Blade-vortex Interaction Noise Research Articles

Numerical investigation on the blade vortex interaction noise reduction using higher harmonic control

Accurately predicting the formation, evolution, and breakdown of helicopter blade tip vortex is crucial for simulating the Blade-Vortex Interaction (BVI) phenomenon. Firstly, the high-order Perturbed polynomial reconstructed Targeted Essentially Non-Oscillatory (TENO-P) scheme proposed by our research group is employed to improve the resolution of the helicopter rotor flowfield solver. The TENO-P scheme, building on the fifth-order TENO5 scheme, achieves one-order of accuracy improvement by adaptively adjusting the values of the free-parameter introduced by perturbed polynomial reconstruction. Subsequently, the AH-1 helicopter model rotor undergoing blade-vortex interaction is analyzed using the improved rotor flowfield solver and the Farassat-1A formula. The implementation of the TENO-P scheme notably enhances the resolution of the rotor flowfield solver in resolving the blade tip vortex structures, and the predicted noise results are in good agreement with the experimental data. Finally, the alteration of the BVI noise and its reduction mechanism of the AH-1 model rotor under different Higher Harmonic Controls (HHC) are analyzed. The findings show that the phase modulation margin increases with a decrease in harmonic order, and the noise reduction effect significantly improves as well. The negative component of the higher harmonic control is beneficial for reducing BVI noise while the positive component of the higher harmonic control exacerbates the BVI noise. The HHC reduces the collective pitch angle in the region where the tip vortex is about to be disturbed, decreasing the strength of the blade tip vortex in this region. This reduction weakens the strength of the parallel and near-parallel interaction on the advancing side, leads to the reduction of the positive peak impulsive of the BVI noise, and consequently lowers the intensity of the BVI noise.

An Experimental Study of Stacked Rotor Tip Vortex Interaction in Hover

Measurements were made using flow visualization to track the blade tip vortices of a 1.108-m radius stacked rotor in hover. Vortex measurements of an isolated, two-bladed rotor were also made and correlated with the Landgrebe wake model. When a second rotor was introduced to the system, the interaction between the upper and lower rotor vortices resulted in deviations from the Landgrebe wake model. Changing the azimuthal spacing revealed large changes in the trajectory of the upper and lower rotor vortices, particularly at small, negative azimuthal spacings. The miss distance, or distance between a rotor blade and vortex, was measured. Positive miss distances where the lower rotor blade passed below the upper rotor tip vortex were associated with greater noise between 1 and 5 kHz, possibly from turbulent boundary layer noise or blade–vortex interaction noise. The greatest performance and lowest broadband noise corresponded with large miss distance and minimal vortex–vortex interaction, which occurred at an azimuthal spacing of −45°. The tip vortex trajectory, and thus miss distance, could be modified by utilizing differential collective. This resulted in a 4.5-dB reduction in broadband noise at negative differential collectives by allowing the lower rotor blade to pass above the upper rotor vortex and achieving negative miss distances. At azimuthal spacings where the miss distance was always negative, the airfoil self-noise dictated the broadband noise level.

Helicopter Noise Source Separation Using an Order Tracking Filter

Due to the importance of understanding the aeroacoustics of rotorcraft with continually changing noise sources, this paper presents a new technique for source separation from ground-based acoustic measurements. The source separation process is based on combining a time-domain de-Dopplerization method with the Vold–Kalman order tracking lter approach. This process can extract rotor harmonic noise even when the sources are continuously changing with time, including impulsive events such as blade–vortex interaction noise. The advantage of this approach over traditional methods such as harmonic averaging is that the phase and amplitude relationship of acoustic signals is preserved throughout the extraction process. The approach is applied to the measured acoustic data from a Bell 430 helicopter. The measured data were separated into main rotor harmonic, tail rotor harmonic, and broadband residual components. For steady-state conditions, the extracted components could be depropagated to form acoustic hemispheres showing the directivity of the separated main and tail rotor components. The source separation process was also applied to a maneuvering ight condition. Each component has different pulse shapes and directivity trends, consistent with aeroacoustic theory.

