Rotor-stator Interaction Noise Research Articles

Smart Blade Count Selection to Align Modal Propagation Angle with Stator Stagger Angle for Low-Noise Ducted Fan Designs

The rotor–stator interaction noise is a major source of fan noise. Especially for low-speed fan stages, the tonal component is typically a dominant noise source. A challenge is to reduce this tonal noise, as it is typically perceived as unpleasant. Therefore, in this paper, we analytically, numerically and experimentally investigate an acoustic effect to lower the tonal noise excitation. Our study on an existing low-speed fan indicates a reduction in tonal interaction noise of more than 9 dB at the source if the excited acoustic modes propagate parallel to the stator leading edge angle. Moreover, a design-to-low-noise approach is demonstrated in order to apply this effect to two new fan stages with fewer stator than rotor blades. The acoustic design of both fans is determined by an appropriate choice of the rotor and stator blade numbers in order to align the modal propagation angle with the stator stagger angle. The blade geometries are obtained from aerodynamic optimization. Both fans provide similar aerodynamic but opposing acoustic radiation characteristics compared to the baseline fan and a significant tonal noise reduction resulting from the impact of the modal propagation angle on noise excitation. To ensure that this effect can also be applied to other low-speed fans, a design rule is derived and validated.

Investigation of the orifice flow of over-the-rotor liner and its interaction with the rotor flow field

The emergence of ultra-high bypass ratio engines calls for advanced noise control technology. A promising technology is installing the acoustic liner at the casing over the rotor (OTR), forming the so-called OTR liner. Experiments have shown that this treatment could compensate for the reduced noise absorption due to the shortened intake length. Compared with the conventional liner receiving a pure acoustic pressure excitation, the OTR liner is exposed to the combined aerodynamic pressure perturbance and high acoustic pressure excitation, leading to complex flow mechanisms in the liner orifices. In this work, the large-eddy simulation method is used to investigate the flow phenomenon of OTR liner where each liner orifice is explicitly modeled to capture the flow behaviors at the blade tip proximity. Interesting findings are as follows: first, OTR liner orifices have an asynchronous dynamic response with a phase lag corresponding to the blade traversing speed; second, oscillating flow, resembling that in a classical Helmholtz resonator, is formed in the upstream liner orifices, while a bias flow appears in the downstream liner orifices. The dramatically different flow behaviors lead to non-uniform energy dissipation among orifices, implying the uniform impedance assumption in conventional acoustic liner definition fails when installing the liner near the rotor blade. Results also show that the orifice flow interacts with the blade tip leakage flow, affecting the development of the tip leakage vortex that may impact the rotor–stator interaction noise.

Three-dimensional effects of cascade perforations on rotor–stator interaction noise

One novel trend in reducing aero-engine noise is to utilize the silent flight mechanism of owls by applying perforations on fan stator vanes. Consequently, the establishment of relevant theoretical models is of particular interest. The current efforts made in this regard are just targeting the features based on two-dimensional models without including the three-dimensionality. In this paper, we present a three-dimensional solution for acoustic scattering by annular perforated cascades, and the dipole source corresponding to the unsteady pressure loading on the vanes is identified as the dominant sound source. By the singularity method, the acoustic response is obtained with the soft boundary condition applied on the vane surfaces. It is found that considerable noise reduction can be achieved for rotor–stator interaction with a modest uniform porosity, and accordingly two mechanisms are proposed to understand the effect of porosity on propagating sound. The first is that the perforations allowing a normal velocity across the vane reduce the unsteady loading induced by the incident disturbances. The second is that the three-dimensional interactions among the dipole sources at different positions are also dampened by the soft boundaries, thus the distribution of the unsteady pressure loading on the vanes will also change significantly compared to hard-vane cases. Non-uniform distributions of porosity are investigated further, indicating that perforations in the vane upstream area are more effective in reducing propagating noise. Our method is fully three-dimensional and capable of investigating non-uniform porosity, and thus is able to provide useful guidance for future soft vane designs.

