Browse Topic: Noise measurement
This study examines the capability of medium-fidelity comprehensive analysis models to predict the acoustics for manned and unmanned rotorcraft configurations. Using the automated tool NDARC2RCAS developed at DEVCOM Army Research Laboratory, multiple configurations including a single main rotor, tilt rotor, coaxial and pusher, quadcopter, and hexacopter are evaluated at various mission segments including hover, advancing climb, and forward flight. Each configuration and condition is evaluated using a range of aerodynamic models from lower to higher fidelity, including uniform inflow, dynamic inflow, prescribed wake, free wake, and viscous vortex particle method (VVPM). These evaluations are then used with another automated tool, RCAS Acoustics, to predict noise on a Voronoi observer sphere. A comparison of the results for the single main showed good agreement between all of the aerodynamic models except VVPM. For the tilt rotor in forward flight, the higher-fidelity models produced changes in rotor loads due to the interaction with the wing. With prescribed and free wake models, this change in load is sharp and causes noise increases of up to 40 dB in front of and behind the vehicle, while the VVPM model produced a smoother change that results in a smaller, 20 dB increase in noise. The quadcopter and hexacopter show similar in-plane noise levels for all models, with alternating cancellation and amplification patterns due to rotor phasing, while out-of-plane noise is increased on the hexacopter when using the higher fidelity models.
Acoustic flight testing of rotorcraft often involves generating noise source hemispheres to gain an understanding about the aircraft's acoustic emissions. However, aerodynamically complex Urban Air Mobility and Future Vertical Lift vehicles may not maintain a steady aerodynamic state during flight, making source hemispheres measured using traditional linear arrays unreliable or difficult to interpret. To address this challenge, all emission angles need to be measured simultaneously. This has lead to the concept of the two dimensional 'snapshot' array layout. A mathematically defined microphone distribution was utilized to achieve uniform coverage on the source hemisphere. Within the chosen distribution, two lower microphone count distributions are embedded, allowing for a comparison of the effects of number of microphones. The array was deployed as part of a joint Army/NASA acoustic research flight test in July of 2024. Data were collected using an MD530F helicopter as the test vehicle, executing both steady and unsteady flight. Analysis of the array resolution is used to determine adequate channel count and spatially varying sensitivity to array density.
In the context of developing new rotorcrafts dedicated to Advanced Air Mobility, aeroacoustic simulations of co-axial rotor systems have been conducted using the lattice Boltzmann method with the ProLB code. Eight configurations, spanning from co-rotating rotors to contra-rotating shrouded rotors, were analyzed in stationary conditions through comparisons with experimental data and flow field analysis. This investigation validates our numerical methodology, based on direct noise simulation, and enhances our understanding of noise generation and propagation of such propulsive systems. Our simulations successfully replicate all measured trends in global aerodynamic performance, average noise levels, and noise directivities. Maximum discrepancies were 1.5 N (9%) and 2.1 dB (averaged noise level on the considered microphones). Based on our analysis, the following observations have been made. The open co-rotating rotors is the quietest configuration due to reduced Blade-Vortex Interaction (BVI). A 30° leading rotor angle increases noise by inducing BVI and flow detachment on the downstream rotor. On the other hand, contra-rotation offers the best performance but generates high tonal noise. Finally, the shrouded configuration is the noisiest due to blade-flow interactions and detachments along the duct, resulting in reduced performance and additional noise sources primarily due to non-adapted blades and duct.
This paper explores a significant step forward, regarding the further detailed understanding of the Fenestron®. Since its patent in 1968 – for the Gazelle helicopter –, the shrouded tail rotor has been resized, inclined, modulated, etc. and has thus been continuously enhanced on different rotorcraft. Half a century after its invention, Airbus is once again exploring in more detail the magic of the Fenestron®, with the objective of optimizing it even further, for future helicopter applications. To grasp and observe properly some specific phenomena, a model (scaled to one third) capable of both unprecedented functions and modularities, was developed. The present paper will describe in detail the novel model and the related challenges and solutions. This model is capable of high rotor speed and dynamic pitch inputs, delivering power levels high enough to reach stall effects, while allowing the measurement of propulsive efficiency and to differentiate rotor vs fairing thrust. Furthermore, the model had to provide aerodynamic-shape modularity, both in the shroud, on the covers and on the tail gearbox supports. The first test campaign performed on this model allowed us to define design drivers and aerodynamic preferences. And since one idea often leads to another, another challenge is being addressed on this scaled mock-up: Reduction of the tail rotor noise level.
