Broadband Noise Generation Research Articles

On Improving the Noise Reduction Performance of Trailing Edge Serrations by Extension

Trailing edge serration is an effective method to reduce broadband noise generation by an airfoil. However, the noise reduction performance can be significantly reduced when there is flow misalignment at the serration. This experimental study investigates the impact of a shift in the serration position downstream of the airfoil trailing edge on the noise reduction performance, referred to in this paper as serration extension. Experiments were performed on a 100-mm-chord NACA 0012 wing model with sawtooth trailing edge serrations. The serration performance was studied at 0° and 7° flap-down configurations at various angles of attack. The serrations with three different extension lengths of 5, 10, and 15 mm were tested and compared with the baseline case without extension. The emitted noise was measured with a phased microphone array. The results show a significant reduction in broadband high-frequency noise by extension under loading conditions. Particle image velocimetry measurements of root flow along the wall-normal plane reveal diminished crossflow across the serration after extension. Furthermore, the extensions cause an up-to-25% increase in the maximum lift coefficient and improved or unaltered lift-to-drag ratios.

Acoustic and psychoacoustic characterisation of small-scale contra-rotating propellers

This paper reports an experimental investigation of noise radiation and psychoacoustic evaluation of contra-rotating propellers with different blade counts over multiple emission angles and thrust settings. A series of aeroacoustic and psychoacoustic tests were performed with a contra-rotating propeller rig at the University of Salford, UK, where both propellers’ blade count and their axial separation z were varied at static conditions and constant thrust. The sounds were analysed using conventional acoustic metrics, such as Sound Pressure Level (SPL) and Sound Power Level for tonal, broadband, and overall noise, and using the psychoacoustic Sound Quality Metrics (SQMs) Loudness, Fluctuation Strength, Roughness, Tonality, and Impulsiveness. Configurations with axial separation-to-diameter ratio z/D between 0.2–0.4 exhibited a balance between tonal and broadband noise generation mechanisms, resulting in reduced noise levels and reduced annoyance for on-axis observers. The number of blades played a secondary role on determining noise levels: higher blade counts resulted in lower overall noise levels at larger axial separations, although at shorter distances (z/D<0.2) increasing the blade count led to higher noise levels. The SQMs Loudness, Fluctuation Strength, and Tonality were found to also reach a minimum at z/D=0.3, although increasing the number of propeller blades led to a decrease in Loudness but not in Fluctuation Strength and Tonality. The dominant noise generation mechanisms observed here present a directivity null perpendicular to the propellers’ axis, and no clear trends were observed for acoustic and psychoacoustic metrics at this emission angle. A listening experiment was set up to validate the results of the acoustic and psychoacoustic analysis, and its results suggest that the reported noise annoyance is driven by Loudness, Fluctuation Strength and Tonality metrics (p<0.01) at on-axis radiation, while at 90° emission angle, Loudness (p<0.01) and Tonality (p<0.05) are the main contributors to noise annoyance.

Influence of orifice thickness and chamfer on broadband noise in a water circuit

An incompressible large-eddy simulation (LES) is used to predict broadband noise generation by an orifice in a water circuit. Flow conditions are chosen in order to avoid cavitation. The model’s results are compared to measured wall pressure fluctuations for three orifices: a thin sharp-square-edge orifice, a thick sharp-square-edge orifice and a thick narrower orifice with an upstream chamfer. Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulations (LES) fail to reproduce the steady-flow drag coefficient of the thick sharp-edge orifice and of the upstream-chamfered orifice. Large differences are found in the steady-flow drag coefficient of different samples of the thick orifice. An edge-tone like instability is observed in the near-field pressure fluctuations of one of the thick sharp-square-edge orifices, which does not appear in the other samples and is suppressed by the upstream chamfer. However, there are only minor differences in far-field radiated sound for these thick sharp-edged orifices. A plane-wave model with acoustic source from the LES predicts the power spectral density (PSD) of the acoustic pressures within a factor 3. At low Strouhal numbers (based on the orifice diameter and the cross-sectional averaged velocity in the orifice) the thin orifice behaves similarly to the thicker strongly chamfered orifices, with thin central cylindrical section. At higher Strouhal numbers the measured and predicted acoustic-pressure fluctuations due to these orifices are two orders of magnitude lower than those of thick orifices with sharp square edges. As orifices are used for the control of the flow distribution through complex water-cooling networks, the good results for thick chamfered narrow orifices call for a further study to find shapes that minimize the sound production at constant static pressure drop.