Zonal Detached Eddy Simulation of the Fan-Outlet Guide Vanes Stage of a Turbofan Engine: Part I—Methodology, Numerical Setup, and Aerodynamic Analysis

Abstract This article deals with the zonal detached eddy simulation of the fan module of a modern turbofan engine for the prediction of the broadband noise due to the interaction of the fan turbulent wakes with the stationary outlet guide vanes. The simulation relies on a hybrid RANS/LES approach with a zonal strategy: the core airflow is treated in RANS, while the bypass airflow is solved with the hybrid approach. The simulation was performed during four revolutions, and statistical convergence was reached. Inspections of the flowfields highlight a consistent behavior of the shielding function (border between RANS and LES solving areas) around the blade walls and at the trailing edge for a such complex flow. The fan module was tested in the AneCom facility in which hot wire measurements were made in-between the fan and the outlet guide vanes. The numerical results are compared to this large dataset of measurements. The flow maps are well retrieved by the simulation for both the time-averaged and the turbulent quantities. Comparison of radial profiles shows an excellent agreement for the three root-mean-square (RMS) components of the velocities between zonal detached eddy simulation (ZDES) and the measurements, particularly in the tip gap flow area, in which RANS results do not reproduce correctly the flow quantities. The wake shape, a key feature in the mechanism of generation of rotor–stator interaction noise), is quite well predicted by the ZDES simulation. These numerical results demonstrate the maturity of the approach for the simulation of complex turbomachinery flows.

A Numerical Study on Aircraft Noise Mitigation Using Porous Stator Concepts

This manuscript presents the application of a recently developed noise reduction technology, constituted by poro-serrated stator blades on a full-scale aircraft model, in order to reduce rotor-stator interaction noise in the fan stage. This study was carried out using the commercial lattice Boltzmann solver 3DS-SIMULIA PowerFLOW. The simulation combines the airframe of the NASA High-Lift Common Research Model with an upscaled fan stage of the source diagnostic test rig. The poro-serrations on the stator blades have been modeled based on a metal foam with two different porosity values. The results evidence that the poro-serrations induce flow separation on the stator blades, particularly near the fan-stage hub. Consequently, the thrust generated by the modified fan stage is lower and the broadband noise emission at low frequencies is enhanced. Nevertheless, the tonal noise components at the blade-passage frequency and its harmonics are mitigated by up to 9 dB. The poro-serrations with lower porosity achieve a better trade-off between noise emission and thrust penalty. An optimization attempt was carried out by limiting the application of porosity near the tip of the stator blades. The improved leading-edge treatment achieves a total of 1.5 dB in sound power level reduction while the thrust penalty is below 1.5%. This demonstrates that the aerodynamic effects of a leading-edge treatment should be taken into account during the design phase to fully benefit from its noise reduction capability.

Potential Amplification Mechanism of Rotor–Stator-Interaction Noise via Spiral-Poiseuille-Flow Instability

This study investigates the instabilities of the interstage flow between a rotor and a stator in turbomachinery and evaluates their impact on the fan noise. By viewing the interstage flow as the so-called spiral-Poiseuille flow, linear stability analysis of the incompressible Navier–Stokes equations reveals two distinct instability regimes at the Reynolds numbers up to : The first mode excites disturbances over wide frequencies, including the rotor speed, and forms wavepacket-like structures above the outer-wall boundary layer. The second mode evolves inside the outer-wall boundary layer with slower velocity and possesses much smaller axial wavenumbers. We extract these modes using an approach analogous to dynamic mode decomposition from a database of our improved–delayed–detached eddy simulation solving NASA’s turbofan-rig test at low speed. The dynamic mode decomposition technique successfully visualizes these two instability modes even at , and their dispersion relations approximately agree with the characteristics predicted by the linear stability analysis. These analyses suggest that the instabilities amplify coherent disturbances in the rotor wakes and vortices near the tip until they interact with the stator; moreover, coherent structures associated with the second instability may act as stationary objects, possibly resulting in an additional fan-noise source.

Mechanisms to reduce the blade passing frequency tone for subsonic low-count OGV fans

Conventionally, fan stages are designed with more stator vanes than rotor blades. These fan stages provide a tonal noise advantage as the blade passing frequency tone is cut-off. Contrary to conventional cut-off designs, low-count OGV (Outlet Guide Vane) designs are fan stages with fewer stator vanes than rotor blades. Low-count OGV designs benefit from lower broadband noise due to the reduced number of stator vanes. However, the blade passing frequency tone may no longer be cut-off. Therefore, we assess tonal noise reduction mechanisms for subsonic low-count OGV fan stages. We use an analytical and a numerical method to predict the rotor–stator interaction noise for different blade count pairings. The results indicate that the mode phase propagation angle and the cut-on factor are two acoustic parameters that dominantly influence the noise excitation. For certain blade count pairings an acoustic benefit exists if the mode phase propagation angle is congruent to the stator leading edge angle and / or if the mode is away from the cut-off limit. Moreover, reducing the rotor tip speed allows, for some configurations, a cut-off design even with fewer stator vanes than rotor blades (inverse cut-off). Overall, the study shows promising possibilities to reduce tonal interaction noise for low-count OGV fan stages.

