Computational Aeroacoustics Research Articles

Aerodynamic and aeroacoustic characterization of an axial fan using Adaptive Mesh Refinement (AMR) based CFD and CAA

This work focuses on numerical analysis of a low-pressure axial fan using Computational Fluid Dynamics (CFD) and Computational Aeroacoustics (CAA) methods. These simulations are performed with the commercial software Hexagon's Cradle scFLOW. The investigation focuses on flow prediction using incompressible Large Eddy Simulation (LES), and acoustic propagation based on the Ffowcs Williams-Hawkings (FW-H) equation. We present a method to evaluate the mesh resolution for LES and a mesh refinement approach to resolve strong flow interactions, particularly near the tip-gap region. Both the aerodynamic and aeroacoustics performance of the fan are compared to experimental measurements for model validation. The pressure rise, velocity profiles and surface pressure fluctuations are captured well. Similarly, the sound propagation results are in good agreement with the experimental measurements. At low-mid frequencies, the blade passing frequencies and subharmonic peaks are captured, and at high frequencies, the broadband energy content is underpredicted, but still reasonably correlated. Additionally, a discussion regarding the mesh quality for LES is presented.

Multirotor drone noise: numerical approach for aeroacoustics and in-flight noise prediction

Nowadays, multirotor drone usage has become widespread for various purposes: amateur or professional videos, building inspection, civil security, military defense, etc. But this usage increase has also seen growing concerns about community noise annoyance in inhabited areas, acoustic detectability for security and military purposes, or remote wildlife monitoring. Predicting the drone noise can now be accurately achieved thanks to computational aeroacoustics (CAA). The present work deals with the aeroacoustic simulation and measurement of a quadcopter drone and in-flight condition prediction. The CAA simulation of a single rotor is first compared with test bench measurements in an anechoic room. Comparison for the full drone is then derived from this single-rotor comparison. Finally, the study is further extended to in-flight comparisons, in order to show that CAA simulation can help design quieter drones in real operating conditions.

Numerical simulation of the flow-induced noise of a ducted diaphragm using the Lattice Boltzmann Method

Acoustic discretion is of prime importance for submarines while operating, in order not to be detected. One of the submarine acoustic sources is the noise induced by water flows passing through singularities in pipes ending at the hull, thus radiating acoustic waves in the vessel surroundings. Predicting the noise induced by singularities in pipes is then fundamental to determine the acoustic signature of an underwater ship. Several Computational Aero-Acoustics methods have been implemented in order to predict the sound induced by singularities such as diaphragms and perforated plates. In this study, the Lattice Boltzmann Method (LBM) coupled to Large Eddy Simulation (LES) is used to simulate the flow passing through a diaphragm and to predict the induced noise. The method is first implemented for an airflow in a rectangular duct, with the final objective of being applied to confined water flows encountered in circular or rectangular ducts.

Effect of Airfoil thickness on low-frequency gust-airfoil interaction noise

Interaction between turbulence and an airfoil is a significant aerodynamic noise source for many engineering applications when turbulence in the wake of upstream blades interacts with the leading edge of downstream blades. Modeling the oncoming turbulence as harmonic gusts is a common approach to studying the noise generated by turbulence-airfoil interaction. The airfoil can be considered a compact noise source, resulting in a dipole-like sound directivity pattern when the acoustic wavelength of gust-airfoil interaction is much larger than the airfoil chord length. However, the interaction becomes more complex when the acoustic wavelength is comparable to or smaller than the airfoil chord, as the airfoil cannot be treated as a compact source anymore and multiple lobes appear in the radiated sound directivity. This paper studies airfoil thickness effects using a computational aeroacoustics approach based on the spectral/hp element method for low-reduced frequency gusts. It is found that the sound pressure decreases with the increase of thickness for low reduced frequency gusts. To reveal its mechanism, a semi-analytical method and the convective FW-H equation are used for low reduced frequency to analyze the radiated noise. The interaction between sound sources on the airfoil surface is crucial for the thickness effect.