Effects of Ambient Conditions on Helicopter Harmonic Noise Radiation: Theory and Experiment

The effects of ambient atmospheric conditions, air temperature, and density on rotor harmonic noise radiation are characterized using theoretical models and experimental measurements of helicopter noise collected at three different test sites at elevations ranging from sea level to 7000 ftabove sea level. Significant changes in the thickness, loading, and blade–vortex interaction noise levels and radiation directions are observed across the different test sites for an AS350 helicopter ying at the same indicated airspeed and gross weight. However, the radiated noise is shown to scale with ambient pressure when the flight condition of the helicopter is defined in nondimensional terms. Although the effective tip Mach number is identi ed as the primary governing parameter for thickness noise, the nondimensional weight coefficient also impacts lower harmonic loading noise levels, which contribute strongly to low-frequency harmonic noise radiation both in and out of the plane of the horizon. Strategies for maintaining the same nondimensional rotor operating condition under different ambient conditions are developed using an analytical model of single main rotor helicopter trim and confirmed using a CAMRAD II model of the AS350 helicopter. The ability of the Fundamental Rotorcraft Acoustics Modeling from Experiments (FRAME) technique to generalize noise measurements made under one set of ambient conditions to make accurate noise predictions under other ambient conditions is also validated.

Far field behaviour of wingtip vortices under synthetic jet-based control

Wingtip vortices have been proven to be detrimental to both aircraft efficiency and safety due to their adverse effects such as wake hazard, blade vortex interaction noise and induced drag. Despite the extensive literature on the subject, the number of experimental works featuring far field velocity measurements under active control is very limited. The present work deals with the experimental investigation of the effectiveness of synthetic jet actuation on the control of wingtip vortices and their wake hazard. In order to preserve the mutual induction of the counter-rotating vortices during their evolution, an unswept, low aspect ratio, squared-tipped, finite-span wing is employed. The synthetic jet actuation is based on triggering the inherent instabilities of Crow and Widnall at different values of the momentum coefficient Cμ with the goal of reducing the vortex strength and obtaining an anticipated vortex break up. Different exit geometries of the synthetic jets have been tested to analyze the effects of the jet velocity and position on the wingtip vortices. Phase-locked measurements of the velocity field in the far wake at a distance from the wing trailing edge from 26 to 80 chord lengths have been performed via stereoscopic particle image velocimetry. The effects of blowing at high momentum coefficient Cμ=1% are demonstrated to be remarkable on the mitigation of the wingtip vortices. On the other hand, both the time and phase-averaged results suggest that, at relatively low values of Cμ (0.2%), using a larger synthetic jet exit section area allows to greatly affect the wingtip vortices' features causing a striking alleviation of the vorticity distribution up to 90% with respect to the baseline reference case. In fact, due to the actuation at low frequency, the vortex instability is prematurely risen up and amplified, leading to early vortices linking and their consequent dissipation.

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

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.

Nonharmonic Control of the Blade-Vortex Interaction Noise of Electrically Controlled Rotor

Electrically controlled rotor, also known as swashplateless rotor, represents an active rotor system that, due to the use of a trailing edge flap system instead of a swashplate, not only enables primary control but also conveniently reduces the blade-vortex interaction (BVI) noise of the rotors through active control. The effect of nonharmonic inputs on the control of the BVI noise of electrically controlled rotors based on their unique trailing edge flap systems has been investigated in this paper. To this end, an analytical model for the vortex interaction-induced load and noise of electrically controlled rotor is first established based on the viscous vortex particle method, the Weissinger-L blade model, and the Ffowcs Williams-Hawkings (FW-H) equation. On this basis, a simulation study of flap nonharmonic control for BVI noise reduction in electrically controlled rotors is carried out. According to the mechanism of the BVI in electrically controlled rotors, the second quadrant flap nonharmonic control is used to reduce the advancing side BVI noise, and the effects of different control waveforms and amplitudes on the peak value and directivity of the BVI noise of the sample electrically controlled rotors are analyzed to reveal the noise reduction mechanism of flap nonharmonic control. Subsequently, the effect of the third quadrant flap nonharmonic control on BVI noise on the retreating side of the sample electrically controlled rotors is investigated. The results show that flap nonharmonic control has little effect on miss distance and that it controls BVI noise mainly by reducing the wake vortex strength on the advancing and retreating sides, which may lead to an increase in rotor noise in other regions; the noise reduction effect of flap nonharmonic control for different blade preindex angles indicates that suitable preindex angles coupled with flap nonharmonic control help optimally reduce noise.

