Broadband Noise Prediction Research Articles

Effects and mechanisms of LES and DDES method on airfoil self-noise prediction at low to moderate Reynolds numbers

The Large Eddy Simulation (LES) method and Delayed Detached Eddy Simulation (DDES) method combined with the Ffowcs Williams and Hawkings (FW–H) analogy are used to predict the self-noise of NACA 65(12)-10 airfoil at low to moderate Reynolds numbers. The computational results have been validated through the comparison with experimental data and NAFNoise predictive data. The results indicate that both LES and DDES methods can output correct sound pressure level (SPL) spectra at a Reynolds number of 2 × 105, but only the LES method is capable of providing reasonable SPL spectra at a Reynolds number of 5 × 105, which the DDES method has failed to do. The broadband noise below 10 kHz obtained from the DDES method is lower than that from NAFNoise prediction by up to 50 dB. Further analysis shows that the DDES method fails to capture the separation and reattachment on the suction side near the trailing edge. Besides, the magnitude of pressure fluctuation on the airfoil surface obtained from the DDES method is far smaller than that from the LES method, providing incorrect acoustic source information for the FW–H equation. In the near-wall region for the DDES case, the Reynolds-averaged Navier–Stokes method is used to compute the unsteady flow field, providing too large viscosity and too small Reynolds stress near the airfoil surface, which makes it difficult to capture the influence of small-scale eddies in the flow field. The characteristics of the DDES method above could cause errors in simulating the unsteady flow field at a Reynolds number of 105 range. On the contrary, the LES method could give better prediction of broadband noise under different incoming flow conditions.

Rapid Prediction Method of Fan Broadband Noise Based on A Three-dimensional Sound Source Model

The fan/compressor broadband noise is an important noise source of civil large bypass ratio turbofan engine.. The empirical method requires massive amounts of test data for support. Adopting the computational aeroacoustic (CAA) method to predict the fan/compressor broadband noise will cost high amounts of computing resources or time. Thus, this study combines flow field calculation with analytical model to study the prediction method of fan/compressor broadband noise, this method can be used for quick and efficient preliminary evaluation and parameter studies. On the basis of the simulation and modeling of the stator – rotor interference phenomenon of fans, rotor wake flow field and the corresponding parameters are obtained. On this basis, the three-dimensional sound source model is used to calculate the blade surface sound source and its propagation under different working conditions. The distribution of sound energy in the duct is obtained. Then, the broadband noise prediction of fan is further acquired. The broadband noise of a typical single-stage TA36 fan is predicted using this method. Study results suggest that (1) turbulence model and turbulence kinetic energy of inlet boundary condition will have an important effect on the prediction results of broadband noise. (2) When the inlet turbulence kinetic energy is small, different turbulence models slightly influence the prediction results of broadband noise; when the inlet turbulence kinetic energy is large, the sound pressure level of sound field obtained by SST model is approximately 5dB larger than that obtained by model. This method can be used in the early stage of fan/compressor design, and the characteristics of blade broadband noise can be rapidly obtained.

Prediction of Broadband Noise Generated from a Low-pressure Fan Based on Pressure Power Spectrum Density of a Flat Plate

Prediction of Broadband Noise Generated from a Low-pressure Fan Based on Pressure Power Spectrum Density of a Flat Plate

Time domain broadband noise predictions for non-cavitating marine propellers with wall pressure spectrum models

The broadband noise can be dominant or important for total characteristics for marine propeller noise representing the minimum base of self-noise. Accurate prediction of such noise is crucial for survivability of underwater military vessels. While the FW-H Formulation 1B can be used to predict broadband trailing edge noise, the method required experiment measurements of surface pressure correlations, showing its limitations in generality. Therefore, in this study, the methods are developed to utilize wall pressure spectrum models to overcome those limitations. Chase model is adopted to represent surface pressure along with the developed formulations to reproduce pressure statistics. Newly developed method is validated with the experiments of airfoils at different velocities. Thereafter, with its feasibility and generality, the procedure incorporating computational fluid dynamics is established and performed for a propeller behind submarine hull. The results are compared with the experiments conducted at Large Cavitation Tunnel, thus showing its usability and robustness.

