Overall Sound Pressure Level Research Articles

A Numerical Investigation on the Aeroacoustic Noise Emission from Offshore Wind Turbine Wake Interference

Offshore wind turbine (WT) wake interference will reduce power generation and increase the fatigue loads of downstream WTs. Wake interference detection based on aeroacoustic noise is believed to solve these challenges in offshore wind farms. However, aeroacoustic noise is closely related to the aerodynamics around WT blades, and the acoustic detection method requires the mastery of noise emission characteristics. In this paper, FAST.Farm, combined with the acoustic model in OpenFAST, is utilized to investigate the acoustic noise emission characteristics from two 3.4 MW-130 WTs with wake interference. Multi-microphone positions were investigated for the optimal reception selection under 8 m/s and 12 m/s wind speeds with a typical offshore atmospheric turbulence intensity of 6%. The numerical simulation results indicate that wake deficit reduces the total noise emission by about 6 dBA in the overall sound pressure level (OASPL) at 8 m/s, while wake turbulence marginally increases it and its fluctuation. There is a mutual influence between these effects, and the wake deficit effect can be 100% compensated for in the OASPL at 12 m/s. Additionally, downstream observer locations are suggested based on comparisons. These investigations provide new insights into wake interference in offshore wind farms.

Data-driven multi-objective optimization of aerodynamics and aeroacoustics in dual Savonius wind turbines using large eddy simulation and machine learning

Savonius rotor is a popular form of vertical-axis wind turbine (VAWT) for small-scale and urban applications because of its straightforward design and self-starting ability. Dual VAWTs present challenges in terms of wake interactions and noise, particularly in urban areas. Optimizing these parameters is essential for future wind energy adoption. This research is the first to analyze how the interaction of wakes from adjacent rotors, combined with a deflector, affects both the aerodynamic performance and noise levels of dual Savonius rotors. Large Eddy Simulation is applied, as it effectively captures detailed turbulent wind flows and their interactions with wind turbines. A multi-objective optimization method combining Machine Learning and Computational Fluid Dynamics (CFD) is developed to optimize rotors for maximum power efficiency and minimum noise, considering their wake interactions with a unique deflector system. First, the influence of geometric parameters on aerodynamics and aeroacoustics characteristics of rotors is analyzed, and the database is generated using Design of Experiment approach. Next, the CFD model is replaced by Artificial Neural Network (ANN) model established for predicting rotor performances. A Multi-Objective Genetic Algorithm method is used to optimize aerodynamics and aeroacoustics characteristics of rotors. Finally, optimal design parameters are identified from the Pareto front using the technique for order of preference by similarity to ideal solution decision-making method. The ANN model demonstrated high accuracy with an RANN2 of 0.995 and 0.971 for the average power coefficient (CP) and overall sound pressure level (OSPL) predictions, respectively. Multi-objective optimization revealed the best configuration of the deflector with bleed jets, improving the average CP up to 57.5% and reducing OSPL to an almost 5.2% compared to the dual rotor case at TSR = 0.8.

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.

Design and characterization of the university of Toronto hybrid anechoic wind tunnel

A detailed overview of the hybrid anechoic wind tunnel at UTIAS is presented, highlighting its design and performance features. The findings demonstrated that the tunnel achieves a uniform flow with very low turbulence intensity, matching the performance of similar open-loop wind tunnel facilities. The anechoic chamber effectively reduces noise with a cutoff frequency of around 160 Hz, providing a suitable environment for a broad spectrum of aeroacoustic measurements. The versatility of the wind tunnel was illustrated through its application in various aerodynamic and aeroacoustic studies, showcasing examples such as the NACA 0012 airfoil, the multi-element 30P30N configuration, and finite-span airfoil investigations. Moreover, the facility's Overall Sound Pressure Level (OASPL) is on par with other prominent global aeroacoustic wind tunnels, indicating its competitive performance and utility in the field.

