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A summary of key discoveries regarding the industrial applications of impinging jets on a vertical plate. The summary explores the link between the dynamics and heat transmission on one hand and the relationship between heat exchange and acoustic coupling on the other hand. The objective of this research work is to investigate the vortex dynamics and heat transfer mechanisms in impinging jets. The jet impingement technique finds widespread use in the industry, it serves purposes such as drying, cooling, and heating. The impinging jet system involves directing a fluid jet with high-velocity onto a surface. The jet impingement results in high heat exchange rates and mass transfer rates, making it an attractive technique in various industrial processes. To investigate these mechanisms, a combination of experimental and computational methods was used, including flow visualization and numerical simulations. The study of vortex dynamics in impinging jets is crucial for understanding the heat transfer mechanisms involved. The flow characteristics of impinging jets, such as the Reynolds number (Re) of the jet, the distance from the blowing mouth to the impinged wall, and the geometry of the blowing mouth, significantly affect the vortex dynamics and heat transfer rates. Therefore, optimizing these parameters can result in significant improvements in heat transfer efficiency. Several methods were proposed to enhance heat transfer, these methods can affect the flow dynamic, the surface of impingement, the nozzle’s shape and size, and the impingement parameters such as the impact distance and the jet angle. For example, investigations have identified that optimal heat transfer took place at an inclination angle between 40 and 90 degrees. In addition, studies have reported enhancement in heat transfer with diamond orifices that reached up to 17%. The key findings of this paperwork include the identification of optimal impingement parameters that maximize heat transmission rates and the understanding of the significant relationship between the dynamics of the flow and the acoustic emissions. For instance, studies showed that synthetic jets can enhance convective heat transfer by 3 times compare to natural convection. The correlation between the flow dynamic and the heat transmission on one side, and the flow dynamics and acoustic emission on the other side, emphasizes the researcher to present acoustic-thermal coupled studies on the impinging jet; this topic needs more effort to understand the relation between the two phenomena. The conclusion highlights the significance of previous findings in industrial applications, suggesting control mechanisms capable of reducing noise and enhancing heat transmission.

Impinging jets are encountered in many industrial applications. Previous studies have considered different parameters that affect the flow and enhance mass and energy transfer of these flows, such as Reynolds number, nozzle geometry, nozzle-to-plate distance, wall material and angle, and the flow regime at the jet exit. Furthermore, the dynamics of these jets could be a source of a high level of noise when the transfer of energy is optimized between the aerodynamic and acoustic fields. This can lead to a resonant jet producing self-sustained tones. Self-sustained tones are related to aero-acoustic coupling and occur in impinging jets when a feedback loop is present between the jet exit and the impinged plate. Few studies that deal with the heat transfer of an impinging jet in the presence of an acoustic resonance have been conducted. This state of the art review and discussion will focus on the heat transfer and flow dynamics in such noisy and resonant configurations. Such investigation will allow us to understand how a similar regime characterized by acoustic instabilities could be useful in enhancing heat transfer at the wall. [ABSTRACT FROM AUTHOR]

A summary of key discoveries regarding the industrial applications of impinging jets on a vertical plate. The summary explores the link between the dynamics and heat transmission on one hand and the relationship between heat exchange and acoustic coupling on the other hand. The objective of this research work is to investigate the vortex dynamics and heat transfer mechanisms in impinging jets. The jet impingement technique finds widespread use in the industry, it serves purposes such as drying, cooling, and heating. The impinging jet system involves directing a fluid jet with high-velocity onto a surface. The jet impingement results in high heat exchange rates and mass transfer rates, making it an attractive technique in various industrial processes. To investigate these mechanisms, a combination of experimental and computational methods was used, including flow visualization and numerical simulations. The study of vortex dynamics in impinging jets is crucial for understanding the heat transfer mechanisms involved. The flow characteristics of impinging jets, such as the Reynolds number (Re) of the jet, the distance from the blowing mouth to the impinged wall, and the geometry of the blowing mouth, significantly affect the vortex dynamics and heat transfer rates. Therefore, optimizing these parameters can result in significant improvements in heat transfer efficiency. Several methods were proposed to enhance heat transfer, these methods can affect the flow dynamic, the surface of impingement, the nozzle's shape and size, and the impingement parameters such as the impact distance and the jet angle. For example, investigations have identified that optimal heat transfer took place at an inclination angle between 40 and 90 degrees. In addition, studies have reported enhancement in heat transfer with diamond orifices that reached up to 17%. The key findings of this paperwork include the identification of optimal impingement parameters that maximize heat transmission rates and the understanding of the significant relationship between the dynamics of the flow and the acoustic emissions. For instance, studies showed that synthetic jets can enhance convective heat transfer by 3 times compare to natural convection. The correlation between the flow dynamic and the heat transmission on one side, and the flow dynamics and acoustic emission on the other side, emphasizes the researcher to present acoustic-thermal coupled studies on the impinging jet; this topic needs more effort to understand the relation between the two phenomena. The conclusion highlights the significance of previous findings in industrial applications, suggesting control mechanisms capable of reducing noise and enhancing heat transmission. [ABSTRACT FROM AUTHOR]