Underwater acoustic signal feature extraction of mixed flow pump based on empirical wavelet transform

The pump converts mechanical energy into potential energy, and a mixed-flow-pump combines the characteristics of an axial flow pump and a centrifugal pump. When the mixed-flow-pump operates at low flow conditions, performance instability in the hump region appears on the performance curve. This study investigates the underwater acoustic signal in this area through experiments, computational fluid dynamics (CFD), and computational aero acoustics (CAA). The hysteresis factor calculated by cross correlation (CC) is utilized to improve the dynamic time warping (DTW) signal comparison verification method. The improved method (CC-DTW) improves the ability of DTW in signal comparison and verification. Compared with the fast Fourier transform (FFT) method, it is more convincing in the comparison of experimental and simulation pressure pulsation signals. After verifying the effectiveness of pressure pulsation signals and underwater acoustic signals, a combined empirical wavelet transform and FFT method is used to analyze underwater acoustic signal in instability regions. The results indicate that the depth of instability aligns with the FFT frequence ratio of the intrinsic mode function. Based on the feature, a criterion for determining instability states in mixed-flow-pumps is proposed.

Computational aeroacoustics of low-pressure axial fans installed in parallel

Abstract Ducted rotor-only Low-Pressure Axial (LPA) fans play an integral role in automotive thermal management of electric vehicles and are the primary source of noise from the underhood region. Multiple LPA fans are often placed in parallel in cooling packages of electric vehicles. There is little scientific work concerning aeroacoustics of ducted LPA fans operating in parallel. This work aims to address this gap through hybrid computational aeroacoustic simulations. Three-dimensional, full-annulus, transient simulations are done using the Delayed Detached Eddy Simulation (DDES) turbulence model. First, a numerical validation study is presented, where aerodynamic and aeroacoustic results from this work are compared to experimental results for a LPA fan issued by the European Acoustics Association (EAA). In the second part, aeroacoustic performance of two-fans placed in parallel is presented. A local diffusion zone is observed in the region where the two-fans are closest to one another. A previously unidentified vortex which envelopes the local diffusion zone is observed. For two-fans in parallel, the amplification of the acoustic spectrum scales in accordance to having two identical equally strong sound sources, i.e, by 6 dB. The scaling of the acoustic spectrum for two-fans in parallel in comparison to a single-fan suggests limited interaction between the acoustic field of the two-fans.

Optimization of Cooling Fan Based on Unequal Angle and Leading-Edge Triangle Blade Design

In order to reduce the noise of the axial fan, an uneven blade spacing was used to optimize tonal noise and blade leading edge triangles were added to optimize the broadband noise. First, different angular distribution schemes were designed on the basis of ensuring the dynamic balance of the fan, and the noise of the different schemes was simulated by using the computational aeroacoustics (CAA) numerical simulation method. Then, the best optimal angular distribution design was selected for experimental verification, which confirmed the reliability of the numerical calculation. The test results also show that the uneven blade spacing has little effect on the fan pressure and efficiency. Additionally, triangular leading edges were added to each blade based on the uneven blade spacing design. Combining these two optimization schemes proved to reduce the fan noise by 13.4 dBA.

A hybrid method for aeroacoustic computation of moving rigid bodies in low Mach number flows

A hybrid method for aeroacoustic computation of moving rigid bodies in low Mach number flows

Towards the prediction of flame transfer functions: Evaluation of a hybrid LES-CAA with compressible LES

