Search Results (37)

To address the difficulty of locating small-hole leaks in buried natural gas pipelines, this study conducted a comprehensive theoretical and numerical analysis of the acoustic characteristics associated with such leakage events. A coupled flow–acoustic simulation framework was developed, integrating gas compressibility via the realizable k-ε and Large Eddy Simulation (LES) turbulence models, the Peng–Robinson equation of state, a broadband noise source model, and the Ffowcs Williams–Hawkings (FW-H) acoustic analogy. The effects of pipeline operating pressure (2–10 MPa), leakage hole diameter (1–6 mm), soil type (sandy, loam, and clay), and leakage orientation on the flow field, acoustic source behavior, and sound field distribution were systematically investigated. The results indicate that the leakage hole size and soil medium exert significant influence on both flow dynamics and acoustic propagation, while the pipeline pressure mainly affects the strength of the acoustic source. The leakage direction was found to have only a minor impact on the overall results. The leakage noise is primarily composed of dipole sources arising from gas–solid interactions and quadrupole sources generated by turbulent flow, with the frequency spectrum concentrated in the low-frequency range of 0–500 Hz. This research elucidates the acoustic characteristics of pipeline leakage under various conditions and provides a theoretical foundation for optimal sensor deployment and accurate localization in buried pipeline leak detection systems. Full article

Slat noise poses a significant challenge during aircraft landing. Slat gap filler (SGF) technology has shown promise in mitigating slat noise, yet its noise reduction mechanisms and characteristics remain unclear. This study numerically investigates the noise reduction mechanisms of SGF and analyzes its noise characteristics using the high-lift common research model (CRM-HL). The lattice Boltzmann solver simulates the unsteady flow field, and the Ffowcs-Williams and Hawkings (FW-H) equation predicts far-field noise. The computed results exhibit a satisfactory concordance with experimental measurements. Furthermore, the near-field flow dynamics have been elucidated through proper orthogonal decomposition. The findings demonstrate that the SGF alters the distribution patterns of flow dynamics and pressure fluctuations, thereby effectively attenuating the mode energy. Moreover, our findings demonstrate that SGF significantly reduces slat noise. The noise reduction mechanism can be attributed to decreased surface pressure fluctuations on the leading edge of the main wing, and a shifted broadband noise peak to a lower frequency due to the enlarged slat cove flow vortex caused by SGF. Finally, a scaling analysis of the slat noise spectra indicates that the SGF noise spectra align well with baseline slat noise spectra when the characteristic length scale is determined by the vortex structure. Full article

In aerospace, flow control techniques have improved the separation flow characteristics around airfoils by various means. In this paper, the delayed detached eddy simulation (DDES) technique is used to simulate the detailed flow field around the NACA0021 airfoil with two different flow control methods (Gurney flaps and leading- and trailing-edge flaps) applied at an angle of attack of 20°. The aerodynamic characteristics around the airfoil under these two flow control methods are investigated, and the results show that both flow control methods lead to a significant increase in the pressure on the suction surface of the airfoil, which contributes to an increase in lift. The aeroacoustic characteristics of the original airfoil, the Gurney flapped airfoil and the airfoil with leading-edge and trailing-edge flaps are then analyzed using a combination of DDES and FW-H acoustic analog equations. The results show that the total sound pressure level of the Gurney flap airfoil and the leading-edge and trailing-edge flap airfoil are improved in most azimuthal angles of the acoustic pointing distribution, among which the degree of improvement of the leading-edge and trailing-edge flap airfoil is greater than that of the Gurney flap airfoil near the trailing edge, and the total sound pressure level of the band leading- and trailing-edge flap airfoil decreases in the azimuthal angles near the leading edge. Compared with the original airfoil, the noise value is thus reduced by up to 4.13 dB. The results of pressure pulsation cloud map, sound pressure level cloud map on the airfoil surface and vortex cloud map distribution show that the two flow controls increase the pressure pulsation near the trailing edge, the range and peak value of sound emission on the airfoil surface increase, and the trailing vortex becomes more finely grained, which leads to an increase in noise. Full article

