Far-field Pressure Research Articles

Annular acoustic black holes to reduce sound radiation from cylindrical shells

Annular acoustic black holes (ABHs) have been recently proposed as a potential means for reducing vibrations of cylindrical shells. The latter are very common structures in the naval, aeronautic and industrial sectors so widening ABH applications from flat plates to curved structures seems worth exploring. This work focuses on the benefits of embedding annular ABH indentations on cylindrical shells to reduce outward sound radiation. The goal of the paper is to propose a semi-analytical method to determine the acoustic power, radiation efficiency, source location and far-field acoustic pressure of ABH shells and compare them with those of uniform thickness shells. The vibration field of the ABH and uniform cylindrical shells is computed by means of the Gaussian expansion method (GEM) within the Rayleigh–Ritz approach. Then, the radiated pressure is obtained by solving the Helmholtz equation in cylindrical coordinates using the Green’s function method. The surface of the cylinder is discretized into small finite size radiators and an impedance matrix is used to obtain the acoustic surface pressure in terms of the shell radial velocity. To determine those regions of the cylinder responsible for the far field radiated sound, use is made of supersonic sound intensity (SSI). A method is proposed to calculate the SSI in the spatial domain for cylindrical shell structures which allows one to make direct use of the previously computed surface pressure and velocity distributions. The whole methodology is validated against finite element method (FEM) simulations and after that, results are presented for an acoustically thick shell. The roles played by the critical and ring frequencies are reported and the spectra of the acoustic power, radiation efficiency and far field acoustic pressure get analysed. It is shown that the annular ABH can become very effective when the cylinder flexural motion dominates over the circumferential one. The slow down of bending waves inside the ABH makes structural supersonic waves (in relation to sound speed) become subsonic at some point, which clearly diminishes the shell radiation efficiency. Overall, it is described why embedding an annular ABH on a cylindrical shell can strongly help reducing the radiated sound to the far-field.

Array Integration and Far-Field Detection of Biocompatible Wireless LC Pressure Sensors.

In this paper, three configurations of LC (inductor-capacitor) pressure sensors are developed, namely series LC pressure sensors, compact LC pressure sensors, and far-field LC pressure sensor tags. The modified silk protein films have been chosen as substrates due to their good biocompatibility and air/water permeability, which is suitable for continuously pasting such substrates on skin. For series LC pressure sensors, conducting wire is used to connect the flexible capacitor and spiral inductor. It exhibits good cycling stability and high sensitivities, suitable for electronic skin. For compact LC pressure sensors, the spiral coil functions as inductor, antenna, and capacitor electrode simultaneously, minimizing the space cost and is suitable for array integration, while the sensitivities remain the same. By tailoring the turn of the spiral coil, the resonate frequency can be regulated continuously. An annular array of compact LC sensors with ten distinct resonate frequencies ranged from 400 to 1000MHz is developed to remotely monitor the press of number 0-9. Finally, far-field LC pressure sensor tags with elongated detection distances are developed in which each compact LC sensor acts as a filter. A wireless in-shoe plantar to detect the sole pressure distribution using the far-field LC sensor configuration is developed.

Dissociation and transport modeling of methane hydrate in core-scale sandy sediments: A comparative study

Better understanding of the dissociation and transport mechanism of methane hydrate in sandy sediments contributes to the future commercial extraction of hydrate. This paper presents a comparative study on the dissociation and transport modeling of methane hydrate in core-scale sandy sediments. The mathematical model is established considering the heat and mass transfer in the hydrate dissociation and the migration process of gas-water flow. The permeability model of the core is modified based on the lab test on the Berea sandstone, which is more feasible and physical than those by historical matching. The theoretical model is verified by comparing with experiments. The following improvements are achieved: 1) The distribution of the synthetic hydrate within the core in the lab is not uniform even when assuming the core is homogeneous. 2) Better agreements are acquired between the simulation and experimental data using the simulated initial distribution of the hydrate, water and methane in Masuda’s experiment. 3) As the first effort in literature, the mini peak of the far-field pressure and the temperature fluctuation in Masuda’s experiments are investigated by simulation, which reveals the physical reason for the phenomena. Moreover, this paper provides new insight into the modeling validation using Masuda’s experiment for numerical simulation benchmarking.