This study investigates the effects of chord-to-radius ratio (c/R) and blade count on the aerodynamic and aeroacoustic performance of cyclorotors through experimental testing and a low-fidelity streamtube model. Cyclorotors with c/R ratios between 0.3 to 0.75 and blade counts ranging from 5 to 9 were tested across pitch amplitudes up to 51°. For a 5-bladed configuration, the pitch amplitude that maximizes the force-to-power coefficient (CF/CP) increases with c/R from approximately 32° at low c/R to around 51° at high c/R. However, the peak attainable CF/CP decreases with increasing c/R, indicating a trade-off between optimal pitch amplitude and aerodynamic efficiency. Increasing blade count enhances the generated force but reduces efficiency in all cases except for the lowest c/R configuration (0.3). Aeroacoustic analysis shows that tonal noise is primarily driven by pitch amplitude and intensifies with increasing c/R, while additional blades effectively mitigate it. In contrast, broadband noise is less sensitive to variations in pitch amplitude, c/R, and blade count. The streamtube model captures key aerodynamic trends, particularly at moderate pitch amplitudes. Scaling studies identify optimal configurations for a given disk loading, balancing power consumption and noise levels and highlighting key trade-offs critical to urban air mobility applications.
ABSTRACT Small-scale rotorcraft exhibit degraded aerodynamic efficiency, which has been linked to non-ideal losses within the wake. Unique high-frequency, broad-band features have also been observed, without a physical verification of their origin. This work seeks to gather insight into the behavior of the rotor wake structures as a function of Reynolds number (Re), relate this to performance capabilities and the corresponding far-field acoustic signature. Two-component particle image velocimetry (PIV), performance, and acoustic measurements were performed using three small-scale, NACA 0012 rotors operated over a range of low-Reynolds number conditions. Rotor geometry and operational speed (Ω) were varied to obtain the desired Re variation. Span-wise PIV has demonstrated an absence of tip vortex formation as the operational thrust coefficient (CT) is increased, suggesting outboard tip stalling. Phase-locked, chordwise PIV has confirmed this hypothesis, showing the development of large-scale separation and a highly turbulent downstream wake. The wake characteristics at moderate collective angles (θ) show the presence of periodic counter-rotating structures attributed to laminar boundary layer vortex-shedding. Significant broad-band frequency content was measured, and found to correlate well with a physical quantification of the shedding phenomenon.
Presented herein is a comprehensive workflow for the aeroacoustic analysis of a tilt-rotor air taxi during cruise and hover flight using high-fidelity numerical methods. Employing a hybrid approach, the near-field flow is resolved by an unsteady Reynolds-Averaged Navier-Stokes (URANS) solver, which is paired with a Ffowcs Williams-Hawkings (FW-H) acoustic solver to compute the far-field noise. Both impermeable and permeable FW-H integration surface approaches are incorporated. To balance computational resources and accuracy, the flow domain is halved, while the acoustic data is processed to reflect the full vehicle acoustics. Isolated acoustic contributions of each rotor are extracted, allowing the investigation into the impact of phase shifts on the acoustic signature of the vehicle. Spectral analysis, directivity maps, and noise hemispheres reveal the resolution of fundamental open rotor characteristics and amplified tonal interaction noise that can be linked to aeropropulsive phenomena. The introduction of phase shifts can lead to reduced noise levels and an unsymmetrical acoustic field development. A direct comparison of the results based on impermeable and permeable FW-H source surfaces shows general agreement on the directivity prediction but significant differences in noise levels, potentially due to numerical dissipation, grid settings and the symmetry boundary.
This paper describes a mathematical framework for determining the optimal sensor set location for adequately capturing the sound generated by rotors. The approach leverages the gappy-POD method proposed by Everson and Sirovich [J. Opt. Soc. Am., Vol. 12, 1995, pp. 1657-1664], which first identifies the various mode constituents that make up the first few rotor blade-pass frequency harmonics of the sound-field. The algorithm is developed using a covariance matrix for the POD problem comprising auto- and cross-spectral densities of spatially and temporally resolved sound waves captured by an array of microphones oriented parallel to the axis of a laboratory-scale hovering rotor. Three different forms of the technique are developed and compared. These comprise a homogeneous form and two heterogeneous forms; the heterogeneous forms are referred to as XX-topos and XX-chronos and depends on which term in the error minimization equation is assigned the gappy sensor set. A greedy algorithm is then employed to determine the optimal location of the limited sensor set. The findings are analyzed for different combinations of POD modes and blade-pass frequency harmonics of the sound generated by the hovering rotor.