Aeroacoustic design and broadband noise predictions of a fan stage with serrated outlet guide vanes

This work, realized in the framework of the European project TurboNoiseBB, presents an advanced aeroacoustic design methodology of Outlet Guide Vanes (OGVs) with leading edge serrations, including details on their broadband noise and aerodynamic performance. The serrated OGV corresponds to a modified stator from an aeroengine fan stage tested at the AneCom AeroTest’s facility (Germany). Sinusoidal leading edge patterns with varying amplitude and wavelength along the span are designed in collaboration with Safran Aircraft Engines. Serrations are adjusted to account for the turbulence characteristics provided by Reynolds-Averaged Navier–Stokes (RANS) calculations. Optimal parameters are found using simple design rules discussed in this paper. Down selection of serrated OGV designs (patent pending) is conducted through a RANS control of aerodynamic performances and in accordance with industrial specifications, ensuring acceptable penalties on the loss coefficient and isentropic efficiency. Broadband noise simulations are performed using a computational aeroacoustic (CAA) code that solves the linearized Euler equations with a synthetic turbulence model. Additionally, the acoustic response of the serrated leading edge airfoils is also estimated using the most relevant analytical formulation in the literature, based on the Wiener–Hopf technique. Numerical predictions at approach conditions are compared to available experimental measurements (on the untreated baseline case) and analytical Amiet-based and Wiener–Hopf solutions, showing satisfactory agreement in the sound power spectrum in the bypass duct. Finally, the acoustic performances estimated at the design stage are numerically assessed by both the CAA and Wiener–Hopf methods, providing around 2.5 dB and 3.5 dB reduction in the overall power level, respectively.

ACAT1 Benchmark of RANS-Informed Analytical Methods for Fan Broadband Noise Prediction—Part I—Influence of the RANS Simulation

A benchmark of Reynolds-Averaged Navier-Stokes (RANS)-informed analytical methods, which are attractive for predicting fan broadband noise, was conducted within the framework of the European project TurboNoiseBB. This paper discusses the first part of the benchmark, which investigates the influence of the RANS inputs. Its companion paper focuses on the influence of the applied acoustic models on predicted fan broadband noise levels. While similar benchmarking activities were conducted in the past, this benchmark is unique due to its large and diverse data set involving members from more than ten institutions. In this work, the authors analyze RANS solutions performed at approach conditions for the ACAT1 fan. The RANS solutions were obtained using different CFD codes, mesh resolutions, and computational settings. The flow, turbulence, and resulting fan broadband noise predictions are analyzed to pinpoint critical influencing parameters related to the RANS inputs. Experimental data are used for comparison. It is shown that when turbomachinery experts perform RANS simulations using the same geometry and the same operating conditions, the most crucial choices in terms of predicted fan broadband noise are the type of turbulence model and applied turbulence model extensions. Chosen mesh resolutions, CFD solvers, and other computational settings are less critical.

Trailing-edge broadband noise prediction of an airfoil with boundary-layer tripping

Hybrid methods for trailing edge noise prediction are applied to a NACA 6512-63 airfoil at zero angle of attack embedded in the actual narrow stream of the Siegen open-jet anechoic wind-tunnel. Incompressible large-eddy simulations (LES) of the airfoil trailing-edge flow yield the noise sources, and different analytical acoustic analogies then predict the acoustic far-field pressure. The chord-based Reynolds number of 1.9 × 105 triggers long laminar flow regions along the airfoil, leading to instability waves with an associated broadband noise radiation. To avoid this laminar-instability noise, the airfoil boundary layer is experimentally tripped on both sides. The actual serration tripping is included in the numerical grid as well as a simplified stair-step trip. Aerodynamic and acoustic results of the performed LES are compared with measurements and compressible direct numerical simulation (DNS) results. No noticeable influence of the trip geometry is found whereas the numerical schemes play a more important role. LES predictions with boundary layer trip compare well with experimental measurements and DNS cases. If the boundary-layer trip is not sufficiently thick in the numerical model, or not present at all, the far-field acoustic pressure is overpredicted and resembles more closely the sound radiation from the untripped airfoil with laminar instability noise.

A hybrid computational aeroacoustic methodology for broadband noise prediction.