Aeroacoustics Assessment of an Hybrid Aircraft Configuration with Rear-Mounted Boundary Layer Ingested Engine

Hybrid electric propulsion is a promising solution to reduce aircraft emissions, thus improving the sustainability of the air transport. In this work, a hybrid aircraft configuration with a rear-mounted boundary layer ingestion (BLI) engine has been investigated. The partial embedding of the engine into the fuselage generates a distortion of the ingested inflow causing additional tonal and broadband BLI noise sources, and, at the same time, alters the existing one, such as the rotor–stator interaction noise (RSI). This work is focused on the tonal RSI noise modeling, with and without the distortion generated by the BLI, and the far-field propagation including the acoustic masking contribution due to the engine–fuselage integration. As the main result, this work shows the contributions of BLI and the engine–aircraft integration on the RSI noise. Both effects should be properly taken into account in the early aircraft design stage for an effective noise reduction even at ground level.

Aerodynamic performance of chevron geometry trailing edge for a transonic axial compressor impeller

Several studies present how serrated rotor blades mix wakes in order to attenuate noise levels. The current paper analyses how this geometry, applied on the trailing edge, affects the global parameters of a transonic axial compressor impeller. Innovative solutions tackling the rotor-stator interaction mechanism in an axial compressor for noise reduction include serrated trailing edges. Inspired by chevron nozzles, serrations can be transferred to the open-rotor concept in order to reduce tonal noise. Throughout the study we will be focusing on aerodynamic loss estimation while being mindful of the mechanisms which lead to rotor-stator interaction noise, without assessing its per-se effectiveness for noise mitigation. Owing to its qualitative experimental data availability, NASA’s Rotor 37 was chosen as a baseline. A set of fully viscous 3D simulations, using the SST k omega turbulence model and RANS, was carried out to this effect. Spatial discretization was made using a fully structured pre-mesh in order to optimize resolution and accelerate convergence. Full-factorial samples were generated for the geometric variations in order to capture the aerodynamic implications of this concept. Overall, the analysed case provides promising perspectives, pending optimization studies and experimental tests thereof.

Influence of Wake and Background Turbulence on Predicted Fan Broadband Noise

Next-generation fan designs featuring large bypass ratios have the potential to reduce rotor–stator interaction noise of fans. However, these designs pose new challenges and long-held beliefs regarding the acoustic benefit of such concepts that need to be evaluated. At off-design operating points, the increase in nozzle area serves to maintain the stall margin by reducing the fan loading. To an extent, this may lower the contribution of wake turbulence to fan broadband noise. Therefore, investigating the relative contribution of other noise sources becomes relevant. In this paper, the authors examine the contribution of wake and ingested turbulence to fan broadband noise levels at approach for the ASPIRE fan, which is a next-generation fan concept. To enable the investigation of background turbulence, a Reynolds stress turbulence model is used. The computational fluid dynamics solution then provides inputs for a two-dimensional cyclostationary synthetic turbulence method. The noise predictions are improved by performing simulations at three spanwise positions and by introducing a correction technique to account for the three-dimensional interstage flow. For the investigated case, it is shown that broadband noise generated at the stator is as high as for conventional concepts. Furthermore, both wake and background turbulence contribute significantly to the overall fan broadband noise.

A rod-linear cascade model for emulating rotor-stator interaction noise in turbofans: A numerical study

This manuscript presents a rod-linear cascade model for emulating rotor-stator interaction noise. The model is intended as a test platform for studying noise mitigation techniques for a turbofan fan stage, while it also extends the classical rod-airfoil configuration by considering a row of blades based on realistic geometrical details. The rod-linear cascade model consists of a rod positioned upstream of a 7-blade linear cascade, such that the rod wake impinges onto the central blade. The rod is scaled to obtain a fundamental shedding frequency equal to the first blade passing frequency of the NASA-Glenn Source Diagnostics Test (SDT) fan stage at approach condition. The cascade blade profile is also based on the OGV of the SDT sampled at 90% of the radial span. Subsequently, numerical simulations are performed using lattice-Boltzmann Method on a computational setup comprised of a contraction and a test section enclosing the rod-linear cascade model. The integral length scales of the rod wake and the mean loading of the central blade have been found to be in good agreement with the trends observed in the SDT fan stage. The primary noise sources are localized at the central blade leading edge, although noise propagation to the far-field is influenced by additional diffraction by the other blades. Furthermore, the acoustic-blade row interaction causes intense pressure fluctuation within the inter-blade channels, including in those that are not directly affected by the rod wake.

