Computational Aeroacoustic Simulations Research Articles

Shock Wave Generation and Radiation from a Turbofan Engine Under Flow Distortion

This paper presents aeroacoustic investigations on a full-scale ultrahigh-bypass-ratio engine with inflow distortion at transonic conditions. Computational fluid dynamics (CFD) simulations are first realized to compute the shocks in the vicinity of the fan. These shocks are then radiated in the near field thanks to computational aeroacoustics (CAA) simulations. The chaining between CFD and CAA simulations is done by injecting the shocks in terms of usual conservative variables using a nonreflecting boundary condition. The CAA solver is based on the nonlinearized Euler equations, which allows definition of the CFD/CAA interface close to the fan where the propagation of shocks is highly nonlinear. Both shock generation and propagation mechanisms are investigated, and the effects of inflow distortion are highlighted by comparison with a baseline case without distortion. The flow distortion is shown to be responsible for a modification of shock amplitudes along the circumferential direction. Thus, azimuthal modes appear in addition to the rotor-locked mode present without distortion. A particular feature of the studied configuration is the presence of a supersonic flow region in the bottom part of the nacelle. This supersonic pocket blocks the shocks in the lower half of the nacelle and modifies the angle at which they leave the nacelle in its upper half. Consequently, the far-field radiation (obtained using Kirchhoff’s integral method) is mainly oriented skyward and toward the side of descending blades. Acoustic power estimates are provided to quantify these effects.

Cabin noise prediction using wave‐resolving aircraft models

AbstractWithin the Coordinated Research Center (CRC) 880, future civil aircraft with short take‐off and landing capabilities are investigated. A main motivation are efficient point‐to‐point connections within Europe. Environmental friendliness is a key factor in the development of such aircraft systems in order to ensure the acceptance of new technologies. Therefore, the sound immission at the ground and in the passenger cabin is a major focus in the project. In this paper, wave‐resolving models of complex aircraft fuselage structures and solution strategies of these large systems in frequency domain are presented.On the basis of preliminary aircraft design data available in the project, a mechanical model of all fluid and structural parts is developed. The model comprises the back partition starting from the wing‐box and assumes a linear behavior in deflection, strain and material. The thickness distribution on the outer surface, the stiffeners, the floor, the bulk heads and the inner panels are considered concerning the structural domain. The insulation is modeled by an equivalent fluid approach. The cabin fluid, the structural domain, and the insulation are fully coupled. All domains are discretized automatically applying coincident nodes at the coupling interfaces. Frequency‐dependent pressure fluctuations distributed on the outer surface as a result of Computational Aeroacoustic (CAA) simulations of jet noise are applied to the model. A realistic consideration of such characteristic noise footprints is only possible due to the wave‐resolving approach. The direct result of the aircraft model is the cabin's pressure field in frequency domain.The finite element discretization leads to a system of equations with 1‐3 mio degrees of freedom to be solved in frequency domain. As one solver run is easily realized, many solutions in the frame of uncertainty analyses or sensitivity studies require a highly efficient solution process. Different strategies applying direct solvers, iterative solvers or its combination are discussed within this contribution as well.

On the effects of fan wake modelling and vane design on cascade noise

This paper investigates simplified and representative configurations of fan wake-cascade interaction noise, which is a major source of broadband noise in ultra-high bypass ratio turbofan engines. To this end, two-dimensional computational aeroacoustic simulations are performed by using synthetic turbulence and solving the linearised Euler equations. Several fan wake modelling assumptions are investigated, including isotropic turbulence and cyclostationary variations in both turbulent kinetic energy and turbulence length scale. Results indicate that broadband noise mainly depends on the circumferentially-averaged turbulence spectrum that is perceived by the cascade. Furthermore, the turbulence length scale plays a significant role on cascade noise. Variations in the circumferential distribution of the turbulence length scale modify the slope of the noise spectra, and the maximum noise level can vary significantly. A parameter study on cascade noise has also been performed, including variations in the vane count, aerofoil thickness, camber, mean flow Mach number, stagger angle, and inter-vane spacing. It is shown that noise reduction due to vane thickness at high frequencies can be estimated as a linear function of the frequency, thickness, and mean flow speed in the upstream and downstream directions. A modal spectral decomposition has been performed to study the contribution of cut-on circumferential modes to the sound power level. Variations in vane design parameters lead to significant changes in the cut-on modes and modal spectral distributions. However, their effect on the amplitude of the sound power level spectra is normally below 2 dB, which reinforces the suitability of the flat plate assumption for the prediction of cascade noise at an early design stage.