Aeroacoustic characterization of multirotor aircraft is a challenging task, especially due to the variability in the rotational speed of rotors. This problem is exacerbated by the use of variable RPM control, because changes in RPM change the noise sources of the aircraft in a time-dynamic way. As a consequence of the constant variations in noise sources, an accurate assessment of the acoustic characteristics of the aircraft can be difficult to obtain. A possible solution to this problem is to separate the noise sources of the aircraft on a rotor-to-rotor basis and understand the variations based on each rotor's operating states. To achieve this, a source separation process based on the Vold-Kalman filtering approach is applied to separate the contributions of the individual rotors from ground-based acoustic measurements of a hexacopter. The source separation process was applied to separate the tonal noise of each rotor based on the rotor speed measurement and the results were analyzed for a level flight flyover case. The rotor speed has a strong correlation to the A-weighted SPL throughout the flyover and the variations in RPM directly affect the tonal noise levels, as expected from theory.
In the context of developing new rotorcraft dedicated to Advanced Air Mobility, the use of non-conventional propulsive systems such as coaxial counter-rotating rotors, equipped or not with a shroud, is often addressed. The present numerical study falls into this framework. We actually propose to analyse the noise generated by such configuration in stationary conditions with a direct noise simulation realized thanks to a lattice Boltzmann Method. In a first step, the predictions are validated by comparisons with measurements realized in an anechoic chamber for the unshrouded case. A good agreement is observed on both tonal and broadband components of the noise radiated by the propulsive system. Then, the effects of using a shroud around the rotors are investigated numerically. It appears that the use of the shroud significantly increases the relative importance of the broadband noise because of interactions between turbulent wakes with the rear propeller and the duct surface. One can consequently conclude that employing a shroud is not necessarily a good option and this architectural choice must be made carefully. Finally, the impact of the rotor-rotor distance is analysed in both cases with and without shroud. As expected, increasing the distance allows reducing the noise levels but this effect seems to be limited to the tonal noise with almost no impact on the broadband component. Those observations are still valid when the duct is used, but placing the front rotor closer to the shroud inlet makes the shrouded configuration even louder. In a near future, this study will be completed by a complementary test campaign that will allow more validations of the present simulations and confirm or not the resulting conclusions.
Small multirotor vehicles, for example, designed for package delivery, are expected to operate in close proximity to populated areas, raising concerns about noise pollution. This study utilizes acoustic flight tests and computational modeling of an instrumented research hexacopter developed at Penn State to investigate noise generation during takeoff and landing maneuvers, considering varying flight path angles and vehicle speeds. Flight tests were conducted at Mid-State Regional Airport and corresponding predictions were made using the Penn State Noise Prediction System. The predicted vehicle states and noise levels are first validated against the flight test data. The validated model and flight test data are then utilized to study the noise emissions of the aircraft. Measurements and predictions of the acoustic characteristics of the vehicle are analyzed using conventional noise metrics, frequency content, and directivity features. Descent maneuvers are found to be noisier than climb maneuvers. Noise generation decreases with an increase in forward speed during both climb and descent and also with an increase in vertical speed during climb. However, during descent, noise generation increases with an increase in sink rate.
The paper describes activities currently run in the frame of the MOTUS project. The developed methodology allows to correct an acoustic measurement database from simulation results in order to model the noise emis-sion of a new helicopter (H/C), accounting for the introduction of low-noise technology bricks (e.g. rotational speed modification, alternate main rotor blade or Fenestron™ design, engine noise control treatment...). The resulting hemisphere database can then be used to simulate various use-cases, from predicting certification noise levels to computing noise footprints on realistic scenarii, including also Low Noise Procedures. All these results can then be auralized in a subsequent step, to address advanced metrics related to noise annoyance.