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.

On a two-dimensional mode-matching technique for sound generation and transmission in axial-flow outlet guide vanes

The present work deals with the analytical modeling of two aspects of outlet guide vane aeroacoustics in axial-flow fan and compressor rotor-stator stages. The first addressed mechanism is the downstream transmission of rotor noise through the outlet guide vanes, the second one is the sound generation by the impingement of the rotor wakes on the vanes. The elementary prescribed excitation of the stator is an acoustic wave in the first case and a hydrodynamic gust in the second case. The solution for the response of the stator is derived using the same unified approach in both cases, within the scope of a linearized and compressible inviscid theory. It is provided by a mode-matching technique: modal expressions are written in the various sub-domains upstream and downstream of the stator as well as inside the inter-vane channels, and matched according to the conservation laws of fluid dynamics. This quite simple approach is uniformly valid in the whole range of subsonic Mach numbers and frequencies. It is presented for a two-dimensional rectilinear-cascade of zero-staggered flat-plate vanes and completed by the implementation of a Kutta condition. It is then validated in sound generation and transmission test cases by comparing with a previously reported model based on the Wiener-Hopf technique and with reference numerical simulations. Finally it is used to analyze the tonal rotor-stator interaction noise in a typical low-speed fan architecture. The interest of the mode-matching technique is that it could be easily transposed to a three-dimensional annular cascade in cylindrical coordinates in a future work. This makes it an attractive alternative to the classical strip-theory approach.

Rotor-stator interaction noise of axial turbo machinery: Minimization by experimental optimization of trailing-edge blowing

The work deals with tonal noise of an axial turbo machinery stage due to rotor-stator interaction. The objective of this study is the reduction of this noise by ejection of a secondary mass flow at the rotor blades' trailing edge (trailing edge blowing, TEB). A widely investigated strategy is a perfect filling of the wake downstream of the blade trailing edge. Recent studies, however, indicated that this state of the art trailing edge blowing strategy is problematic since it may stimulate undesired secondary flows which overcompensate for the benefits of trailing edge wake filling. Hence, in this paper a novel strategy of trailing edge blowing is described. The key idea is the application of an evolutionary optimization algorithm to trailing edge blowing experiments with the measured far field sound pressure level as target function. As a result spanwise trailing edge blowing distributions that reduce the fundamental tone at blade passing frequency (BPF) by 2.4 dB, and at its first harmonic by 21.4 dB (the former representing a small benefit, but the latter corresponding to a complete elimination of the tone), have been found. The required blowing mass flow rate is 2% of the mass flow rate through the stage and hence relatively moderate. 3D hot wire measurements proved that the acoustically relevant upwash velocity fluctuations as seen by the stator are not only caused by the wakes from the rotor blades but also by vortex structures at the rotor's hub and tip. TEB, which is obtained by optimization as proposed in this contribution, either does not enhance these secondary flow structures or puts them out of action.

Active Control of Rotor–Stator Interaction Noise Using Stator-Mounted Actuators

Active control of rotor–stator interaction noise is studied numerically using a locally oscillating stator surface. A stator of 24 blades inside an infinite coaxial duct is considered. The governing equations are linearized three-dimensional Euler equations and solved with an explicit numerical technique based on a high-order approximation of spatial derivatives. A number of parameters governing actuation-induced sound field and control performance are investigated. It is found that the actuation-induced sound field and noise reduction achieved depend on the actuation amplitude and frequency. The side and position, where actuation is imposed, of the blade as well as actuator geometry are also crucial for the control performance. The downstream-radiated sound power may be reduced by 50% or more over a range of frequencies given an optimum combination of control parameters.

Prediction of Noise Sources in Axial Compressor from URANS Simulation

As part of a larger aeroacoustic study focused on rotor–stator interaction noise, the unsteady flow in a single-stage axial compressor is achieved by a numerical Reynolds-averaged Navier–Stokes simulation. The flowfield is precisely analyzed and assessed by comparing it with available experimental and numerical data. Specifically, the rotor wakes correctly agree with previous laser Doppler anemometry measurements. The unsteady flow, especially the potential effects created by the stator vanes, is then investigated, showing that the stator potential field is responsible for a major secondary tonal noise source on the rotor blades. The rotor wake velocity and turbulent content correspond to the excitations for tonal and broadband noise, respectively. They are greatly influenced by the vane potential effects and, to a lesser extent, by their convection with the mean flow. Two methods of extraction are surveyed along the whole span, showing the need to consider the blockage induced by the stator on the wakes. Finally, the realistic tonal acoustic sources of rotor–stator interaction extracted in time on the stator vane surfaces show a noise mechanism mainly driven by the leading edge and some significant variations from hub to tip.