Noise Sources of Lean Premixed Flames

The thermoacoustic sound generation mechanisms of lean premixed laminar and turbulent flames are investigated by a two-step approach. First, the conservation equations of a reacting compressible fluid are solved. This solution is used to determine the acoustic source terms of the acoustic perturbation equations (APE). Second, the contributions of the different source terms to the amplitude and phase of the acoustic pressure signal are analyzed by solving the APE in a computational aeroacoustics (CAA) simulation. The results show that it is not sufficient to only consider the unsteady heat release rate fluctuations which occur in the substantial pressure-density relation. The acceleration of density gradients occurring at the flame front is a significant contributor to the overall sound emission. For the investigated laminar and turbulent flames the amplitude and phase of the acoustic pressure signal can only be predicted accurately if both source terms are included in the acoustic analysis.

Development of immersed boundary computational aeroacoustic prediction capabilities for open-rotor noise

A sharp immersed boundary computational aeroacoustic simulation approach for open-rotor direct noise predictions is presented. A specific feature of the finite-difference based immersed boundary method is that the stencil coefficients in the vicinity of the immersed boundary are determined in such a way that the stability of the numerical scheme is improved. The characteristics of this immersed boundary method are discussed for acoustic scattering and the treatment of moving boundaries, such as accounting for freshly cleared and dead cells, efficient geometry queries and efficient computation of irregular boundary stencils and point clouds in the vicinity of the immersed boundary. It is shown that the numerical error when considering moving boundary problems consistently scales with the order of accuracy of the boundary discretization. Finally, the method is applied to simulate the flow around a contra-rotating open rotor at take-off and cruise conditions where experimental data is available for comparison. Excellent agreement for the noise predictions of the contra-rotating open rotor system are obtained between the numerical simulations and the experimental noise measurement.

Parallel Algorithm of the NOISEtte Code for CFD and CAA Simulations

This paper describes the parallel algorithm of the NOISEtte code for computational fluid dynamics and aeroacoustics simulations. It is based on a family of higher-accuracy numerical schemes for unstructured hybrid meshes. The multilevel MPI + OpenMP parallelization is described in detail. Performance results are presented for various supercomputers and applications.

Numerical analysis and acoustic optimization of a detached splitter plate applied for passive cylinder wake control

Computational aeroacoustics simulations require a considerable amount of time, which makes the comparison of a large number of different geometric designs a difficult task. The goal of the present study were to provide a suitable methodology for aeroacoustic optimization. By means of numerical analyses using commercial computational fluid dynamics tools (Ansys Fluent), the application of a detached splitter plate in the turbulent wake of a circular cylinder as a passive control method for noise generation was investigated. We have employed two-dimensional URANS simulations, so this case can be considered a toy problem, which contains the main challenges of the physical problem and could indicate trends correctly, but was simplified in order to decrease the computational cost to an affordable level, at the cost of making comparison with experimental data not possible. The irradiation of noise caused by the interaction between the flow and both bodies was evaluated using computational aeroacoustics tools based on the Ffowcs-Williams and Hawkings method. Using a commercial optimization software package (modeFRONTIER), various design optimization methodologies were applied to this flow in order to achieve a possible optimal configuration, i.e., one which is capable of reducing the far field noise level without increasing the aerodynamic forces. Using a multi-objective optimization tool, it was possible to evaluate the behavior of heuristic optimization algorithms and the major advantage of algorithms based on response surface methods when applied to a nonlinear aeroacoustics problem, since they require a smaller number of calculated designs to reach the optimal configuration. In addition, we applied a partitive clustering technique to identify and group the cases simulated into five clusters based on their geometric parameters, overall sound pressure level and RMS of the drag coefficient, confirming the efficiency of the application of long detached splitter plates placed next to the cylinder in stabilizing the turbulent wake, whereas the positioning of splitter plates at a distance larger than a critical gap increased the overall sound pressure level radiated due to the formation of vortices in the gap.

