Aeroacoustic Interactions Research Articles

A numerical investigation of the aerodynamic and aeroacoustic interactions between components of a multi-rotor vehicle for urban air mobility

In this work, we conduct a numerical investigation of a hexacopter for urban air mobility, emphasizing the interactional effect between different components of the vehicle on the aerodynamic and aeroacoustic performance. The nearfield turbulent flow fields are resolved by delayed detached eddy simulations, and the acoustic waves at far-field observers are computed using an integral solution of the Ffowcs-Williams and Hawkings equation. The results show that the close placement of rotors can lead to remarkable thrust fluctuations and slightly reduce the average thrust. Consequently, the sound pressure level for the tonal components is considerably increased over a wide frequency range, especially at the observers in the upstream and downstream directions. The rotor–rotor interaction also results in a contracted wake geometry towards the vehicle centre and intensifies the dissipation of the vortices in the downstream wake. The presence of connecting arms can cause significant thrust fluctuations on the surfaces of both rotor blades and arms, which produces additional tonal noise at high-order harmonics of the blade passing frequency. In contrast, for current configuration, the existence of the fuselage only slightly reduces the total thrust and contributes little to noise generation.

Aeroacoustic Interactions of Blade Skew and Leading Edge Serrations Applied to Low-Pressure Axial Fans

Abstract Leading edge serrations are well known for their ability to reduce turbulence-induced noise of single aerofoils while also providing aerodynamic advantages under certain operating conditions. Continuatively, applying leading edge serrations to rotating machinery such as axial fans proved the validity to generally transfer the obtained aeroacoustic benefits of single aerofoils. However, for the rotating applications the noise reduction potential highly depends on the point of operation. The current work aims at assessing the aeroacoustic effects of serrated leading edges under the increased geometrical complexity of the fan blades through blade skew. Therefore, the question is whether combining two potentially effective noise-reducing treatments through blade skew and leading edge serrations results in leveraging or obstructing effects. By varying the skew angle from 0 deg to 38 deg, four different prototypes of the fan impeller are tested experimentally in a test rig according to ISO 5136 and ISO 5801. All configurations are tested with original blades of straight leading edges plus five sets of serrations each, parameterised by the serration amplitude and the serrations wavelength. The intensity of the incoming turbulence ranges from 2.6% to 12.1%. The results obtained show the skewed blades to improve both the aerodynamic performance and the noise radiation after exceeding an initial skew angle, complemented by a significant onset of stall. Moreover, no contraindication between blade skew and serrated leading edges is encountered, showing the potential to further extend the noise reduction capabilities by combining effects of blade skew and leading edge treatment.

Steady-state statistics, emergent patterns and intermittent energy transfer in a ring of oscillators

Networks of coupled nonlinear oscillators model a broad class of physical, chemical and biological systems. Understanding emergent patterns in such networks is an ongoing effort with profound implications for different fields. In this work, we analytically and numerically study a symmetric ring of N coupled self-oscillators of van der Pol type under external stochastic forcing. The system is proposed as a model of the thermo- and aeroacoustic interactions of sound fields in rigid enclosures with compact source regions in a can-annular combustor. The oscillators are connected via linear resistive coupling with nonlinear saturation. After transforming the system to amplitude-phase coordinates, deterministic and stochastic averaging is performed to eliminate the fast oscillating terms. By projecting the potential of the slow-flow dynamics onto the phase-locked quasi-limit cycle solutions, we obtain a compact, low-order description of the (de-)synchronization transition for an arbitrary number of oscillators. The stationary probability density function of the state variables is derived from the Fokker–Planck equation, studied for varying parameter values and compared to time series simulations. We leverage our analysis to offer explanations for the intermittent energy transfer between Bloch waves observed in acoustic pressure spectrograms observed of real-world gas turbines.