Numerical Investigation of an Unsteady Blade Surface Blowing Method to Reduce Rotor Blade-Vortex Interaction Noise

This paper presents a numerical investigation of unsteady surface blowing using periodic variations of jet velocity with azimuthal angle to reduce helicopter rotor blade-vortex interaction (BVI) noise. The unsteady blowing is modeled as the mass flow outlet boundary condition of time-varying jet velocity on the blade surface grid using computational fluid dynamics. The same high-resolution overset grid system and flow/noise solver are used to perform a detailed flow field simulation and noise prediction for the nonblowing baseline case and the steady/unsteady blowing cases under the rotor BVI condition, and a grid convergence study for the steady and unsteady blowing cases is carried out. The BVI noise reduction and rotor thrust coefficient results of the unsteady blowing method and the previously published steady blowing with constant jet velocity are then compared. The noise reduction level of unsteady blowing is approximately equivalent to that of steady blowing (noise reduction is more than 3 dB). However, the loss in rotor thrust coefficient caused by unsteady blowing (3.3%) is only half of that by steady blowing (6.3%); the air mass cost by unsteady blowing is only 63.7% of that by steady blowing per rotation revolution. The results show that unsteady blowing can effectively reduce BVI noise with lower cost and less thrust loss.

Application of High-Order WENO Scheme in the CFD/FW–H Method to Predict Helicopter Rotor Blade–Vortex Interaction Tonal Noise

The accurate prediction of helicopter rotor blade–vortex interaction (BVI) noise is challenging. This paper presents an implementation of the seventh-order improved weighted essentially non-oscillatory (WENO-Z) scheme for predicting rotor BVI noise using a high-resolution numerical method based on the Reynolds-averaged Navier–Stokes and the Ffowcs Williams–Hawkings equations. The seventh-order improved WENO-Z scheme is utilized to minimize the inherent numerical dissipation of the reconstruction method in the monotone upstream-centered scheme for conservation laws (MUSCL), thereby improving the rotor wake resolution and the BVI noise-prediction accuracy. The three-layer dummy cell method is used to ensure that the flux at the boundary maintains seventh-order accuracy. The effectiveness of the flow solver and the acoustic solver is validated using the Helishape-7A rotor and the UH-1H rotor, respectively. The flow field and BVI noise characteristics of the OLS rotor obtained from the fifth- and seventh-order WENO-Z schemes are compared with that of the third-order MUSCL for coarse and fine background grids. The wake resolution, noise-prediction accuracy, and computational cost of the three schemes are compared. The results show that the high-order WENO scheme provides higher accuracy for flow field simulation and BVI noise prediction than the MUSCL, but the computational cost of the WENO scheme increases substantially as the grid resolution increases. However, the WENO scheme can predict BVI using a coarser grid than the MUSCL. The computational cost of the WENO scheme is relatively low under the same flow field simulation resolution.