Optimized simulation of multiscale problems on high‐performance computers

AbstractClassic hybrid computational fluid dynamics ‐ computational aeroacoustics simulations rely on disk I/O to exchange large volumes of data between the flow solver and the aeroacoustics solver, which considerably limits the scalability of the overall scheme. A direct‐hybrid method is presented that eliminates this restriction by executing both solvers simultaneously on a joint hierarchical Cartesian mesh. Simulation results for a pair of co‐rotating vortices show the parallel efficiency of the approach. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computational Aeroacoustics Simulation of an Air Micro-Injector with Dimples Shape on the Surface of Divergent Nozzle

As for jet noise reduction of an air micro-injector with divergent nozzle, dimples shape in the inner surface of divergent nozzle is proposed, in the paper, the jet noise reduction effect of dimples shape is numerically confirmed at delivery pressure 1.33 atm. and 2.0 atm. (fully expanded jet Mach number 0.65 and 1.05), using Ffows Williams and Hawkings (FW-H) method base on time-accurate solutions of Large-Eddy Simulation (LES). In addition, the influence of dimples nozzle on the jet flow field is numerically investigated.

Synthetic turbulence methods for computational aeroacoustic simulations of leading edge noise

A leading edge noise prediction methodology that uses an advanced digital filter method to generate synthetic turbulence is presented for efficient two- and three-dimensional simulations. The digital filter method combines the advantages of the Random Particle-Mesh method, for the mathematical background, and synthetic eddy methods, for the numerical implementation. This allows the generation of non-periodic turbulence without explicitly filtering white noise signals, and gives a significant reduction in the number of constraint parameters and random numbers involved in comparison with previous methods. A new eddy profile is defined through a superposition of Gaussian eddies that matches a target isotropic energy spectrum. The method is used in a linearised Euler equation solver to predict turbulence-aerofoil interaction noise from a number of configurations, including variations in aerofoil thickness, angle of attack and Mach number. A comparison with stochastic turbulence based on Fourier modes indicates that noise predictions are independent of the choice of synthetic turbulence method, provided that streamwise and transverse turbulent velocity components are included. Nevertheless, the advanced digital filter method is advantageous due to its reduced computational cost. This paper also extends the advanced digital filter method to realise a two-dimensional turbulent flow with the key statistics of three-dimensional turbulence, which is suitable to perform low-cost leading edge noise predictions that can be compared with experiments. Tests show that this approach is capable of reproducing experimental noise measurements to within an accuracy of 3 dB, and predicts similar noise levels to fully three-dimensional simulations.

Biomimetic optimization research on wind noise reduction of an asymmetric cross-section bar.

Background In this paper, we used the principle of biomimetics to design two-dimensional and three-dimensional bar sections, and used computational fluid dynamics software to numerically simulate and analyse the aerodynamic noise, to reduce drag and noise.Methods We used the principle of biomimetics to design the cross-section of a bar. An owl wing shape was used for the initial design of the section geometry; then the feathered form of an owl wing, the v-shaped micro-grooves of a shark’s skin, the tubercles of a humpback whale’s flipper, and the stripy surface of a scallop’s shell were used to inspire surface features, added to the initial section and three-dimensional shape.ResultsThrough computational aeroacoustic simulations, we obtained the aerodynamic characteristics and the noise levels of the models. These biomimetic models dramatically decreased noise levels.

