Ffowcs Williams-Hawkings Research Articles

Swirling Flow Effects on the Aeroacoustic Signature of an Aerospike Nozzle

Abstract Supersonic nozzles are not always operated at design conditions. The total pressure, temperature and velocity distributions at the nozzle inlet plane are often characterized by inhomogeneities, conditions dictated by the operating regime of the turbine or combustion chamber. In particular, a swirling flow motion can be induced by these components. While homogeneous inflow conditions are well documented for a large range of supersonic nozzles, data on the aeroacoustics of supersonic swirling jets is scarce. Large Eddy Simulations are deployed to simulate the swirling flow of a non-ideally expanded three-dimensional, cold, axisymmetric aerospike nozzle at a Nozzle Pressure Ratio (NPR) = 3. Three swirl numbers are considered and compared with the baseline case. Near-field acoustic analyses are completed by far-field acoustic computations based on the Ffowcs Williams-Hawkings (FWH) equation. Swirling flow shortens the potential core of the jet and leads to an annular shock cell length increase. Two-point space-time cross-correlations of pressure data acquired in the annular shear layer indicate an enhancement of the azimuthal modes. Similar cross-correlations in the circular jet shear layer further downstream show that screech tones are suppressed. Power spectral density of the radial velocity at monitoring points in the vicinity of the nozzle trailing edge allows to identify the oscillation modes of the annular shock-cell structure. The far-field spectra exhibit lower mixing noise with increasing swirl number. The global SPL decreases while the nozzle thrust remains at 99% of the baseline thrust at low swirl numbers.

STUDY OF AERODYNAMIC NOISE OF PROPELLER CONSIDERING THE INFLUENCE OF BOUNDARY SURFACE USING CFD METHOD

This article focuses on the aerodynamic and acoustic characteristics of both an isolated propeller model and a propeller - boundary surface model. The study uses the Reynolds-averaged Navier-Stokes (RANS) method combined with Computational Fluid Dynamics techniques to analyze the flow field and wake flow behind the propeller and to determine its basic aerodynamic coefficients. The pressure on the surface of the propeller and boundary surface is used as input for the noise calculations. An acoustic analogy method, based on the Farassat 1A formulation derived from the Ffowcs Williams-Hawkings (FW-H) equation, is employed to calculate the propagation of noise from the surfaces of the propeller and boundary surface to designated positions (microphones). The research results lead to some significant conclusions drawn by the authors.

Acoustic and aerodynamic evaluation of DLR small-scale rotor configurations within GARTEUR AG26

AbstractThis paper presents the activities performed in the GARTEUR Action Group HC/AG-26 to study the acoustic and aerodynamic characteristics of small rotor configurations, including the influence of the rotor-rotor interactions. This paper will focus on comparisons between numerical activities and wind tunnel results on a small rotor provided by DLR. The wind tunnel models included a Rotor/Rotor/Pylon in isolated, tandem and coaxial configuration. The wind tunnel experiments for acoustics were performed in DLR’s Acoustic Wind Tunnel Braunschweig (AWB) and PIV test were performed in CIRA within a joint CIRA/DLR test program. For simulations, the numerical approaches from each partner are applied. The aerodynamic simulations necessary for the aeroacoustic predictions are conducted with various fidelity numerical methods, varying from lifting line to CFD. The acoustic values on the microphone positions are evaluated using Ffowcs Williams/Hawking (FW-H) formulation by all partners. The acoustic and aerodynamic predictions are compared to test data, including performance, PIV and acoustic directivity.