Blade-Tip Vortex Noise Mitigation Traded-Off against Aerodynamic Design for Propellers of Future Electric Aircraft

We study noise generation at the blade tips of propellers designed for future electric aircraft propulsion and, furthermore, analyze the interrelationship between noise mitigation and aerodynamics improvement in terms of propeller geometric designs. Classical propellers with three or six blades and a conceptual propeller with three joined dual-blades are compared to understand the effects of blade tip vortices on the noise generation and aerodynamics. The dual blade of the conceptual propeller is constructed by joining the tips of two sub-blades. These propellers are designed to operate under the same freestream flow conditions and similar electric power consumption. The Improved Delayed Detached Eddy Simulation (IDDES) is adopted for the flow simulation to identify high-resolution time-dependent noise sources around the blade tips. The acoustic computations use a time-domain method based on the convective Ffowcs Williams–Hawkings (FW-H) equation. The thrust of the 3-blade conceptual propeller is 4% larger than the 3-blade classical propeller and 8% more than the 6-blade one, given that they have similar efficiencies. Blade tip vortices are found emitting broadband noise. Since the classical and conceptual 3-blade propellers have different geometries, especially at the blade tips, they introduce deviations in the vortex development. However, the differences are small regarding the broadband noise generation. As compared to the 6-blade classical propeller, both 3-blade propellers produce much larger noise. The reason is that the increased number of blades leads to the reduced strength of tip vortices. The findings indicate that the noise mitigation through the modification of the blade design and number can be traded-off by the changed aerodynamic performance.

Prediction of broadband noise generated from low-pressure fan based on experimental pressure power spectral density

Low-pressure fans are widely used in industrial machinery as well as in home-use electrical apparatus. Consequently, their improved aerodynamic performance and reduced fan noise are important technical issues. This study developed a methodology for the experimental determination of the pressure power spectral density (PSD) of a flat plate based on a wind tunnel experiment and the broadband noise generated from a low-pressure fan predicted based on its pressure PSD. In the proposed methodology, the broadband noise of the fan was evaluated by superimposing the predicted broadband noise of the divided blade elements in the impeller. Using this methodology, the broadband noise of the objective fan could be predicted via the wind tunnel experiment and the representative internal flow properties of the fan such as relative velocity and deviation angle. Moreover, the relationship between the broadband noise in the design and off-design conditions and the flow regime were analyzed by referring to the parameters for the prediction. In the off-design low flow rate condition, the flow around the impeller became a swirling flow owing to the increasing back pressure, and the fan noise in the off-design condition increased, especially in the low-frequency domain. The broadband noise was generated by leakage flow in the vicinity of the leading edge at the blade tip which interfered with the adjacent blade. Therefore, the mechanism of broadband noise generation in the off-design condition distinguished the mechanism of trailing edge noise. As indicated by the results of the predicted broadband noise comparison, the suggested experimental methodology best represented the measured noise spectra in the design condition. The experimental pressure PSD showed that the broadband noise in the design condition was affected by the trailing edge noise generated at the blade tip.

Trailing Edge Thickness Effect on Tonal Noise Emission Characteristics from Wind Turbine Blades