Aeroacoustics of Darrieus Wind Turbine

The objective of this paper is to validate two different numerical methods for noise prediction of the H-Darrieus wind turbine using a complementary approach consisting of experimental measurements and numerical simulations. The acoustic measurements of a model scale rotor were performed in an anechoic wind tunnel. This data is the basis for the validation of the computational aeroacoustic simulations. Thereby, we have applied two different numerical schemes for noise prediction using hybrid methods. As usual in hybrid aeroacoustic approaches, flow field and acoustic calculations are carried out in separate software packages. For both schemes the time-dependent turbulent flow field is solved with Scale-Adaptive Simulation. The two schemes then differ in how the location of the acoustic sources and their propagation is calculated. In the first scheme the acoustic source terms are computed according to Lighthill's acoustic analogy which gives source terms located on the original CFD grid. These source terms are projected onto a coarser acoustic grid on which Lighthill's inhomogenous wave equation is solved by the Finite Element (FE) method. The second scheme uses the Ffowcs Williams-Hawkings (FW-H) method which is based on a free field Green's function. The scheme uses a porous integration surface and implements an advanced time formulation. Both methodologies are compared with experimental data.

Numerical Investigation of Flow Acoustic Coupling in a Half-Dump Combustor

This paper reports on the investigation of combustion instabilities in a methane–air non-premixed half-dump combustor for different flow Reynolds number () using computational aero-acoustic (CAA) simulation. To simulate the flow physics under turbulent flow conditions, detached-eddy simulation in conjunction with generalized eddy-dissipation model is adopted, while a CAA formulation based on Lighthill’s acoustic analogy is used for computing the acoustic field. It is observed that the unsteady pressure signals predominantly arise from either the natural acoustic modes of the duct or the local flow fluctuations in the vortex shedding process downstream of the dump plane, giving rise to different dominant frequencies at different spatial locations at lower and a single dominant locked-on frequency at higher . The nondimensional numbers, that is, Helmholtz and Strouhal numbers, are used to characterize the duct acoustic modes from the vortex shedding modes. Under reacting flow conditions, unsteady heat release and pressure oscillations are monitored to compute Rayleigh index in order to verify if the instability is driving (positive net value) or damping (negative net value) at different frequency levels. Moreover, the predicted Helmholtz and Strouhal numbers are found to be in excellent agreement with the experimental data available in the open literature for a wide range of .

Numerical and Experimental Study on the Tonal Noise Generation of a Radial Fan

The main focus of this work is on the geometrical modifications can be made to the fan wheel and to the volute tongue of a radial fan to reduce the tonal noise. The experimental measurements are performed by using the in-duct method in accordance with ISO 5136. In addition to the experimental measurements, CFD (computational fluid dynamics) and CAA (computational aeroacoustics) simulations are carried out to investigate the effects of different modifications on noise and performance of the fan. It is shown that by modifying the blade outlet angle, the tonal noise of the fan can be reduced without impairing its aerodynamic performance. Moreover, it is indicated that increasing the number of blades leads to a significant reduction in the tonal noise and also an improvement in the aerodynamic performance. However, this trend is only valid up to a certain number of blades, and a further increment might reduce the aerodynamic performance of the fan. Besides modifying the impeller geometry, new volute tongues are designed and tested on the rig. It is demonstrated that the shape of the volute tongue plays an important role in the tonal noise generation of the fan. Moreover, in order to find out whether or not it is possible to reduce the tonal noise level through a destructive phase-shift generation, stepped tongues are comprehensively investigated by means of numerical simulations and experimental measurements.