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The present study experimentally and numerically investigates the flow past flat plates and NACA0012 airfoils with the different trailing edge (i.e., blunt, sharp, and rounded) and both end-rounded configurations on the far-field acoustic emissions and complex flow phenomena to determine the best edge geometry for low noise. The studies are conducted for various jet speeds of 20, 30, and 40 m/s (chordwise Reynolds numbers of 1.812 × 105, 2.72 ×105, and 3.62 ×105). It is observed that all the edge-modified flat plates show the highest directivity at an emission angle of 60° for all jet velocities studied. The comparison of power directivities between realistic and flat plate airfoils reveals that the realistic airfoils radiate lower acoustic emissions as compared to flat plate airfoils, albeit they show a common feature of downstream directivity. In general, the far-field acoustic emissions of both end-rounded plates are observed to be lower as compared to blunt, sharp, and rounded trailing edge geometries. The leading edge modification is also observed to reduce airfoil self-noise. The likely reason for the lower far-field acoustic emissions provided by both end-rounded trailing edged foils is due to the modification in the boundary layer characteristics owing to the presence of smooth flow around the rounded leading edge as well as reduced vortex strength as compared to the other foil geometries. Thus, the present study demonstrates that passive modifications of leading edge profiles are essential along with trailing edge for achieving lower acoustic emissions and higher noise reductions. [ABSTRACT FROM AUTHOR]

Impinging jets are encountered in ventilation systems in order to ensure air mixing and thermal comfort such as in residential buildings. These jets represent, under certain configuration, a source of noise which degrades the acoustic comfort. This source can be reinforced by the presence of self-sustaining tones which occurs when the impinged wall has a slot or other abrupt geometry. In order to reduce the acoustic level, many techniques of passive or active control could be adapted. In this work a cylinder is introduced between the exit nozzle and the impinged wall. This aims to disturb the vortex dynamics of the flow and to break the self-sustaining tones loop what will drastically decrease the acoustic generation as it is shown in this study. Thus, in this paper we present the influence of the introduced cylinder on the acoustic field generated by a flow impinging on a slotted plate for a Reynolds number Re=4600. An acoustic cartographic is illustrated to present of the noise level with respect to the cylinder position between the nozzle and the impinged wall. A total number of 1085 spatial positions for the cylinder with a step of 1 mm were experimentally investigated to find out the optimal position to adapt for the best noise attenuation.

Abstract An analytical framework to compute the acoustic response of one-dimensional planar waves in a compound-compressible nozzle is presented. Using the steady-state conservation equations, a set of algebraic relations governing compound flow is solved numerically. In each stream, the linearized Euler equations are recast using the invariant formulation and solved with a Magnus expansion following Duran & Moreau (Journal of Fluid Mechanics 723 (2013) 190–231). The modeled acoustic response is compared to the direct solution of the non-linear Euler equations with a maximum error of 5%. A parametric investigation of the acoustic response of a two-stream compound nozzle is presented. The effects of the bypass inflow Mach number and bypass ratio on the magnitude and phase of the transmitted and reflected acoustic waves are studied. It is shown that indirect noise generation is maximum in the peripheral bypass stream, not the core jet and that the initial Mach number of the bypass greatly impacts the phase shift of the resulting acoustic waves. The work provides an insight for acoustic response mitigation of high-speed confined jets. [ABSTRACT FROM AUTHOR]

The path to the World's energy system decarbonisation requires the contribution of a mix of renewable energy sources. In particular, wave energy and oscillating-water-column (OWC) devices can be significant contributors. However, they are still in an early stage of development, and their levelized cost of energy (LCOE) is still too high to compete in the utility market. The Wells turbine is an attractive solution to reduce the LCOE of OWC devices. Completing the available information in the literature on the performance of different Wells arrangements is a necessary step towards decision-making on which configuration is most cost-efficient given the hydrodynamic characteristics of a particular OWC wave energy converter (WEC). Furthermore, there is a research gap on these devices' acoustics, which needs to be filled, given the impact noise might have on surrounding ecosystems. This study investigated the aero-acoustics performance of four Wells turbine geometries, including monoplane and biplane turbines with and without guide vanes. Experimental tests of the turbine models were conducted in an open-circuit test rig under design and off-design flow conditions. The research provides accurate aerodynamic data on very high flow rate (pressure) coefficients, essential to turbine-generator control algorithms and OWC WECs wave-to-wire numerical models. A specific noise level definition was inferred by analysing the aerodynamic noise sources in Wells turbines. This allows the prediction of noise emission from geometrically similar turbines of different sizes and rotational speeds from the experimental noise data collected with the tested Wells turbines. • Aero-acoustic characterisation of Wells turbine in design and off-design conditions. • Rotor blade trailing edge noise emission is dominant in stall-free operation. • A sharp increase in noise emission is observed with the stall onset. [ABSTRACT FROM AUTHOR]