Numerical Investigation of Unsteady Flow Characteristics of Planar Plug Nozzle in Over-Expanded Conditions

Abstract Plug nozzles are frequently regarded as a strong contender for futuristic single-stage-to-orbit space missions due to their inherent altitude-compensating properties. Optimizing the design of these nozzles to achieve peak performance during the initial ascent phase is of preeminent importance. The current research aims to deepen our comprehension of the unsteady behavior of plug nozzles during this ascent phase, commonly referred to as the overexpanded flow regime. It utilizes a full-spike plug nozzle and employs a finite volume-based solver within the OpenFOAM framework to investigate flow features across Nozzle Pressure Ratios (NPRs) ranging from 3 to 8. Additionally, the Ffowcs Williams Hawkings (FW-H) analogy is employed to assess the influence of acoustics in the nozzle's flow field at different NPRs.Spectral analysis and flow visualizations are employed to study the frequency content and the respective flow features of the system. It is observed that the flow exhibits separation beyond NPR 4 and is highly unsteady at NPR 6, which is inferred from the prominence of the recirculation bubble. However, beyond NPR 6, the flow tends to reattach the plug surface, dampening the disturbance.

Computational fluid dynamics driven surrogate model to predict hydrodynamic and acoustic properties of propeller boss cap fins

A novel method is proposed to predict the hydrodynamic and acoustic properties of propeller boss cap fins (PBCF) based on computational fluid dynamics (CFD) and surrogate models. The propulsive performance and radiated noise of the B-series propeller equipped with PBCF under open-water and hull-propeller coupling conditions are predicted and analyzed. The transient calculations are performed to generate accurate sample data for the surrogate model based on the Unsteady Reynolds Averaged Navier–Stokes and Ffowcs Williams–Hawkings equations. The response surface, polynomial, and Kriging models are used to learn sample data and output predictions. The relationship between inputs and outputs can be established from local data, which enables to obtain arbitrary response results in the global range. The hydrodynamic performance and radiated noise are compared for configurations with and without PBCF. PBCF improves the open-water efficiency by more than 1.5% at medium to high advance velocities. For the self-propulsion efficiency, over 5% improvement is achieved under ideal working conditions. In addition, PBCF has a directional effect on the radial and axial radiated noise, with better noise reduction in the axial direction. The difference between the axial and radial spectrums is significant, especially near the first blade passing frequency.

The Influence of bogie cavity bottom plate on the aerodynamic noise characteristics of high speed trains

The bogie area with a complex structure is an extremely important source of noise for trains. Based on the large-eddy simulation(LES) and Ffowcs Williams-Hawkings(FW-H) analogy, aero-acoustic simulation was conducted on a 3-car high-speed train of a certain type with a bogie cavity bottom plate installed on the front and rear cars. The results show that bottom plate changes the flow field around the bogie, weakens the flow separation of airflow at the leading edge of the bogie cavity, mitigates the flow impact of airflow on various components of the bogie, suppresses the formation and detachment of large-scale vortices in the bogie area, and reduces the formation of pressure pulsations on the geometric surface of the bogie area. The bogie cavity bottom plate scheme reduces the far-field aerodynamic noise of the train by 1.63dBA, and the noise reduction effect is significant. The aerodynamic noise of the measurement point equipped with a bogie cavity bottom plate is lower than that of the conventional bogie cavity in all frequency bands, and the energy reduction is more significant at medium and low frequencies.

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.

A Correlation Study Between Aerodynamic Structural Changes and Aerodynamic Noise Characteristics of Archimedes Spiral Wind Turbines

This paper investigates the impact of aerodynamic structural changes on the aerodynamic noise of Archimedes spiral wind turbines (ASWTs) using experimental and numerical research methods. Both parts of the experimental work are validated based on the prototype ASWT, with the working conditions consistent with the numerical calculation conditions. The aerodynamic noise of the ASWT before and after modification is numerically simulated by combining Large Eddy Simulation (LES) with the Ffowcs Williams-Hawkings (FW-H) method, which converts internal flow field information into acoustic field information. The results demonstrate that the improved ASWT exhibits reduced high-frequency discrete noise in the 400 to 1000 Hz range at $TSR=2$. Specifically, the Sound Pressure Levels (SPLs) of the quadrupole noise source near 471 Hz and 671 Hz for the prototype ASWT are 100.15 dB and 112.90 dB, respectively, while the corresponding SPLs near 471 Hz and 679 Hz for the retrofitted ASWT are 57.68 dB and 63.29 dB, respectively. These represent reductions of 42.47 dB and 49.61 dB, respectively. Additionally, the predicted Overall Sound Pressure Levels (OASPLs) of the ASWT at locations 8 and 9 are 16.82 dB and 38.22 dB lower before and after modification, respectively. These findings not only improve the aerodynamic efficiency of ASWTs but also lay a solid foundation for the subsequent study of their acoustic propagation characteristics. This is of significant importance for future research on wind turbine miniaturization.

