Showing papers on "Computational aeroacoustics published in 2023"

Weighted compact nonlinear hybrid scheme based on a family of mapping functions for aeroacoustics problem

Abstract: The study of differential schemes with high accuracy and low dispersion is of great significance for the numerical simulation of aeroacoustics over complex geometries. The midpoint-and-node-to-node explicit finite difference is employed to solve the flux derivatives of the linear Euler equations for improving the robustness. To obtain the numerical flux at the center of the grid element, we adopted a hybrid interpolation method of center and upwind interpolations, combined with a symmetrical conservative metric method, to achieve high-resolution discretization of the acoustic field variables and geometric variables of the structural grid. To suppress spurious oscillations and improve the resolution of discontinuous regions, a family of mapping functions is developed to establish different smoothness indicators and applied to the sixth-order weighted compact nonlinear hybrid scheme (WCNHS), forming a new WCNHS scheme based on piecewise exponential mapping functions (WCNHS-Pe). The approximate dispersion relation shows that the dispersion error and numerical dissipation of WCNHS-Pe are smaller than those of WCNHS with a simple mapping function to the original weights in Jiang and Shu and other mapping function-weighted WCNHS schemes. We have applied various WCNHS schemes to the numerical simulation of the Shu–Osher problem, propagation of Gaussian impulses on two-dimensional wavy grids, sound transmission at discontinuous interfaces, propagation of Gaussian impulses on three-dimensional wavy grids, etc. Numerical results indicate that the WCNHS-Pe scheme has better discontinuity capture capability, higher resolution, and lower dispersion under the same differential stencil, and is suitable for computational aeroacoustics of complex geometries.

Integrated Evaluation of the Aeroacoustics and Psychoacoustics of a Single Propeller

Abstract: Aeroacoustic noise in multiple rotor drones has been increasingly recognized as a crucial issue, while noise reduction is normally associated with a trade-off between aerodynamic performance and sound suppression as well as sound quality improvement. Here, we propose an integrated methodology to evaluate both aeroacoustics and psychoacoustics of a single propeller. For a loop-type propeller, an experimental investigation was conducted in association with its aerodynamic and acoustic characteristics via a hover stand test in an anechoic chamber; the psychoacoustic performance was then examined with psychoacoustic annoyance models to evaluate five psychoacoustic metrics comprising loudness, fluctuation strength, roughness, sharpness, and tonality. A comparison of the figure of merit (FM), the overall sound pressure level (OASPL) and psychoacoustic metrics was undertaken among a two-blade propeller, a four-blade propeller, the loop-type propeller, a wide chord loop-type propeller, and a DJI Phantom III propeller, indicating that the loop-type propeller enables a remarkable reduction in OASPL and a noticeable improvement in sound quality while achieving comparable aerodynamic performance. Furthermore, the psychoacoustic analysis demonstrates that the loop-type propeller can improve the psychological response to various noises in terms of the higher-level broadband and lower-level tonal noise components. It is thus verified that the integrated evaluation methodology of aeroacoustics and psychoacoustics can be a useful tool in the design of low-noise propellers in association with multirotor drones.

Acoustic Predictions for Side-by-Side Rotor with Ground Effect

Abstract: Influence of the ground plane on the performance, aerodynamics, and aeroacoustics of NASA's side-by-side Urban Air Mobility (UAM) reference vehicle rotors in hover is investigated using high-fidelity computational fluid dynamics (CFD) and aeroacoustics simulations. CFD simulations are carried out with Helios, while acoustics calculations are conducted using PSU-WOPWOP. Two overlap cases, each at two different rotor heights, are considered. The results demonstrate a significant influence of the ground plane on rotor performance, with the figure of merit (FM) increasing as the distance between the rotors and the ground is reduced. The 0% overlap configuration shows an increase in blade sectional thrust, pressure fluctuations, and resulting noise levels compared to the 25% overlap configuration, primarily due to lower induced velocity as a result of having no rotor overlap, allowing upwash to reach the rotor disk plane. Aerodynamic interactions intensify with the presence of the ground plane and become more pronounced as the rotor-ground distance decreases. The 25% overlap configuration is found to provide a noise reduction benefit of 3-4 dBA near the ground compared to the 0∖% overlap configuration. Furthermore, a considerable difference is observed in the noise directivity between A-weighted and unweighted OASPL. A-weighting notably reduces regions of acoustic interference.