The demand for micro-air vehicles is increasing as well as their potential missions. Whether for discretion in military operations or noise pollution in civilian use, the improvement of aerodynamic and acoustic performance of micro-air vehicles propeller is a goal to achieve. Micro- and nano-air vehicles operate at Reynolds numbers ranging from 103 to 105. In these conditions, the aerodynamic performance of conventional fixed and rotary wings concepts drastically decreases due to the increased importance of flow viscous forces that tend to increase drag and promote flow separation, which leads to reduced efficiency and reduced maximum achievable lift. Reduced efficiency and lift result in low endurance and limited payloads. The numerical simulation is a potential solution to better understand such low Reynolds number flows and to increase the micro-air vehicles’ performance. In this paper, it is proposed to review some challenges related to micro-air vehicles by using a Lattice-Boltzmann method. The method is first briefly presented, to point out its strengths and weaknesses. Lattice-Boltzmann method is then applied to three different applications: a DNS of a single blade rotor, a large eddy simulation of a rotor operating in-ground effect and a large eddy simulation of a rotor optimised for acoustic performance. A comparison with reference data (Reynolds Averaged Navier-Stokes, DNS or experimental data) is systematically done to assess the accuracy of lattice-Boltzmann method-based predictions. The analysis of results demonstrates that lattice-Boltzmann method has a good potential to predict the mean aerodynamic performance (torque and thrust) if the grid resolution is chosen adequately (which is not always possible due to limited computational resources). A study of the turbulent flow is conducted for each application in order to highlight some of the physical flow phenomena that take place in such rotors. Different designs are also investigated, showing that potential improvements are still possible in terms of aerodynamic and aero-acoustic performance of low-Reynolds rotors. [ABSTRACT FROM AUTHOR]

As the running speed increases, aerodynamic noise gradually becomes the main noise source for high speed trains, and may act as a bottleneck for the design of new-designed high speed trains. It ss very crucial to perform the study on aerodynamcc nosse of high speed trains, to uncover the mechanssm of the aerodynamcc nosse and to develop high speed trains with lower aerodynamcc nosse. In the present paper, research results on the aerodynamcc nosse of hggh speed trains from 2010 jn China have been overviewed. Firstly, the investigation methods adopted for the study of aerodynamcc nosse of hggh speed trains are introduced in detall. Real vehcle tests, wjnd tunnel test, and numeiccal '。！！化丄、are the three main approaches adopted by the researchers and industry. Consequenlly, these three approaches and some key procedures are dsscussed. Secondly, based on the research results, the research status on the aerodynamcc nosse of h g h speed trains are dsscussed, and the following aspects are summarized: the identitication of the aerodynamcc nosse sources, the mechanssm and of the main nosse sources, and the •卩！！过化口 of the aerodynamcc nosse sources. S o m e clear conclusjons among these results are also introduced. In the end, the possible trends for the study on the aerodynamcc nosse of hggh speed trains in the future are dsscussed. [ABSTRACT FROM AUTHOR]

Abstract: The present study focuses on the correlation between the flow structures evolving during the dynamic stall processes of a two-dimensional NACA64-618 airfoil, which performs a sinusoidal movement about its quarter chord axis, and their aeroacoustic response in the far field. Experiments are conducted in an anechoic wind tunnel at a Reynolds number of 8×105 based on the chord length and include simultaneous velocity field measurements in the vicinity of the airfoil and microphone measurements in the acoustic far field. A causality correlation method based on phase locked snapshots of the velocity field allows for the identification of specific structures at different phases of the dynamic stall life cycle that contribute to the sound generation process. The sound emission during the stall development and flow reattachment phases is attributed to coherent structures evolving downstream of the trailing edge. When the flow is fully stalled, the region that contributes to the sound emission increases. The position of the sound emitting coherent structures also fluctuates stronger between oscillation cycles during full stall.Graphical abstract: [ABSTRACT FROM AUTHOR]