Near-Field Aeroacoustic Shape Optimization at Low Reynolds Numbers

We investigate the feasibility of gradient-free aeroacoustic shape optimization using the flux reconstruction (FR) approach to study two-dimensional flow at low Reynolds numbers. The overall sound pressure level (OASPL) is computed via the direct acoustic approach, and optimization is performed using the gradient-free mesh adaptive direct search (MADS) algorithm. The proposed framework is assessed across three problems. First, flow over an open cavity is investigated at a Reynolds number of Re=1500 and freestream Mach number of M∞=0.15, resulting in a 7.9 dB noise reduction. The second case considers tandem cylinders at Re=200 and M∞=0.2, achieving a 16.5 dB noise reduction by optimizing the distance between the cylinders and their diameter ratio. Finally, a NACA0012 airfoil is optimized at Re=10,000 and M∞=0.2 to reduce trailing edge noise. The airfoil’s shape is optimized to generate a new four-digit NACA airfoil at an appropriate angle of attack to reduce OASPL while maintaining the baseline time-averaged lift coefficient and preventing an increase in the baseline time-averaged drag coefficient. The optimized airfoil is silent at 0 dB and the drag coefficient is decreased by 24.95%. These results demonstrate the feasibility of shape optimization using MADS and FR for aeroacoustic design.

Mechanism analysis of the influence of rotor-to-rotor interactions on global rotor noise

Existing research mainly investigated the influence of rotor-to-rotor interactions on the aerodynamics performance and noise level, but did not reveal the mechanism by which loading fluctuation significantly increases the noise below the rotor. In this study, first, an acoustic modal analysis method was established by mapping the source modal domain to acoustic modal domain, to investigate the global noise generated by a twin rotor considering the rotor-to-rotor interactions. Then, it was concluded from the simulation that the rotor-to-rotor interactions have little influence on the distribution of the sound sources, while they obvious change the noise in region at high elevation angle. Next, the method proposed was verified through experiments in an anechoic chamber, and the prediction error of overall sound pressure level (OASPL) was less than 7%. Finally, acoustic modal analysis reveals that the mechanism is attributed to the high radiation efficiency of the higher-order source modal components and dominant vertical directivity pattern of new acoustic modal components. Moreover, the initial azimuth angle of the rotors has an amplitude modulation effect on those acoustic modal components. When the difference in initial azimuth angle is π/L for rotors with L blades, the new modal components generated by different rotors cancel out each other for the odd-order blade passing frequency (BPF) noises.

Data-driven prediction of aerodynamic noise of transonic buffeting over an airfoil

Accurately predicting buffet frequency and aerodynamic noise level is crucial in transonic buffet noise reduction studies. In this study, the Random Forest (RF) algorithm is employed to predict the Power Spectral Density (PSD) and Overall Sound Pressure Level (OASPL) distribution over the supercritical airfoil RAE2822. The study indicates that the RF algorithm exhibits greater advantages over the Multi-Layer Perceptron (MLP). This algorithm does not suffer from the problems of high-frequency divergence or inflection distortion, maintaining a reduction in prediction errors for OASPL and PSD by approximately three to four orders of magnitude. Additionally, this paper proposes a priori criterion for evaluating the accuracy of PSD prediction. When there is a strong correlation between PSD at adjacent points, a data-driven modeling approach can achieve higher prediction accuracy. However, when the root mean square error of the cross-correlation functions, auto-correlation functions, and the PSD between adjacent points are both high, the generalization ability of pure data-driven modeling is insufficient, necessitating the additional monitoring points to ensure prediction accuracy.

Flow and noise control of a cylinder using grooves filled with porous material

In the present numerical study, we propose a new passive flow control mechanism at the Reynolds number of 3900. The novel method benefits from making grooves in the cylinder wall while the grooves are filled with porous materials of a specific permeability. According to the literature survey, while the porous medium is potentially an effective noise control method, it has serious drawbacks, mainly significant pressure drop. In the present study, instead of a porous coating, porous fillers are introduced offering substantial reduction of the noise level, in addition to managing the hydrodynamic parameters. To find a suitable design for the grooves and porous fillers, a systematic parametric study is performed on the number, sequence and size of the grooves, as well as the porous fillers' permeabilities. Based on the results, the newly proposed method dominated the traditional full porous coating by limiting the turbulent kinetic energy (TKE). The results of the parametric study indicated that grooves at an angle of 90° relative to the front stagnation point reduced the overall sound pressure level (OASPL) by 1.25 dB; meanwhile, the high-intensity TKE region shrunk. Further reductions were achieved by deeper grooves and porous fillers, as the drag coefficient, the lift coefficient, the Strouhal number, and the OASPL reduced up to 40.2%, 27.4%, 10.6%, and 3 dB, respectively. The proposed passive control method will be helpful for various industrial applications of cylinders through rigorous control of aerodynamic parameters and the noise level.

