Jerome Anthoine

Computation of Wall-Pressure Spectra from Steady Flow Data for Noise Prediction

DOI: 10.2514/1.J050206 A method is proposed to calculate the trailing-edge broadband noise emitted from an airfoil, based on a steady Reynolds-averaged Navier–Stokes solution of the flowfield. For this purpose, the pressure spectrum on the airfoil surfacenearthetrailingedgeiscalculatedusingastatisticalmodelfromtheReynolds-averagedNavier–Stokesmean velocity and turbulence data in the airfoil boundary layer. The obtained wall-pressure spectrum is used to compute the radiated sound by means of an aeroacoustic analogy, namely, Amiet’s theory of airfoil sound. The statistical model for wall-pressure fluctuations is validated with two test cases from the literature, a boundary layer with an adverse pressure gradient, and a flat plate boundary layer without a pressure gradient. The influence of specific model assumptions is studied, such as the convection velocity of pressure-producing structures and the scale anisotropy of boundary-layer turbulence. Furthermore, the influence of the Reynolds-averaged Navier–Stokes simulation on the calculated spectra is investigated using three different turbulence models. The method is finally applied to the case of a Valeo controlled-diffusion airfoil placed in a jet wind tunnel in the anechoic facility of Ecole Centrale de Lyon. Reynolds-averaged Navier–Stokes solutions for this test case are computed with different turbulencemodels,thewall-pressurespectrumnearthetrailingedgeiscalculatedusingthestatisticalmodel,andthe radiated noise is computed with Amiet’s theory. All intermediate results of the method are compared with experimental data.

Trailing Edge Noise of a Controlled-Diffusion Airfoil at Moderate and High Angle of Attack

This paper proposes to study the flow and corresponding noise around a segment of an automotive blade (CD-airfoil) at design conditions, namely a 8 ◦ angle of attack, and a higher angle of attack of 15 ◦ for which there is experimental evidence that the flow regime has significantly changed. This study encompasses two aspects, the flow resolution around the profile using the commercial solver Fluent 6.3 and the noise propagation using Amiet’s theory based on the trailing edge wall-pressure spectrum or Curle’s analogy using the pressure distribution along the blade. All along, computational results are compared to experiments taken in the large anechoic chamber from Ecole Centrale de Lyon (ECL). For the 8 ◦ case, two mesh refinements in the spanwise direction and two boundary conditions in spanwise direction, periodic and symmetric, and combination of both are studied. It reveals that both improvement of mesh and use of periodic spanwise boundary condition, have an influence on the trailing edge spectrum, decreasing the low frequency content to approach experimental results. Furthermore, these numerical parameters help to predict correctly the spanwise coherence measured experimentally. On the contrary, this numerical configuration is not improving the mean and rms velocity profiles in the wake of the airfoil and can even have a negative effect on the energy spectrum of the wake velocity. Acoustic results show satisfactory agreement using Curle’s analogy especially in the low frequency range with experimentally measured noise, the sound radiated being under-predicted abobe 1 kHz. Amiet’s theory is over-predicting the sound radiated compared to experiments due to the over-prediction of the wall-spectrum at trailing edge. The first attempt to compute the flow around the same airfoil at 15 ◦ angle of attack reveals that, again, the wall-pressure spectrum is over-predicted at low frequencies. Furthermore, it appears that the computational domain in the spanwise direction is too restrited to capture correctly the low frequency content of the flow. The computation is then largely over-predicting the spanwise coherence at low frequencies.

Prediction of Incoming Turbulent Noise Using a Combined Numerical / Semi-Empirical Method and Experimental Validation

