Ffowcs Williams-Hawkings Acoustic Analogy Research Articles

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

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

Characteristics of the flow around finite wall-mounted square cylinders and the mechanism of tonal noise

Motivated by the need to reduce the noise from train pantographs, numerical simulations are carried out for the flow over finite wall-mounted square cylinders with different aspect ratios at a Reynolds number of 1.5×104. Five aspect ratios (height-to-width ratios) are taken into account, namely, 1.4, 4.3, 7.1, 10, and 12.9. The effect of the aspect ratio on the aerodynamic coefficients, the near-wall flow topologies, and the pressure distributions are studied in detail to give insight into the noise generation mechanisms. The pressure rate of change dp/dt on the cylinder surfaces is adopted to evaluate the dipole noise source. It turns out that distributions of dp/dt are closely related to flow evolutions near the free ends and the wall-mounting junctions of cylinders with different aspect ratios. High levels of dp/dt are found close to lateral trailing edges of the cylinder, while the strength grows quickly as the aspect ratio is increased. The far-field noise emitted from these cylinders is predicted using the Ffowcs Williams–Hawkings acoustic analogy and validated with wind tunnel measurements available in the literature. For receivers located in the cross-flow direction, a single acoustic tone near a Strouhal number of 0.1 is observed for cylinders with aspect ratios not greater than seven, while an additional tone at a higher Strouhal number occurs as the aspect ratio is further increased. The underlying mechanism of the tonal noise emitted to the far field is also investigated by combining the noise source localization and dynamic mode decompositions.

Suppression effect of the leading-edge groove on deep cavity noise at low speeds: Groove length

Detached Eddy Simulations combined with Ffowcs Williams–Hawkings acoustic analogy of subsonic flows over a deep cavity preceded by deep grooves with different lengths were conducted to investigate the effects of groove length on the flow characteristics and the noise suppression in the vicinity of the deep cavity. The length-to-depth ratio of the deep cavity is 2/3, and the length-to-depth ratios of the grooves are 1/2. The distance between the grooves and the cavity remains constant at twice the groove length. The groove length ranges from 5 mm to 30 mm. The Reynolds number based on the cavity length is 8 × 10−5. The analysis results indicate that all grooves studied in this paper have some effect on the distribution of sound energy of near-field noise and near-/far-field noise amplitude, but have no effect on the characteristic frequencies of cavity noise and far-field noise directivity. When the groove length is 15 mm and 20 mm, the deep grooves are most effective in suppressing deep cavity noise. The time-averaged velocities in the incoming boundary layer, shear layer, and wake region decrease, and the height of the boundary layer downstream of the cavity increases, which reduces the intensity of the interaction between the flow structure and solid wall downstream of the cavity, resulting in a reduction in flow-acoustic feedback noise. Meanwhile, the suppression effect of the groove flow on the cavity flow effectively alters the flow characteristics of the entire cavity mouth, the rear edge wall, and part of the bottom surface region, reducing the energy at these locations. This is the primary reason for the reduction in broadband noise and acoustic resonance noise.

Numerical study on aerodynamic and noise performance of bionic asymmetric airfoil with surface grooves

Based on the asymmetric NACA4412 baseline airfoil, a bionic airfoil with surface grooves is presented. For the bio-inspired airfoil, non-smooth grooves are placed on the trailing edge of NACA4412 airfoil. To reveal the effects of non-smooth structures of the trailing edge on the aerodynamic and noise performance of airfoil, large eddy simulation and Ffowcs Williams–Hawkings acoustic analogy are adopted to investigate the aerodynamic performance and acoustic characteristics of the baseline NACA4412 airfoil and bionic airfoil at the chord-based Reynolds number, Re = 1.2 ×105. The numerical results show that the aerodynamic performance of the bionic airfoil is better than that of the baseline airfoil when the angle of attack is 14°. For all the sound frequencies studied in this study, the overall sound pressure level of the bionic airfoil is reduced by 2.0 dB at angle of attack is 14°. At the same time, the mechanisms of flow control and noise reduction of non-smooth surface grooves at the trailing edge are also revealed. As a result, the presence of surface grooves near the trailing edge of the airfoil can effectively improve the aerodynamic performance and reduce the aerodynamic noise of the traditional asymmetric airfoil, especially at high angles of attack.