Effect of blade number on rotor efficiency and noise emission at hovering condition

The configuration of rotors significantly impacts the aerodynamic efficiency and noise emission of multicopters. To date, there are no general guidelines regarding how many blades a rotor should use for optimal aerodynamic performance and minimum noise emission. From the perspectives of aerodynamics and acoustics during the hovering condition, two key parameters, i.e., figure of merit (FM) and overall sound pressure level (OASPL), are evaluated to determine the optimal blade number (BN). The number of blades chosen in this study is BN = 2–6, which is largely observed in commercial multicopters. A genetic algorithm was developed to optimize blade design for each BN-rotor configuration. The individuals are evaluated by steady computational fluid dynamics (CFD) simulations and acoustic analogy for optimizations, and the detailed analyses of optimal ones are further explored by unsteady CFD simulations. The planform of the baseline blade is maintained, and the radial distribution of twist angles is the parameter for optimization. While generating the same thrust, the value of FM keeps increasing as the number of blades increases from 2 to 4, after which the FM value reaches a plateau. The value of OASPL keeps decreasing as the number of blades increases. The reason for the FM and OASPL value trends vs blade number is explained with the numerical simulation results, and a general design rule is suggested at the end.

Acoustic measurements in single-rotor/wing interaction at low disk loading and Reynolds number

The tiltrotor design is favored for urban air mobility (UAM) prototypes due to the combination of vertical takeoff and landing (VTOL) capability and efficient forward flight. With rising UAM air traffic at low altitudes, noise from these aircraft is a crucial design factor. Most tiltrotor noise research focuses on high disk loading and Reynolds number setups, leaving smaller aircraft configurations less explored. This study investigates aero-acoustic trends from rotor-wing interaction at low disk loading ([Formula: see text]100 N/m2) and Reynolds number (Re < 100,000). While prior literature suggests lowering disk loading and reducing rotor wake interference can mitigate rotor noise, such ideas lack empirical validation. The setup involves an anechoic chamber housing a two-blade rotor, along with flat and NACA 0012 airfoil wings. Microphones and a rotation stage capture acoustic data for analysis. Factors like flow recirculation, isolated rotor noise, rotor height, rotation direction/rate, and wing curvature are assessed for impact on noise signature. It is found that the deflected rotor wake in rotor-wing interaction significantly increases low-frequency broadband noise and overall sound pressure level (OASPL), compared to an isolated rotor. Dominant tonal noise diminishes based on the strength of the deflected rotor wake. These findings offer insights into reducing noise from rotor wake impingement on the wing.

A New Method Using Pseudovelocity-Based Fatigue Analysis to Determine the Effectiveness of Acoustic Fields Used for Acoustic Testing

Abstract This paper summarizes results from the use of advanced adaptive multiple-input, multiple-output (i.e., MIMO) control for direct field acoustic testing to excite an aluminum-honeycombed 1.22 m × 2.44 m (4 ft × 8 ft) test panel with MSI-DFAT's near-diffuse acoustic field and four nondiffuse acoustic fields using three panel orientations, and two speaker configurations. The five fields have the same overall sound pressure level (OASPL) and 1/3-octave sound-pressure-level (SPL) spectrum. Test and analysis results demonstrate a dependence of the test panel's response characteristics to each acoustic-field excitation, showing that the near-diffuse field excites experimental-modal-analysis-identified test panel modes effectively, whereas nondiffuse fields excite these modes ineffectively. Differences are attributed to the relative diffuseness of fields. Partial fatigue damage values from 20 accelerometer locations are determined from pseudovelocity (PV) outputs from single-degree-of-freedom (SDOF) models at each modal frequency excited by their response time histories, for each acoustic field, to quantify the relative ability of acoustic fields to detect workmanship flaws of typical test articles. Fatigue-damage values are obtained from PV response time histories of SDOF models at identified modal frequencies, which are rainflow-cycle counted with the partial-damage coefficients calculated using the Palmgren-Miner rule. A new form of vibro-acoustic gain based on PV power spectral densities (PSDs) is also introduced and correlated with these experimental results, since the PV response PSDs of a test article undergoing random vibration are proportional to modal stress at their frequencies. This paper shows that the modal stress caused by acoustic-field types is an important consideration for acoustic testing of satellites and other spacecraft, since using a nondiffuse field may result in an acoustic test that doesn't detect workmanship flaws in a test article, even though the acoustic environment is achieving the same specified OASPL and average acoustic 1/3-octave SPL spectrum.