Broadband noise generation from wind turbine blades is one of the fundamental aspects of flow-induced noise. Besides the turbulent boundary layer flow over the blades, factors such as the angle of attack, the turbulence intensity, the trailing edge thickness of the blade and their shapes strongly influence the overall sound power levels at high frequencies, i.e. f &gt; 8 kHz. In large operating wind farms, a trade-off between noise generation and power production is considered by power utility firms to maximize the return on investment (ROI) and minimize the fatigue damage on wind turbine components. The present work deals with the analysis of the thickness effect on trailing edge bluntness noise level at hub height average wind speeds of 7 m/s, 10 m/s. A semi-empirical BPM model was used to predict the sound pressure levels from the 37 m blade length of a 2MW wind turbine. The receiver configuration was fixed at a distance of 120 m from the source height of 80 m. The results demonstrated that as the trailing edge height increased from 0.1 % to 0.5 % of the local chord, the sound power level increased by ~ 17 dB for frequencies &gt; 200 Hz, but decreased by 16 dB when the thickness is 0.1 % local chord. The computed results of the sound power level using the BPM model have been validated using experimental data and showed a good agreement for the tonal frequencies, f ~ 10 kHz, where the trailing edge bluntness noise becomes dominant.

Characterization of pressure fluctuations within a controlled-diffusion blade boundary layer using the equilibrium wall-modelled LES

In this study, the generation of airfoil trailing edge broadband noise that arises from the interaction of turbulent boundary layer with the airfoil trailing edge is investigated. The primary objectives of this work are: (i) to apply a wall-modelled large-eddy simulation (WMLES) approach to predict the flow of air passing a controlled-diffusion blade, and (ii) to study the blade broadband noise that is generated from the interaction of a turbulent boundary layer with a lifting surface trailing edge. This study is carried out for two values of the Mach number, {{rm Ma}}_{infty } = 0.3 and 0.5, two values of the chord Reynolds number, {{rm Re}}=8.30 times 10^5 and 2.29 times 10^6, and two angles of attack, AoA =4^circ and 5^circ. To examine the influence of the grid resolution on aerodynamic and aeroacoustic quantities, we compare our results with experimental data available in the literature. We also compare our results with two in-house numerical solutions generated from two wall-resolved LES (WRLES) calculations, one of which has a DNS-like resolution. We show that WMLES accurately predicts the mean pressure coefficient distribution, velocity statistics (including the mean velocity), and the traces of Reynolds tensor components. Furthermore, we observe that the instantaneous flow structures computed by the WMLES resemble those found in the reference WMLES database, except near the leading edge region. Some of the differences observed in these structures are associated with tripping and the transition to a turbulence mechanism near the leading edge, which are significantly affected by the grid resolution. The aeroacoustic noise calculations indicate that the power spectral density profiles obtained using the WMLES compare well with the experimental data.

Mechanisms of broadband noise generation on metal foam edges

The turbulent flow over a porous trailing edge of a NACA 0018 airfoil is experimentally investigated to study the link between the hydrodynamic flow field and the acoustic scattering. Four porous trailing edges, obtained from open-cell metal foams, are tested to analyze the effects on far-field noise of the permeability of the material and of the hydrodynamic communication between the two sides of the airfoil. The latter is assessed by filling the symmetry plane of two of the porous trailing edges with a thin layer of adhesive that acts as a solid membrane. Experiments are performed at a zero degree angle of attack. Far-field noise measurements show that the most permeable metal foam reduces noise (up to 10 dB) with respect to the solid trailing edge for Strouhal numbers based on the chord below 16. At higher nondimensional frequencies, a noise increase is measured. The porous inserts with an adhesive layer show no noise abatement in the low frequency range, but only a noise increase at higher frequency. The latter is, therefore, attributed to surface-roughness noise. Flow field measurements, carried out with time-resolved planar particle image velocimetry, reveal correlation of near-wall velocity fluctuations between the two sides of the permeable trailing edges only within the frequency range where noise abatement is reported. This flow communication suggests that permeable treatments abate noise by distributing the impedance jump across the foam in the streamwise direction, promoting noise scattering from different chordwise locations along the inserts. This is further confirmed by noise source maps obtained from acoustic beamforming. For the frequency range where noise reduction is measured, the streamwise position of the main noise emission depends on the permeability of the insert. At higher frequencies, noise is scattered from upstream the trailing edge independently of the test case, in agreement with the roughness-generated noise assumption.

