face milling cutters. The effects of milling cutter geometry on the generation of aerodynamic noise are analyzed. Based on the computational fluid dynamics (CFD) method, the Ffowcs Williams–Hawkings (FW–H) equation is used to predict the sound pressure level (SPL) of aerodynamic noise in face milling cutters. The accurate calculation of time-varying flow variables along with the rotation of cutter is very important for the prediction of aerodynamic noise. In this case, the Navier–Stokes (N–S) equation is employed to evaluate the pressure and velocity fields around the milling cutters, first in a steady mode with the Multiple Reference Frames (MRF) model, and then in an unsteady mode with sliding mesh technique (SMT) by introducing the steady flow variables as its initial fields. It is found that both the overall aerodynamic noise due to the entire cutter and the aerodynamic noise only due to the cutter gullet regions are significantly affected by the number of cutter teeth/gullet regions. Moreover, six representative milling cutters with different tooth numbers and geometries of gullet regions are chosen to study the effects of gullet configuration on aerodynamic noise generation, and the characteristics of noise spectra generated by the cutters are analyzed. The aerodynamic noise generated only by the cutter gullet regions is found to be strongly dependent on the gullet design-volume and shape. The results also reveal that the gullet design advantage of Cutter C in reducing noise generation among the eight-tooth designs, and the gullet design advantage of Cutter A in reducing noise generation among the five and seven-tooth designs in this investigation.     

Noise Sources of Aerodynamic Origin in Air Blowers

The paper presents the results of theoretical and experimental studies of the occurrence and locations of aerodynamic noise sources in air blowers related to air flow around stationary and moving elements inside a machine body. These studies were based on basic research by Lighthill and Curle and used a developed method for measuring pressure pulsations on rotating blades and stationary elements of a machine body. The most significant sources of discrete and broadband components of aerodynamic noise were revealed. The role of blades in an impeller in the emission of discrete noise components was studied. It was established that broadband peaks in the emitted noise are associated with acoustic resonances of the internal volume of the air blower. It was shown that the turbulence and velocity of the incoming flow influence the intensity of aerodynamic sources inside the body. Our studies spurred both deeper research into the nature of aerodynamic noise sources that form in air blowers and recommendations for reducing the noise produced by these sources.     

Investigation of a hybrid approach combining experimental tests and numerical simulations to study vortex-induced vibration in a circular cylinder

In this study, a hybrid approach based on computational fluid dynamics (CFD) was used to investigate the aerodynamic forces associated with vortex-induced vibration (VIV) in a circular cylinder. The circular cylinder and the flow field were considered as two substructures of a system. Circular cylinder motion was produced in a wind tunnel test of the VIV prior to the numerical simulation; this motion was used as a known cylinder boundary condition and applied to the flow field. The flow field with the known moving boundary condition was then numerically simulated by the ANSYS CFX code. The transient aerodynamic coefficients of the circular cylinder with predetermined motion were obtained from the numerical simulation. To verify the feasibility and accuracy of the proposed hybrid approach and to calculate cylinder vibrations, the transient aerodynamic coefficients were applied to a single degree of freedom (SDOF) model of the circular cylinder. The oscillation responses of the circular cylinder from the calculated (SDOF model) and experimental results were compared, and the results indicate that the hybrid approach accurately simulated the transient aerodynamic coefficients of the circular cylinder. For further comparison, a nonlinear aerodynamic coefficient model based on a nonlinear least square technique was applied to the SDOF model. The nonlinear aerodynamic model can predict well the amplitude and lock-in region of the VIV of the circular cylinder model.     

Aeroacoustic interaction in a corrugated duct

The sound generation by an air flow in a corrugated tube is studied experimentally for different values of the corrugation pitch and different tube lengths. The Strouhal numbers of sound generated in different tubes with different flow velocities lie within 0.4–0.6. As the flow velocity increases, the Strouhal number decreases. The effect of sound absorption by an air flow in a corrugated duct is described: in a corrugated tube with a flow, at frequencies below the generation frequency, the absorption of sound produced by an external source is observed. A semiempirical model of aeroacoustic interaction in a corrugated tube is proposed. The model provides a qualitative agreement with the experiment.     

