Flow-generated Noise Research Articles

Aeroacoustic Investigation of Airfoil Profiles in Turbulence at Moderate Reynolds Numbers Involving the Phased Array Microphone Technique and the Anova Statistical Method

Due to the growing number of drones and unmanned aerial vehicles, the aeroacoustic noise radiated by airfoils in moderate Reynolds-number flows increasingly becomes the focus of interest. In the present paper, the flow-generated noise of two basic (a flat plate and a cambered plate) and two conventional (RAF6 and NACA 0012) airfoil profiles is investigated in an open-jet wind tunnel in the interval of Reynolds numbers between 60,000 and 140,000. The measurements were carried out with a phased array microphone. The measurement data were evaluated using frequency–domain beamforming and the analysis of variance statistical method. The results show that the emitted noise is nearly independent of the angle of attack but dependent on the airfoil geometry and the chordwise location.

A generalized computational approach to predict high-frequency acoustic pressure response of cavity structures for structural health monitoring of wind turbine blades

This paper details the development of a generalized computational approach that enables prediction of cavity-internal sound pressure distribution due to flow-generated noise at high frequencies. The outcomes of this research is of particular interest for development of an acoustics-based structural health monitoring system for wind turbine blades. The methodology builds from existing reduced-order aeroacoustic modeling techniques and ray tracing based geometrical acoustics and is demonstrated on the model NREL 5 MW wind turbine blade as a case study. The computational predictions demonstrated that damage could be successfully detected in the first half of the blade cavity near the root and that the change in frequency content may be indicative of the type of damage that has occurred. This study provides a foundation to analyze specific blades and likely damage cases for determining key factors of system design such as number and placement of sensors as well as for hardware selection.

The Effects of Hydrophone End-Cap Extension on the Flow-Generated Noise

The Effects of Hydrophone End-Cap Extension on the Flow-Generated Noise

Generalized flow-generated noise prediction method for multiple elements in air ducts

Unavoidable discontinuities due to in-duct elements in a ventilation ductwork system result in the generation of localized turbulence. Some of the turbulence energy is converted into noise. The 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. Prediction of the flow-generated noise at the design stage is important in engineering, since it is almost impossible to solve the problem after the installation of a ventilation ductwork system. The measurement and computational fluid dynamics simulation can provide an accurate prediction but are expensive in terms of resources. There is an urgent need for developing a practical prediction method for the flow-generated noise in air ducts. Current design guides only provide a prediction method for aerodynamic sound produced by a single in-duct element in the ventilation system, which is usually not in accord with actual systems. An interaction factor βm is therefore proposed to take account of the interaction effects of multiple in-duct elements. Experimental measurements were conducted to verify the proposed prediction results. The predicted results show an acceptable agreement with the measurements. The proposed method provides engineers a practical generalized technique for predicting noise produced by multiple in-duct elements.

Effect of standoff distance on the reconstruction of in-duct velocity field and regeneration of pressure field

Identification of in-duct acoustic source characteristics is essential in the design of fluid machinery system for reducing and predicting the flow-generated noise. To this end, the inverse estimation method can be employed by using the measured sound field and matrix formulation for wave propagation within a duct. In this paper, the effect of the distance between source and measurement plane is investigated. At each standoff distance, pressures are measured at three planes with two different spacings to widen the estimation frequency range, and measurements are conducted with three different standoff distances. Modal decomposition is applied to estimate modal amplitudes, and the result is used to reconstruct the velocity field at the source plane and to obtain the regenerated pressure field at the measurement planes. It is shown that the modal amplitude identified by measured pressure field at the short standoff distance, i.e., at nearfield, can yield an accurate reconstructed velocity field of the source and regenerate the pressure field with smaller error, which is similar to the other inverse techniques such as equivalent source method and nearfield acoustical holography. A field reduction example by suppressing some parts of source velocity field is shown for demonstrating the effectiveness of the method.