The present paper investigates the case of a NACA0012 airfoil placed in a turbulent jet. The nozzle outlet diameter is equal to D = 0.041 m. The airfoil is placed at zero angle of incidence and with its leading edge located at 6D from the jet outlet. The chord of the airfoil is equal to D, and has a constant section over its span of 10D. The outlet velocity magnitude U0 is fixed to 13.2 m/s resulting in a Reynolds number based on the chord length of 36,000 and a Mach number of 0.04. The unsteady, three-dimensional incompressible flow around the airfoil is first computed with the LES module of the commercial solver FLUENT Rev. 6.2. The numerical flow results are compared with statistics on the velocity field (mean, RMS and spectra) obtained experimentally with hot wire anemometry on the same geometry and for the same operating conditions. This comparison reveals, as expected, that the mesh refinement influences the cut-off frequency resolution. Besides, an innovative procedure is proposed to compute from the same CFD computation, the noise radiated for the all frequency spectrum. The SYSNOISE Rev.5.6 solver, integrating Curle’s analogy, is used to predict the low frequency part of the noise spectrum, while Amiet’s theory is used to predict the high frequency range. The first one, limited to the computation of sound radiated by compact sources and then to the low frequency range, uses the unsteady pressure fluctuations on the airfoil stored during the CFD flow computation. The second one is a theory specially developed for airfoil sound radiation at high frequency and taking then into account, in an explicit way, non-compactness effects. The statistical flow data, needed by Amiet’s model, are fitted on the CFD data.

Numerical issues in the application of an amiet model for spanwise-varying incoming turbulence

The present paper investigates the possible application of Amiet’s theory in spanwise varying conditions. This study encompasses two different aspects. Firstly, all the parameters of the turbulent flow upstream of the airfoil have to be known to rescale the theoretical turbulence spectrum used in the turbulence interaction theory. In that framework, a methodology to compute the turbulence length scale upstream of the airfoil has been developed through the use of Fourier transforms of the upwash velocity of the incoming flow. Secondly, Amiet’s theory has been developed for airfoil noise radiation in uniform flow. In case of spanwise varying conditions, a Segmentation Method is proposed to predict the radiated noise. This method consists in cutting the airfoil in segments having each its own upstream flow conditions and to sum up the resulting individual emitted noise to obtain the total radiated noise in the far-field. This direct method reveals that spurious effect due to radiation angle and finite size of segments can appear. A rescaling based on listener position and a new Inverse Segmentation Method using a combination of large span airfoils are proposed to solve both problems. This method has been tested for several flow parameters and has shown its potential to reproduce correctly the radiated noise. The methods proposed in this paper have been combined and used on the jet-airfoil interaction case, for which acoustic prediction using Amiet’s theory has shown a satisfactory agreement compared to experimental noise measurements. Noise pollution is encountered in various industrial applications : noise emitted by landing gear in transport industry; noise from wings and high-lift devices in aeronautics; noise due to windshield wipers, rear-view mirrors, engine cooling systems and mufflers in car industry. It also exists in other domains of applications, as wind turbines or fans. In most of the cases, the noise produced is spread among all the frequency spectrum leading sometimes to a significant participation of high frequencies to the overall sound produced. The definition itself of the limit between low and high frequencies is not straightforward but a common rule is to consider the non-compactness limit of the problem. In such situations of non-compactness, the influence of the higher frequencies is an important factor to take into account in the noise predictions and specific methods for these frequencies have to be developed. Among the methods available and widely used to predict the low frequency components are the hybrid methods (indirect methods) in which the computation of the flow is decoupled from the computation of the sound. The computational cost to obtain the acoustic field is highly reduced compared to direct methods, computing the flow and its sound field together, when high Reynolds number and low Mach number are considered, as in most of industrial applications of interest. These hybrid methods consist in two steps : a) firstly, near the noise source, the flow field is obtained from an unsteady computation ; b) secondly, the acoustic source radiation is computed in the far-field by the use of an analogy. 1–3 This methodology is based on the substitution of the real flow by equivalent sources, computed as a post-processing of the flow data.

Assessment of Slag Accumulation in Solid Rocket Boosters: Part II, Two-Phase Flow Experiments

In solid propellant rocket motors incorporating a submerged nozzle, the entrapment of liquid residues of the combustion in the cavity formed in the surrounding of the nozzle integration part can lead to the accumulation of slag with a considerable mass. The long-term goal of the present study is to characterise experimentally the driving parameters of the slag accumulation in a stagnant area modelling the nozzle cavity. The experimental database will be used for validation of a numerical tool. In the present article measurements are shown in two-phase flow condition using a coldgas simplified model to determine effectively the main parameters that are influencing the droplet entrapment. Furthermore, the deformation of the accumulated liquid surface is analysed and presented as well.