Aerodynamic and aeroacoustic sensitivities of contra-rotating open rotors to operational parameters

Abstract Open rotors can play a critical role towards transitioning to a more sustainable aviation by providing a fuel-efficient alternative. This paper considers the sensitivity of an open-rotor engine to variations of three operational parameters during take-off, focusing on both aerodynamics and aeroacoustics. Via a sensitivity analysis, insights to the complex interactions of aerodynamics and aeroacoustics can be gained. For both the aerodynamics and aeroacoustics of the engine, numerical methods have been implemented. Namely, the flowfield has been solved using unsteady Reynolds Averaged Navier Stokes and the acoustic footprint of the engine has been quantified through the Ffowcs Williams-Hawking equations. The analysis has concluded that the aerodynamic performance of the open rotor can decisively be impacted by small variations of the operational parameters. Specifically, blade loading increased by 9.8% for a 5% decrease in inlet total temperature with the uncertainty being amplified through the engine. In comparison, the aeroacoustic footprint of the engine had more moderate variations, with the overall sound pressure level increasing by up to 2.4dB for a microphone lying on the engine axis and aft of the inlet. The results signify that there is considerable sensitivity in the model and shall be systematically examined during the design or optimisation process.

Influence of distribution parameters on acoustic radiation from bubble clusters

Cavitation noise is the major noise in underwater, and the study of acoustic radiation from bubble clusters is the primary means to reveal the mechanism of cavitation noise. In this study, direct numerical simulation (DNS) of bubble clusters with volume fractions of 20–40 % with different bubble sizes and bubble position distributions are performed, and the far-field sound pressure is calculated using the Ffowcs Williams–Hawkings (FW-H) method. Then, we compare the collapse and acoustic radiation of bubble clusters with equivalent bubble. The results show that the collapse times of bubble clusters at the same volume fraction are identical and close to equivalent bubble, despite the different bubble sizes and positions in the bubble cluster. Further, in terms of acoustic radiation, the layered arrangement of bubble positions results in bubble clusters exhibiting layer-by-layer collapse and emitting multiple sound pressure pulses. In contrast, a random arrangement of bubble positions lacks this feature, resulting in the collapse of the bubble cluster without a layered phenomenon and radiating only a single primary sound pulse, which is consistent with the equivalent bubble. Additionally, the distribution of bubble sizes in the bubble cluster has almost no effect on the acoustic radiation of the bubble cluster. Notably, when the volumetric fraction exceeds 25 %, the sound pressure levels of bubble clusters with different distributions in the frequency domain are nearly identical, with differences from the equivalent bubble within 5 dB.

The influence of jet on aerodynamic noise in the pantograph area at different sinking heights

The influence mechanism of jet on aerodynamic noise control in the pantograph region at different sinking heights was numerically studied using an Improved Delayed Detached Eddy Simulation (IDDES) model and the Ffowcs Williams-Hawkings (FW-H) equation. Active flow control was achieved by setting jet slots at the leading edge of the cavity to predict the noise generated by the jet itself. The results showed that in the pantograph region with a sinking height of 500 mm, the shear layer was lifted by the jet, which prevented high-speed turbulence caused by flow separation from entering the cavity. Therefore, the model with jet control device reduced the overall far-field noise on the side of the pantograph by 1.6 ∼ 3.9 dB. Through flow field analysis, for the pantograph region where flow separation occurs in the front of the cavity, the jet needs to lift the shear layer enough to cross the front of the pantograph and prevent flow separation, thereby reducing the aerodynamic noise. For the pantograph region without flow separation in the front of the cavity, the jet may introduce additional noise sources, deteriorating the aerodynamic noise in the pantograph region.