Data-Driven Multiobjective Optimization of Wave-Packets for Near-Field Subsonic Jet Noise

Abstract: Aeroacoustics of innovative aircraft cannot disregard the development of low-order models for the jet noise source, which are essential to assess the propulsion–airframe interactions from the conceptual design stage. The main scope of this work is to provide a noise source model that can be coupled with relatively low computational cost methods for aeroacoustic scattering. To this end, this paper presents for the first time a multiobjective optimization of the zeroth-mode wave-packet in the jet near field. The importance of calibrating the model with near-field pressure data stems from the fact that in new aircraft, the engine nacelles are typically positioned at a few diameters from the wing or fuselage. In this work, the near field of a high subsonic jet at a Mach number of 0.9 is represented as a cylindrical surface radiating the pressure disturbances of a wave-packet source, which is optimized using large-eddy simulation data from three lines at different radial distances. The optimized model has been tested at Strouhal numbers between 0.25 and one, and the optimized solutions have been chosen using a Pareto-ranking criterion considering the wave-packet prediction over an extra line. A good agreement is achieved between the reference data and model predictions for multiple near-field radial distances.

A weakly-coupled simulation for HVAC duct aeroacoustics including propagation into the cabin

Abstract: This paper focuses on a weakly-coupled solution between a near-field time domain aeroacoustics source generation and its propagation into car cabin in frequency domain. Such a weak coupling enables the aeroacoustics field to be resolved efficiently together with the aerodynamic field without concerning the cabin geometry and absorbing materials, which are easier to define in frequency domain. The resulting permeable Kirchhoff radiating surface is then used in a frequency domain acoustic solver together with the cabin geometry including absorber and porous materials in order to account for the installation effects. The weak coupling is first validated with a reference strongly-coupled numerical acoustic solution. The near field aeroacoustics field is also validated with experiments using a simplified L-shaped duct. Finally, the weak-coupling solution is applied to the industrial HVAC duct attached to the cabin.

Development of active noise control simulation with virtual controller based on computational aeroacoustics.

Abstract: Noise reduction and control research are actively conducted as increasing noise problems compel the stringent noise requirement. Active noise control (ANC) is constructively used in various applications to reduce low-frequency noise. In previous studies, ANC systems were designed based on experiments, requiring extensive effort for effective implementation. In this paper, a real-time ANC simulation in a computational aeroacoustics framework based on the virtual-controller method is presented. The aims are to investigate sound field changes following ANC system operation and gain more insight into ANC system design through a computational approach. Using a virtual-controller ANC simulation, the approximate shape of the acoustic path filter and changes in the sound field when ANC is either "on" or "off" at the target domain can be obtained, enabling practical and detailed analyses. Then, the computational results of the duct and open space cases are predicted and compared with the experimental results to validate the prediction capability of the proposed method. In addition, the ANC system design parameters and their effects on sound fields with unintended phenomena can be predicted. Through case studies, the ability to design, optimize, and predict the performance of the ANC system using the computational method is also demonstrated.

Noise Characterisation of Multi-Propeller in Hovering and Transitional Configurations Under Static Condition

Abstract: This paper represents an experimental study on the aeroacoustics behaviours of multi propellers. Two propellers are distributed in various spatial configurations to mimic some of the configurations that resemble a typical eVTOL (electrical Vertical Take-Off and Landing) aircraft. The interdependence for various influencing parameters for the acoustic radiations, such as the longitudinal and transverse displacements between a reference propeller and another propeller, as well as the different rotational speeds, was investigated in a specially constructed test rig inside an anechoic chamber under a quiescent condition. Statistical quantities such as the power spectral density and overall sound pressure level are presented.

Development of URANS and Fast Multipole Boundary Element Method Workflow for Installed Propeller Noise

Abstract: For the simulation of isolated propeller noise, a well-established computational aeroacoustics (CAA) approach relies on the simulation of the unsteady propeller blade loads with the unsteady Reynolds-Averaged Navier-Stokes (URANS) method for the aerodynamic part. The porous Ffowcs-Williams and Hawkings (FW-H) integral method is then applied for propagating sound waves to a far-field. Under realistic propeller installation conditions, one has to account for the propeller wake and airframe interaction, as well as for the shielding effects. The URANS / FWH approach needs to be extended to accomplish that, meaning that the FW-H surface must enclose significant parts, if not all of the aircraft. The problem arises where the URANS simulation lacks quality on distant surfaces due to excessive numerical dissipation and dispersion. If these data are treated as unknown the surface integral is best tackled by means of a fast multipole boundary element method (FM-BEM). One challenging part is to verify that the FW-H integral and the Kirchhoff’s formulation used in the FM-BEM are compatible. In terms of the application, the DLR’s own Dornier 228 STOL aircraft (registration D-CODE) that features five-bladed twin propellers has been chosen as a test bed. Several potent noise mechanisms are considered: propeller induced noise, vortex-surface interaction noise and hydrodynamic near-field interference with the airframe. The latter could be eliminated by improved propulsion system integration. Thus, a design study is carried out where the distance between the propeller plane and engine is increased to evaluate the noise reduction potential.