Time-domain inflow boundary condition for turbulence–airfoil interaction noise prediction using synthetic turbulence modeling

The present paper deals with development of the synthetic turbulence inflow boundary condition (STIBC) to predict inflow broadband noise generated by interaction between turbulence and an airfoil/a cascade of airfoils in the time-domain. The STIBC is derived by combining inflow boundary conditions that have been successfully applied in external and internal computational aeroacoustics (CAA) simulations with a synthetic turbulence model. The random particle mesh (RPM) method based on a digital filter is used as the synthetic turbulence model. Gaussian and Liepmann spectra are used to define the filters for turbulence energy spectra. The linearized Euler equations are used as governing equations to evaluate the suitability of the STIBC in time-domain CAA simulations. First, the velocity correlations and energy spectra of the synthesized turbulent velocities are compared with analytic ones. The comparison results reveal that the STIBC can reproduce a turbulent velocity field satisfying the required statistical characteristics of turbulence. Particularly, the Liepmann filter representing a non-Gaussian filter is shown to be effectively described by superposing the Gaussian filters. Each Gaussian filter has a different turbulent kinetic energy and integral length scale. Second, two inflow noise problems are numerically solved using the STIBC: the turbulence–airfoil interaction and the turbulence-a cascade of airfoils interaction problems. The power spectrum of noise due to an isolated flat plate airfoil interacting with incident turbulence is predicted, and its result is successfully validated against Amiet׳s analytic model (Amiet, 1975) [4]. The prediction results of the upstream and downstream acoustic power spectra from a cascade of flat plates are then compared with Cheong׳s analytic model (Cheong et al., 2006) [30]. These comparisons are also in excellent agreement. On the basis of these illustrative computation results, the STIBC is expected to be applied to investigate more complicated inflow noise problems including the effects of non-uniform mean flow, nonlinear interaction, and real airfoil shapes.

Petascale large eddy simulation of jet engine noise based on the truncated SPIKE algorithm

We apply the truncated SPIKE algorithm to petascale simulation of jet engine noise.We derive a tridiagonal linear system solver from the truncated SPIKE algorithm.The new solver has optimal weak scalability not found in traditional methods.Experimental data show the new solver to be much faster than traditional methods. With the emergence of petascale computing platforms, high-fidelity computational aeroacoustics (CAA) simulation has become a feasible, robust and accurate tool that complements theoretical and empirical approaches in the prediction of sound levels generated by aircraft airframes and engines. Differentiating itself from the broader discipline of computational fluid dynamics, CAA is particularly challenging as it demands high accuracy, good spectral resolution, and low dispersion and diffusion errors from the underlying numerical methods. Large eddy simulation based on space-implicit high-order compact finite difference schemes has been shown to meet such stringent requirements. In this paper, we discuss a new, scalable parallelization scheme with a three-dimensional computational space partitioning. Unlike many traditional multiblock computational fluid dynamics (CFD) methods, our partitioning is non-overlapping. We use the truncated SPIKE algorithm to solve the governing equations accurately and limit one-sided biased differentiation to just the physical boundaries. We present experimental performance data collected on Kraken and Ranger, two near-petascale computing platforms.

Hybrid computational fluid dynamics and computational aero-acoustic modeling for liftoff acoustic predictions

This paper presents development efforts at the NASA Marshall Space flight Center to establish a hybrid computational fluid dynamics and computational aero-acoustics (CFD/CAA) simulation system for launch vehicle liftoff acoustics environment analysis. Acoustic prediction engineering tools based on empirical jet acoustic strength and directivity models or scaled historical measurements are of limited value in efforts to proactively design and optimize launch vehicles and launch facility configurations for liftoff acoustics. CFD based modeling approaches are now able to capture the important details of vehicle specific plume flow environment, identify the noise generation sources, and allow assessment of the influence of launch pad geometric details and sound mitigation measures such as water injection. However, CFD methodologies are numerically too dissipative to accurately capture the propagation of the acoustic waves in the large CFD models. The hybrid CFD/CAA approach combines the high-fidelity CFD anal...