Power-based application of frequency-averaged 𝓁1-norm regularisation technique for the synthesis of accelerating indoor tyre pass-by noise

Pass-by noise contribution analysis is an engineering procedure employed to estimate the contributions from various noise sources on a vehicle to the overall sound pressure level. This can be realised by placing a set of microphones close to the various sources to estimate their source strengths and then synthesising the response at a far-field linear array in the presence of the remaining sources. The results described in this paper rely on measured near-field pressure data close to the tyres of an electric vehicle under accelerating conditions. The number and position of the estimated virtual source strengths used is a compromise between complexity and accuracy, which has previously been addressed mostly empirically. A power-based, frequency-averaged 𝓁1-norm regularisation technique is investigated to optimise the equivalent source position and strength for one operating tyre and, subsequently, the far-field pass-by noise pressure estimates. It is shown that for the tyre under investigation, optimising the positions of only two equivalent sources over the frequency range of interest gives a good representation of the measured far-field spectra.

Experiment on aeroacoustic characteristics of a NACA0012 airfoil with rime ice model on the leading edge

Ice accretion changes the shape of the airplane aerodynamic surface, resulting in not only the degradation of aerodynamic performance but also the variation of the aeroacoustic characteristics. In order to explore the effect of the ice accretion on the aerodynamic noise, a NACA0012 airfoil with rime ice model on the leading edge was investigated experimentally in an aeroacoustic wind tunnel using far-field microphone array measurements. Two rime ice models of different ice thickness were attached on the airfoil leading edge. The results were compared with the baseline airfoil at various airflow speeds and different angles of attack. The experiments reveal that the ice model increases the maximum far-field sound pressure level (SPL) by 9.5 dB in the frequency band of 7–25 kHz. And the averaged OASPL difference between airfoil with ice model and baseline airfoil is linear with the 1/2 power of ice thickness. Higher angle of attack results in larger increase of the noise level. Steady RANS simulation as well as large eddy simulation was also performed to uncover the mechanism of the noise increase by the rime ice model. The presence of the rime ice model altered the local flow field around the leading edge of airfoil, resulting in the variation of airfoil surface pressure fluctuation and far-field acoustic characteristics. It would enhance the noise radiation of the airfoil, which was verified by the sound power level (PWL) of airfoil with ice model. It may provide an idea to develop a novel ice detection method based on the far-field noise increment by the ice model.

A wave and finite element method for calculating sound transmission through rectangular panels

A wave based methodology is proposed to investigate sound transmission through rectangular, rigidly-baffled panels, including the modal behaviour of the structure. The panel is homogeneous in-plane, but the through-thickness construction can be of arbitrary complexity. The fluids are modelled analytically, the structure by the wave and finite element method: a small segment of the structure is meshed to predict the behaviour of an infinite panel with the same properties. The transverse displacement of the panel is modelled as a sum of sine terms, each associated with a specific mode. Neglecting fluid loading, the blocked pressure of the incident wave is decomposed into space harmonics with given trace wavenumbers. An equivalent, infinite plate, excited by a periodic continuation of the loading on the finite panel, is considered. The natural frequencies of the finite panel follow from free wave propagation, its response estimated from that of the infinite panel. The far-field acoustic pressure and sound transmission loss are presented. Numerical results agree well with analytical solutions, other numerical solutions and measurements. The proposed method takes the modal effects fully into account, and is efficient and straightforward to implement.

Investigation of the acoustic particle velocity using synchronized non-invasive measurements in the near and far field of a turbulent flow

The sound generation of an aeroacoustic source with dominating tonal components and the propagation into the acoustic far field is investigated based on measurements of the flow around a square rod - mounted horizontally in an open-jet closed-circuit anechoic wind-tunnel - at a Reynolds number of 6×104. Pressure time series are measured with microphones in the far field while velocity field measurements are performed simultaneously with the non-invasive particle image velocimetry (PIV) technique in the flow field. It is shown that the propagation of acoustic waves from the source region to the far field can be resolved analyzing additionally flow fields recorded in vertically shifted fields of view, if the time evolution is synchronized properly with the far-field pressure signal. Furthermore, a comparison with two analytical solutions shows that the cross-correlation function of far-field pressure and near-field velocity fluctuations, normalized by the standard deviation of the pressure signal, corresponds to the acoustic particle velocity. However, this only applies to areas where the acoustic fluctuations dominate, i.e. not in the wake or in the free shear layer between the wind tunnel flow and the quiescent air where acoustic fluctuations are superimposed by hydrodynamic ones.