To study the interference effects of the fuselage/rear-mounted propeller on the aerodynamic and aeroacoustic characteristics at a forward speed of Ma = 0.323, a multi-component flowfield simulation and an aeroacoustic prediction method were employed. Firstly, hybrid grids were adopted in the embedded grid system, and a new boundary identification method was developed to address the overlap problem by adjusting the grid boundary based on entities. The simulations were based on the URANS and FW-H equations. The employed numerical analysis methods were validated through comparisons with experimental data. Then, the aerodynamic and aeroacoustic characteristics of the propeller were analyzed, and the interference of the fuselage with the propeller was discussed in detail. Key findings included the following. Under fuselage interference, the sound pressure level (SPL) of the propeller at those observers near the forward flight direction increased dramatically, by more than 10 dB, especially in the range of two to six times the fundamental frequency. A downward vertical velocity reduced the SPLs beneath the fuselage, while an upward one had the opposite effect. The flat/vertical tails’ deceleration effect caused a thrust surge in the propeller, with most magnitudes around 20%. At different forward speeds, the thrust surge and SPL changes were similar. Full article

Rotor-to-rotor interactions have been found to significantly contribute to acoustic characteristics. The present research presents a novel idea to conduct comparative studies on the aeroacoustics characteristics of conventional, coaxial, and hybrid octocopter configurations in hover and steady forward flight to identify the configuration that has minimal noise levels. Moreover, the influence of vertical spacing in the coaxial octocopter is explored, and an aeroacoustics comparison of coaxial and hybrid configurations with larger-diameter rotors in the same confined vehicle space for missions that require a high thrust is presented at the end. The virtual blade method (VBM) is employed herein for aerodynamic analysis due to its high computational efficiency, and a numerical analysis code based on FWH equations is developed for the acoustics analysis. The hover results show that coaxial configurations produce a peak SPL value of 93 dB, which is 5 dB louder than the conventional configuration and 3 dB louder than the hybrid configuration. The coaxial configuration with 0.125 R vertical spacing produced a peak SPL of 92 dB, which is 5 dB louder than the 0.5 R and 10 dB louder than the 1.5 R. In steady forward flight with normal-sized rotors, the hybrid configuration outperformed the others with a peak SPL value of 85 dB, which is 7 dB lower than that of the conventional configuration; meanwhile, the other configurations had similar noise values. The noise attributed to larger rotors in a confined vehicle space during hover flight in the coaxial configuration is 5 dB less than that of the hybrid configuration at almost all elevation angles in the farfield; meanwhile, a 5 dB difference was observed in the front half of the vehicle in forward flight and minute differences were found in the rear half. Full article

This review presents an overview of advanced air mobility broadband noise (BBN) prediction and control techniques, highlighting significant advancements in various prediction models. Methods such as the semi-empirical Brooks–Pope–Marcolini (BPM) model, analytical Amiet model, and time-domain models based on the FW-H equation have been extensively studied. Machine learning (ML) shows promise in BBN prediction but requires extensive data training and application to noise source mechanisms. Passive control methods, such as leading and trailing edge serrations and blade tip designs, have been partially successful but often compromise the aerodynamic performance. Active control methods, like suction and blowing control, trim adjustments, and dielectric barrier discharge (DBD) plasma actuators, show great potential, with the latter two being particularly effective for reducing BBN in thin propeller structures. Overall, while progress has been made in understanding and predicting BBN, further research is needed to refine these methods and develop comprehensive noise control strategies. These advancements hold significant promise for effective and efficient noise mitigation in future AAM vehicles. Full article

The study of aquatic organisms’ ectomorphology is important to understanding the mechanisms of efficient swimming and drag reduction in fish. The drag reduction mechanism in fish remains unknown yet is needed for optimising the efficiency of bionic fish. It is thus crucial to conduct drag tests and analyses. In this paper, an optimal dolphin morphological model is constructed taking the beakless porpoise as the research object. A numerical simulation of the dolphin body model is carried out for different combinations of pitch angle and speed adopting computational fluid dynamics, and the flow field noise of the dolphin body model is solved for different speeds using the FW-H equation. When the dolphin model is oriented horizontally, the differential pressure drag accounts for approximately 20–25% of the total drag as airspeed increases. As both the pitch angle and airspeed increase, the differential pressure drag and friction drag decrease with increasing airspeed. Moreover, the acoustic energy is mainly concentrated at low frequencies for both the dolphin and Bluefin-21 models. The dolphin body model has better noise performance than the Bluefin-21 model at the same speed. The optimisation of the external morphology of the bionic underwater submarine and the analysis of the shape drag are thus important for revealing the drag reduction mechanism, reducing noise in the flow field and provide guidance for research on bionic fish. Full article