The prediction of flame transfer functions, particularly in practically relevant systems, remains challenging and computationally demanding. Numerical approaches are a valuable addition to experimental acoustic characterizations of industrial configurations. Conventionally, fully compressible numerical simulations are used that naturally include acoustic fluctuations in their computations, but can be computationally expensive depending on the configuration. Therefore, a convenient approach to use tailored numerics for the underlying physics is considered in this work. In this work, this is realized by applying a runtime-coupled method of computational fluid dynamics and computational aeroacoustics to a single-sector aero-engine combustor. This hybrid computational fluid dynamics and computational aeroacoustics method captures fluid flow behavior and combustion dynamics in a low-Mach computational fluid dynamics domain while allowing for acoustic perturbations in the computational aeroacoustics. Runtime exchange of hydrodynamic and acoustic quantities between the two solvers allows for a bidirectional coupling and, by extension, a complete description of the combustion system. In this work, the hybrid computational fluid dynamics and computational aeroacoustics is applied in a high-fidelity large eddy simulation configuration. The flame transfer function is evaluated for both compressible and hybrid simulations. The results for both numerical approaches are validated with each other and compared to the experimentally obtained flame transfer function. Finally, the computational effort for the numerical approaches is considered. This article presents the first application of a high-fidelity computational fluid dynamics framework using large eddy simulation with bidirectional coupling with the acoustic solver to an industry-relevant configuration. The aim is to provide a roadmap towards investigating thermoacoustic instabilities in a real gas turbine engine at reduced computational costs.

A very fast high-order flux reconstruction for Finite Volume schemes for Computational Aeroacoustics

AbstractGiven the small wavelengths and wide range of frequencies of the acoustic waves involved in Aeroacoustics problems, the use of very accurate, low-dissipative numerical schemes is the only valid option to accurately capture these phenomena. However, as the order of the scheme increases, the computational time also increases. In this work, we propose a new high-order flux reconstruction in the framework of finite volume (FV) schemes for linear problems. In particular, it is applied to solve the Linearized Euler Equations, which are widely used in the field of Computational Aeroacoustics. This new reconstruction is very efficient and well suited in the context of very high-order FV schemes, where the computation of high-order flux integrals are needed at cell edges/faces. Different benchmark test cases are carried out to analyze the accuracy and the efficiency of the proposed flux reconstruction. The proposed methodology preserves the accuracy while the computational time relatively reduces drastically as the order increases.

High-fidelity computational study of aerodynamic noise of side-by-side rotor in full configuration

This paper presents a numerical study of side-by-side rotor aircraft in full configuration during hover using high-fidelity computational fluid dynamics (CFD) and aeroacoustic simulations. CFD simulations are performed using HPCMP CREATE™-AV Helios, while acoustic calculations are conducted with PSU-WOPWOP. Overset structured grids are applied using the SA-DES turbulence model along with adaptive mesh refinement in the near- and off-body meshes. Four overlap cases at a specific collective pitch angle of 8° are considered. Results reveal a substantial influence of the fuselage on rotor performance, with improvement in the figure of merit being observed for each overlap configuration due to a partial ground effect. There is considerable increase in blade sectional thrust, pressure fluctuations, and noise levels seen in the 0% overlap configuration compared to the 25% overlap configuration. This increase is primarily due to the absence of rotor overlap, resulting in a lower induced velocity allowing for flow to travel upward, and reaching the rotor disk plane. The acoustic spectrum appears at all harmonics of the blade passing frequency for the 0% and 5% overlap configurations. In contrast, the acoustic spectrum is distinctly present only at even harmonics for the 15% and 25% overlap cases. A noise reduction benefit of 3-5 dBA is noticed in the full configuration cases having rotor overlap compared to the 0% overlap configuration. Acoustic destructive interference is observed in the directivity plot only for the 15% and 25% overlap configurations. Overall, this paper reveals that the fuselage has a significant impact on the performance, aerodynamics, and aeroacoustics of the side-by-side UAM vehicle in hover.

Flow and Aeroacoustic Characteristics of Underexpanded Supersonic Jets Exhausting from a Conical Converging Nozzle

Ensuring the safety of space flights and solving the problems of reducing acoustic loads during the launch of space vehicles requires not only the development of new technical systems for launch complexes, but also methods for the numerical simulation of fluid and aeroacoustic fields generated by supersonic jets. The growing regulations for space vehicle noise also explain the interest in developing models and techniques that anticipate flow and the aeroacoustic characteristics of supersonic jets. Together with integral techniques for computing far-field noise, development of relevant mathematical models and implementation of numerical tools, the concepts of computational fluid dynamics (CFD) and computational aeroacoustics (CAA) are covered. The noise generated by a supersonic underexpanded jet is used to illustrate the capabilities of current numerical modelling and simulation tools. The jet structure, flow properties, and aeroacoustic quantities are affected by the nozzle pressure ratio. The outcomes of numerical simulation are contrasted with existing experimental and computational data. The available numerical modelling and simulation tools facilitate the development of novel computational methods and methodologies for challenges in CFD and CAA, in addition to solving research and engineering problems.