Aeroacoustics noise prediction for the airfoil-rod benchmark using high-order large eddy simulation on unstructured grids and the acoustic analogy approach in frequency-domain

Abstract: An implicit large eddy simulation (ILES) is coupled with a frequency-domain Ffowcs Williams-Hawkings (FWH) acoustic analogy formulations for the noise prediction of the airfoil- rod problem. Higher order Flux-Reconstruction method is used for spatial discretisation. The airfoil-rod case is simulated based on the compressible Navier-Stokes equations for a relatively high Reynolds number (Re = 480,000) and a Mach number of (M = 0.2) in accordance with the experimental setup. The FWH acoustic analogy solver is implemented in the open-source massively parallel software PyFR which is a Python based computational fluid dynamics (CFD) solver developed for heterogeneuous computing systems. The main goal of this work is to investigate the use of high-order methods on unstructured grids for such acoustic problems and assess the suitability of the hybrid approach in computational aeroacoustics with high-order discretization methods. The near-field aerodynamics based on averaged pressure and friction coefficients is computed and validated before the farfield acoustics are collected and results show good agreement with experimental and numerical literature.

Aerodynamic Calibration for Aeroacoustics Validation of an Open Fan Configuration

Abstract: The necessity of alleviating adverse climate effects has reinvigorated the drive to assess open fan technology as a promising option for aircraft engines due to its high propulsive efficiency. Nevertheless, maintaining its noise levels within acceptable limits has been a challenge. Assessing prediction tools in aeroacoustics then plays an important role for im�proving the design of future open fan configurations, especially those in the high-fidelity category. Complete agreement between simulations and experiments is difficult to achieve due to an uncertainty of several simulated aspects such as the underlying physics, model parameters, and accounting for all information from experiments. In order to assess the agreement between numerical predictions and experiments, a study consisting first of aero�dynamic calibration and then of aeroacoustic validation of lattice Boltzmann simulations against experiments on the F31/A31 open rotor is carried out. Numerical results show that calibrated pitch settings result in averaged discrepancies, in overall sound pressure level, ranging from 2.5 to 1.6 dB for the lowest to the highest rotor speed. The overall acoustic power follows a similar trend with discrepancies of 2.5 and 0.36 dB with rotor speed

Multi-Fidelity Numerical Approach to Aeroacoustics of Tandem Propellers in eVTOL Airplane Mode

Abstract: The present paper deals with an innovative multi-fidelity framework for characterizing the aerodynamics and the aeroacoustics of multirotor systems with application to advanced eVTOL aircraft. In this framework, the goal of the activity is a multi-fidelity investigation of two propellers in tandem with rotor disks overlap, reproducing a typical feature of eVTOL architectures in airplane mode flight conditions. A Ffowcs Williams–Hawkings equation solver is used to compute the aeroacoustic footprint of the test case in different configurations. The solver is run with both mid-fidelity aerodynamic simulation data obtained with DUST and high-fidelity simulation data obtained with SU2. The comparison between the two different approaches highlights the suitability of the DUST mid-fidelity simulations for the aeroacoustic investigation of a propeller in airplane mode configuration with a much lower computational cost with respect to high-fidelity CFD simulations. On the other hand, the co-axial propellers tandem configuration shows some differences in the sound pressure level computed with the two approaches, mainly due to the contribution to noise emission given by the rear propeller.

Validation of the Computational Aeroacoustic Simulation of a Subsonic Jet tailpipe Flow Noise

Abstract: A case study in the application of computational aeroacoustics in the prediction of flow noise from a subsonic jet generated at the tailpipe of a straight pipe. The simulation is completed using a hybrid approach. The results from a Large Eddy Simulation completed using are used to formulate acoustic sources. These acoustic sources are formulated using Lighthill's analogy and then mapped onto an acoustic finite element mesh using the tools built into the acoustic FEM software Actran. The total predicted sound power of the sources inside the pipe and the turbulent jet region are then validated to completed acoustic measurement using the University of Kentucky's muffler flow test rig.