Experimental and Computational Aeroacoustic Investigation of Small Rotor Interactions in Hover

This paper investigates the aeroacoustic interactions of small hovering rotors, using both experiments and computations. The experiments were conducted in an anechoic chamber with arrays of microphones setup to evaluate the azimuthal and polar directivity. The computational methodology consists of high fidelity detached eddy simulations coupled to the Ffowcs-Williams and Hawkings equation, supplemented by a trailing edge broadband noise code. The aerodynamics and aeroacoustics of a single rotor are investigated first. The simulations capture a Reynolds number effect seen in the performance parameters that results in the coefficient of thrust changing with the RPM. The acoustic analysis enables the identification of self-induced noise sources. Next, dual side-by-side rotors are studied in both counter-rotating and co-rotating configurations to quantify the impact of their interactions. Higher harmonics appear due to the interactions and it is verified that the counter-rotating case leads to more noise and a less uniform azimuthal directivity. Difficulties that arise when trying to validate small rotor calculations against experiments are discussed. Comparisons of computational and experimental results yield further insight into the noise mechanisms that are captured by each methodology.

High-Fidelity Modeling of Multirotor Aerodynamic Interactions for Aircraft Design

Electric aircraft technology has enabled the use of multiple rotors in novel concepts for urban air mobility. However, multirotor configurations introduce strong aerodynamic and aeroacoustic interactions that are not captured through conventional aircraft design tools. This paper explores the capability of the viscous vortex particle method (VPM) to model multirotor aerodynamic interactions at a computational cost suitable for conceptual design. A VPM-based rotor model is introduced along with recommendations for numerical stability and computational efficiency. Validation of the individual rotor is presented in both hovering and forward-flight configurations at low, moderate, and high Reynolds numbers. Hovering multirotor predictions are compared with experimental measurements, evidencing the suitability of the proposed model to capture the thrust drop and unsteady loading produced by rotor-on-rotor interactions.

Accurate numerical simulation on the structural response of the VEGA payload fairing using modal coupling approach

Structural aeroacoustic interactions are of great interest in the design and manufacture of aerospace structures. During the lift off and the early phases of the launch various external loads like steady accelerations, random and broadband frequency vibrations, acoustic loads due to jet noise and fluid–structure interactions act on the lightweight panel structure of the payload fairing. Thereby internal vibrational loads and instability effects may be caused by the interaction of various mechanical loads and acoustic noise acting on the cylindrical shell structure. Within the project “Prediction of Acoustic Loads on Space Structures” funded by the European Space Agency (ESA) an aeroelastic coupling approach has been built up. Therein, the aerodynamic loads on the VEGA payload fairing (PLF) have been determined for different flight conditions by the use of the open-source CFD solver SU2. Due to the different discretization between the computational fluid dynamics (CFD) and computational structural dynamics (CSD) mesh the approach of Radial Basis Function (RBF) has been used to interpolate the resulting pressure distribution onto the structural model. According to that the dynamic response of the PLF has been analyzed by taking the basic aerodynamic forces, structural vibration and acoustic pressure fluctuations into account. An intrinsic part of this work is the numerical simulation and assessment of the interaction between structural vibrations, transonic flow and acoustic pressure fluctuations during the early launch phase.

Wall-resolved large eddy simulation for aeroengine aeroacoustic investigation

ABSTRACTThe work presented here forms part of a larger project on Large-Eddy Simulation (LES) of aeroengine aeroacoustic interactions. In this paper, we concentrate on LES of near-field flow over an isolated NACA0012 aerofoil at zero angle-of-attack and a chord based Reynolds number ofRec= 2 × 105. A wall-resolved compressible Numerical Large Eddy Simulation (NLES) approach is employed to resolve streak-like structures in the near-wall flow regions. The calculated unsteady pressure/velocity field will be imported into an analyticallybased scheme for far-field trailing-edge noise prediction later. The boundary-layer mean and root-mean-square (rms) velocity profiles, the surface pressure fluctuation over the aerofoil, and the wake flow development are compared with experimental data and previous computational simulations in our research group. It is found that the results from the wall-resolved compressible NLES are very encouraging as they correlate well with test data. The main features of the wall-resolved compressible NLES, as well as the advantages of such compressible NLES over previous incompressible LES performed in our research group, are also discussed.