Residual Distribution Method for Aeroacoustics

This article deals with the discretization of linearized Euler equations by multidimensional upwind residual distribution methods. Linearized Euler equations are applied to model the propagation of sound in the domain where no source of sound is present and where the analogy methods such as Ffowcs―Williams can not be used because of gradients in the mean flow. The residual distribution method leads to a class of schemes that shares properties of both finite element method and finite volume method. In particular, the schemes used here are multidimensional upwind, which make them very attractive because of their low cross-dissipation. First, the discretization method is introduced as an alternative method for computational aeroacoustic applications on unstructured grids. The residual distribution method is then analyzed analytically for wave propagation. Next it is applied to linearized Euler equations with proper acoustic boundary conditions, and finally verifiedon test cases having exact solution.

Assessment of Slag Accumulation in Solid Rocket Boosters: Summary of the VKI research

In solid propellant rocket motors incorporating a submerged nozzle, the entrapment of liquid residues of the combustion in the cavity formed in the surrounding of the nozzle integration part can lead to the accumulation of slag with a considerable mass. The goal of the present study is to characterize experimentally the driving parameters of the slag accumulation in a stagnant area modelling the nozzle cavity. Furthermore, the experimental database is used for validating a numerical tool as well. In the present article a summary of the VKI (von Karman Institute) research carried out during the last four years is shown. Single-phase and two-phase measurements are performed using a cold-gas simplified model. Simultaneously, the experimental configuration is simulated numerically with the help of a commercial solver (CFD-ACE+). The final aim of the article is to point out the driving forces of the droplet entrapment process.

Measurement and analysis of radiated sound from a low speed fan with a large tip gap

The wake flow field and radiated sound from a low speed axial fan is studied experimentally. The fan geometry uses controlled diffusion blades and is designed with a low aspect ratio (0.9). The fan is installed with a large tip gap, approximately 10% of the blade span. The radiated sound field is analyzed using a known trailing edge noise formulation. First, the model is compared to an experiment of a single airfoil in a wind tunnel to assess the predictive capabilities. Second, measurements of the fan are made at two different blade loading conditions. Hot wire measurements are made in the near wake of the fan to assess the extent of the tip leakage flow for each condition. The radiated sound fields are compared with the trailing edge noise theory. Use is made of the wake measurements as an input to a surface pressure model. When the fan is operated with the optimal blade loading, the influence of the tip leakage flow is found to be of secondary acoustic impact. When the fan is operated at a high loading condition for the blades, a more significant leakage flow develops and is found to be responsible for the dominant radiated sound.

Design and evaluation of an aeroacoustic wind tunnel for measurement of axial flow fans

An anechoic wind tunnel dedicated to fan self-noise studies has been designed and constructed at the von Karman Institute The multi-chamber, mass flow driven design allows for all fan performance characteristics, aerodynamic quantities (e.g., wake turbulence measurements), and acoustic properties to be assessed in the same facility with the same conditions. The acoustic chamber performance is assessed using the optimum reference method and found to be within the ISO 3745 standards down to 150 Hz for pure tone and broadband source mechanisms. The additional influence of installation effects of an aerodynamic inlet was found to create a scattered sound field only near the source location, while still providing good anechoic results at more distant sound pressure measurement positions. It was found to have inflow properties, span-wise uniformity, and low turbulence intensity, consistent with those desired for fan self-noise studies.

Application of Residual Distribution Method for Acoustic Wave Propagation

This article deals with the discretization of Linearized Euler Equations (LEEs) by multidimensional upwind Residual Distribution methods. Linearized Euler equations are applied in the domain where there is no source of sound and the analogy methods such as Ffowcs-Williams can not be used because of gradients in the mean flow. Residual distribution method is a class of schemes that is in between finite-element and finite-volume. In particular, the schemes that we use are multidimensional upwind which make them very attractive because of their very low cross-dissipation. First, we define the formulation of the LEEs that we choose to use. Then, we focus on the residual schemes and we describe two ways of discretizing unsteady problems. The third part presents a wave number of those schemes. Finally, we show the advantage of these schemes on several acoustic problems.