Enhancing the Characterisation of Ducted Noise Sources Using a Spectral Formulation

The characterization of ducted sound sources is very useful to the aeronautical and automotive industries. It provides inputs to models used for predicting flow-generated noise at the design stage. Conventional inverse approaches, such as the Equivalent Source Method, offer accurate reconstruction results. But they require either a large number of in-duct field pressure measurements or a suitable location of a few number of equivalent sources. This research proposes an alternative approach formulated in the spectral domain: the Spectral Decomposition Method. It directly provides a cross-sectional imaging of the source velocity distribution from pressure data measured at small or large stand-off distance to the source. Well-resolved imaging results of the axial acoustic velocity generated by wall-mounted drivers have been obtained from either induct or wall-pressure measurements acquired respectively without or with flow.

Normalised spectrum for flow-generated noise prediction using computational fluid dynamics

Flow-generated noise from in-duct strip spoilers was radiated from an open exhaust termination unit into a 70 m3 reverberation chamber and have been measured in 1/3 octave bands using sound power spectra. Computational fluid dynamics (CFD) software package was adopted to model the strip spoilers in the air duct. Based on the results of CFD simulation of relevant configurations, the technique of Mak and Au and Mak and Oldham was adopted to normalise the experimental data. A normalised spectrum has been produced for predicting the sound power level of flow-noise produced by the strip spoilers in a rectangular air duct. The data collapse for the strip spoilers is generally good at higher Strouhal numbers, but is less efficient at lower Strouhal numbers where considerable scatter is observed. Together with the normalised spectrum, the predictive equations of Mak and his co-investigators can be used to predict the flow-generated noise produced by in-duct strip spoilers in a ventilation system at design stage using CFD. Practical application: Together with the normalised spectrum, the predictive equations of Mak and his co-investigators can be used to predict the flow-generated noise produced by in-duct strip spoilers in a ventilation system at design stage using CFD.

Prediction of flow-generated noise produced by an in-duct spoiler in a ventilation system using CIBSE Guide B5 methods

Measurements of flow-generated noise produced by two different types of strip spoilers in the air duct and radiated from an open exhaust termination unit into a reverberation chamber have been made in 1/3 octave bands using sound power spectra. The measured values are compared with the predicted values using CIBSE Guide B5 method based on selected values of the constant C. It is found that the measured values agree well with the predicted values. The measured values are also compared with the predicted sound power levels using the generic equations given in the Appendix 5.A2 of the CIBSE Guide. It is found that the predicted values using the generic formulae and the single normalised spectrum given in the Appendix 5.A2 of CIBSE Guide do not agree well with the measured values, particularly at low frequencies. For very different in-duct elements, a single normalised spectrum for flow-generated noise may not be sufficient for all configurations of the in-duct elements. It is suggested that several representative normalised spectra for different types of in-duct elements should be given. Practical application: This study provides an evaluation of the accuracy of the CIBSE Guide B5 method and the CIBSE Appendix 5.A2 generic formulae for the prediction of flow-generated noise produced by in-duct spoilers in a ventilation system. It also gives comments on the method for improvements.

A turbulence-based prediction technique for flow-generated noise produced by in-duct elements in a ventilation system

A number of investigators have tried to develop a generalized prediction method for flow-generated noise produced by in-duct elements in a ventilation system. Most of these prediction methods relied on limited data obtained from conventional measurement techniques that require the use of an expensive and specially combined acoustic and aerodynamic experimental facility. An alternative to using a specialised and aerodynamic facility that is currently gaining favour in building engineering is using computational fluid dynamics (CFD) software packages. CFD is a powerful design tool that is able to predict the behaviour of fluid flow regimes. With the aid of CFD, Mak and Oldham have developed a predictive technique that is based on the relationship between the acoustic power radiated, due to the interaction of air flow and a spoiler, and the turbulent kinetic energy generated in the region of the spoiler. Based on the results of CFD simulation of relevant configurations, the technique has been adopted to normalize the published experimental data of Nelson and Morfey, who produced a normalized spectrum for predicting the sound power level of flow-noise produced by the strip spoilers in a rectangular air duct. In this paper, the theoretical basis of this technique was reviewed and revised. A collapse of data from the simulation models were obtained against the experimental data of Oldham and Ukpoho. The data collapse for a damper were generally excellent at most Strouhal numbers. The data collapse for an orifice plate were generally excellent at lower Strouhal numbers but was less efficient at higher Strouhal numbers where considerable scatter was observed.