Aerodynamic and aeroacoustic characteristics of rocket sled under strong ground effect

Rocket sled test avoids boundary effects in wind tunnel test and inconvenience of flight test, which is one of the feasible options for future single-stage-to-orbit. To analyze potential safety issues during payload separation and optimize the arrangement of testing sensors, different structural layouts and operating speeds of the rocket sled are conducted based on computational fluid dynamics and computational aeroacoustics. The hypersonic winged standard model is utilized as the load for these simulations. The analysis encompasses the evolution of shock waves, the forces exerted on the payload and noise propagation. Variations in flow field and aeroacoustic characteristics of rocket sled are analysed, revealing underlying physical mechanisms. It is observed that placing the payload in front of the thruster can reduce the head pressure, excessive wingspan may have an impact on the structural safety of the wingtip, and the regions of high sound pressure levels mainly exists in the middle and rear sections of the rocket sled. Moreover, as the Mach number increases, the characteristic frequency initially rises and then declines. These researches can serve as a valuable reference for the development of ground test systems and single-stage-to-orbit.

Numerical Prediction of Tonal Aeroacoustic Noise Produced by Small Wind Turbines

The aeroacoustic design of small wind turbines (SWTs) can be challenging due to the possibility of the low Reynolds number (Re) flow over the blades generating tonal noise. The numerical prediction of this tonal noise using computational aeroacoustics can improve the understanding of the flow mechanisms behind the tonal noise to improve SWT blade design. In this study, wall-resolved incompressible large eddy simulation (LES) and the Ffowcs-Williams and Hawkings (FW-H) acoustic analogy are applied to a low Re airfoil, SD 7037, at Re = 4.1 × 104 to assess the ability of this method to predict tonal noise. The tonal prediction at 1° angle of attack aligned with experimental measurements and further analysis confirmed that the Kelvin-Helmholtz (K-H) rolls in the suction side laminar separation bubble (LSB) are the source of the aeroacoustic tone. The tone is due to the K-H rolls passing the trailing edge of the airfoil, and a secondary tone intermittently appears due to a 3D instability in the K-H roll. The accurate prediction of tonal noise using LES and FW-H opens the possibility of incorporating this method into the aeroacoustic design of low Re airfoils used for SWTs.

Hybrid aeroacoustic investigation of turbulent 90° pipe bend flow with source terms from Large-Eddy Simulation

The internal, low Mach number, turbulent flow-induced acoustic field of a cylindrical 90° pipe bend with a curvature radius Rm/D=1.02 is investigated using hybrid Computational Aero-Acoustics (CAA). Incompressible Large-Eddy Simulation (LES) is used to computationally describe the turbulent flow field at bulk Reynolds numbers ReB=5000,12000,26000 and 66000. The incompressible LES predicts all salient flow features, like the separation from the inner wall of the bend followed by a turbulent recirculation zone, in good agreement with literature. The internal flow-induced noise is predicted by Acoustic Propagation Simulations (APS) applying Lighthill’s Equation (LE), Ribner’s Dilatation Equation (RDE) and the Perturbed Convective Wave Equation (PCWE), provided with acoustic source terms obtained from the incompressible LES. The turbulent recirculation zone is the most active region for generating acoustic sources. Among the two constituent components of the PCWE source term, the local pressure time derivative term turned out as clearly dominant in this region, so that PCWE and RDE predicted similar acoustic fields. The sub-grid scale (SGS) model applied in LES notably contributes only at high frequencies to the total LE source term, starting from f=2000 Hz for the lower Reynolds numbers and beyond for the higher. The mapping of source terms from the boundary layer resolving LES grid onto the typically coarser preferably uniform APS grid leads to lower high-frequency signal amplitudes for coarser APS mesh size in regions with small turbulent structures. The impact of this amplitude reduction on the predicted acoustics sound power levels was still very small, exhibiting relative differences less than 0.7% for the different APS meshes. Despite the LES-specific limitations expected from the decrease of directly resolved content for higher Reynolds numbers, the use of increasingly underresolved source terms did not significantly lower the accuracy of finally predicted acoustics. The comparison of the acoustic results against data from dedicated measurements generally showed very good agreement for all Reynolds numbers.