Unsteady flow and multi-scale vortex transformation inside a cavity of L/ D = 6 (ratio of length to depth) at Ma = 0.9 and 1.5 were studied using the numerical simulation method of modified delayed detached eddy simulation (DDES) in this paper. Aero-acoustic characteristics for the cavity at same flow conditions were obtained by the numerical method and 0.6 m by 0.6 m transonic and supersonic wind-tunnel experiments. The analysis on the computational and experimental results indicates that some vortex generates from flow separation in shear-layer over the cavity, and the vortex moves from forward to downward of the cavity at some velocity, and impingement of the vortex and the rear-wall of the cavity occurs. Some sound waves spread abroad to the cavity fore-wall, which induces some new vortex generation, and the vortex sheds, moves and impinges on the cavity rear-wall. New sound waves occur. The research results indicate that sound wave feedback created by the impingement of the shedding-vortices and rear cavity face leads to flow oscillations and noise generation inside the cavity. Analysis on aero-acoustic characteristics inside the cavity is feasible. The simulated self-sustained flow-oscillation modes and peak sound pressure on typical frequencies inside the cavity agree well with Rossiter's and Heller's predicated results. Moreover, the peak sound pressure occurs in the first and second flow-oscillation modes and most of sound energy focuses on the low-frequency region. Compared with subsonic speed (Ma = 0.9), aerodynamic noise is more intense at Ma = 1.5, which is induced by compression wave or shock wave in near region of fore and rear cavity face. [ABSTRACT FROM AUTHOR]

In computational aero-acoustics, the lattice Boltzmann method (LBM) has demonstrated a number of advantages such as low dissipation and high scalability. To recover the thermal equation and energy transport in a physically consistent way, the fourth order moments of LBM distribution function must be accurately captured, which in turn demands a multispeed velocity set. Accurate and economic realization of various boundary conditions with multispeed velocities has been a challenge in LBM. Here we present a boundary condition for open boundaries with arbitrarily specifiable sound reflection properties by embedding the characteristic non/partially reflecting boundary condition of finite difference (FD) method into multispeed LBM. The local one-dimensional inviscid analysis for Euler equations is used to build boundary treatment method for FD, and distribution functions are reconstructed from FD solution at open outlets as inner-domain LBM's boundary condition. The numeric scheme is validated in a number of test cases, including one-dimensional (1D) and two-dimensional (2D) propagation of acoustic, entropy and vorticity waves, and sound generation in flow past a 2D cylinder. Results show our method can achieve desired reflecting coefficient and reduce non-physical reflecting waves to a satisfactory level. • Open boundary condition for a multi-speed high-order lattice Boltzmann method. • Recover full compressible Navier-Stokes equation in a physically consistent manner. • Boundary condition with arbitrarily specifiable sound reflection properties. • Validation in one and two-dimensional acoustic waves propagation problems. [ABSTRACT FROM AUTHOR]

Ventilation strategies play an important role of enhancing energy conservation, Indoor Air Quality (IAQ) and acoustic comfort in any conditioned space. Impinging jets are used in such application and should be understood to be controlled. This paper focuses on the flow physics and the aero-acoustic coupling of an impinging jet ventilation system. Experimental data coupling are obtained thanks to simultaneous measurements of the velocity and the acoustic fields using respectively time-resolved particle image velocimetry (PIV) and a microphone. Spatio-temporal cross-correlations between the transverse velocity and the acoustic signals give us a better understanding of the self-sustained tones generated in impinging jets. A pre-whitening technique is used to investigate the coupling between the acoustic and the velocity signals. This method is useful for analysing small random signals superimposed on a high amplitude pure tone. [ABSTRACT FROM AUTHOR]

Current method for airframe acoustic study can be categorized into prediction methodologies and experimental measurements. The experimental measurements include flight test and wind tunnel experiment. The noise prediction methodologies consist of fully analytic method, semi-empirical method. computational fluid dynamics (CFD) combining with acoustic analogy method, and fully numerical method. Over the last decades, a considerable amount of work has been conducted on noise source identification, noise mechanism,noise reduction techniques and so on. A high-lift device noise is generally categorized into three parts : lowfrequency tones generated from the slat cove,mid-frequency broadband noise generated from the flap side edge,and high-frequency tones generated from the slat trailing edge. The noise reduction techniques for the high-lift device have active and passive types, including slat cove cover, slat cove filler,and continuous mold line for the passive one and blowing-suction air and plasma actuation for the active one. [ABSTRACT FROM AUTHOR]