Development and Validation of Generic Maneuvering Flight Noise Abatement Guidance for Helicopters

An extensive flight-test campaign has been conducted to look into developing actionable advice for pilots of today's vehicles to reduce their acoustic footprints. Ten distinct vehicles were tested at three different test ranges, with nine of the vehicles' data being documented here. Twelve pairs of turning conditions were tested to determine their effect on blade–vortex interaction noise. Each turning flight condition was evaluated using the peak A-weighted, band-limited (50–2500 Hz), sound pressure level measured throughout the maneuver. This metric was a surrogate for blade–vortex interaction (BVI) noise, and the difference between the peak values of each turning pair was investigated. That peak value difference was subsequently corrected by the offset from the intended vehicle altitude at turn initiation from the actual altitude at initiation. The corrected amplitudes were investigated and grouped into six validated actionable guidance principles that can be given to pilots to immediately reduce their acoustic footprint during operations. This generic guidance works by keeping the rotor well away from the wake throughout the maneuver, thus increasing miss distance and reducing the occurrence of objectionable BVI noise.

Parametric Effect of Blade Surface Blowing on the Reduction of Rotor Blade–Vortex Interaction Noise

AbstractReducing rotor blade–vortex interaction (BVI) noise is a major challenge in rotorcraft research. This paper presents a high-resolution numerical study of the effects of blade surface blowin...

Transient analysis of blade-vortex interaction noise

In this work, we propose the using of a transient acoustic imaging technique to analyze blade-vortex interaction (BVI) noise, which is considered as one of the dominant noise sources from rotorcraft. A simplified but representative set-up with a two-bladed rotor in the presence of a moving flow is considered herein. First, we conducted a delayed detached-eddy simulation to reveal the associated flow physics and to identify the distinctive flow structures during the BVI process. The numerical analysis complements the physical understanding gained from the following acoustical imaging analysis, where a recently developed phase-averaged wavelet-based beamforming method in the time-frequency domain is adopted. Compared to transient flow structures, which have been visualized in the numerical study, the acoustic imaging results clearly demonstrate the transient analysis capability of the proposed approach. It is impossible to achieve such a transient analysis capability by simply using the classical beamforming method. In particular, the transient analysis shows that the dominant noise sources would move along the blade in the radial direction, by essentially following the corresponding BVI positions on the blade, which is also consistent with the numerical results. Overall, the proposed method can help achieve a deepened physical understanding of transient aerodynamic and aeroacoustic process and could find applications in low-noise rotorcraft designs.

Fundamental investigation using active plasma control to reduce blade–vortex interaction noise

Blade vortex interaction noise is a problematic and dominant component of rotor noise. Plasma actuators strategically placed at the tip of the rotor blades can reduce the strength of the tip vortices. This reduction has the potential to significantly reduce blade vortex interaction noise. A combined experimental, numerical, and theoretical program shows supporting evidence that low power plasma actuators can effectively lower coherence of the blade tip vortex and reduce blade vortex interaction noise over-pressure by up to 80%. For a nominal small five-bladed unmanned aerial vehicle, we predict an approximate 8.88 maximum ΔdB reduction for a 150 m/s tip speed. Experimental, computational, and acoustic modeling support these predictions. This study represents a fundamental investigation in the fixed-frame, which provides evidence for higher level research and testing in a rotating framework.

Numerical investigation on noise reduction of rotor blade-vortex interaction using blade surface jet blowing

The effects of blade surface jet blowing on the reduction of rotor blade-vortex interaction (BVI) noise are investigated using the compressible Reynolds-averaged Navier-Stokes equations and the Ffowcs Williams-Hawkings equations. A detailed analysis of the accuracy of the Weighted Essentially Non-Oscillatory (WENO) scheme and Monotone Upstream-Centered Scheme for Conservation Laws for simulating the flow field and predicting the BVI noise is performed using a two-bladed scaled AH-1 helicopter main rotor undergoing BVI as the baseline case. Additionally, a grid convergence study is conducted. The BVI noise is accurately predicted using the fifth-order WENO scheme and high-resolution grid system (the prediction errors of the peak sound pressure are less than 4%). Surface boundary conditions are introduced to model the effect of jet blowing on the blade surface on the flow control of the baseline case. Blade surface blowing is analyzed near the trailing edge and the leading edge near the rotor tip. The results show that blowing near the blade trailing edge is effective for BVI mitigation by altering the tip vortex strength and increasing the blade-vortex miss distance. The sound pressure level is reduced by 2.6 dB, and the peak amplitude is attenuated by more than 30%, with a 9.8% loss in rotor thrust. Therefore, blowing near the trailing edge may be considered a promising approach for active BVI noise control.