Analysis of Sound Propagation Behaviors for Supersonic Jet on Far Field Region by Computational Aero-Acoustics (CAA) Simulation

Noise emission is an essential issue for the aviation industry, as it harms health and induces various physiological responses. The noise generated by supersonic jets is very intense. It will cause fatigue and even damage the human hearing system in the surrounding area of jet operation. Besides, the experimental and prototyping cost for the jet model is prohibitive, and it is a vast project and process that takes a lot of time to run. The purpose of this study is to determine the sound propagation behaviours of a supersonic jet in the far-field region and to analyse the consequences of the velocity of a supersonic jet on sound propagation of supersonic jet by using computational fluid dynamics (CFD) and computational aeroacoustics (CAA) simulations. This study focused on the perspective of observing the distance of the receiver when receiving the sound propagation of a supersonic jet, the observed angle of the receiver when receiving the sound propagation of a supersonic jet, and the velocity of the supersonic jet. The CFD and CAA analyses were performed in transient state simulation and the 2-inch Acoustics Reference Nozzle (ARN2). The result shows that the overall SPL throughout the frequency is proportional to the jet velocity of the supersonic jet. However, the distance and angle of the receiver gave different results in sound propagation behaviour. The results also conclude that as the distance between the receiver and jet nozzle exit increases, the overall SPL trend will decrease throughout the frequency increase. As the vertical distance between the receiver and the axisymmetric line of the jet nozzle increases, the frequency of the receiver starts to observe will decrease.

Numerical prediction of mean flow and acoustic field of a supersonic impinging jet

The three-dimensional turbulent mean flow and acoustic field of a supersonic jet impinging on a solid plate is studied computationally using the general purpose CFD code Ansys Fluent. A pressure-based coupled solver formulation with the second order weighted central-upwind spatial discretization is applied to compute transient solutions. Cold and hot jet thermal conditions are considered. Mean flow characteristics are investigated by a steady-state modeling approach. Acoustic radiation of impingement tones is simulated using a transient time-domain formulation. The effects of turbulence in steady-state are modeled by the SST k-ω turbulence model. The Wall-Modeled Large-Eddy Simulation (WMLES) model is applied to compute transient solutions. The near-wall mesh on the impingement plate is fine enough to resolve the viscosity-affected near-wall region all the way to the laminar sublayer. Nozzle-to-plate distance is parameterized in the model for automatic re-generation of the mesh and results. Steady-state predictions of hover lift loss and mean jet velocity distributions are compared with experimental data, and favorable agreement is reported. The transient solution reproduces the mechanism of impingement tone generation by the interaction of large scale vortical structures with the impingement plate. The acoustic near-field is directly resolved by Computational Aeroacoustics (CAA) to accurately propagate impingement tone waves to near-field microphone locations. Calculated impingement tone frequencies and sound pressure levels agree with experimental values.