Ever-rising fuel costs necessitate design of fuel-efficient vehicles. Consequently, modern vehicle manufacturers are focused on designing low aerodynamic drag vehicles which would in-turn reduce the fuel consumption. This study analyses the contribution of external rear-view mirrors to the total drag force and the overall sound pressure level at the A, B and C pillars, while optimising the external rear-view mirror design accordingly. Solid Works renditions of external rear-view mirror models mounted on a reference luxury sedan were analysed using a commercially available computational fluid dynamic package ANSYS FLUENT. A different approach was followed to carry out the empirical flow visualisation and predict sound pressure levels. The aerodynamic characterisation of the vehicle was done utilising the widely used shear stress transport turbulence model, while the analysis of wind noise and the contributing vortices employed a large eddy simulation. This approach significantly reduced computational time without compromising on accuracy. [ABSTRACT FROM AUTHOR]

Three surface integral approaches of the acoustic analogies are studied to predict the noise from three conceptual configurations of three-dimensional high-lift low-noise wings. The approaches refer to the Kirchhoff method, the Ffowcs Williams and Hawkings (FW-H) method of the permeable integral surface and the Curle method that is known as a special case of the FW-H method. The first two approaches are used to compute the noise generated by the core flow region where the energetic structures exist. The last approach is adopted to predict the noise specially from the pressure perturbation on the wall. A new way to construct the integral surface that encloses the core region is proposed for the first two methods. Considering the local properties of the flow around the complex object-the actual wing with high-lift devices-the integral surface based on the vorticity is constructed to follow the flow structures. The surface location is discussed for the Kirchhoff method and the FW-H method because a common surface is used for them. The noise from the core flow region is studied on the basis of the dependent integral quantities, which are indicated by the Kirchhoff formulation and by the FW-H formulation. The role of each wall component on noise contribution is analyzed using the Curle formulation. Effects of the volume integral terms of Lighthill's stress tensors on the noise prediction are then evaluated by comparing the results of the Curle method with the other two methods. [ABSTRACT FROM AUTHOR]

Classical Beamforming arithmetic is the basic method for aero-acoustics test in wind tunnel. Many new optimization techniques up to date are developed based on it, for example CLEAN, NNLSC Non-Negative Least-Squares) and DAMASC Deconvolution Approach for the Mapping of Acoustic Sources). But Classical Beamforming arithmetic has its own obvious shortcoming, which is that the resolving power of Classical Beamforming result is not high enough for both low frequency and high frequency signal. And especially when background noise exitsing, it has low Signal-Noise Ratio(SNR) due to background noise. A new method based on the classical Beamforming arithmetic is developed in this paper, which changes the width of beam by controling the phase of sound and varies the speed for beam focusing. The resolving power improves obviously for both low frequency and high frequency source when the method applied to noise source identifying. The skill of mechanism filtering is also proposed here. The filtering method is tending to reduce the effect of background noise by controlling the observation angle of microphone and improve the SNR of test. [ABSTRACT FROM AUTHOR]

In this paper, the effect of the backlight angle on the aerodynamic and acoustic parameters of the Ahmed model is investigated using numerical methods. The numerical results are validated by the experimental results available in the literature. The comparison reveals good agreement between the simulations and experiments. After validation of the aerodynamic model, aerodynamic noise on the c-pillar is studied and aerodynamic noise sources are identified. The critical backlight angles considering the aerodynamic and acoustic parameters are obtained. [ABSTRACT FROM AUTHOR]

The CLEAN arithmetic based on Spatial Source Coherence is developed and then applied to the wind tunnel test in the paper. The mechanism for this method is introducing a clean beam to instead the main lobe of beam picture iteratively, and getting rid of side lobe for background noise of wind tunnel or microphones distribution in phase array. The data from test in wind tunnel is optimized with this method. The results show that it improves the resolving power of low frequency noise and increases the signal-to-noise of the aero-acoustics test greatly. In the paper, coefficient K for judging the performance of phase array is also introduced, which including the effect to the resolving power both for frequency and distance from array to sound source plane. To some point, it is helpful to improve the efficiency for the coefficient K inducted. [ABSTRACT FROM AUTHOR]

- Background/Aim: There have been many reports on the discomfort of speech when wearing oral appliances. Fricatives articulated in an oral cavity can be difficult to pronounce when oral appliances are worn, because the oral cavity is partially changed by their installation. Sibilant /s/, one fricative, is especially difficult to pronounce when wearing oral appliances. This study investigates the effect of the difference in the setting positions of the palatal margin of custom-made mouthguards on the aeroacoustic characteristics of sibilant /s/. Materials and methods: Eighteen subjects (11 women and seven men) participated. The palatal margin of mouthguards was set at the gingival line for nine subjects and 4 mm from the line for another nine subjects. Acoustical analyses examined the difference of the palatal margins of the mouthguards on the autocorrelation coefficient, the zero crossing count, and the spectral peaks of sibilant /s/. Results: The results showed that the zero crossing count of the waveforms and the spectral peaks of sibilant /s/ were significantly broadened and shifted toward the low-frequency range with the mouthguard whose palatal margin extended 4 mm from the gingival line than the mouthguard whose palatal margin was set at the gingival line. Conclusion: We believe that a more appropriate palatal mouthguard design for custom-made mouthguards can be made by considering the aeroacoustical effects. Our study supported the mouthguard whose palatal margin was set at the gingival line by considering the influence on pronouncing sibilant /s/. We believe that a more appropriate palatal mouthguard design for custom-made mouthguards can be made based on the balance of aeroacoustical effects and mechanical requirements. [ABSTRACT FROM AUTHOR]