Modal Analysis of Acoustic Directivity in Turbulent Jets

The directivity of noise from three large-eddy simulations of turbulent jets at Mach 0.7, 0.9, and 1.5 is investigated using spectral proper orthogonal decomposition (SPOD). The most energetic patterns of acoustic radiation are extracted using the far-field pressure 2-norm. Specialization of the norm to the far field is accomplished through localized spatial weighting. Radiation patterns to specific jet inlet angles are isolated by further restricting the spatial weighting to small rectangular regions in the far field. The most energetic radiation pattern for all cases and relevant frequencies is a single superdirective acoustic beam in the downstream direction. The source region of these beams is traced back to the end of the potential core for low frequencies and the shear-layer region for higher frequencies. In the sideline direction, to low angles, the acoustic patterns consist of beams that propagate upstream or perpendicular to the jet axis. The sideline radiation patterns are found to originate from the same source locations as the dominant superdirective beams. Inspection of the SPOD modes reveals that sideline radiation is directly linked to directive downstream radiation. Within the restricted radial extent of the computational domain, these results indicate that the sources of sideline and downstream radiation are intimately linked.

Stochastic modelling of leading-edge noise in time-domain using vortex particles

The interaction of a wing/blade profile with turbulent inflow is one of the main sources of sound generation in turbomachines. There are already several noise prediction methodologies available which either appear not to account for the influence of geometrical and flow parameters on noise generation or have been able to account for a remarkably limited extent due to the requirement of high-performance computing for flow calculations. This leads to the motivation of the current paper, which presents a low-cost and easy-to-use noise prediction methodology based on the statistical modelling of the inflow turbulence and Lookup Table (LUT) approach for aeroacoustic design and optimisation. The development of the statistical method is divided into three parts; namely – i) calculating the background flow, ii) modelling of statistically optimised inflow disturbance, iii) computing the far-field sound pressure for individual vortex passages and superpose them linearly – this step involves repeated computation of identical vortex passages and can be therefore easily sped up using a database approach. In the framework of this work, a new approach to model the inflow turbulence using vortex particles characterised by shape functions, based on waveforms, is presented. The idea is to not conduct a time-dependent unsteady calculation of the flow field in real-time, instead to consider the mean flow around the profile in the computational domain, in which the vortex particles are convected to realise a statistical turbulent signal. The convection of these vortex particles, also, does not take place in the real-time calculation, instead, vortices of every possible size and strength are convected in a similar domain with a specific airfoil, and the acoustic radiation due to their interaction with the airfoil are computed and stored in a database. The far-field noise is predicted using Curle’s formulation. The generated database is accessed using the LUT approach to rapidly extract the acoustic signals. Through this approach, the influence of geometrical as well as flow parameters on the noise generated by airfoils can be quantified without requiring to conduct a numerical simulation every time for a new set of geometrical and flow variables. In the article, the application of the method for different blade profiles is shown, and the results obtained are compared with the standard literature.

Aeroacoustic Analysis of a Circulation-Controlled High-Lift Flap by Zonal Overset Large-Eddy Simulation

Circulation-controlled wings using the Coanda effect over the high-lift device (i.e., the flap) potentially enhance the wing’s lift coefficient by delaying or avoiding separation of the strongly deflected flow on the flap. However, such a Coanda-jet-equipped flap contributes significantly to the airframe noise. Hence, identifying the sound sources on the flap in the aircraft landing conditions is of paramount importance. In this context, the current paper reports the outcomes of zonal overset–large-eddy-simulation (Overset-LES) computations carried out on such a Coanda-jet-equipped flap of a DLRF16 profile. The results show that the Overset-LES computation quantitatively corrects the Reynolds-averaged Navier–Stokes baseflow. The effect of Coanda jet scales on the noise generation is studied by stochastically reconstructing them. As a validation of the computations performed, the power spectral densities of surface pressure fluctuations show a good agreement with the experimental data. This validation further emphasizes the necessity of considering the Coanda jet scales in the Overset-LES. Furthermore, aeroacoustic source ranking is performed based on far-field overall sound pressure levels. Results show that the curvature noise arising from the flow acceleration over the flap’s curvature is dominant at all angles except between 180 and 280°, where the flap’s trailing-edge noise contribution is higher. The effect of the Coanda jet scales on the far-field noise can be ignored when the complete flap is considered. However, the fluctuating jet scales have the tendency to reduce the noise arising from the curvature in the low-frequency regime of the spectrum when individual sources are accounted for, due to enhanced mixing aided by the smaller turbulent scales. This suggests that ignoring the jet scales will result in overpredicting the curvature noise in the low-frequency regime. Including the Coanda jet scales results in a flow-straightening effect near the flap trailing edge and hence alters the magnitude and directivity pattern of the flap trailing-edge noise. The current investigation reveals an additional, previously not recognized sound source at play, resulting from the strong flow deceleration at the region between the curvature and the flap’s trailing edge.