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ABSTRACT
An extensive flight test campaign was recently completed, which aims to reduce the operational noise generated by helicopters in an effort to improve community acceptance. Using a ground-based microphone array, acoustic measurements were acquired on helicopters in the medium-sized vehicle weight class over a number of flight conditions including steady level flight, steady descents, and approaches. While data were collected across four helicopters, the Leonardo AW139 and Sikorsky S-76D will be the focus of this paper. Source noise hemispheres are computed for the steady test points, but the ground noise contours measured during approach conditions cover only a small portion of the area impacted by noise because of practical constraints on the size of the deployed array. Thus, the Noise Informed Community Environment Operations Planning System (NICEOPS) in conjunction with the flight test data was used to estimate how changes in the approach procedures impact noise exposure over a larger ground area. It was found that even small longitudinal accelerations can have a substantial influence on the noise generation processes during approach and must be modeled appropriately to develop new flight procedures which minimize the acoustic impact on the ground.
This paper presents an efficient prediction of coaxial rotor broadband noise, particularly trailing-edge noise. The method combines a newly developed iterative coaxial rotor BEMT, a viscous panel method, an empirical wall pressure spectrum, and Amiet's trailing-edge noise model. Aerodynamic data including the induced velocity and angle of attack on each rotor are calculated by the iterative BEMT. Then, turbulent boundary layer flows, such as the boundary layer thickness, skin friction coefficient, pressure gradient, etc., are computed by a viscous panel code, XFOIL. Based on these boundary layer parameters, the wall pressure spectrum near the trailing edge is computed by Lee's semi-empirical model. Finally, trailing-edge noise is predicted by Amiet’s model from the wall pressure spectrum. This method provides fast computations for aerodynamics and acoustics for coaxial rotors. Acoustic predictions can be performed for various design and operating conditions including the effect of rotor-to-rotor separation distance. In addition to the overall noise of the combined rotor system, each rotor's contributions to noise can be analyzed. A small-scaled untwisted rotor is selected to analyze aerodynamics and aeroacoustics. It is found that the noise contribution from each rotor is about the same at small separation distances. At large separation distances, the lower rotor generates higher noise levels than the upper rotor mainly due to the change in rotor distance with respect to the observer. The detailed boundary-layer flow properties are investigated on both rotors.
This paper investigates the acoustics of a side-by-side Urban Air Mobility (UAM) aircraft with 0%, 5%, 15%, and 25% rotor overlaps in forward flight based on high-fidelity Computational Fluid Dynamics (CFD) simulations. The CFD and acoustics simulations are carried out using the HPCMP CREATETM-AV rotorcraft simulation and analysis tool Helios and the acoustic prediction tool PSU-WOPWOP. The influence of the finest wake-grid spacing size on acoustic prediction of the side-by-side rotor with 0% overlap is studied based on two wake-grid spacing cases: 5% Ctip and 10% Ctip. No significant difference in overall sound pressure level (OASPL) is found between the two cases. The effect of rotor overlap on rotor acoustics is also assessed, and it is shown that the 25% overlap case yields higher OASPL than the other overlap cases particularly due to stronger rotor-to-rotor blade-vortex interactions (BVIs). Furthermore, the noise of the side-by-side rotor with 0% and 25% overlaps is compared against similar-sized helicopter noise and various background noise levels. It is shown that the side-by-side rotor noise is comparable with the noise of a similar-sized four-bladed helicopter in cruise. The OASPL difference of the overhead case is less than 10 dB, which does not meet the guideline that the UAM noise should be 15 dB lower than similar-sized helicopter noise. The results also show that the side-by-side rotor noise could not be fully masked by the freeway background noise at an altitude of 1,500 ft. Thus, noise reduction technology should be developed to overcome the barrier of public acceptance of UAM noise.
A comprehensive noise prediction system is required to predict helicopter noise and assess potential noise mitigation strategies. It is necessary to validate and understand the limitations of the noise prediction system before it can be used to develop noise abatement procedures. Here, the validation is carried out by direct comparison with the flight test data. This paper gives a brief introduction to the noise prediction system, processing of the flight test data, and development of the different helicopter models with parametric sensitivity. The validation process is carried out by comparing the sound exposure level (SEL) noise contours on the ground plane of the acoustic flight test data with that predicted by the noise prediction system. The examples considered are: level flight; descent flight; level turn maneuver in left and right direction; level, decelerating turn maneuver; and a descending turn maneuver. This range of flight conditions was necessary to analyze the prediction system and understand its capabilities and deficiencies for future work. Overall the predicted noise levels were able to match the trends and levels within a few SELdBA of that measured during the flight test.