Many problems with large length scale disparities are amenable to spectral modelling. In this paper, two models are described to deal with multi-scale problems with a distinctive scale disparity in time and space, respectively. The first one makes use of a hybrid time-frequency domain formulation to realign hugely mismatched timescales in the fluid and solid domains for solving coupled convection–conduction heat transfer problems. An emphasis here is on the interface condition for multi-harmonic turbulent disturbances in conjunction with large eddy simulations. The second model deals with the spatial length scale disparity by using a block-spectral formulation. A small subset of discrete mesh blocks is solved with high local mesh resolution. The pointwise block-to-block variation is established by a discrete Fourier spectrum. An instantaneous whole domain solution can then be obtained efficiently by a simple spectral mapping. The validity and effectiveness of the models are illustrated with relevant computational examples. [ABSTRACT FROM PUBLISHER]

Abstract: Predicting broadband fan noise is key to reduce noise emissions from aircraft and wind turbines. Complete CFD simulations of broadband fan noise generation remain too expensive to be used routinely for engineering design. A more efficient approach consists in synthesizing a turbulent velocity field that captures the main features of the exact solution. This synthetic turbulence is then used in a noise source model. This paper concentrates on predicting broadband fan noise interaction (also called leading edge noise) and demonstrates that a random particle mesh method (RPM) is well suited for simulating this source mechanism. The linearized Euler equations are used to describe sound generation and propagation. In this work, the definition of the filter kernel is generalized to include non-Gaussian filters that can directly follow more realistic energy spectra such as the ones developed by Liepmann and von Kármán. The velocity correlation and energy spectrum of the turbulence are found to be well captured by the RPM. The acoustic predictions are successfully validated against Amiet’s analytical solution for a flat plate in a turbulent stream. A standard Langevin equation is used to model temporal decorrelation, but the presence of numerical issues leads to the introduction and validation of a second-order Langevin model. [Copyright &y& Elsevier]

Abstract: Noise emission is one of the major concerns in wind turbine industry and especially for small scale wind turbines, which are mostly erected to the urban areas; the concern is turning into a problem. This paper focuses on the optimization of six airfoils which are widely used on small scale wind turbines in terms of the noise emission and performance criteria and the numerical computations are performed for a typical 10 kW wind turbine. The main purpose of this optimization process is to decrease the noise emission levels while increasing the aerodynamic performance of a small scale wind turbine by adjusting the shape of the airfoil. The sources of the broadband noise emission are defined and their dominancy is investigated with respect to the operating conditions. While redesigning, together with the principals of reducing the airfoil self-noise, the aerodynamic prospects of increasing the performance have been taken into account. The codes which are based on aero-acoustic empirical models and a collection of well-known aerodynamic functions are used in this study. The results obtained from the numerical analysis of the optimization process have shown that, the considered commercial airfoils for small scale wind turbines are improved in terms of aero-acoustics and aerodynamics. The pressure sides of the baseline airfoils have been manipulated together with the trailing edge and redesigned airfoils have lower levels of noise emission and higher lift to drag ratios. [Copyright &y& Elsevier]

The homogeneous nonlinear Euler equations are linearized around the mean flow. The functions describing the perturbation produced by a given frequency time harmonic source are added as right hand members to the homogeneous linearized Euler equations. For the obtained non homo- geneous equations solution corresponding to the zero initial value is researched. It is required that the solution be defined for all positive values of the time variable t, to have Laplace transform with respect to t and Fourier transform with respect to the spatial variable x and satisfy the boundary condition corresponding to the liner-perturbation interaction at the duct wall. Conditions assuring existence and uniqueness of bounded solution and dispersion relations are derived. The Lyapunov stability of the gas flow with respect to the considered perturbations is discussed. [ABSTRACT FROM AUTHOR]

Abstract: A non-expensive insulation box for aero-acoustic experiments at moderate Reynolds numbers Re <2×104 and low Mach numbers M <0.2 is presented. Its performance is evaluated with particular attention to unwanted noise sources inherent to the flow facility. Objective acoustic parameters of the insulation box are assessed. [Copyright &y& Elsevier]