Suppression of deep cavity aeroacoustics at low Mach number by localized surface compliance

Abstract: A unique concept of utilizing localized surface compliance is proposed to suppress deep cavity aeroacoustics at a low Mach number. The core idea is to provide local absorption of the energy of aeroacoustic processes supporting cavity flow self-sustained feedback loop responsible for tonal noise generation. The concept is studied with a flow past cavity of length-to-depth ratio of 0.4 at freestream Mach number 0.09 and Reynolds number based on cavity length 4 × 104 using high-fidelity, two-dimensional direct aeroacoustic simulation. Having confirmed the replication of key aeroacoustic processes in the numerical solution through careful validation, localized surface compliance in the form of an elastic panel is strategically introduced to modify every process for cavity noise suppression. The panel natural frequency is set equal to the feedback loop characteristic frequency to facilitate its flow-induced structural resonance for energy absorption. Suppression of cavity noise pressure and power levels by 3.8 and 4.8 dB, respectively, is successfully achieved, together with an unforeseen cavity drag reduction by almost 19%. Comprehensive wavenumber–frequency analyses of the coupled aeroacoustics and flow-induced panel vibration are conducted to uncover the physical mechanism of noise suppression. The results show that the same type of aeroacoustic feedback loop occurs, but its efficacy is significantly reduced due to the exhaustion of aeroacoustic process energy to the flow-induced vibrating panel. The proposed concept is confirmed to be feasible in terms of giving remarkable cavity noise and drag suppression, yet it retains the basic problem geometry intact, which are considered important in many practical applications.

Optimized Combined Compact Differencing (CCD) Methods for Computational Aeroacoustics

Abstract: The sixth order Combined Compact Differencing (CCD) scheme of Chu and Fan is optimized for improved dispersion and dissipation performance. Results indicate that the best of these optimized schemes can accurately propagate linear simple harmonic waves with fewer than three points per wavelength, while using a three point differencing stencil. Stable and accurate boundary treatments for these schemes are given, and the performance of these schemes is assessed using a benchmark problem.

Assessment of the Vortex Feature-Based Vorticity Confinement Method Applied to Rotor Aerodynamics and Aeroacoustics

Abstract: The accurate prediction of helicopter rotor aerodynamics and aeroacoustics using Computational Fluid Dynamics (CFD) techniques still remains a challenge, as the over-dissipation of numerical schemes results in a higher diffusive rate of rotor wake and vortices than what can be expected from the fluid governing equations. To alleviate this issue, a vortex feature-based vorticity confinement (FVC2-L2) method that combines the locally normalized λ2 vortex detection method with the standard second vorticity confinement (VC2) scheme is presented to counterbalance the truncation error introduced by the numerical discretization of the convective term while avoiding the over-confinement inside the boundary layer. The FVC2-L2 scheme is adopted for helicopter rotor aerodynamic and aeroacoustic predictions through its implementation in the multi-block structured grid CFD solver ROSITA and coupling with the aeroacoustic code ROCAAP based on the permeable surface Ffowcs Williams–Hawkings (PS-FWH) equation. This approach is assessed in helicopter rotor flows via three databases. Firstly, the well-documented HART-II rotor in the baseline condition is used to evaluate the capability of the presented VC scheme in blade–vortex interaction (BVI) phenomena prediction. Subsequently, the UH-1H non-lifting hovering rotor and the AH-1/OLS low-speed descending flight rotor are adopted for assessment of such a method in aeroacoustics. The benefits of the FVC2-L2 scheme in terms of aerodynamics prediction, wake preservation, and noise signal prediction are well demonstrated by comparison with the experimental data and the results obtained without VC schemes. Particularly, the FVC2-L2 scheme mainly improves the highly unsteady airloads prediction, and results in an improvement of BVI noise prediction by more than 5 dB with respect to the case without VC schemes for AH-1/OLS rotor case. Additionally, some shortcomings of the approach are noticed in engineering applications. On the basis of a simplified convective vortex, some provisional guidelines on the required εo value in terms of number of cells per vortex diameter are provided: an εo value ranging from 0.01 to 0.04 for grids which may represent the vortex core diameter with 6 to 12 cells.