This paper attempts to characterize the high-frequency components of in-flight, near-field noise measurements of a Bell 206B helicopter. This noise is shown to be largely broadband, and displays a pattern of modulation that appears to be an audible and salient feature of the noise. The development of a frequency domain technique is given that uses very short (in time) discrete Fourier transforms with windowing and overlap in order to produce a representation of how the 1/3rd octave band spectrum of the noise varies over time scales shorter than a blade passage. The output of this method can be used to resynthesize the source noise. This resynthesis evidences natural variations that are perceptually similar to those seen in the original recording, and which would not be predicted by contemporary rotorcraft noise analysis techniques. These concepts are then applied in an attempt to synthesize helicopter source noise from a physics-based prediction that includes such modulating high frequency components. The elevated significance of broadband noise on future prediction efforts is discussed.
Piasecki Aircraft Corporation (PiAC) and Continuum Dynamics, Inc. (CDI) are investigating noise levels of the PA890, an emergent eVTOL slowed rotor winged compound (SRWC) configuration using the CHARM/PSU-WOPWOP aeroacoustic analysis. While the adoption of electric drive eliminates engine noise, this design allows several operational degrees of freedom that permit it to further mitigate noise mechanisms associated with conventional single rotor helicopters, including: lift compounding to offload the main rotor; reduction of main rotor RPM in cruise; and a dual mode (antitorque/pusher) tail rotor that allows alternate modes of operation for targeted forward flight conditions, with significant potential for noise reduction. This paper provides an overview of an acoustics analysis of the PA890 eVTOL SRWC configuration, including a comparison to an existing single rotor civil helicopter; and the principal noise mechanisms affecting each type of aircraft and the opportunities offered by the SRWC configuration to mitigate them. Ongoing work to further clarify important noise mechanisms is also outlined.
The Tiltrotor Test Rig (TTR) was tested in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel from 2017 to 2018. The primary goal of the test was to understand the operational capabilities of the TTR, while also acquiring research data, including acoustic data. Four microphones were placed around the TTR: two on the wind tunnel floor and two on struts. Acoustic measurements of the TTR rotor were acquired to 1) understand the acoustic testing capabilities of the TTR in the NFAC 40- by 80-FootWind Tunnel, 2) compare to previous XV-15 rotor acoustic data acquired in the NFAC 80- by 120-Foot Wind Tunnel, and 3) provide data for future validation studies. A data quality study revealed that the NFAC 40- by 80-Foot Wind Tunnel is an adequate acoustic environment to test the TTR rotor. For a given thrust and advance ratio, a shaft angle sweep was performed and acoustic measurements were compared against 1996 and 1999 XV-15 data in the NFAC 80- by 120-Foot Wind Tunnel; differences between the three tests are discussed.
High-fidelity simulations performed with the CFD code elsA have been used to improve a fast prediction code named ARHIS and dedicated to the prediction of the aerodynamic response of a blade in interaction with a vortex. To do so, comparisons have been made on a well documented blade-vortex interaction configuration. After validation of the computational methodology against available measurements, discrepancies between elsA and ARHIS solutions have been observed close to the blade tip due to the bidimensional approach used in the latter. Consequently, a new model of tip loss factor has been developed in ARHIS providing a better agreement between fast and high-fidelity simulations. The maximal error in terms of amplitude of the lift jump due to the blade-vortex interaction goes from 20:1% to 5:2%. This new model has been applied to several cases to assess the impact of the blade shape by considering modification of the aspect ratio, sweep angles, parabolic tip and tapered blade. Even if not designed specifically to account for modification of the blade shape, it is clear that the new tip loss factor model still improves the agreement between AHRIS and elsA results. Finally, the improved model has been tested for the evaluation of the noise emission of complete rotors in descent flight considering a straight and a double swept rotor blade. In all cases, the new model induces a reduction of the predicted noise level of about 0:5 to 1 dB. This noise reduction appears to be an improvement in one case but a deterioration in the other one. However, this worsening is maybe due to a compensation of errors in the entire computational chain. This new modeling needs consequently to be applied on a larger database to conclude on its efficiency in terms of noise predictions.
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