An analytical investigation of stator lean on rotor–stator interaction noise

This paper concentrates on the noise reduction effect of stator lean in rotor–stator interaction. The compressor with a serial stator-blade lean angle has been employed to acoustically test and numerically calculate. The experiment results show that stator-leaned positive has better effect on noise reduction than leaned negative; the tone noise is determinant on total sound pressure level, and the lean angle of the stator should exceed 10°. Based on the results of unsteady calculation about the compressor, the amplitude of the unsteady loading of stator decreases with the increase in the lean angle. Leaned positive stator has lower unsteady force and loading than leaned negative. The phase parameters q and pcle of wakes are almost proportionate to the stator-blade lean angle. The distribution of q and phase close-leading edge ( pcle) shows that the compressor with 25° and −20° stator-blade lean angles has the maximum and minimum phase variation, respectively.

Numerical prediction of the interaction noise radiated from an axial fan

The rotor–stator interaction noise in axial fans is mainly caused by the periodic wake from the rotating fan impinging on the stator rows downstream. In order to accurately predict the noise, a thin-body BEM/Curle method is developed in this paper. It is a hybrid method combining computational aeroacoustic with Boundary Element Method (BEM) and can be used to predict the propagation of sound wave in the duct. The calculation includes three steps: firstly, the unsteady viscous flow around the blades is calculated using the CFD method to acquire the noise source information; secondly, the radiated sound pressure is calculated using the acoustic analogy Curle equation in the frequency domain; lastly, the scattering effect of the duct wall on the propagation of the sound wave is expressed using the thin-body BEM method. In comparison with the experimental results, the predicted sound pressure levels (SPLs) are correct in the Blade Passing Frequency (BPF) and its harmonics. The sound pressure directivities of sound source and scattering effect are shown in the paper. The conclusion is that the scattering effect of the duct wall cannot be ignored in the prediction of rotor–stator interaction noise and can increase the accuracy of the prediction in comparison with the results based on the free-field assumption.

Sound generation in centrifugal compressors

An experimental study is described to explore the dominant sound generation mechanisms of the spectral components governing the overall noise level of centrifugal compressors. At the design speed with supersonic flow conditions in the rotor blade channels, blade tone noise and buzz-saw noise are the main contributors. On the inlet, rotor-alone noise is the main source while rotor–stator interaction noise dominates on the outlet side in case of vaned outlet diffusers. Over a large range of rotor speeds with subsonic flow conditions, radial compressor noise is dominated by tip clearance noise which is produced by the secondary flow through the gap between rotor blade tips and the casing wall which in turn gives rise to the rotating instability phenomena observed earlier in axial-flow machines.

Numerical investigation on radiation characteristics of discrete-frequency noise from scarf and scoop aero-intakes

Numerical studies have been carried out to investigate the detailed geometrical effects of full three-dimensional aero-intakes on the radiation of the discrete-frequency rotor–stator interaction noise. The near-field acoustic radiation characteristics and the far-field directivity patterns from the scarf and scoop aero-intakes with three different scarf/scoop angles are quantitatively analyzed and compared. The near-field predictions were obtained by solving the linearized Euler equations with computational aeroacoustic techniques consisting of high-order finite difference scheme, non-reflecting boundary conditions, overset grids and parallel computational methods. For the prediction of far-field directivity pattern, the Kirchhoff integral method was applied. By comparing the directivities of discrete-frequency noise radiating from the scarf and the scoop aero-intakes with that from an axisymmetric aero-intake, it is shown that the 7 dB noise reduction at downward peak radiation angle can be achieved by using the scoop aero-intake with scoop angle of 15°, and the 5 dB noise reduction by the scarf aero-intake with the scarf angle of 15°. The scattering of the radiating acoustic wave by the background mean flow around the aero-intakes shifts the peak lobe radiation angle toward ground and increases the amplitude of the acoustic pressure compared with the cases without mean flow effect. Overall, the scoop aero-intake was found to be more effective than the scarf and the axisymmetric aero-intakes in view of the lower noise radiation toward ground.