We present physical/mathematical models base on partial differential equations (PDEs) and efficient numerical simulation schemes based on the Finite Element (FE) method for multi-field problems, where the acoustic field is the field of main interest. Acoustics, the theory of sound, is an emerging scientific field including disciplines from physics over engineering to medical science. We concentrate on the following three topics: vibro-acoustics, aero-acoustics and high intensity focused ultrasound. For each topic, we discuss the physical/mathematical modeling, efficient numerical schemes and provide practical applications. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a necessary and sufficient condition is found for the existence of non-zero modes, which satisfy the Pridmore–Brown equation and the mass-spring-damper impedance boundary condition. The flowing fluid is assumed to be inviscid, non-slipping and compressible. The mean flow velocity profile in the equation is assumed to be function of y only. The condition which is found defines in fact a dispersion relation, which has to be used in the linear stability analysis of the flow also by Briggs–Bers method. As far as we know, the dispersion relation reported in the present paper is new and it is not an obvious consequence of other results already reported in the literature. The numerical illustration shows that the dispersion-relation is effective and for the considered numerical data reveals the existence of mode type initial perturbations whose amplitude increase exponentially in time showing linear instability. In the same time the numerical illustration reveals the existence of mode type initial perturbations whose amplitude decrease exponentially tending to zero for t tending to plus infinity. [ABSTRACT FROM AUTHOR]

Abstract: Assessing solution error continues to be a formidable task when numerically solving practical flow problems. Currently, grid refinement is the primary method used for error assessment. The minimum grid spacing requirements to achieve design order accuracy for a structured-grid scheme are determined for several simple examples using truncation error evaluations on a sequence of meshes. For certain methods and classes of problems, obtaining design order may not be sufficient to guarantee low error. Furthermore, some schemes can require much finer meshes to obtain design order than would be needed to reduce the error to acceptable levels. Results are then presented from realistic problems that further demonstrate the challenges associated with using grid refinement studies to assess solution accuracy. [ABSTRACT FROM AUTHOR]

Abstract: The sound and underwater sound emitted from a water drop colliding with a water surface are simulated by a new model of the finite difference lattice Boltzmann method. The two-particle immiscible fluid model is modified to simulate sound in the gas phase and underwater simultaneously. In the very early stage after the collision, sounds propagating into the gas and liquid phases are successively detected, and the effects of drop shape and gas bubbles are also observed. [Copyright &y& Elsevier]

Abstract: Assessing solution error continues to be a formidable task when numerically solving practical flow problems. Currently, grid refinement is the primary method used for error assessment. The minimum grid spacing requirements to achieve design order accuracy for a structured-grid scheme are determined for several simple examples using truncation error evaluations on a sequence of meshes. For certain methods and classes of problems, obtaining design order may not be sufficient to guarantee low error. Furthermore, some schemes can require much finer meshes to obtain design order than would be needed to reduce the error to acceptable levels. Results are then presented from realistic problems that further demonstrate the challenges associated with using grid refinement studies to assess solution accuracy. [Copyright &y& Elsevier]

Abstract: This study proposes an effective numerical model based on the Computational Fluid Dynamics (CFD) approach to obtain the flow structure around a passenger car with wing type rear spoiler. The topology of the test vehicle and grid system is constructed by a commercial package, ICEM/CFD. FLUENT is the CFD solver employed in this study. After numerical iterations are completed, the aerodynamic data and detailed complicated flow structure are visualized using commercial packages, Field View and Tecplot. The wind effect on the aerodynamic behavior of a passenger car with and without a rear spoiler and endplate is numerically investigated in the present study. It is found that the installation of a spoiler with an appropriate angle of attack can reduce the aerodynamic lift coefficient. Furthermore, the installation of an endplate can reduce the noise behind the car. It is clear that the vertical stability of a passenger car and its noise elimination can be improved. Finally, the aerodynamics and aero-acoustics of the most suitable design of spoiler is introduced and analyzed. [Copyright &y& Elsevier]

Abstract: A new Hermite Least-Square Monotone (HLSM) reconstruction to calculate accurately complex flows on non-uniform meshes is presented. The coefficients defining the Hermite polynomial are calculated by using a least-square method. To introduce monotonicity conditions into the procedure, two constraints are added into the least-square system. Those constraints are derived by locally matching the high-order Hermite polynomial with a low-order TVD or ENO polynomial. To emulate these constraints only in regions of discontinuities, data-depending weights are defined; those weights are based upon normalized indicators of smoothness of the solution and are parameterized by a O(1) quantity. The reconstruction so generated is highly compact and is fifth-order accurate when the solution is smooth; this reconstruction becomes first-order in regions of discontinuities. By inserting this reconstruction into an explicit finite-volume framework, a spatially fifth-order non-oscillatory method is then generated. This method evolves in time the solution and its first derivative. In a one-dimensional context, a linear spectral analysis and extensive numerical experiments make it possible to assess the robustness and the advantages of the method in computing multi-scales problems with embedded discontinuities. [Copyright &y& Elsevier]