Impact of optimized variable rotor speed and active blade twist control on helicopter blade–vortex interaction noise and environmental impact

Combined variable rotor speed and active blade twist helicopter technologies have the potential to reduce the environmental and acoustic impact of modern rotorcraft. This paper explores the aeroacoustics, environmental and ground noise impact of combined variable rotor speed and active blade twist helicopter rotors, optimized for simultaneous mitigation of source noise and NOx emissions. An integrated approach is employed including free-wake aeroelastic rotor modeling and time-domain aeroacoustic formulations fundamentally based on Acoustic Analogy. The overall method is incorporated into a multi-disciplinary design space exploration and surrogate-model-assisted optimization algorithm. The developed framework is deployed towards devising optimal variable rotor speed and active blade twist control for a representative twin-engine light helicopter operating in a realistic descent trajectory. The obtained rotor control schedules achieve overall ground noise reductions of up to 7 dB with concurrent NOx reductions of 11%, relative to the conventional rotor. The proposed approach provides new insight into rotor wake behavior and blade–vortex interactions, as well as environmental impact and ground noise footprint of combined variable rotor speed and active blade twist helicopter concepts.

Instability and merging of a helical vortex pair in the wake of a rotor

We present results from an experimental investigation of two closely spaced helical vortices. The two co-rotating vortices are generated in a water channel by a one-bladed rotor, whose tip geometry is modified by the addition of a perpendicular fin on the pressure side. The fin parameters are tuned so that the two vortices have approximately equal circulation. Dye visualisations and flow field measurements from Particle Image Velocimetry show that the vortices merge into a single tip vortex within less than one rotor rotation. This fast merging is caused by a rapid increase of the core radii of the initial vortices, which indicates the presence of an instability phenomenon. Analysis of the vortex velocity profiles identifies it as a centrifugal instability, linked to the presence of opposite-signed vorticity in the blade tip region. We further explore the origin of this vorticity by modifying the streamwise fin position. Measurements suggest that the interaction between the secondary fin vortex and the blade surface is partly responsible for the generation of the unstable velocity profile. The vortex resulting from the merging has a significantly larger core size than the one developing without a fin, indicating that this configuration could be beneficial in the context of Blade-Vortex Interaction noise for helicopters, or for the reduction of fatigue loads on wind turbine rotors caused by the interaction with concentrated tip vortices in the wake of upstream turbines.

A Ffowcs Williams-Hawkings numerical aeroacoustic study of varied and fixed-pitch blades of an H-Rotor vertical axis wind turbine

The effects of sound pressure level at two observation positions of a fixed and varied blade pitch angle at Low-Mach unsteady incompressible Reynolds-Average Navier-Stokes flow approach, on an H-rotor Vertical Axis Wind Turbine. The objective of the study is to compare the noise dissipation and output power/energy of the airfoil blades design of the vertical axis wind turbine in residential zones. The Ffowcs Williams-Hawkings (FHWH) techniques were applied to validate the output noise and vortex shedding of the different angles of attacks (AoA). The study postulated that the time history of the calculated sound pressure level at two observers positions: the aeroacoustic, blade vortex interaction noise, flow separations, dynamic stall experience from varied angled of attacks are found to produces less noise and vortex shedding compared to the fixed angle of attack.