Current hybrid computational aeroacoustic methodologies for broadband noise prediction rely on large eddy simulation (LES) for noise source computation, and integral methods for noise propagation. In this paper, LES of a benchmark controlled-diffusion airfoil was conducted, utilizing the rotation rate based Smagorinsky model (RoSM). The RoSM is seen to provide flow results of equal or improved accuracy as compared to the dynamic Smagorinsky model, with 35% less computational cost, with computational benefits of the RoSM increasing with mesh size. Current integral methods for noise propagation, including Formulation 1A of Farassat, require numerical differentiation of highly turbulent input flow data coming from LES, introducing numerical inaccuracies. Here, Formulation 1 of Farassat is modified to entirely avoid the numerical differentiation of flow field data. The ability of the methodology for broadband noise prediction is demonstrated, with the prediction at high frequencies being considerably closer to experimental data than Formulation 1A. For low discrete frequency noise prediction, Formulation 1A is still recommended. The use of two innovative approaches in conjunction for broadband noise prediction is seen to be efficient and easy to implement, without compromising on accuracy.

Analytical prediction of broadband noise from mini-RPA propellers with serrated edges

This paper is dedicated to the analysis of the broadband noise of an innovative mini-remotely piloted aircraft propeller geometry with straight or serrated edges. New analytical models for predicti...

Prediction of Rotorcraft Broadband Trailing-Edge Noise and Parameter Sensitivity Study

This paper investigates the effects of rotorcraft design and operating parameters on trailing-edge noise. A rotor trailing-edge noise prediction method is first developed where the aerodynamics and the turbulence wall pressure spectrum near the trailing edge on airfoils are predicted by a combination of the standard blade element momentum theory , a viscous boundary-layer panel method, and a recently developed empirical wall pressure spectrum model. The coordinate transformations are combined with the Amiet model to predict far-field noise. Compared to experimental data, the validation of this method demonstrates its advantages and validity for airfoil and rotorcraft broadband noise predictions. Then, this method is used to study the effects of rotorcraft design and operating parameters on rotor trailing-edge noise. It is found that helicopter broadband noise scales with the 4.5th to 5.0th power of the tip Mach number in which the range is determined by the typical helicopter collective pitch angle in operation. Detailed trend analyses of noise levels as a function of frequency are presented in terms of the collective pitch angle, twist angle, rotor solidity, rotor radius, disk loading, and number of blades. It is found that the collective pitch angle, twist angle, and chord length make noticeable impacts on low- and midfrequency noise. Finally, a semianalytic model is presented to predict the directivity and geometric attenuation of rotor trailing-edge noise.

Prediction of Broadband Noise in an Automotive Centrifugal Compressor with Three-Dimensional Computational Fluid Dynamics Detached Eddy Simulations

<div class="section abstract"><div class="htmlview paragraph">Centrifugal compressors for automotive turbochargers must operate over wide speed and flow ranges to provide the required air pressure and mass flow rate to the intake manifold of the internal combustion engines. At a fixed rotational speed, the flow field near the inducer of the impeller becomes increasingly unstable with decreasing flow rate, as the incidence angle grows between the air flow approaching the impeller, relative to the tangent of the main impeller blades at the leading edge. Flow field measurements conducted earlier have revealed that once the incidence angle exceeds a critical value (nearly independent of rotational speed) of approximately 15°, reversed flow near the periphery (blade tips) starts penetrating upstream of the impeller, with a high tangential velocity in the direction of impeller rotation. As the incidence angle is increased towards this critical value, whoosh noise elevates, where it remains high for a significant portion of the mid-flow operating range, before decreasing at further elevated incidence angles. To understand this phenomenon further, a detailed, three-dimensional (3D) computational fluid dynamics (CFD) model of the experimental setup was constructed, and simulations were completed at four flow rates along a constant rotational speed. Predictions from this 3D CFD model agree reasonably well with experiments, including the steady-state performance, time-averaged flow field, and noise as captured from the pressure transducer installed in the compressor inlet duct. Near the peak whoosh noise of the studied speed, the impeller flow field was closely examined. Predictions reveal the highest total sound pressure level in the whoosh frequency range occurs near the inducer plane, within the shear layer between the concentric, bi-directional flow structure, with forward flow closer to the axis and reversed flow around the periphery.</div></div>