Coupling of an aeroacoustic model and a parabolic equation code for long range wind turbine noise propagation

This study proposes to couple a source model based on Amiet's theory and a parabolic equation code in order to model wind turbine noise emission and propagation in an inhomogeneous atmosphere. Two broadband noise generation mechanisms are considered, namely trailing edge noise and turbulent inflow noise. The effects of wind shear and atmospheric turbulence are taken into account using the Monin-Obukhov similarity theory. The coupling approach, based on the backpropagation method to preserve the directivity of the aeroacoustic sources, is validated by comparison with an analytical solution for the propagation over a finite impedance ground in a homogeneous atmosphere. The influence of refraction effects is then analyzed for different directions of propagation. The spectrum modification related to the ground effect and the presence of a shadow zone for upwind receivers are emphasized. The validity of the point source approximation that is often used in wind turbine noise propagation models is finally assessed. This approximation exaggerates the interference dips in the spectra, and is not able to correctly predict the amplitude modulation.

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.

Jet noise prediction via coupling large eddy simulation and stochastic modeling

A novel method for efficient and accurate jet noise prediction is developed and tested in the framework of coupled LES and stochastic noise modeling. In the proposed method, the low frequency range of the acoustic spectrum, which corresponds to large turbulent structures, is resolved with an implicit LES model by employing higher-order spatial and temporal discretizations. The high frequency range, which corresponds to the fine-scale turbulence, is modeled via a stochastic broadband noise generation model. The inputs to the stochastic model are represented by statistics from an axisymmetric RANS jet simulation. The smaller synthetic scales are convected by larger scales, accounting for the effects of sweeping from the larger LES-resolved turbulent scales. The farfield acoustic data is obtained using either the Linearized Euler Equations, or an acoustic analogy based on the Ffowcs-Williams Hawkings method. The method is evaluated on cold and heated jets at different Reynolds numbers.

Effect of atmospheric stability and low level jets on wind turbine noise

Numerous studies from the literature have shown that strong wind shear occurs frequently in stable atmospheres, typically at night. This phenomenon can be associated to low level jets, characterized by a wind speed profile with a maximum at a few hundred meters above the ground. This study investigates the effect of such wind speed profiles on wind turbine noise using a physically-based model. The predictions are obtained by coupling an aeroacoustic source model based on Amiet’s theory and a parabolic equation code for acoustic propagation in an inhomogeneous atmosphere. Two important broadband noise generation mechanisms are considered, namely trailing edge noise and turbulence inflow noise, and the coupling method takes into account the fact that wind turbine blades are moving and extended sources. In order to obtain realistic wind speeds for the simulations, a simple model of nocturnal low-level jet is used, with input parameters based on measurements acquired in Cabauw observatory in the Netherlands. Predictions of the overall sound pressure levels and of the third octave band noise spectra will be presented up to a distance of 1km to show the noise variations due to the wind shear effect.

Scattered noise prediction using acoustic velocity formulations V1A and KV1A

Aeroacoustic scattering prediction generally relies on boundary integral methods which require evaluation of the impermeability condition on the scattering surface. The boundary condition implies zero normal velocity relative to the scattering surface. This condition has been expressed by relating the acoustic velocity to the acoustic pressure gradient, allowing indirect evaluation of the boundary condition by existent acoustic pressure gradient formulations. In the present paper, a direct evaluation of the hardwall boundary condition in scattering problems is demonstrated by time-domain analytic acoustic velocity formulae. Acoustic velocity formulations V1A and KV1A are implemented for acoustic scattering prediction, by hybrid approaches based on the FW–H equation and the Kirchhoff method These formulations can be coupled to any scattering solver, allowing time-domain prediction of the incident acoustic field when broadband noise generation is concerned. Formulation V1A offers mathematical simplicity and computational efficiency, which can be advantageous for realistic scattering applications. Implementation of formula KV1A enables acoustic scattering prediction by existing solvers based on the Kirchhoff method. The validity of the suggested methodology is assessed through the analytical test case of harmonic sound scattered by a rigid sphere. Sound propagation and scattering effects are analyzed by examination of the acoustic velocity field characteristics.