Hybrid control of aerofoil self-noise by coupling air blowing and trailing-edge serration

Active and passive control methods have been extensively studied and employed for reducing turbulent-boundary-layer (TBL) trailing-edge (TE) noise of aerofoils, for example air blowing and TE serration. To further explore the potential of noise reduction, this paper proposes a hybrid method coupling blowing and serration at the trailing edge, and investigates the effects of air blowing, TE serration, and the hybrid blowing-serration (HBS) on the TBL TE noise of a NACA0012 aerofoil at a Reynolds number of Rec=4×105. Large eddy simulation and Ffowcs Williams–Hawkings acoustic analogy were used to analyse these effects and the respective noise reduction mechanisms. The numerical results show that the HBS method exhibits superior acoustic performance to the blowing and serration methods, with a maximum reduction of 20.5 dB in broadband noise and 5.7 dB in overall sound pressure level. It is revealed that the HBS method functions primarily through three noise reduction mechanisms. Firstly, blowing eliminates the outward flow motion at the serration root and tip, resulting in a forced flow alignment. Secondly, it weakens the turbulence energy in the near wake as for air blowing. Finally, this method mitigates spanwise coherence at the trailing edge, thereby causing destructive interference of noise sources, similar to the effect of TE serration.

Modulation of wake evolution, separation, and radiated noise by a cylinder with porous media cladding

In this paper, the effects of porous media parameters on circular cylinder wake flow and radiation noise are investigated using large eddy simulations and Ffowcs Williams–Hawkings acoustic analogy. We performed three-dimensional numerical simulations for flow around the cylinder coated with a porous layer of different pores per inch in a subcritical flow regime (ReD=4.7×104) to explore the control mechanism of porous media on wake and radiation noise. The results show that the application of porous media significantly alters the separation pattern behind the cylinder and stabilizes the shear layer detached from the cylinder. The existence of porous layers leads to the transformation of chaotic and irregular vortex structures into more orderly vortices. Moreover, this study also reveals that the cylinder coated with high pore density can provide the desired noise reduction. The analysis of vortex sound theory indicates that porous media reduces the interaction area and magnitude of the positive and negative Lamb vector divergences, which is beneficial for drag reduction and noise attenuation. In addition, the comparison of sound pressure contours shows that the application of porous media does not change the radiation mode of noise, but the porous media with high pore density helps to decrease the generation of noise and intensity of the sound source.

Aerodynamic and aeroacoustic performance of a pitching foil with trailing edge serrations at a high Reynolds number

Abstract The aerodynamic and aeroacoustic performance of a low-aspect-ratio ($$\hbox {AR}=0.2$$ AR = 0.2 ) pitching foil during dynamic stall are investigated numerically with focus on the effects of trailing edge serrations. A hybrid method coupling an immersed boundary method for incompressible flows with the Ffowcs Williams–Hawkings acoustic analogy is employed. Large eddy simulation and turbulent boundary layer equation wall model are also employed to capture the turbulent effects. A modified NACA0012 foil with a rectangular trailing edge flap attached to the trailing edge (baseline case) undergoing pitching motion is considered. Trailing edge serrations are applied to the trailing edge flap and their effects on the aerodynamic and aeroacoustic performance of the oscillating airfoil are considered by varying the wave amplitude ($$2h^*= 0.05, 0.1$$ 2 h ∗ = 0.05 , 0.1 , and 0.2) at a Reynolds number of 100,000 and a Mach number of 0.05. It is found that the reduction of the sound pressure level at the dimensionless frequency band $$St_{b}\in [1.25,4]$$ S t b ∈ [ 1.25 , 4 ] can be over 4 dB with the presence of the trailing edge serrations ($$2h^*=0.1$$ 2 h ∗ = 0.1 ), while the aerodynamic performance and its fluctuations are not significantly altered except a reduction around 10% in the negative moment coefficient and it fluctuations. This is due to the reduction of the average spanwise coherence function and the average surface pressure with respect to that of the baseline case, suggesting the reduction of the spanwise coherence and the noise source may result in the noise reduction. Analysis of the topology of the near wake coherent structure for $$2h^*=0.1$$ 2 h ∗ = 0.1 reveals that the alignment of the streamwise-oriented vortex with the serration edge may reduce the surface pressure fluctuation. Graphical abstract