Effects of propeller boss cap fins on hydrodynamics and flow noise of a pump-jet propulsor

As an underwater thruster, the pump-jet propulsor (PJP) exhibits low radiation noise but generates significant line spectral noise in the low-frequency band. In this paper, we equipped the PJP hub with two types of propeller boss cap fins (PBCF): one fixed and the other rotating with the rotor. The hybrid large eddy simulation and Reynolds-averaged Navier–Stokes method, along with the Ffowcs Williams-Hawkings (FW-H) equation, are employed to systematically analyze the hydrodynamics, exciting force, flow noise, and flow field of PJPs. The results indicate that the fixed PBCF improves the hydrodynamic performance and reduces the exiting force, raising the rotor's thrust coefficient by 9.22%–14.99%. The fixed PBCF also modifies the characteristics of line spectrum noise but causes an increase in the flow noise. The rotating PBCF increases the rotor's thrust coefficient by 2.03%–3.15%, decreasing both exciting force and line spectrum noise. For instance, at the advance coefficient of 0.8, its sound pressure level at the rotor frequency drops to 49.6%. Additionally, the rotating PBCF increases the pressure of the hub wake and effectively reduces the hub vortices' strengths. This paper provides a theoretical foundation for designing PJPs that enhance concealment and minimize vibrations and noise.

Acoustic optimization of a tee via a Helmholtz resonant cavity and noise prediction via a genetic algorithm coupled with the grey model

As a key local component in heating, ventilation and air conditioning (HVAC) systems, the acoustic characteristics of the tee largely determine the indoor acoustic environment quality. The aim of this study was to determine the impact of the Helmholtz resonant cavity (HRC) on the tee noise and sound transmission loss (STL). The noise was predicted using the Ffowcs Williams-Hawkings (FW-H) equation and large eddy simulation (LES), and an analysis was conducted on four design variables: the cavity width, cavity height, outlet flow ratio, and hole width. The results indicated that increasing the hole width had a positive effect on reducing the noise of the tee. HRC could result in a tee peak noise attenuation up to 34.81 dB. In addition, based on grey model (GM) and genetic algorithm (GA) theory, the noise of the Helmholtz resonant tee (HRT) was predicted, and a noise prediction formula for resonant cavity structure paraments was derived. This study provides a new perspective for optimizing the STL of the tee and a simplified prediction method for low-noise tee design.

Hydroacoustic analysis of a full-scale marine vessel: Prediction of the cavitation-induced underwater radiated noise using large eddy simulations

This numerical study provides insight into the mechanism of noise generation by a cavitating flow in the wake of a marine propeller under realistic operating conditions, which poses a significant threat to marine ecosystems. We examined a full-scale vessel with an entire hull and an isolated model-scale marine propeller (INSEAN E779A) with a maneuverable rudder under various highly turbulent inflow conditions that strongly affect the spectral characteristics of the radiated noise. Insight into the acoustic behavior was gained by employing a combination of the large eddy simulation (LES) treatment of turbulence and the Schnerr–Sauer volume of fluid cavitation model. The hydrodynamic solution was coupled with the Ffowcs Williams-Hawkings (FW-H) strategy for noise and vibration identification. We focused on the interactions between the characteristic cavitation patterns of marine propellers (sheet, tip, and hub cavities) and the dominant structures of the turbulent wake (tip, root, trailing edge, and hub vortices, as well as the distributed small-scale vorticity). The small-scale topological structures in the swirling wake of a propeller directly manifest in the radiated sound level and affect the intensity of multiple frequency ranges. Quantitative analysis of thrust, pressure fluctuations, and sound pressure levels (SPLs) demonstrates significant effects of blade loading, wake distribution, and cavitation development. The peak and average SPL distributions obtained through LES show lower dominant and higher average frequencies compared to those obtained by the FW-H method. The overall SPL obtained by LES were higher than those calculated using the FW-H acoustic analogy at all microphone locations. The overall noise was dominated by the low-frequency broadband noise, attributed to energetic helical vortices, and narrow-band peaks in the medium-high frequency range that originated from other sources, like cavitation structures.