This study seeks to develop a fundamental comprehension of the noise challenges encountered by commercial aircraft fuselage surface attachments, such as blade antennas and pitot tubes. The study examines the flow characteristics and far-field noise directivity of a wall-mounted NACA0012 airfoil with various sweep angles (−35°, −15°, 0°, +15°, and +35°) and an aspect ratio of 1.5. The Mach numbers of the incoming flow range from 0.8 to 0.9 with a Reynolds number of about 7 × 10⁵. Delayed Detached Eddy Simulation (DDES) and the Ffowcs Williams–Hawkings (FW-H) equation are utilized. The results show that the shock wave intensity at the junction between the airfoil and the bottom wall is enhanced by the forward-swept angle. The shock wave moves and changes into a λ-type structure, while the boundary layer separates and produces shedding vortices in the junction at a smaller Mach number on the forward-swept airfoil compared to the straight airfoil and the backward-swept airfoil. These phenomena cause significant surface pressure fluctuations in the junction and result in a significant dipole noise in the far field, which is the primary source of noise in the far field. In addition, the normal Mach number and the absolute sweep angle also contribute to the far-field noise. Full article

This paper deals with large onshore wind turbine aeroacoustics. Noise from the NREL 5 MW device is predicted by the permeable-surface Ffowcs Williams–Hawkings equation (FWH-P), starting from the postprocessing of LES data on different acoustic surfaces $\mathcal{S}$. Their size and placement is aimed at embedding most of the aerodynamic sources of sound surrounding rotor and nacelle. Due to the presence of eddies that inevitably cross $\mathcal{S}$, this paper compares results from open and closed acoustic surfaces, and the outflow disk averaging technique. The issues related to the interpolation process of LES data on $\mathcal{S}$ is discussed as well. In order to assess the LES/FWH-P aeroacoustic platform, LES and FWH-P pressures are compared in the very-near field. It is shown that, within the limits of the discretization settings imposed by the interpolation procedure and for the Reynolds number working condition investigated herein, the lack of quadrupole sources outside the permeable surface(s) deeply affect the quality of FWH-P acoustic pressures with respect to direct LES signals. Full article

Research on helicopter transient maneuvering flight noise is a hotspot and challenging topic in the fields of helicopter design and application. A new time-accurate free-wake (TAFW) method and the Fowcs Williams-Hawkings (FW–H) equations are applied to analyze the aerodynamic and noise responses of a rotor subjected to a ramp increase in collective pitch, in hover, and in forward flight. First, a TAFW algorithm suitable for rotor aerodynamic simulation in steady-state flight and transient maneuvers is developed using modified third-order upwind backward differentiation formulas. Then, to verify the effectiveness and accuracy of the proposed method, various parameters are calculated for two scenarios and compared with corresponding results from experiments by the University of Maryland: the Langley 2MRTS rotor and the NACA rotor with ramp increases in collective pitch. Finally, the influence of collective pitch increase rate, the total increase of collective pitch, and the start and stop azimuth of ramp increase on the aerodynamic and loading noise responses of the rotor are analyzed in hover and forward flight conditions. The results show the ramp increase in collective pitch will affect the loading noise in three timescales: short-term, medium-term, and long-term. The change of the loading noise is greater when the collective pitch increase rate is greater, and the start and stop azimuth angles of the ramp increase are also important factors affecting the aerodynamic load distribution and directionality of the noise. Full article

Podded propulsors have become a focal point of research in the field of marine propulsion in recent years due to their high efficiency, low noise, and excellent maneuverability. To investigate the acoustic characteristics induced by the flow field of podded propulsors, a high-precision unsteady numerical simulation was conducted using the Delayed Detached Eddy Simulation (DDES) coupled with Ffowcs Williams–Hawkings (FW-H) equations. Multiple spatial acoustic receiving arrays were employed, and analysis methods including Proper Orthogonal Decomposition (POD) and Fast Fourier Transform (FFT) were utilized to determine the spatial distribution of the acoustic field of the podded propulsor. The results show that the blade passing frequency and the shaft frequency consistently dominate as the primary characteristic frequencies. On the plane of the propeller disk, the distribution of sound pressure levels is uniform without distinct directivity. Across the space curved surface, approximately the first ten POD modes encompass 99.8% of the total energy, and their spatial distribution characteristics of sound pressure are closely related to the pod structure. Additionally, these modes exhibit characteristic frequencies such as the blade passing frequency and shaft frequency. The spatial distribution of sound pressure at a single frequency on the spatial surface corresponds well with the results obtained from the POD analysis. Full article