Comparative study of surface integral methods in aeroacoustic prediction

Abstract: Introduction: Surface integral methods based on the acoustic analogy and Kirchhoff formulation are widely employed in computational aeroacoustics. The computational accuracy is usually highly dependent on the selections of the acoustic prediction method and of the integral surfaces. Methods: This paper analyzes the pros and cons of each aeroacoustic prediction method and studies numerically sound generated from flow past a circular cylinder by employing different surface integral methods. The acoustic analogy based on the impermeable solid surfaces either ignores the quadrupole contribution or needs high computational cost to calculate the quadrupole contribution, and the acoustic analogy based on the permeable integral surfaces usually suffers from the spurious source issue. Results: Both the pressure-based or density-based Kirchhoff formulations can be used in aeroacoustic prediction, however, the numerical results indicate that the pressure-based Kirchhoff formulation also suffers from the issue of the spurious sound because the pressure fluctuations at the permeable integral surfaces are contaminated by hydrodynamic component. Discussion: It seems that only the density-based Kirchhoff formulation does not suffer from the issue of the spurious sound, but this formulation requires the acoustic sources should be extracted from compressible flow simulations.

Special Issue “Aeroacoustics and Noise Mitigation”

Abstract: Aerospace, an open access journal operated by MDPI, recently released a Special Issue entitled “Aeroacoustics and Noise Mitigation” [...]

A scale-aware dispersion-relation-preserving finite difference scheme for computational aeroacoustics

Abstract: The numerical schemes for computational aeroacoustics (CAA) should have minimal dispersion and proper dissipation in order to accurately capture the amplitude and phase of waves. In this paper, we propose a scale-aware dispersion-relation-preserving (SA-DRP) finite difference scheme based on an improved scale sensor and a new dispersion control strategy. The scale sensor quantifies the local length scale of the solution in the form of the effective scaled wavenumber. The new feature of this scale sensor is the accurate prediction of the wavenumber for a pure sine wave. The new dispersion control strategy determines the dispersion parameter of the scheme in terms of the scale sensor. In contrast to the traditional dispersion-relation-preserving (DRP) scheme that minimizes the integral dispersion error, the new strategy directly solves the dispersion parameter by requiring the numerical dispersion relation to be equal to the exact one. As a result, precise dispersion relation can be realized within a very broad wavenumber range. The approximate dispersion relation analysis shows that the SA-DRP scheme maintains an accurate dispersion relation up to the scaled wavenumber k = 2.5. Moreover, the overshoot in the dispersion relation of the DRP scheme is not presented in that of the SA-DRP scheme. To suppress nonphysical oscillations, we also add proper dissipation that is adjusted automatically according to the effective scaled wavenumber. Several CAA benchmark test cases are presented to demonstrate the higher resolution and higher efficiency achieved by the proposed scheme compared with the conventional spectrally optimized schemes.

Empirical Rotor Broadband Noise Prediction Using CFD Boundary Layer Parameter Extraction

Abstract: In this study, for small unmanned aerial vehicle aeroacoustic predictions, an in-house acoustic code was extended for broadband noise using the empirical Brooks–Pope–Marcolini (BPM) method. The acoustic code was coupled with a comprehensive analysis tool (FLIGHTLAB) or Reynolds-averaged Navier–Stokes (RANS) method to predict the noise from a DJI 9443 CF rotor. The effects of two input parameters of the BPM method on the broadband noise were investigated, including the effective angle of attack and boundary layer parameters. The empirical formula for the boundary layer parameters was replaced with predictions using two-dimensional RANS, allowing arbitrary airfoils other than NACA0012. The boundary layer parameters were also predicted using three-dimensional RANS to capture a three-dimensional flow effect, which can be dominant in modern propellers operating at high rotational speeds. As a result, the three-dimensional effect on the boundary layer was confirmed to conflict with the BPM method, which was developed based on a two-dimensional chordwise flow database. Finally, mid- and high-frequency noise spectra were predicted from the three-dimensional hybrid RANS/LES simulation to be combined with the BPM results, thus improving the noise spectra predictions at midrange frequencies.

The Influence of Forebody Topology on Aerodynamic Drag and Aeroacoustics Characteristics of Squareback Vehicles using CAA

Abstract: This study numerically investigates the aerodynamic forces and flow-induced noise generated by SAE-T4, Ahmed, and Hybrid forebody shapes with a squareback vehicle configuration using SBES-FW-H. The results show significant differences in lift coefficients and the presence of a horseshoe vortex at the mirror, with smaller eddies that interact with A-pillar vortices, resulting in pronounced pressure fluctuations and noise generation on the side window for the three configurations. Surprisingly, negligible differences in aerodynamic drag and radiated sound are predicted despite these effects.