Development of a FW-H tool coupled with CFD for infrasound noise emissions from wind turbines

Noise remains an obstacle to the advancement of onshore wind energy. Despite the ongoing debate on the effects of infrasound on human health, especially from wind turbines, there is consensus that the blade passing frequency (BPF) in the infrasound range modulates the audible higher-frequency sound and creates a rhythmic “swishing” sound at farther distances. This paper presents a simulation framework for assessing infrasound noise (<20 Hz) from wind turbines, which was designed to capture the relevant physics without incurring in overwhelming computational costs. The flow is modelled using large eddy simulation (LES) and is coupled through an actuator line method (ALM) to the aeroelastic model of a wind turbine. The use of an ALM significantly reduces the computation effort compared to blade-resolved CFD approaches. Moreover, the Ffowcs Williams-Hawkings (FW-H) acoustic analogy is applied on a permeable integration surface that surrounds the entire wind turbine, thereby capturing the coupled effects of rotor, tower, and the near-wake turbulence shed by the turbine. The present approach is shown to be capable of resolving rotor-tower interactions as tonal peaks at BPF and its harmonics, enabling the evaluation of design and control measures to mitigate infrasound noise emissions from wind turbines.

Aerodynamic noise reduction of high-speed pantograph by introducing planar jet on leeward surface of panhead

Combining the improved delayed detached eddy simulation (IDDES) model and Ffowcs Williams-Hawkings (FW–H) equation, the potential of panhead leeward-side jet in pantograph aerodynamic noise control is numerically investigated. A typical double-strip pantograph is used as the baseline model. Two identical planar jets are arranged on the leeward side of the two strips to achieve active flow control. The results show that the planar jet can reduce the aerodynamic drag of the upstream strip and suppress its lift fluctuation, while the jet does not produce equivalent level of positive effects on the downstream strip. Benefiting from the suppression of lift fluctuation of the upstream strip, the far-field noise of the model equipped with jet achieves a 7 dB reduction in the main direction of panhead lift fluctuation, and the noise on the pantograph side is also reduced by 1–2 dB. The flow-field analysis reveals that the planar jet pushes the coherent flow structures in the panhead's wake downstream, which makes the high-intensity flow-field fluctuation away from the upstream strip. The absence of the positive effect of the jet on wake flow modification of the downstream strip can be attributed to the distinct inflow conditions of the two strips.

Multi-Physics Coupled Acoustic-Mechanics Analysis and Synergetic Optimization for a Twin-Fluid Atomization Nozzle

Fine particulate matter produced during the rapid industrialization over the past decades can cause significant harm to human health. Twin-fluid atomization technology is an effective means of controlling fine particulate matter pollution. In this paper, the influences of the main parameters on the droplet size, effective atomization range and sound pressure level (SPL) of a twin-fluid nozzle (TFN) are investigated, and in order to improve the atomization performance, a multi-objective synergetic optimization algorithm is presented. A multi-physics coupled acoustic-mechanics model based on the discrete phase model (DPM), large eddy simulation (LES) model, and Ffowcs Williams-Hawkings (FW-H) model is established, and the numerical simulation results of the multi-physics coupled acoustic-mechanics method are verified via experimental comparison. Based on the analysis of the multi-physics coupled acoustic-mechanics numerical simulation results, the effects of the water flow on the characteristics of the atomization flow distribution were obtained. A multi-physics coupled acoustic-mechanics numerical simulation result was employed to establish an orthogonal test database, and a multi-objective synergetic optimization algorithm was adopted to optimize the key parameters of the TFN. The optimal parameters are as follows: A gas flow of 0.94 m3/h, water flow of 0.0237 m3/h, orifice diameter of the self-excited vibrating cavity (SVC) of 1.19 mm, SVC orifice depth of 0.53 mm, distance between SVC and the outlet of nozzle of 5.11 mm, and a nozzle outlet diameter of 3.15 mm. The droplet particle size in the atomization flow field was significantly reduced, the spray distance improved by 71.56%, and the SPL data at each corresponding measurement point decreased by an average of 38.96%. The conclusions of this study offer a references for future TFN research.