Noise characteristic analysis and sound sources identification for rod–airfoil interaction using different subgrid-scale models

Four subgrid-scale models based on large eddy simulation (LES), such as Smagorinsky–Lilly (SL), dynamic Smagorinsky–Lilly (DSL), wall-adapting local eddy-viscosity (WALE), and dynamic kinetic-energy transport (KET) were used and couple Ffowcs Williams–Hawkings equation to accurately analyze and identify the characteristics and position of the sound sources of rod–airfoil interaction. The results of four models were compared with experimental data. It was found that the DSL model was the optimal subgrid-scale model for the study of the interaction noise considering the calculation accuracy. Therefore, the DSL model was selected for analyzing and identifying the characteristics and location of the interaction noise source. During the calculation, solid and permeable data surfaces were used for acoustic integral surfaces. The results show that the impact of the quadrupole source is negligible at a low Mach number, and the dipole noise coming from the pressure fluctuations is dominant. Meanwhile, the dipole noise from the airfoil is louder than that from the rod; the leading edge of about 30% chord length of airfoil the is the main sound source of interference effect. Above results can provide guidance for research of blade-vortex interaction noise.

Impact of Tip-Vortex Modeling Uncertainty on Helicopter Rotor Blade–Vortex Interaction Noise Prediction

Free-wake models are routinely used in aeroacoustic analysis of helicopter rotors; however, their semiempiricism is accompanied with uncertainty related to the modeling of physical wake parameters. In some cases, analysts have to resort to empirical adaption of these parameters based on previous experimental evidence. This paper investigates the impact of inherent uncertainty in wake aerodynamic modeling on the robustness of helicopter rotor aeroacoustic analysis. A free-wake aeroelastic rotor model is employed to predict high-resolution unsteady airloads, including blade–vortex interactions. A rotor aeroacoustics model, based on integral solutions of the Ffowcs Williams–Hawkings equation, is utilized to calculate aerodynamic noise in the time domain. The individual analytical models are incorporated into an uncertainty analysis numerical procedure, implemented through nonintrusive Polynomial Chaos expansion. The potential sources of uncertainty in wake tip-vortex core growth modeling are identified and their impact on noise predictions is systematically quantified. When experimental data to adjust the tip-vortex core model are not available the uncertainty in acoustic pressure and noise impact at observers dominated by blade–vortex interaction noise can reach up to 25% and 3.50 dB, respectively. A set of generalized uncertainty maps is derived, for use as modeling guidelines for aeroacoustic analysis in the absence of the robust evidence necessary for calibration of semiempirical vortex core models.

Investigation into the Effects of Blade Tip Twist on Noise Reduction for a NACA 0012 Rotor Blade

The advancement of the urban air mobility concept is heavily dependent on the public acceptance of the aircraft vehicles used for air taxis. The importance of the perception of noise by humans is crucial to the passing of legislation and proposals working to implement the new transport system. The investigation into how the noise perception can be reduced is vital to the success of the personal air travel industry. The air taxi design has been geared towards the rotorcraft models. Therefore, the investigation into the rotor blade designs is necessary because, similar to helicopters, the main component in noise generation is the rotors and its blades. The blade-vortex interaction creates noise which humans perceive as the most annoying and disruptive type of noise produced by rotorcraft. The blade-vortex interaction noise is due to the interaction between the advancing blade and the vortexes generated by the tips of the previous rotor blade. This study looks into the effect of tip twist on rotor blades. Understanding how twist in a blade will affect thrust production and noise production will aid in the progress of developing the acceptance of the urban air mobility movement. This investigation is carried out through the use of high-fidelity rotorcraft modeling software developed by Continuum Dynamics Incorporated called the Comprehensive Hierarchical Aeromechanics Rotorcraft Model (CHARM) coupled with the aeroacoustic processing package PSU-WOPWOP to produce sound data files at the wake of the rotor blades. The noise reducing parameter under investigation is the tip twist of the rotor blades. Particularly, interest is on how this change affects the frequency of sound pressure levels. The reduction of this type of noise will aid in determining the design of efficient and safe air taxis that will utilize a system of smaller porting stations as drop off zones, similar, only in concept, to those ride-sharing services.