Prediction of Broadband Noise from Symmetric and Cambered Airfoils

Aerodynamic sound generation and self-noise mechanisms from lifting surfaces such as airfoil involve the fields of classical acoustics and fluid mechanics. In this paper, trailing edge noise production is evaluated using empirical model for NACA 0012 and NACA 6320 airfoils. The sound pressure levels from trailing edge surface are calculated for different flow configurations. The growth of boundary layer thickness and displacement thickness, for different chord lengths and Mach numbers with varying angles of attack, is illustrated for NACA 0012. The sound pressure levels were computed numerically between 00 to 60 angles of attack and at constant chord length of 1.2m using Brookes Pope Marcolini method. The results showed a change of ~2-5dB in peak amplitude for mid frequencies region of spectrum. The effects of varying chord length and Mach number on sound pressure levels are illustrated for both airfoils. The relative velocity field for airfoils was computed using the boundary element method. The combined effect of thickness and camber on sound power level is demonstrated at a 40 angle of attack and for a Mach number of 0.191. Validation of sound pressure levels is done based on the results obtained for NACA0012 for similar flow conditions.

Experimental Study on Prediction of Broadband Noise Generated from a Cooling Fan

Experimental Study on Prediction of Broadband Noise Generated from a Cooling Fan

Comparison of two low-order models for the prediction of fan broadband noise

Two low-order methods for predicting the broadband noise downstream of a fan exit guide vane (FEGV) due to interaction with rotor wake turbulence are compared. The noise is predicted with a two-step approach in each method. The methods share the exact same first step. The unsteady vane surface pressure is computed using a strip approach. At each strip, the 2D flat-plate cascade response to a gust in the wave-number, frequency domain is calculated. An integral approach is used for this calculation. The two methods then differ in the second step. One method integrates the unsteady surface pressure together with the Green's function for a cylindrical annulus to obtain the downstream duct acoustic pressure modes. The other method computes rectilinear acoustic modes relevant to the cascade at each strip. In the second method, the total power is obtained by adding the acoustic power downstream of each strip. Both methods neglect mean flow swirl effects on acoustic propagation. The fan broadband noise workshop RC1 and FC2 cases are used as a platform to compare the two methods. When the FEGV inflow turbulent parameters are taken from different sources (e.g. hot-wire, CFD) the predictions from both methods respond similarly. The trends predicted with both methods based on turbulence level or rotor speed are similar. The trends with vane count predicted by the two methods do not match and it is unclear which prediction is correct. The actual spectrum predicted by the Green's function method for all of the cases is shown to be slightly closer to experimental results.

Impact of cyclostationarity on fan broadband noise prediction

One of the dominant noise sources of modern Ultra High Bypass Ratio (UHBR) engines is the interaction of the rotor wakes with the leading edges of the stator vanes in the fan stage. While the tonal components of this noise generation mechanism are fairly well understood by now, the broadband components are not. This calls to further the understanding of the broadband noise generation in the fan stage. This article introduces a new extension to the Random Particle Mesh (RPM) method, which accommodates in-depth studies of the impact of cyclostationary wake characteristics on the broadband noise in the fan stage. The RPM method is used to synthesize a turbulence field in the stator domain using a URANS simulation characterized by time-periodic turbulence and mean flow. The rotor-stator interaction noise is predicted by a two-dimensional CAA computation of the stator cascade. The impact of cyclostationarity is decomposed into various effects, which are separately investigated. This leads to the finding that the periodic turbulent kinetic energy (TKE) and periodic flow have only a negligible effect on the radiated sound power. The impact of the periodic integral length scale (TLS) is, however, substantial. The limits of a stationary representation of the TLS are demonstrated making this new extension to the RPM method indispensable when background and wake TKE are of comparable level. Good agreement of the predictions with measurements obtained from the 2015 AIAA Fan Broadband Noise Prediction Workshop are also shown.