Aerodynamic-whistles-based ultrasonic tone generators for bat deterrence

Novel ultrasonic bat deterrents based on aerodynamic whistles are proposed and investigated experimentally and numerically for their ability to generate ultrasound. The baseline deterrent, a single-whistle design inspired by Beeken [“Fluid ultrasonic generator,” U.S. patent 3,432,804 (1969)], is examined first. It consists of two resonating cavities/chambers. The whistle is “powered” by a regulated high-pressure air supply, and the performance of the whistle is examined for a range of supply pressures. Far-field acoustic measurements in the 20 Hz–50 kHz frequency range are made in an anechoic chamber. The noise measurements are supplemented with two- and three-dimensional unsteady Reynolds-averaged Navier–Stokes (uRANS) simulations to investigate the mechanisms of ultrasound generation. The Ffowcs Williams–Hawkings acoustic analogy is used with the three-dimensional uRANS results to predict the far-field radiation. The far-field acoustic predictions are in good agreement with the measurements in the anechoic chamber. The peak frequency (fundamental) of the radiated ultrasound of the baseline deterrent is approximately 23 kHz. Harmonics and a sub-harmonic of the fundamental tone are also observed. The numerical simulations show that the two resonating chambers of the baseline deterrent operate out-of-phase and Helmholtz resonance determines the whistling frequency over the range of supply air pressure considered. A six-whistle deterrent targeting a broad spectral coverage in the 20–50 kHz frequency range is designed, fabricated, and tested in the anechoic chamber. Each whistle in the deterrent is obtained by geometrically scaling the baseline whistle. Measurements show that the six-whistle ultrasound deterrent generates ultrasound with six dominant peaks in the designed frequency range when the supply air pressure exceeds 5 psig. The proposed ultrasound devices can be used for a variety of purposes including bat deterrence at wind turbines.

Active control of airfoil turbulent boundary layer noise with trailing-edge blowing

Large Eddy Simulation (LES) and Ffowcs Williams-Hawkings acoustic analogy are performed to study the effect of trailing-edge blowing on airfoil self-noise. Simulations were conducted using a National Advisory Committee for Aeronautics 0012 airfoil at zero angle of attack and a chord-based Reynolds number of 4 × 10 5. The aerodynamic and aeroacoustic characteristics of the baseline airfoil were thoroughly verified by comparison with previous numerical and experimental data. The noise reduction effects of continuous and local blowing with different blowing ratios and blowing momentum coefficients were compared. A maximum noise reduction of 20 dB was achieved via trailing-edge blowing and the noise reduction mechanisms of the two blowing methods were discussed. The LES results show a pair of recirculation bubbles in the airfoil wake which are suppressed by trailing-edge blowing. As the blowing vortices convect into the wake, they stretch and stabilize the shear flows from airfoil surfaces. Instantaneous vorticity and root mean square velocity fluctuations are also weakened. There is a decrease in the spanwise coherence and an increase in the phase difference, which contribute to noise reduction. It is concluded that the suppression of turbulence fluctuations in the near wake is the main mechanism of noise reduction for airfoil trailing-edge blowing.

Numerical study on cavitation–vortex–noise correlation mechanism and dynamic mode decomposition of a hydrofoil

The large eddy simulation model coupled with the modified Schnerr–Sauer cavitation model has been used to numerically simulate the unsteady cavitation and noncavitation flow of the three-dimensional NACA66 (National Advisory Committee for Aeronautics) hydrofoil under different operating conditions. The results show that the magnitude of the cavitation number plays a decisive role in the hydrofoil cavitation quasiperiodic phenomenon. The cavitation number of 1.25 is used as a typical working condition for analysis. Using the Ffowcs Williams–Hawkings acoustic analogy approach accompanied by the vorticity transport equation splitting, the growth and shedding of cavitation also lead to the growth and shedding of the vortex structure. The cavitation–vortex interaction is mainly influenced by the vortex stretching term and vortex dilatation term and amplitude of them are larger than 500. The baroclinic torque term may be responsible for generating vorticity during the cloud cavitation collapse and has a lower amplitude about 200. The cavity volume acceleration is the main influencing factor of the low-frequency pressure fluctuation around the cavitating hydrofoil. Moreover, the NACA66 hydrofoil surface-pressure data are collected for dynamic mode decomposition to locate the hydrofoil surface noise sources. The alternate high and low amplitude regions in the mode results overlap highly with the cavitation transformation regions. The cavity transformation and pressure fluctuations are the main reason for the generation of periodic low-frequency noise source regions on the hydrofoil surface. Moreover, the corresponding frequencies of each order mode are linearly correlated with the cavitation shedding frequency of 5.70 Hz. Combined with the results of the multiple mode comparisons, it can be inferred that the hydrofoil suction surface under the cavitation effect will generate quasiperiodic waves starting from upstream and moving downstream.