Noise reduction for high-lift devices on a swept wing model by droop nose

The noise of slat in the high-lift device of large aircraft is one of the main noise sources of large aircraft during take-off and landing. This paper investigates a noise reduction measure which consists in droop nose for high-lift device of three-dimensional sweep wing instead of leading-edge slat. The unsteady flow field and the far-field noise are predicted by a hybrid SA-IDDES method coupled with the integration of the Ffowcs Williams–Hawkings equation (FW-H). Based on the optimal droop nose configuration from 2D airfoil, the results of the 3D swept wing model with the droop nose on the leading edge of the inner wing are given. Numerical results show that the droop nose configuration can reduce the overall sound pressure level (OASPL) of the intermediate frequency noise, and the peak noise can be reduced by 15 dB, and the sound pressure level of the broadband noise at low and high frequencies is also partially suppressed. The maximum lift-to-drag ratio of the droop nose configuration increased by 2.7 %. In addition, the noise reduction mechanism of leading-edge droop is analyzed by combining the vortex sound theory and the generation and evolution process of the vortex structure. The leading-edge droop nose configuration fundamentally not only eliminates the vortex structure of the slat cove of the leading-edge slat configuration, but also avoids the interaction between the slat slot flow and the upper surface of the main wing. These directly lead to a thinner boundary layer vortex near the top of the main wing, and the wall pressure pulsation is smaller than that of the leading-edge slat high-lift device, thereby reducing noise. The reduction of the BEF distribution on the upper surface boundary shows again that the droop nose inhibits the vortex on the upper surface of the main wing. The change of the Lamb vector divergence reveals the noise reduction mechanism of the drooping leading edge configuration directly from the sound source.

Noise Prediction and Plasma-Based Control of Cavity Flows at a High Mach Number

Cavity flows are a prevalent phenomenon in aerospace engineering, known for their intricate structures and substantial pressure fluctuations arising from interactions among vortices. The primary objective of this research is to predict noise levels in high-speed cavity flows at Mach 4 for a rectangular cavity characterized by an aspect ratio of L/D = 7. Moreover, this study delves into the influence of the plasma actuator on noise control within the cavity flow regime. To comprehensively analyze acoustic characteristics and explore effective noise reduction strategies, a computational fluid dynamics technique with the combination of a delayed detached eddy simulation (DDES) and plasma phenomenological model is established. Remarkably, the calculated overall sound pressure level (OASPL) and plasma-induced velocity closely align with the experimental data, validating the reliability of the proposed approach. The results show that the dielectric barrier discharge (DBD) plasma actuator changes the movement range of a dominating vortex in the cavity to affect the OASPL at the point with the maximum noise level. The control of excitation voltage can reduce the cavity noise by 2.27 dB at most, while control of the excitation frequency can only reduce the cavity noise by 0.336 dB at most. Additionally, the increase in excitation frequency may result in high-frequency sound pressure, but the influence is weakened with the increase in the excitation frequency. The findings highlight the potential of the plasma actuator in reducing high-Mach-number cavity noise.

Reduced-order models of aeroacoustic sources for sound radiated in twin subsonic jets