A continuous adjoint-based aeroacoustic optimization, based on a hybrid model including the Ffowcs Williams–Hawkings (FW–H) acoustic analogy, to account for the multidisciplinary design of aero-engine intakes with an axisymmetric geometry, is presented. To optimize such an intake, the generatrix of its lips is parameterized using B-Splines, and the energy contained in the sound pressure spectrum, at the blade passing frequency at receivers located axisymmetrically around the axis of the engine, is minimized. The engine is not included in the optimization and manifests its presence through an independently computed time-series of static pressure over the annular boundary of the simulation domain that corresponds to the inlet to the fan. Taking advantage of the case axisymmetry, the steady 3D RANS equations are solved in the rotating frame of reference and post-processed to compute the flow quantities’ time-series required by the FW–H analogy. The numerical solution of the unsteady flow equations and the otherwise excessive overall cost of the optimization are, thus, avoided. The objective function gradient is computed using the continuous adjoint method, coupled with the analytical differentiation of the FW–H analogy. The adjoint equations are also solved in the rotating frame via steady solver. Full article

Propulsion noise is an enduring problem of significant military and environmental importance. Hence, it is crucial to investigate propeller noise characteristics. In this study, the hydrodynamic performance and noise level of the DARPA (Defense Advanced Research Projects Agency) Suboff submarine with the E1619 propeller were analyzed. The hull resistance and propeller hydrodynamics were studied separately, and the numerical results were validated using available experimental values. The self-propulsion point was determined by matching the hull resistance and propeller thrust following ITTC (International Towing Tank Conference) convention. Based on hydrodynamics and acoustic Ffowcs Williams–Hawkings (FW–H) models, the underwater-radiated noise characteristics in the self-propulsion state were simulated. The calculations indicated that the contribution of the quadrupole term in the FW–H equation is not negligible in the high-frequency band. Compared with the noise of open-water propellers, the spectrum of the E1619 propeller in its self-propulsion state is more complex, and the upstream noise is amplified. Full article

Flat roofs and platform sinking are two common installation configurations for high-speed pantographs. The cavity formed by the platform sinking is a potential source of aerodynamic drag and noise. In this paper, the shape of the rectangular cavity is optimized, and the aerodynamic performance of the high-speed pantograph with or without platform sinking is compared and discussed based on the optimized cavity results. The flow field and sound propagation are predicted by the improved delayed detached eddy simulation (IDDES) method and the FW-H equation. The results show that the rectangular cavity produces the largest aerodynamic drag and radiation noise. The upstream, downstream, and bottom surfaces of the cavity can be optimized by rounded and sloped edges to reduce aerodynamic drag and noise. The unstable shear flow and recirculation zone formed by flow separation and reattachment can be reduced by modifying the upstream and downstream surfaces of the cavity. In addition, the vortex in front of the downstream surface of the cavity can be reduced or even eliminated by modifying the bottom surface. When the upstream and downstream surfaces of the cavity are rounded and the bottom surface is sloped (R/H = 0.8), the aerodynamic performance of the cavity is better. Compared with the pantograph installed on the flat roof, the aerodynamic drag and noise of the pantograph with platform sinking are significantly reduced due to the shielding of the lower structure by the cavity, and the total drag and noise are reduced by 5.22% and 1.45 dBA, respectively. Full article

Based on the shear stress transfer (SST) k-ω model, Ffowcs-Williams and Hawkings (FW–H) equation, and Lilley sound source equation, the flow and sound field of high-temperature and high-pressure steam ejectors are simulated. The entrainment performance, near-field sound source, and far-field noise of the steam ejector are discussed. The influences of working parameters including the primary steam pressure, the secondary steam pressure, and the back pressure are analyzed. The results show that under the design conditions, the steam ejector has two shock waves and three sound source regions. A shear layer at the boundary of the first shock wave generates the Sound source-I, and the flow separation at the boundary of the second shock wave causes the Sound source-III. The Sound source-II is located near the mixing chamber wall and the sound pressure levels around the ejector depend on the distances from the Sound source-II. In terms of the entrainment performance, with the increasing primary pressure or the decreasing secondary pressure, as the driving pressure difference of the secondary steam decreases, so does the entrainment ratio. As the back pressure increases, the entrainment ratio firstly remains constant, and then rapidly decreases when the back pressure exceeds the critical value at p_b = 5.5 MPa. In terms of the noise characteristics, the sound pressure level and the intensity of the second shock wave have a positive correlation. When the primary or secondary pressure increases, the sound pressure level increases. Moreover, with the increasing back pressure, the sound pressure level firstly decreases, reaches the minimum of 98.2 dB at the critical back pressure, and then slowly increases. Full article