Aerodynamic and aeroacoustic performance assessment of a vertical axis wind turbine by synergistic effect of blowing and suction

The power efficiency of Darrieus wind turbines significantly deteriorates during rotation caused by periodic dynamic stall at low tip speed ratios. This leads to a strong fluctuation in torque and a reduction in energy acquisition. This paper aims to improve the aerodynamic performance of an H-type Darrieus wind turbine using an innovative fluidic flow control technique based on the synergistic effect of blowing and suction. The aeroacoustic noise emissions accomplished with this enhancement are evaluated. The Improved Delayed Detached Eddy Simulation turbulence model and the Ffowcs Williams-Hawkings acoustic analogy method are adopted to simulate the instantaneous flow field and predict the far field noise. Following validation of the numerical approach using wind tunnel experimental data on a static airfoil, an orthogonal experimental design method was used to analyze and optimize the operating factors. The factors include suction position relative to leading-edge (Ls), blowing position relative to trailing-edge (Lb) and jet coefficient (Cμ). The results indicate that Cμ plays crucial role in determining the airfoil performance, while the role of Lb is almost negligible. Furthermore, the impact of optimal combination of these factors was analyzed based on three different control strategies. It is found that appropriate application of the active control solution can eliminate the wind turbine’s negative torque, avoid excessive alternating load on the rotor and improve the energy extraction efficiency. The aeroacoustic noise estimation shows that the active device can reduce the noise emission by moderating pressure fluctuation, stabilizing the flow field and influencing the vortex shedding. Similarly, the proposed active control solution can reduce the wind turbine noise level by up to 6.56 dB by modifying the sound pressure spectra at frequencies between 100 and 1000 Hz.

Experimental investigation of broadband thrust and loading noise from pumpjet due to turbulence ingestion

Noise emitted from a rotor due to its interaction with ingested turbulence is dominant for modern submarine propulsors. An experimental study of broadband thrust from a couple of representative pumpjet configurations is conducted in this work. The associated loading noise is further predicted by incorporating the Ffowcs Williams–Hawkings acoustic analogy, which enables the projection of the local unsteady thrust measurements to the far-field noise predictions. Both the unsteady thrust measurements and the loading noise predictions are validated by comparing to the asymptotic relations from acoustic analogy and the Blake’s spectral method, respectively. The good agreement justifies the manipulations, such as the far-field dipole approximation and compact source assumption for the pumpjet, which have been adopted for the current problem set-up. Moreover, the spectra of the unsteady thrust and the loading noise are characterized by the haystacking phenomenon around the blade passing frequencies, where the tonal peaks are broadened due to the ingestion of turbulent structures. Overall, the experimental results given in this paper should be helpful in extending the noise testing capability of the current experimental facilities, as well as in deepening the understanding of the inherent fluid mechanism relevant to haystacking phenomenon and will eventually benefit low-noise pumpjet designs.

Predicting cavitating propeller noise in off-design conditions using scale-resolving CFD simulations

There is increasing awareness about the harmful impact of underwater radiated noise of shipping on the marine environment, with propeller cavitation being a major contributor thereof. In order to allow low-noise propeller design, reliable and validated numerical tools are necessary. The combined use of viscous computational fluid dynamics (CFD) and Ffowcs Williams–Hawkings acoustic analogy has long been suggested as a potential frontrunner that could address this need. However, few studies presented in the open literature have shown detailed validation focused on farfield radiated noise of propellers in cavitating conditions. Present work aims to address this by applying the methodology to two thrusters operating in off-design conditions and tested at model scale. Flow is computed using scale-resolving CFD simulations and a mass-transfer cavitation model. This allows for part of the turbulence spectrum and cavitation dynamics to be resolved. It is shown that peak sound pressure levels, corresponding to the low-frequency underwater radiated noise source, may be predicted to within 5 dB of experimental results. In addition, key features of the noise spectra, such as centre frequency of the peak broadband noise level and decay slope, are also well represented in the computations. The results are supplemented by analysis of the numerical signal-to-noise ratio.