Effect of pressure pores size on hydrodynamic and hydroacoustic marine propeller performances under cavitating case

The numerical work presented in the paper investigates the effect of pressure pores on hydrodynamic and hydroacoustic performances. This research aims to reduce cavitation area and underwater noise by mitigating the tip vortex cavitation. Compared to the few studies devoted to the pressure pores technique, several configurations based on the E779A marine propeller have been tested by considering different azimuthal and radial step values, a wider pore region concentrated at the top of the blade, and several pore diameter values. In addition, a numerical simulation was started to verify the effectiveness of the theoretical models in detecting the effect of pressure pores on the acoustic propagation generated by the propellers tested. The numerical approaches combining cavitating flow and noise propagation are performed using a hybrid method, which solves the Ffowcs Williams-Hawkings (FW–H) equation. A validation of the numerical simulation is carried out for cavitating and non-cavitating cases. Open water performances, cavitation area, sound pressure levels, and thrust distributions are analysed for two cavitation numbers. σ= 1,763 and σ= 1,029. The obtained results reveal that the cavitation area decreases as the pressure pore radius increases, but a slight reduction in propulsive efficiency accompanies this. Particularly for the pores radius of 0,00264D, propeller efficiency loss doesn't exceed 2,6 % and 4,05 % for the two cavitation numbers investigated. Nevertheless, this configuration showed better acoustic performances with a diminution of 10 dB in overall sound pressure level compared to the propeller without pressure pores.

Research on finite element model of rotor aerodynamicnoise in hovering state

The helicopter has been widely used in many fields, and the rotor aerodynamic noise has gradually attracted attention. The non-homogeneous wave equation, Ffowcs Williams-Hawkings equation (FW-H equation) derived from hydrodynamic N-S equation, combined with RANS turbulence simulation, was used to simulate the sound field of NACA0012 airfoil-shape model in hovering state. On this basis, by changing the thickness and camber of the blade, the aerodynamic noise of the rotor under different airfoil profile thickness and camber is calculated. The results show that the change of airfoil thickness and camber will affect the aerodynamic noise of the rotor in hovering state.

Numerical investigation on the use of various blade tips for the helicopter's main rotor blade in forward flight regarding aerodynamic performance and HSI noise considerations

The goal of this research is to investigate aerodynamic performance and high-speed impulsive (HSI) noise of various blade tips for the helicopter's main rotor in forward flight. For this purpose, three-dimensional compressible flow field around the rotor blades is numerically simulated by the solution of the unsteady Reynolds averaged Navier-Stokes (URANS) equations with the "SST k-ω" turbulence model. Solution is quantified by the calculation of torque, drag, and thrust coefficients of CQ, CD, CT, and L/D ratio. HSI noise on the rotor plane is assessed by the calculation of sound pressure level at certain points of the flow field using the Ffowcs Williams-Hawking (FW-H) equation. Blade tips are eleven and include anhedral, dihedral, Eagle and combinations of them, and also Blue Edge and rectangular. The combinations are inspired by the tip configurations of the fixed wings. In this regard effects of the tip geometry parameters including curvature and height of its parts have been considered as well. All of the tips are examined for two flight conditions of Caradonna and Tung two-blade rotor with fixed pitch angle and time-varying pitch angles. Based on the present results, it is found that blade tips of dihedral family do not improve the aerodynamic performance of the helicopter blade. Also, addition of anhedral part to the blade tip improves the L/D ratio. According to the present acoustic analysis, it was seen that after the Blue Edge, the Eagle tip stands in order of priority. For the condition of Caradonna and Tung two-blade rotor with tip Mach number of 0.8 and advance ratio of 0.2, HSI noise of the Eagle-tip blade decreases by 2.39 % compared to that of the two-blade rotor with rectangular tip. However, the aerodynamics performance of the Eagle tip is much better than that of the Blue Edge.