We construct reduced-order models of aeroacoustic sources for single and twin subsonic jets ( $M_j=0.9$ , $Re=3600$ ), with the goal of accurately recovering the far-field sound over a wide band of frequencies $St=[0.07,1.0]$ and directivity angles $\phi = [30^{\circ },120^{\circ }]$ within a subdecibel level accuracy. These models are realized via combining spatio-temporally coherent spectral proper orthogonal decomposition (SPOD) modes extracted directly from Lighthill's stress tensor, itself calculated using large-eddy simulation (LES). We consider two sets of twin subsonic jets of diameter $D$ each, with spacings of $0.1D$ and $1D$ , where the jets merge upstream and downstream of breakdown, respectively. The closely spaced twin jet decays the slowest due to reduced turbulent stresses which are, however, more broadband due to early merging. Such jets show strong shielding in the plane of jets, especially at shallow directivity angles where sound levels may drop below that of the single jet. The farther spaced twin jets have dynamics more akin to the constituent single jet with turbulent fluctuations peaking here at $St=0.34$ , but showing very little shielding, with their overall sound pressure level (OASPL) mostly linked to the nature of extra flow structures created during merging. Three-dimensional, energy-ranked, coherent structures for twin jets exhibit rather poor low-rank behaviour, especially at the far-field spectral peak $St=0.14$ . At $St \gtrsim 0.3$ , the SPOD wavepackets show strong visual coherence, resembling Kelvin–Helmholtz instability modes upstream of breakdown, while at the lower frequencies, there is very little spatial coherence with wavepackets peaking downstream of breakdown. Although the leading SPOD modes radiate poorly, reduced-order models using a subset of them, up to $45$ SPOD modes per frequency, show a remarkable match (within $1$ dB) against the LES-predicted sound over $0.1 \lesssim St \lesssim 0.5$ , at all angles investigated. At other frequencies, the closely spaced twin jet shows more error, due to its greater hierarchy of spatio-temporal structures, showing slower convergence at the shallower angles.

Experimental investigations on aerodynamic and psychoacoustic characteristics of three-blade looprop propeller

Aeroacoustic noise in multiple rotor drones has been increasingly recognized as a crucial issue. This study focuses on addressing this challenge by introducing a novel low-noise looprop propeller design with three blades. Unlike traditional propellers, the looprop propeller utilizes three closed-loops to generate thrust. Through comprehensive experimental investigations conducted in an anechoic chamber using a hover stand test, we conducted an integrated study of the propeller's aerodynamic, aeroacoustic, and psychoacoustic characteristics. The results demonstrate that the three-blade looprop propeller achieves a notable reduction in the overall sound pressure level (OASPL), particularly in tonal noise at the blade passing frequency where the tone reduction amounts to approximately 15 dB. Additionally, we employed Zwicker psychoacoustic annoyance models to evaluate the propellers' psychoacoustic performance and discussed the environmental noise mask effect. The findings indicate that the three-blade looprop propeller exhibits an improved sound quality while maintaining aerodynamic performance comparable to that of conventional propellers. Overall, our study suggests that the looprop propeller design offers significant advancements in acoustic performance, particularly in terms of psychoacoustic characteristics. These findings hold promising implications for the field of drone technology and the potential for notable improvements in noise reduction.

Aeroacoustic investigation of transonic flow behavior in M219 deep cavity with passive flow control configurations

This study aims to investigate the effect of front and aft wall modifications and their combinations on the sound pressure level response of the deep configuration of the M219 cavity at transonic speed. Experimental results for the baseline geometry of the cavity were validated with the high-fidelity lattice Boltzmann method computational fluid dynamics solver, SIMULIA PowerFLOW®. In this study, inclined and parallel cuts were applied to the front and aft walls of the M219 cavity to determine their efficiency in reducing the overall sound pressure level (OASPL). The most effective configurations on reducing OASPL at the front and aft walls were combined to form mixed configurations. All front wall and aft wall configurations as well as mixed configurations were validated through grid independence studies. Simulations showed that modifications to the aft wall were more effective in reducing noise than modifications to the front wall. The spectral analysis of pressure oscillations provided information about the broadband and tonal pressure responses of the mixed configurations. The findings showed that the mixed configurations were effective in reducing the frequency and amplitude of the Rossiter modes. The most successful configurations on noise reduction also showed a decrease in the drag coefficient and the average static pressure distribution along the cavity.