Fast prediction of aerodynamic noise induced by the flow around a cylinder based on deep neural network

Accurate and fast prediction of aerodynamic noise has always been a research hotspot in fluid mechanics and aeroacoustics. The conventional prediction methods based on numerical simulation often demand huge computational resources, which are difficult to balance between accuracy and efficiency. Here, we present a data-driven deep neural network (DNN) method to realize fast aerodynamic noise prediction while maintaining accuracy. The proposed deep learning method can predict the spatial distributions of aerodynamic noise information under different working conditions. Based on the large eddy simulation turbulence model and the Ffowcs Williams–Hawkings acoustic analogy theory, a dataset composed of 1216 samples is established. With reference to the deep learning method, a DNN framework is proposed to map the relationship between spatial coordinates, inlet velocity and overall sound pressure level. The root-mean-square-errors of prediction are below 0.82 dB in the test dataset, and the directivity of aerodynamic noise predicted by the DNN framework are basically consistent with the numerical simulation. This work paves a novel way for fast prediction of aerodynamic noise with high accuracy and has application potential in acoustic field prediction.

Finite element hybrid and direct computational aeroacoustics at low Mach numbers in slow time-dependent domains

In this work, we address the finite element computation of flow noise in the presence of arbitrarily slowly moving rigid bodies at low Mach numbers, by means of hybrid and direct computational aeroacoustics (CAA) strategies. As regards the former, the problem could be dealt with by means of the Ffowcs Williams–Hawkings acoustic analogy. That reduces to Curle’s analogy for a static body, which is analogous to a problem of diffraction of sound waves generated by flow eddies in the vicinity of the body. Acoustic analogies in CAA first demand computing the flow motion to extract an acoustic source term from it and then use the latter to calculate the acoustic pressure field. However, in the case of low Mach number flows, the Ffowcs Williams–Hawkings and Curle analogies present a problem as they require knowing the total pressure distribution on the body’s boundary (i.e. the aerodynamic pressure plus the acoustic one). As incompressible computational fluid dynamics (CFD) simulations are usually performed to determine the flow motion, the acoustic pressure distribution on the body surface is unavoidably missing, which can yield acoustic analogies inaccurate. In a recent work, it was proposed to tackle that problem for static and rigid surfaces, by keeping the incompressible CFD and then splitting the acoustic pressure into direct and diffracted components. Two separate wave equations were solved for them, in the framework of the finite element method (FEM). In this article, we extend that work to compute the aerodynamic sound generated by a flow interacting with a slowly moving rigid body. The incompressible Navier–Stokes equations are first solved in an arbitrary Lagrangian–Eulerian (ALE) frame of reference to obtain the acoustic source term. Advantage is then taken from the same computational run to separately solve two acoustic ALE wave equations in mixed form for the incident and diffracted acoustic pressure components. For validation of the total acoustic pressure field, an ALE formulation of a direct CAA approach consisting of a unified solver for a compressible isentropic flow in primitive variables is considered. The performance of the exposed methods is illustrated for the aeroacoustics of flow past a slowly oscillating two-dimensional NACA airfoil and for flow exiting a duct with a moving teeth-shaped obstacle at its termination.

NoiseNet: A neural network to predict marine propellers’ underwater radiated noise

A dedicated neural network architecture called noiseNet has been developed to predict URN (Underwater Radiated Noise) of cavitating marine propellers. The noiseNet predicts the sound pressure level at the first three blade passing frequencies with knowing the propeller geometry, ship hull wake field and working conditions. The physical mechanism of the URN generation is firstly analyzed. Thereafter, the physical knowledge about the hydrodynamics and hydroacoustics of marine propellers are used to develop the noiseNet architecture. A dataset obtained with the boundary element method and Ffowcs Williams–Hawkings acoustic analogy is used for the training and evaluation. The evaluation conducted on fully unseen cases shows a mean absolute error of 7.34 dB.

Numerical simulation regarding flow-induced noise in variable cross-section pipelines based on large eddy simulations and Ffowcs Williams–Hawkings methods