Rotor Interaction Noise in Counter-Rotating Propfan Propulsion Systems

Due to their inherent noise challenge and potential for significant reductions in fuel burn, counter-rotating propfans (CRPs) are currently being investigated as potential alternatives to high-bypass turbofan engines. This paper introduces an integrated noise and performance assessment methodology for advanced propfan powered aircraft configurations. The approach is based on first principles and combines a coupled aircraft and propulsion system mission and performance analysis tool with 3D unsteady, full-wheel CRP computational fluid dynamics computations and aeroacoustic simulations. Special emphasis is put on computing CRP noise due to interaction tones. The method is capable of dealing with parametric studies and exploring noise reduction technologies. An aircraft performance, weight and balance, and mission analysis was first conducted on a candidate CRP powered aircraft configuration. Guided by data available in the literature, a detailed aerodynamic design of a pusher CRP was carried out. Full-wheel unsteady 3D Reynolds-averaged Navier-Stokes (RANS) simulations were then used to determine the time varying blade surface pressures and unsteady flow features necessary to define the acoustic source terms. A frequency domain approach based on Goldstein’s formulation of the acoustic analogy for moving media and Hanson’s single rotor noise method was extended to counter-rotating configurations. The far field noise predictions were compared to measured data of a similar CRP configuration and demonstrated good agreement between the computed and measured interaction tones. The underlying noise mechanisms have previously been described in literature but, to the authors’ knowledge, this is the first time that the individual contributions of front-rotor wake interaction, aft-rotor upstream influence, hub-endwall secondary flows, and front-rotor tip-vortices to interaction tone noise are dissected and quantified. Based on this investigation, the CRP was redesigned for reduced noise incorporating a clipped rear-rotor and increased rotor-rotor spacing to reduce upstream influence, tip-vortex, and wake interaction effects. Maintaining the thrust and propulsive efficiency at takeoff conditions, the noise was calculated for both designs. At the interaction tone frequencies, the redesigned CRP demonstrated an average reduction of 7.25 dB in mean sound pressure level computed over the forward and aft polar angle arcs. On the engine/aircraft system level, the redesigned CRP demonstrated a reduction of 9.2 dB in effective perceived noise (EPNdB) and 8.6 EPNdB at the Federal Aviation Regulations (FAR) 36 flyover and sideline observer locations, respectively. The results suggest that advanced open rotor designs can possibly meet Stage 4 noise requirements.

Towards Numerical Simulation of Fan Broadband Noise Aft Radiation from Aeroengines

A first attempt to numerically simulate broadband noise transmitted through and radiated from a turbofan bypass duct is presented in this paper. Turbulent source assessment on the fan rows is left out here, and a generic power spectral density of in-duct acoustic pressure is imposed as an input to a computational aeroacoustic simulation performed using a time-domain Euler solver. The acoustic field is expanded over Fourier―Bessel modes, and the broadband noise behavior is modeled by introducing a random phase and by performing a quadratic averaging over independent runs. This broadband source generation approach is first validated in a semi-infinite annular duct by comparison with analytical solutions. The exhaust radiation problem from an idealized nozzle is then validated (for a single mode) by comparison to a Wiener―Hopf solution. In a second step, refraction effect through the shear layer is investigated for broadband noise. Computed results appear to be stable even in the presence of vorticity waves created in the shear layer. Then, the computation method is applied to a realistic nozzle. The only problem arising is the computational cost resulting in a limitation of high-frequency range limited here to 2―3 kHz (with around 600 propagating modes). Computed far-field directivities relative to idealized or realistic nozzle with similar input conditions are found to be rather close up to 90° of radiation angle.

Surface integration method used on semi-solid surface for fan noise prediction.

As a continuation of previous work, the Ffowc-Williams and Hawkings model was applied to an axial fan that was operated under unfavorable inflow condition. A thin aluminum foam screen that is used as a treatment for disturbed inflow condition was included as a noise source in computational aeroacoustic simulation. The incompressible URANS model was used for the CFD simulation and the inflow foam was modeled as a homogeneous porous zone with permeability and inertial resistance factors found experimentally. Since the homogeneous porous zone defined in this way does not have a solid structure that could be used as a dipole noise source, the control surface integration method was applied to the surface of the homogeneous porous zone: i.e., to a surface right between the normal fluid zone and the porous zone. The unsteady pressure data on the control surface under the incompressible assumption was treated as representing pressure fluctuations on a semi-solid surface rather than as a representation of acoustic sources within the volume surrounded by the control surface.