Influence of inhomogeneities of the plasma density and electric field on the generation of electrostatic noise in the auroral zone

In order to study instabilities caused by inhomogeneities of the electric field and plasma density in the auroral zone, numerical algorithms are developed and numerical simulations are performed for different conditions in the background plasma. To this end, a nonlocal dispersion relation for a given type of wave is analyzed. It is shown that the dispersion relation has unstable solutions in a wide range of frequencies and wavenumbers. These solutions manifest themselves in satellite observations as a broadband spectrum of electrostatic perturbations. Two mechanisms of broadband noise generation related to the gradients of the density and electric field are compared.

Tandem cylinder aerodynamic sound control using porous coating

This study is concerned with the application of porous coatings as a passive flow control method for reducing the aerodynamic sound from tandem cylinders. The aim here is to perform a parametric proof-of-concept study to investigate the effectiveness of porous treatment on bare tandem cylinders to control and regularize the vortex shedding and flow within the gap region between the two bluff bodies, and thereby control the aerodynamic sound generation mechanism. The aerodynamic simulations are performed using 2D transient RANS approach with k−ω turbulence model, and the acoustic computations are carried out using the standard Ffowcs Williams–Hawkings (FW–H) acoustic analogy. Numerical flow and acoustic results are presented for bare tandem cylinders and porous-covered cylinders, with different porosities and thicknesses. Experimental flow and acoustic data are also provided for comparison. Results show that the proper use of porous coatings can lead to stabilization of the vortex shedding within the gap region, reduction of the vortex shedding interaction with the downstream body, and therefore the generation of tonal and broadband noise. It has also been observed that the magnitude and the frequency of the primary tone reduce significantly as a result of the flow regularization. The proposed passive flow-induced noise and vibration control method can potentially be used for other problems involving flow interaction with bluff bodies.

유입난류와 평판 캐스케이드 상호작용에 따른 광대역 소음 해석을 위한 효율적인 시간영역 수치기법의 개발

An efficient time-domain numerical method for the analysis of broadband noise generation and propagation due to turbulence-cascade interaction is developed. The core algorithm of the present method is based on the B-periodicity of the acoustic response function of the flat-airfoil cascade to the ingesting gust(B denotes the number of airfoils in the cascade). To confirm this periodicity, gust-cascade interaction problem are solved by using the time-domain method, which shows that the incident gust with the circumferential mode number having the same remainders when divided by the airfoil number excites the same acoustic response of the cascade. Using the proposed fast algorithm with this periodicity, we show that the total computation time for the model broadband problem using the total 525 incident gust modes can be reduced to about 1/4 of that taken in using the previous time-domain program.

Indirect Combustion Noise: Investigations of Noise Generated by the Acceleration of Flow Inhomogeneities

The entropy noise mechanism was experimentally investigated under clearly defined flow and boundary conditions on a dedicated test setup. Previous experimental research on the topic of entropy noise could draw only indirect conclusions on the existence of entropy noise due to the complexity of the physical mechanism. In order to reduce this complexity, a reference test rig has been set up within this work. In this test rig well controlled entropy waves were generated by electrical heating. The noise emission of the entropy waves accelerated in an adjacent nozzle flow was measured accurately and therewith a qualitative and quantitative determination of the entropy noise source mechanism could be experimentally accomplished. In addition to this, a parametric study on the quantities relevant for entropy noise was conducted. The results were compared to a one-dimensional theory by Marble & Candel. In a next step investigations on a combustor test rig showed a broadband noise generation mechanism in the frequency range between 1 and 3.2 kHz. The combustor rig was set up with a similar outlet-nozzle geometry like the reference test rig (EWG) and provided therefore outlet-boundary conditions like in real-scale aero-engines (outlet Mach number = 1.0). It was found that this broadband noise has a strong dependency on the nozzle Mach number in the combustor outlet. The summed-up broadband sound pressure level increases exponentially with the nozzle Mach number. However, investigations of comparable cold flow conditions did not show this behavior. Since the results of the reference experiment with artificially generated entropy waves did not show this exponential increase with the nozzle Mach number, this leaves the conclusion, that this additional noise is generated by the interaction of small-scale fluctuations, e.g. in entropy or vorticity, with the turbulent nozzle flow in the combustion chamber outlet nozzle.