Sound radiation accompanying perturbation generation in turbulent jets under aeroacoustic interactions

The shadow method is used to study the sound radiation that accompanies the action of finite-amplitude saw-tooth sound waves on supersonic jets.

Impact of turbulence on the prediction of linear aeroacoustic interactions: Acoustic response of a turbulent shear layer

A powerful model to predict aeroacoustic interactions in the linear regime is the perturbed compressible linearized Navier–Stokes equations. Thus far, the frequently employed derivation suggests that the effect of turbulence and its associated Reynolds stresses is neglected and a quasi-laminar model is employed. In this paper, dynamic perturbation equations are derived incorporating the effect of turbulence and its interaction with perturbation quantities. This is done by employing a triple decomposition of the instantaneous variables. The procedure results in a closure problem for the Reynolds stresses for which a linear eddy-viscosity model is proposed. The resulting perturbation equations are applied to a grazing flow in a T-joint for which strong shear layer instabilities at certain frequencies are experimentally observed. Passive scattering properties of the grazing flow are validated against the experiments performed by Karlsson and Åbom and perturbation equations being quasi-laminar. We find that prediction models must include the effect of Reynolds stresses to capture the aeroacoustic interaction effects correctly. Neglecting its effect naturally results in the over prediction of vortex growth at the frequencies of shear layer instability and therewith in an over prediction of aeroacoustic interactions.

Identification of aero-acoustic scattering matrices from large eddy simulation. Application to a sudden area expansion of a duct

A methodology is presented which allows to determine the coefficients of transmission and reflection of plane acoustic waves at flow discontinuities in piping systems by combining large eddy simulation (LES) of turbulent compressible flows with system identification. The method works as follows. At first, an LES with external, broadband excitation of acoustic waves is carried out. Time series of acoustic data are extracted from the computed flow field and analyzed with system identification techniques in order to determine the acoustic scattering coefficients for a range of frequencies. The combination of broadband excitation with highly parallelized LES makes the overall approach quite efficient, despite the difficulties associated with simulation of low-Mach number compressible flows. The method is very general, here it is applied to study the scattering behavior of acoustic waves at a sudden change in cross-section in a duct system. The complex aero-acoustic interactions between acoustic waves and free shear layers are captured in detail by high resolution compressible LES, such that the scattering coefficients can be determined accurately from first principles. In order to demonstrate the reliability and accuracy of the method, the results for the scattering behavior and the acoustic impedance are presented and physically interpreted in combination with several analytical models and experimental data.

Diagnostics and reduction of propulsion airframe aeroacoustic interactions

In the past, significant reduction in aircraft noise has been achieved by studying engine noise and airframe noise in isolation as two separate components. However, when an engine is installed on an aircraft the flow and acoustic interactions between them produce a total noise signature which is often different than the sum of the component noise signatures. While take-off conditions are typically dominated by engine noise, approach conditions are characterized by both engine and airframe noise. Nevertheless, propulsion airframe aeroacoustic (PAA) interactions are present under all conditions and gain in importance when engine or airframe noise is further reduced. This paper focuses on the diagnostics and reduction of the flow-acoustic interaction effects between an engine exhaust and the airframe, especially the high-lift system on wings. Flow and acoustic results are presented for an isolated scale-model nozzle in a free jet, and also when it is installed under a wing at take-off and approach conditions. The difference between these two noise signatures, in the far field and at the source, captures the PAA effect. In particular, the PAA effect due to changes in nozzles, pylons, and flaperons is presented and the overall noise reducing mechanisms are postulated.

Effect of aeroacoustic interactions on ejector performance

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