Abstract: The flow around a rectangular cylinder involves various fluid-dynamic phenomena, such as separation, reattachment and vortex shedding. The fluid-dynamic characteristics of a cylinder change according to the side ratio D/H, where cylinder height is defined as H and depth as D, and Reynolds number Re=U ∞ H/ν, where U ∞ is uniform flow velocity and ν is the kinetic viscosity coefficient. In the present study a three-dimensional turbulent flow calculation was carried out using large eddy simulation (LES) to solve the flow around rectangular cylinders with side ratios D/H of 0.6, 1.0, 2.0, 2.5 and 3.0, and quantitative predictions were made of the sound generated by dipoles involved in pressure fluctuations on the cylinder surface. The calculated results for flow characteristics and sound radiation were compared with the experimental results and showed good quantitative agreement. Then the variation in sound pressure level was studied as a function of side ratio and the relationship between the flow patterns and the sound radiation, indicating that the Strouhal number and the coherence length in the spanwise direction are very important parameters for predicting sound radiation. Notably, a drastic change in Strouhal number between D/H=2.5 and 3.0 causes discontinuous change in the sound radiation. [Copyright &y& Elsevier]

Background. A sound generator based on a double-membrane design that fits into a regular tracheoesophageal shunt valve may improve voice quality after total laryngectomy in patients rehabilitated with surgical voice prostheses. Methods. Voice-producing element (VPE) prototypes were manufactured using medical grade biocompatible materials and tested in vitro under physiological conditions. Results. Basic sound, containing multiple harmonics, was successfully produced under physiologic air pressure and airflow conditions. The fundamental frequency and sound pressure level (SPL) is controlled by changing the driving pressure, thus enabling sufficient intonation for day-to-day speech. The obtained frequency range (190–350 Hz) is appropriate for producing a female voice. The low noise-to-harmonics ratio (mean 0.15) and also the efficiency of sound production (5.5 × 10-5 at 80 dB(A) and 0.15 m microphone distance) is comparable to that of normal vocal folds. Conclusions. Functional restoration of the voice after laryngectomy with a double-membrane VPE appears to be a feasible concept for female laryngectomized patients with a hypotonic, or atonic pharyngoesophageal segment. © 2007 Wiley Periodicals, Inc. Head Neck, 2007 [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the high-order discontinuous Galerkin (DG) method for solving wave propagation problems. First, we develop a one-dimensional DG scheme and numerically compute dissipation and dispersion errors for various polynomial orders. An optimal combination of time stepping scheme together with the high-order DG spatial scheme is presented. It is shown that using a time stepping scheme with the same formal accuracy as the DG scheme is too expensive for the range of wave numbers that is relevant for practical applications. An efficient implementation of a high-order DG method in three dimensions is presented. Using 1D convergence results, we further show how to adequately choose elementary polynomial orders in order to equi-distribute a priori the discretization error. We also show a straightforward manner to allow variable polynomial orders in a DG scheme. We finally propose some numerical examples in the field of aero-acoustics. [Copyright &y& Elsevier]

Abstract: The flow field around a symmetrical NACA airfoil in the uniform flow under generation of noise was numerically studied. The numerical simulation was carried out by a large-eddy simulation that employs a deductive dynamic model as the subgrid-scale model. The results at small angle of attack α =3–6° indicate that the discrete frequency noise is generated when the separated laminar flow reattaches near the trailing edge of pressure side and the strong instability thereafter affects positive vortices shed near the trailing edge. The quasi-periodic behavior of negative vortex formation on the suction side is affected by the strength and the periodicity of positive vortices near the trailing edge. The computation using aero-acoustic analogy indicates the primary discrete peak at the Strouhal frequency (=2f · δ/U 0) of 0.15 by the vortex shedding from the trailing edge, which is in a close agreement with the experiment. [Copyright &y& Elsevier]

The noise of aerodynamics nature from modern transonic fan is examined from its sources with the perspective of noise reduction through aero-acoustics design using advanced Computational Fluid Dynamics (CFD) tools. In particular the problems associated with the forward propagating noise in the front is addressed. It is identified that the shock wave spillage from the leading edge near the fan tip is the main source of the tone noise. Two different approaches have been studied to reduce the forward arc tone noise and two state-of-art transonic fans are designed using the strategies developed. The following rig tests show that while the fans exhibit other noise problems, the primary goals of noise reduction have been achieved through both fans and the novel noise reduction concept vindicated. [ABSTRACT FROM AUTHOR]