An equivalent-source model for simulating noise generation in turbofan engines

Nowadays, computational aeroacoustics (CAA) is used for simulating wave propagation in ducted turbofans, especially as computational fluid dynamics (CFD) is increasingly employed to model the identified noise sources. An efficient way to match the CFD and CAA domains is to make some assumptions on flow and duct geometry, so that disturbance fields can be expanded over incoming/outgoing acoustic modes. Based on this approach, this paper presents an original matching model in which the outgoing modes are generated by means of equivalent monopole distributions defined as source terms in the equations governing the acoustic propagation, instead of a conventional inflow boundary condition (BC). Advantages and limits of the method are discussed. The process to get back to the sources and its numerical implementing are described. Although initially focused on tones, an extension of the method to broadband noise generation is tackled too. The method then is validated on a simplified turbofan exhaust configuration. Numerical solutions obtained by implementing the source terms in a high-order time-domain Euler code are compared to analytical solutions, either in a uniform or in a radially shear mean flow (provided by RANS). The parallel shear flow solution is obtained by solving the Pridmore–Brown equation. The ability to accurately simulate the standing waves due to acoustic reflections at duct ends is also assessed by comparing the numerical solutions computed using both source-based and BC-based options in the Euler solver.

Experimental Investigation of the Acoustic Field in a Bluff-Body Combustor

Experimental results on the longitudinal acoustic field generated by a methane-air turbulent flame stabilized downstream of a bluff body (V-gutter) confined in a rectangular duct are presented. The V-gutter runs straight across the width of the rectangular cross-section of the duct. The fuel is injected at some distance upstream of the bluff body, as in practical combustors. Varying the distance between the fuel injection and the bluff body alters the degree of fuel-air mixing and hence influences the flame structure, and consequently the nature of the generated acoustic field. Other geometrical variations considered are the length of the combustor duct, the axial location of the bluff body, and the width of the bluff body. The flow rates of air and fuel are varied widely to traverse conditions of generation of low-amplitude broadband noise to those of excitation of high-amplitude discrete tones. The observed dominant frequencies are expressed in terms of non-dimensional numbers such as the Helmholtz number and the Strouhal number, representing the natural acoustic modes and the free-field vortex shedding process, respectively. It is seen that, as the flow conditions are varied, the amplitude of the dominant frequency steeply rises, accompanying an abrupt jump in the dominant frequency from a constant Helmholtz number regime to a constant Strouhal number regime. High-speed images of CH* chemiluminescence of the flame zone exhibit periodic shedding of vortical structures at the same dominant frequencies as in the observed acoustic spectra. These results show that the natural acoustic modes shift to follow the vortex shedding trend with the Reynolds number of the air-flow, under conditions of excitation of loud discrete tones. Higher fuel flow rates are observed to be capable of exciting higher acoustic modes to exhibit the above behaviour. A larger location of the bluff body away from the fuel injection location, and a lesser width of the bluff body, tend to excite acoustic oscillations of lower amplitude.

The EU FP6 research project PROBAND ‐ Objectives and first results

Fan broadband noise is a major aircraft noise challenge now and in the future will be even more important. Novel low‐noise engine architectures, such as ultra‐high‐bypass‐ratio engines and lower‐speed fans, can help address jet noise and fan tone noise, but previous EC‐funded programmes have shown they are unlikely to reduce significantly fan broadband noise without improved understanding of the source mechanisms. PROBAND will accomplish a major technical leap in providing industry with an improved understanding of the broadband noise source mechanisms, with validated broadband noise prediction methods, and with low fan broadband noise concepts. PROBAND will exploit the noise technology and methodology acquired in EC‐funded projects and national programmes, to develop methods to allow the design of a fan system that will generate sufficiently low broadband noise to meet the EU noise level targets. This will be achieved by: 1. Developing a better understanding of broadband noise generation mechanisms using advanced experimental and computational techniques. 2. Developing and validating improved prediction methods using conventional computational fluid dynamics, and integrating them into industrial codes. 3. Exploring new prediction strategies using advanced computational techniques 4. Developing low broadband fan noise concepts.