Analysis of the near-field and far-field sound pressure generated by high-speed trains pantograph system

Due to the limited understanding of spatial sound field and contribution of pantograph, a deep analysis is presented to extract understanding the aerodynamic noise characteristics of high-speed trains pantograph system. The near-field and far-field noise are predicted by the acoustic perturbation equations and Ffowcs Williams-Hawkings equation, respectively. The spatial sound propagation is analyzed by proper orthogonal decomposition and cluster-based reduced-order modelling. The flow field results predicted by large eddy simulation show that the flow behind pantograph is governed by hierarchical structures, which occurs to be featured with three layers as well as periodic evolution. The dipole source is dominated in far-field radiated noise while the quadrupole source is negligible, since the intensity of the quadrupole source is less than that of the dipole source. The contribution rates of base-frame, pan-head, groove, upper-arm, horn, lower-arm and rod-insulator are higher than the other components, and the radiated sound energy of them accounts for approximately 90% of the total energy. The noise contribution of pan-head exceeds 10% at the frequency of 1000 Hz. Base-frame is the largest contributor, and more than 6% of noise contribution occurs at the frequency of 630 Hz. The spatial sound propagation includes two major aspects: one is mainly reflected upwind from groove, and the other is propagated with the center of groove. The results highlight the possibility to develop a new design of high-speed trains pantographs, in order to obtain an aerodynamic noise reduction.

Shape stability of a gas cavity surrounded by linear and nonlinear elastic media

A number of recent medical procedures such as histotripsy rely on inertially-dominated oscillating gas cavities (i.e. bubbles) inside soft tissue. As a first approximation, soft tissue can be modeled by linear or nonlinear elasticity models and the equations describing bubble dynamics in such media have been derived in previous works. However, these models assume that the bubble remains perfectly spherical at all times for all initial conditions, which is in contradiction with any practical setting or experiments. Ignoring non-spherical behavior could for instance lead to inaccurate predictions for the extent of tissue damage generated during these procedures. The use of such models in practice thus requires one to predict departures from spherical behavior. In this article, departures from sphericity are expressed by non-spherical perturbations. Two sets of equations describing the dynamics of all non-spherical modes are derived for a bubble surrounded by a medium described using linear elasticity and Neo-Hookean hyperelasticity. For both elasticity models and for given initial conditions, bubble shape stability is shown to be controlled by five dimensionless parameters: the Weber number We, the Cauchy number Ca, the dimensionless vapor pressure inside the bubble, the dimensionless initial non-condensible gas pressure inside the bubble and the dimensionless far-field pressure. A growth criterion indicating whether the amplitude of a given non-spherical mode increases exponentially with time is also derived for both models. Bubble shape stability is then compared for both elasticity models during a Rayleigh collapse. Overall, it is found that shape stability is promoted when the shear modulus of the surrounding medium is increased and when the initial step increase in the external pressure is reduced. It is also established that the bubble shape during a Rayleigh collapse is stable over a much wider range of parameters for a surrounding medium described using Neo-Hookean hyperelasticity as opposed to linear elasticity with a similar shear modulus. This could lead to the overprediction of the occurrence of bubble shape instabilities if the surrounding medium is described using linear elasticity, which is particularly problematic during violent bubble collapse.

Analytical and experimental study of the vibro-acoustic behavior of a semi-submerged finite cylindrical shell

The vibro-acoustic behavior of a semi-submerged finite cylindrical shell is studied theoretically and experimentally. An analytical form of the vibro-acoustic coupling equation is developed using the wavenumber transformation and separation of variables for the sound pressure and the mode expansion for the shell's motion. The far-field radiation sound pressure is derived by utilizing the stationary-phase approximation. The radial velocity, radiated sound power, and sound pressure directivities (including the circumferential and axial directivities) from analytical models are compared with numerical and experimental results to verify the method. The assumption that the shell is terminated by semi-infinite rigid baffles is the main reason for the deviations between the analytical and experimental results. Below the ring frequency, acoustic radiation from a semi-submerged finite cylindrical shell occurs primarily because the air–liquid demarcation points on the shell surface act as radiation sources. Therefore, the circumferential and axial directivity patterns of a semi-submerged finite cylindrical shell can be approximated as a superposition of two in-phase dipoles formed by the air–liquid demarcation points and as a superposition of two in-phase dipole line sources, respectively. Simple formulas are derived to predict the circumferential and axial directivity patterns. This is a new and simplified approach for predicting the directivity patterns of a semi-submerged finite cylindrical shell. The vibro-acoustic behavior of a semi-submerged finite cylindrical shell differs from that of a submerged shell, especially the circumferential directivity pattern due to the reflection from the free surface.