Analysis of the co-rotating vortex pair as a test case for computational aeroacoustics

The co-rotating vortex pair arrangement is a frequently applied test case in computational aeroacoustics. Its flow quantities and far-field radiated sound pressure can be computed analytically, assuming delta-distribution potential vortices. When this configuration is used for verification in a numerical computation, the singular vortex cores cannot be modeled directly; they have to be desingularized, which affects the associated flow and acoustical fields. Closed-form expressions for the aeroacoustic source terms are derived for the Scully vortex model and used for detailed numerical computations of the radiated sound pressure. The effects of desingularization and finite source region size are analyzed in the frequency domain for Lighthill’s equation and the perturbed convective wave equation by means of an efficient numerical approach that computes the convolution of the aeroacoustic source terms and the 2D free-field Green’s function. It is shown that the deviations of the far-field radiated sound pressure from the idealized analytical solution depend on the Mach number, vortex core radius, and truncation limit of the source region. A significant increase in the effective size of the source region due to the desingularization is demonstrated. Minimal error bounds compared to the analytic far-field pressure solution are established. The presented converged numerical results serve as reference solutions for validating implementations of hybrid computational aeroacoustic workflows.

Theory of the momentum source method for synthetic turbulence

The interaction between turbulence and blade leading edges is known to have a significant impact on the aerodynamic and aeroacoustic performance of propellers. In addition to directly simulating turbulence, synthetic turbulence, such as the momentum source method, has been developed as a popular method for studying this interaction process in computational fluid dynamics and computational aeroacoustics. However, it is found that for non-periodic disturbances, although the induced velocity field is divergence-free, spurious noise may be generated in the source region and contaminate simulation results. To address this issue, the present work proposes adding a correction term so that the divergence-free condition is satisfied globally and the unwanted acoustic waves are suppressed, as an extension to our previous work for time-periodic gusts [H. Jiang, Phys. Fluids 35, 096115 (2023)]. The strength of the proposed approach lies in its simplicity, flexibility, and generality. First, it derives explicit source terms, which are straightforward for numerical implementations, to generate unsteady flow fluctuations. Second, the sources can be added inside the computational domain, saving computational costs for turbulence convection and being compatible with most existing boundary conditions. Third, the proposed method can obtain analytical expressions for the needed momentum source of the Navier–Stokes equation subject to any desired isotropic or anisotropic divergence-free turbulence fields. The method has been verified by examples of synthesizing harmonic gusts, Gaussian eddies, and random turbulence. The synthetic velocity results characterized by different spectral components are directly compared to target velocity fields, verifying the proposed approach and showing its capability. Parameters that influence the distribution of added sources are systematically investigated to identify an optimal combination for different scenarios. Finally, the model is employed to evaluate the aerodynamic interaction between an incoming turbulence and a thin airfoil. The obtained results exhibit good correspondence with analytical solutions.

A mathematical‐boundary‐recognition domain‐decomposition Lattice Boltzmann method combined with large eddy simulation applied to airfoil aeroacoustics simulation

AbstractBeing a direct computational aeroacoustics method, Lattice Boltzmann method (LBM) has great potential and broad application perspective in the field of numerical simulation of aerodynamic noise due to its low dispersion and low dissipation. A series of numerical algorithms and the related improvements based on the standard LBM method are proposed and developed in this paper to adapt to the airfoil noise calculation with complex grid at middle‐high Reynolds number. First, a new mathematical‐boundary‐recognition algorithm based on Green's formula is proposed to deal with complex curved geometric models, which is validated by three‐element airfoil 30P30N benchmark. Then, in order to reduce grid redundancy and improve computing efficiency, the grid refinement technique of domain decomposition model (DDM) is adopted and also improved, which is verified by calculating the flow and sound fields around 2D and 3D cylinders at Reynolds number equal to 90,000. Finally, three different LES turbulence models are combined with the standard MRT‐LBM method, where different finite difference schemes are used to solve Reynolds stress tensor which is different from the traditional one. Through the direct acoustic numerical simulation of NACA0012 airfoil at Reynolds number equal to 200,000, the effects of Smagorinsky models and Wall‐adapting local eddy‐viscosity (WALE) model on aerodynamic noise prediction are compared and analyzed. Overall, the proposed methodology is shown to be appropriate for predicting the aerodynamic noise at low Mach number and can successfully simulate the generation and propagation of far field acoustics.