Computational Aero-Acoustic Using High-order Finite-Difference Schemes

In this paper, a high-order technique to accurately predict flow-generated noise is introduced. The technique consists of solving the viscous incompressible flow equations and inviscid acoustic equations using a incompressible/compressible splitting technique. The incompressible flow equations are solved using the in-house flow solver EllipSys2D/3D which is a second-order finite volume code. The acoustic solution is found by solving the acoustic equations using high-order finite difference schemes. The incompressible flow equations and the acoustic equations are solved at the same time levels where the pressure and the velocities obtained from the incompressible equations form the input to the acoustic equations. To achieve low dissipation and dispersion errors, either Dispersion-Relation-Preserving (DRP) schemes or optimized compact finite difference schemes are used for spatial discretizations of the acoustic equations. The classical fourth-order Runge-Kutta time scheme is applied to the acoustic equations for time discretization.

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.

Development of Computer Software for Predicting Flow-generated Noise Produced by In-duct Elements in Ventilation Systems

The accurate prediction of flow-generated noise in a ventilation system at the design stage is of importance. The prediction method recently derived by Mak for predicting flow-generated noise produced by multiple elements in an air duct is introduced in this paper. Based on the prediction method of Mak, new computer software is developed to enable designers to predict flow-generated noise produced by multiple in-duct elements. The development of the program is introduced. Three numerical examples computed by the software are presented and discussed.

Generalised Prediction Methods for Flow-generated Noise Produced by Multiple In-duct Elements

Generalised Prediction Methods for Flow-generated Noise Produced by Multiple In-duct Elements

Problems Encountered in the Prediction of Flow-Generated Noise in HVAC Air Distribution Systems

In order to attain a satisfactory indoor environmental quality, the design of a HVAC air distribution system is not only determined by thermal requirements but also the acoustical requirements. Current design guides such as CIBSE guide and ASHRAE handbook have provided the design method for predicting the flow-generated noise in a HVAC air distribution system. However, these guides are based on the experimental works of a number of investigators who worked on a limited range of in-duct components and a limited range of duct sizes. Moreover, these guides are applied to an isolated element in a HVAC air distribution system. It is known that they underestimate the levels off low-generated noise in practical systems. In this paper, problems encountered in the prediction off low-generated noise will be discussed. Recent development of prediction methods for flow-generated noise will also be introduced.

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.

Flow-Generated Noise in Expansion-Chamber-Type Silencers (2nd Report, The Effect of Chamber Resonance on Noise Characteristics).

This paper describes the relationship between the flow noise generated secondarily in cylindrical expansion chambers and chamber resonance. In a sound pressure spectrum measured inside each chamber tested, the resonance components appear at the frequency determined by the chamber dimensions and the mode of vibration in the cylindrical sound field produced in the chamber. The resonance in connection with the radial vibration mode is intensified when its frequency approaches that of the dominant component of flow-generated noise caused by the impact of the jet to the exit of the chamber of which the length is less than four times of pipe diameter. In this case, the sound pressure level of flow noise emitted from the tail-pipe end excels by a very large amount at the frequency close to the resonance frequency, so that its overall value increases. The ranges of chamber size and mean flow velocity, in which the amplified chamber resonance occurs, may be presumed by using Fig. 11 arranged with a Strouhal number and the formula for resonance frequency on the circular sound field.

Flow-Generated Noise in Circular Cavities or How Teapots Whistle

Flow-Generated Noise in Circular Cavities or How Teapots Whistle

Estimation on Acoustic Power of Flow-Generated Noise and on Head Losses of Silencers

Estimation on Acoustic Power of Flow-Generated Noise and on Head Losses of Silencers