Large eddy simulations and Ffowcs Williams–Hawkings acoustic analogy methods have been adopted to simulate the flow-induced noise for variable cross-section pipelines under variable flow velocity conditions in this paper, and the main influencing factors of flow-induced noise are analyzed numerically, including the flow velocity and variable diameter angle. Results manifested that the flow field distribution, sound source characteristics, and frequency spectrum characteristics of the sound pressure level (SPL) at different flow velocities follow similar trends. The average acoustic source intensity increases gradually with the increase in flow velocity. The maximum of the acoustic source intensity is located near the outlet of the variable diameter angle due to the vortex effect. The flow-induced noise in variable cross-section pipelines is mainly low-frequency noise, and its energy is mainly concentrated below 200 Hz according to the frequency spectrum characteristics. Additionally, the SPL increases with the increase in flow velocity. Compared with v = 1 m/s, the SPL at v = 2 m/s and v = 3 m/s increases by 9.4% and 22.1%, respectively. In addition, there is an approximate linearly increasing relationship between the SPL and the variable diameter angle. The minimum of the SPL appears at φ = 15.2°, and the maximum appears at φ = 25.7° at different flow velocities, which is up to 70.18 dB. Briefly, the flow-induced noise characteristics, including the average acoustic source intensity, the SPL, and the frequency spectra, are revealed in this paper. This provides a theoretical basis for the optimization of variable cross-section piping systems and the investigation of flow-induced noise control techniques.

A nexus between active and passive control methods for reduction of aerodynamic noise of circular cylinder

Today, the status of wildlife is extremely worrying because of noise pollution caused by aerodynamic noise of various industries. Circular cylinders are the most used devices in industries. Therefore, in this study, the aerodynamic noise prediction of circular cylinders is performed using Ffowcs Williams-Hawkings acoustic analogy. The simulation is performed using a 2D URANS model and combines active and inactive methods to control the flow. Our study encompasses the novelty of implementing passive-jet as a passive method with varying numbers of passive suction/blowing holes. Furthermore, in order to play the role of active methods, a rotating cylinder at different rotation frequencies is used, which is combined with the proposed passive method. Results show that the active method reduces the aerodynamic noise by 22 dB, when the frequency is twice the vortex shedding frequency; whereas, the passive jet reduces the noise by 50 dB when the ring is completely composed of suction/jet holes. As a more interesting idea, the two methods are combined to achieve a notable noise reduction of about 56 dB. Meanwhile, results show that the mean drag coefficient, lift fluctuation and Strouhal number are dramatically reduced. The presented methods may not be economically ideal, but they can be used as attractive ideas to optimize applications in the marine and wind industries.

Investigation into the Tip-Gap Flow and Its Influence on Ducted Propeller Tip-Gap Noise Using Acoustic Analogies

Ducted propellers are commonly used on naval vessels such as submarines and increasingly more in omnipresent autonomous underwater vehicles. Understanding the noise signature of these vessels is of critical importance from both detection and concealment perspective, but the detailed physics of how the tip of the propeller blade interacts with the boundary layer on the duct is not sufficiently well understood. The present study introduces a series of simulations in which tip vortices formed over finite-span lifting surfaces are investigated with the aim of better understanding interactions of tip-gap flow and its interaction with incident boundary layers on radiated noise. Particular attention is devoted to characterizing the nature of the incipient complex flow features and their effect on the force coefficients, surface pressure fluctuations. These reveal that variations of force coefficients with tip-gap height may be used as an early indicator of tip vortex being suppressed as the gap is reduced. Use of the λ2 criterion identifies the formation of strong coherent vortical structures in the tip-gap region. It is suggested that the interaction of these structures with the tip edges is likely to be an important source of noise, with a similar generation mechanism to trailing edge noise. Preliminary analysis of the acoustic signatures computed using the Ffowcs Williams-Hawkings acoustic analogy indicates high levels of directivity, with most of the noise being generated by the foil rather than the duct. The presence of substantial amounts of vorticity in the flow also suggests that accounting for the nonlinear quadrupole noise sources within the fluid might be necessary to fully describe the noise pattern developed.

Investigation on rod-airfoil noise with high-order cell-centered finite difference method and acoustic analogy

This paper focuses on the radiation noise of a rod-airfoil configuration based on a high-order cell-centered finite difference method (CCFDM) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy. To reduce numerical dissipation and dispersion, a class of optimized compact schemes is firstly proposed and then validated with benchmark cases from computational aeroacoustic workshops. For turbulence, the delayed detached eddy simulation (DDES) with Spalart-Allmaras model is adopted and is validated through a canonical periodic hill problem. For the far-field radiated acoustics, the FW-H acoustic analogy is utilized with validation of laminar flow past NACA0012. Finally, a rod-airfoil configuration is investigated. The results show reasonable agreement with the experimental data in terms of both near-field aerodynamics and far-field acoustics. The interaction between the upstream turbulence wake from the rod and the downstream airfoil is evidenced to be the dominant contributor to the far-field noise.