平面叶栅气动噪声的高精度谱元方法求解

本文建立了一种用于气动噪声预测的高精度三角谱元方法。结合CBS法(Characteristic-based split method)求解Navier-Stokes方程获取伪声压声源后,基于波动方程求解声传播问题。谱元法的谱收敛特性满足了气动声学问题的高精度需求,而CBS法的引入保证了高雷诺数问题的计算稳定性。通过两组基准解问题求解验证了本文方法的正确性。将本文方法应用于平面叶栅气动噪声计算,并考察了不同攻角下噪声的变化规律。研究内容为进一步探究各类流体机械的气动噪声提供了一种新的途径。     

基于转捩模型及声比拟方法的高精度圆柱分离涡/涡致噪声模拟

基于七阶加权紧致非线性格式(WCNS-E8T7),结合延迟分离涡模拟(DDES)和Ffowcs WilliamsHawkings声比拟方法,对亚临界雷诺数下单圆柱、圆柱-翼型的分离涡/涡致噪声问题进行了数值模拟.针对亚临界雷诺数下圆柱尾迹中的转捩问题,发展了基于γ-Reθ模型高精度转捩-延迟分离涡模拟(Tran-DDES)方法,并与传统基于全湍流剪切应力输运(SST)模型的高精度DDES方法进行了对比.单圆柱模拟结果表明:传统SST-DDES方法会造成平均流场的回流区增大,压差阻力偏小等问题;而添加转捩模型的Tran-DDES方法与实验符合得很好.圆柱尾迹中添加翼型后,翼型对圆柱附近流场产生影响,使SST-DDES方法造成的圆柱后回流区偏大的问题减弱,并与Tran-DDES模拟结果差异变小.但在脉动量预测以及脉动产生的噪声预测方面, Tran-DDES方法仍与实验符合得更好.     

Contributions to the acoustic excitation of bubbles released from a nozzle

It has recently been demonstrated that air bubbles released from a nozzle are excited into volume mode oscillations by the collapse of the neck of air formed at the moment of bubble detachment. A pulse of sound is caused by these breathing mode oscillations, and the sound of air-entraining flows is made up of many such pulses emitted as bubbles are created. This paper is an elaboration on a JASA-EL paper, which examined the acoustical excitation of bubbles released from a nozzle. Here, further details of the collapse of a neck of air formed at the moment of bubble formation and its implications for the emission of sound by newly formed bubbles are presented. The role of fluid surface tension was studied using high-speed photography and found to be consistent with a simple model for neck collapse. A re-entrant fluid jet forms inside the bubble just after detachment, and its role in acoustic excitation is assessed. It is found that for slowly-grown bubbles the jet does make a noticeable difference to the total volume decrease during neck collapse, but that it is not a dominant effect in the overall acoustic excitation.     

Calibration of a γ-Re_θ transition model and its application in low-speed flows

The prediction of laminar-turbulent transition in boundary layer is very important for obtaining accurate aerodynamic characteristics with computational fluid dynamic(CFD)tools,because laminar-turbulent transition is directly related to complex flow phenomena in boundary layer and separated flow in space.Unfortunately,the transition effect isn’t included in today’s major CFD tools because of non-local calculations in transition modeling.In this paper,Menter’sγ-Re_θtransition model is calibrated and incorporated into a Reynolds-Averaged Navier-Stokes(RANS)code-Trisonic Platform(TRIP)developed in China Aerodynamic Research and Development Center(CARDC).Based on the experimental data of flat plate from the literature,the empirical correlations involved in the transition model are modified and calibrated numerically.Numerical simulation for low-speed flow of Trapezoidal Wing(Trap Wing)is performed and compared with the corresponding experimental data.It is indicated that theγ-Re_θtransition model can accurately predict the location of separation-induced transition and natural transition in the flow region with moderate pressure gradient.The transition model effectively imporves the simulation accuracy of the boundary layer and aerodynamic characteristics.     

吸气式高超声速飞行器气动力气动热的数值模拟方法及应用

对吸气式高超声速飞行器而言,物面热流和摩阻的准确预测对飞行器设计及安全十分关键.介绍采用CFD准确预测气动力和气动热的方法,包括流动的控制方程、湍流模型及湍流的先进壁面函数边界条件,介绍流动的数值求解方法.对典型超声速层流和湍流流动的摩擦阻力和热流进行详细的验证与确认,考察CFD工具在使用先进壁面函数边界条件后,湍流计算的法向网格无关性能力.对设计的一种吸气式高超声速飞行器的气动力和气动热进行数值模拟,为飞行器的气动设计及热防护提供了可靠的数据.