Performance, Aerodynamics, and Aeroacoustics of Side-by-Side Rotors Using High-Fidelity Computational Fluid Dynamics

This paper investigates the performance, aerodynamics, and aeroacoustics of NASA’s side-by-side urban air mobility (UAM) aircraft in hover conditions, utilizing high-fidelity computational fluid dynamics (CFD) simulations. High Performance Computing Modernization Program CREATE™-AV Helios is employed for the CFD simulations, while acoustic predictions are performed by PSU-WOPWOP. The simulations apply overset structured grids, the Spalart–Allmaras detached eddy simulation turbulence model, and adaptive mesh refinement (AMR) on the off-body mesh. The investigation covers four rotor overlap scenarios and five collective pitch angles, all maintaining a consistent rotational speed. Results indicate that as rotor overlap distance increases, a minor improvement in the figure of merit is noticed. Nevertheless, a substantial reduction in blade loading within the overlap region becomes apparent. The blade–vortex interaction (BVI) intensifies between both rotors as the overlap region broadens. The 25% overlap configuration is shown to yield the highest overall sound pressure level (OASPL) among all configurations due to the more pronounced BVIs at the entrance and exit of the overlap area. It is observed that the OASPL exceeds 62 dBA at an altitude of 500 ft (152.4 m) for all overlap cases, exceeding the suggested UAM aircraft noise guideline. Additionally, noise generated by all overlap scenarios is compared against various background noise levels. Results suggest that the noise produced by the side-by-side rotor configuration is not fully masked by any of the different background noise levels at an altitude of 500 ft (152.4 m).

Minimum Quantity Lubrication Jet Noise: Passive Control.

Jet noise is a common problem in minimum quantity lubrication (MQL) technology. This should be given great attention because of its serious impacts on the physical and mental health of the operators. In this study, a micro-grooved nozzle is proposed based on the noise reduction concept of biological micro-grooves. The flow field and acoustic characteristics of an original nozzle and a micro-grooved nozzle were investigated numerically to help better understand the noise reduction mechanism. The reasons for noise generation and the effects of the length (L), width (W) and depth (δ) of the micro-grooves on noise reduction were analyzed. It was found that jet noise is generated by the large-scale vortex ring structure and the pressure fluctuations caused by its motion. The overall sound pressure level (OASPL) decreased with the increases in W and δ, and increased with the increase in L. Among of them, δ has the greatest effect on noise reduction. The maximum noise reduction achieved was 6.66 dB, as verified by the OASPL test. Finally, the noise reduction mechanism was discussed in terms of the flow field, vorticity and the frequency characteristics. Micro-grooves can enhance the mixing of airflow inside the nozzle and accelerate the process of large-scale vortices breaking into smaller-scale vortices. It also reduces the sound pressure level (SPL) of middle frequencies, as well as the SPL of high frequencies on specific angles.

Geometry optimization of axial cyclone for high performance and low acoustic noise

Axial flow cyclones are widely used to separate gas-particle mixtures in many industrial applications. This study aims to optimize the axial cyclone pressure drop, removal rate, and sound levels. Single and multi-objective optimizations using a surrogate model were used to minimize the Euler number, Stokes number, and overall sound pressure level (OASPL). A radial basis function artificial neural network approach was also applied to generate more reliable data. The performed three single-objective genetic algorithms optimization studies led to the minimized Euler, Stokes, and OASPL numbers values of 5.082, 0.333, and 3.607, respectively. Then, the multi-objective NSGA-II optimization technique was used to generate a Pareto front that decision-makers can use for optimal performance. It was found that the concave guide vane, the guide vane revolution, and swirler geometry significantly affect the performance of axial cyclones. The low values of acoustic noise were recorded at low tangential and axial velocities values.

Noise reduction on low Reynolds number rotors by boundary layer transition

Following force balance and acoustic measurements in an anechoic chamber, we have shown that promoting earlier boundary layer transition on an idealised low Reynolds number rotor using 3-D cylindrical roughness elements could lead to an interesting decrease in the noise emitted by the rotor. For most tripped cases, the aerodynamic performance of the rotor was impaired by tripping, but a reduction of the broadband noise and the high amplitude blade passing frequency harmonics were observed. On the contrary, the blade passing frequency and its first harmonics did not seem to be significantly affected by the presence of the tripping. The tripping was more effective at low rotational speeds, where the broadband noise contribution dominates the overall sound pressure level (OASPL), whereas the effect on OASPL is almost negligible at high rotational speeds, where the blade passing frequency noise becomes dominant. Careful selection of the roughness height with respect to the boundary thickness and inter-roughness spacing showed that aerodynamic performance loss can be minimised while preserving the gain in noise reduction.