Novel Method for Investigating Broadband Velocity Fluctuations in Axisymmetric Screeching Jets

The noise spectrum of screeching jets consists of broadband and discrete components referred to as turbulent mixing noise and screech tones, respectively. The screech tones make the application of classical noise models to screeching jets difficult for modern problems. These problems involve an additional treatment of the near-field dynamics before the broadband noise prediction schemes can be implemented. This paper proposes an a posteriori approach for studying the velocity fluctuations associated with the broadband turbulent mixing noise in screeching jets. The broadband and screeching fluctuations associated with the broadband and discrete noise components are calculated using proper orthogonal decomposition to decouple the velocity variables. The methodology is demonstrated on velocity fields of screeching jets measured with high-resolution particle image velocimetry. Jets dominated by the precessing flapping (B) and helical (C) instability screech modes are considered, which correspond to ideally expanded Mach numbers of , 1.45, and 1.59, respectively. Axially averaged spatial wavelength spectra of the broadband velocity fluctuations are compared to illustrate the decomposition. The proper-orthogonal-decomposition-based approach is shown to be highly dependent upon the dominant instability regime of the jets. Further analysis shows that the integral length scales of the broadband velocity fluctuations normalized by the incompressible momentum thickness in the screeching jets considered agree well with values reported in studies on nonscreeching jets despite the absolute values differing.

Broadband noise prediction using large eddy simulation and a frequency domain method

A new LES-acoustic analogy method for accurate flow and broadband noise prediction is proposed. A frequency domain method for the generalized Lighthill acoustic analogy theory is derived in detail and the final equations for code is provided which can help to bridge the gap between the flow field prediction and the acoustic field prediction for those who are interested in acoustic results but lack of acoustic prediction ability. The hybrid method (LES and acoustic analogy method) for broadband noise prediction is validated using the rod-airfoil interaction problem. Both flow field results and acoustic field results are compared with experimental results and other numerical results in detail. The flow field results agree very well with the experimental results and other numerical results. The predicted acoustic results and experimental results also reach good agreement both in far field acoustic pressure Power Spectral Density (PSD) and noise directivity. The method developed in the paper proves to be an effective tool for broadband noise prediction. In addition, the different span correction methods for the acoustic pressure spectrum are also discussed.

Prediction of the Broadband Noise Radiated from a Fan Rotor of a High-Bypass Ratio Turbofan Engine: Take-off Condition

The purpose of this study is to predict the broadband noise generated by the fan rotor of a high bypass ratio turbofan engine at take-off condition. A numerical investigation of the unsteady aerodynamics of a fan rotor was described, with emphasis on acoustics. Three case studies were examined; namely, design point, a choked point and a near surge point. It was found that the dominant broadband noise mechanisms are due to interaction of turbulence of the incoming flow with the engine casing and nose, interaction of the turbulent boundary layers on the rotor blades with their trailing edges, and interaction of the rotating blade with the turbulence in the incoming flow. The most important noise source was the interaction of the turbulent boundary layers on the rotor blades with their trailing edges. Choked case scored the highest noise level as it has the maximum turbulent kinetic energy levels, maximum mass flow rate and thus maximum relative Mach number. Near-surge case has the lowest noise level except at the lower part of the suction side of the rotor where the reverse flow and vortices caused higher levels of noise.

Advanced open rotor noise prediction

AbstractThe purpose of this paper is to describe the current status of open rotor noise prediction methods and to highlight future challenges in this area. A number of analytic and numerical methods are described which can be used for predicting ‘isolated’ and ‘installed’ open rotor tonal noise. Broadband noise prediction methods are also described and it is noted that further development and validation of the current models is required. The paper concludes with a discussion of the analytical methods which are used to assess the acoustic data collected during the high-speed wind-tunnel testing of a model scale advanced open rotor rig.

Frequency-domain acoustic pressure formulation for rotating source in uniform subsonic inflow with arbitrary direction

Abstract This paper presents a frequency-domain formulation for predicting noise radiated from the rotating thickness and loading sources in uniform subsonic inflow with arbitrary direction. The proposed frequency-domain formulation is an extension of the recently published frequency-domain formulation for the stationary medium. It avoids the singular integral and numerical interpolation problems encountered in the time-domain numerical method. Three test cases, i.e., noise radiation from the rotating monopole and dipole point sources and the Isom thickness noise of a transonic rotor in the subsonic uniform flow, have been carried out to validate the proposed formulation. Both the acoustic pressure spectrum and directivity pattern computed with the present frequency-domain method are in good agreement with those obtained from the time-domain method, thus validating the correctness of the present formulation. Furthermore, the numerical results indicate that the frequency-domain formulation is suitable for tonal noise prediction, while it is inefficient for broadband noise prediction.