Trailing-edge broadband noise prediction of an airfoil with boundary-layer tripping

Hybrid methods for trailing edge noise prediction are applied to a NACA 6512-63 airfoil at zero angle of attack embedded in the actual narrow stream of the Siegen open-jet anechoic wind-tunnel. Incompressible large-eddy simulations (LES) of the airfoil trailing-edge flow yield the noise sources, and different analytical acoustic analogies then predict the acoustic far-field pressure. The chord-based Reynolds number of 1.9 × 105 triggers long laminar flow regions along the airfoil, leading to instability waves with an associated broadband noise radiation. To avoid this laminar-instability noise, the airfoil boundary layer is experimentally tripped on both sides. The actual serration tripping is included in the numerical grid as well as a simplified stair-step trip. Aerodynamic and acoustic results of the performed LES are compared with measurements and compressible direct numerical simulation (DNS) results. No noticeable influence of the trip geometry is found whereas the numerical schemes play a more important role. LES predictions with boundary layer trip compare well with experimental measurements and DNS cases. If the boundary-layer trip is not sufficiently thick in the numerical model, or not present at all, the far-field acoustic pressure is overpredicted and resembles more closely the sound radiation from the untripped airfoil with laminar instability noise.

Linear superposition method (LSM) for solving stress tensor fields and displacement vector fields: Application to multiple pressure-loaded circular holes in an elastic plate with far-field stress

This study presents a novel, linear superposition method (LSM) to compute the stress tensor field and displacement vector field in a homogeneous elastic medium with an unlimited (but finite) number of circular cylindrical holes. The displacement field and the associated stress concentrations are due to a far-field stress. The method allows for the hole-centers to occur in arbitrary locations, and the hole-radii may vary over a wide range (but holes may not overlap). The holes may also induce additional elastic displacement due to internal pressure loading that will affect the local stress field, which is fully accounted for in the method. Each hole may be loaded by either equal or individual pressure loads. The underlying algorithms and solution methodology are explained and examples are given for a variety of cases. Selected case study examples show excellent matches with results obtained via independent methods (photo-elastics, complex analysis, and discrete volume solution methods). The LSM provides several advantages over alternative methods: (1) Being closed-form solutions, infinite resolution is preserved throughout, (2) Being grid-less, no time is lost on gridding, and (3) fast computation times. The specific examples of LSM applications to the multi-hole problem developed here, allow for an unlimited number of holes, with either equal or varying radii, in arbitrary constellations. The solutions further account for variable combinations of far-field stress and pressure loads on individual holes. The method can be applied for either plane strain or plane stress boundary conditions. A constitutive equation for linear elasticity controls the stress field solutions, which can be scaled for the full range of Poisson's ratios and Young moduli possible in linear elastic materials.

A Long-Range and Self-Powered Intelligent Food Monitoring System Based on Far-Field Energy Harvesting and Pressure Measurement

Introduction Radiofrequency (RF) energy harvesting has recently become a promising solution to power wireless sensors networks. As this emerging technology allows low-power sensors and systems to be charged wirelessly, it is advantageous in various applications with low cost and fewer requirements for periodical maintenance [1]. On the other hand, food quality is one of the most concerned problems of our society, foodborne illnesses caused by eating spoilt food has been a serious threat to human health. Therefore, development of a low cost, rapid and sensitive monitoring system is essential to ensure food safety and security. However, conventional food monitoring methods, which are usually based on measuring gas concentration show some drawbacks in terms of power consumption, cost, and reliability [2]. In this study, we developed a high-efficient battery-less system that can be used for food monitoring based on the far-field RF energy harvesting technique. Methods Our proposed system consists of an RF energy harvester and a sensing circuit. The RF energy harvester can harvest energy from radio waves at an ultra-high frequency (UHF) of 915 MHz to supply the power for the operation of all tag components. To improve the received power and enhance the distance of power transfer, a Yagi type structure antenna with three elements (driver, director, and reflector) was implemented to achieve a high gain and fit into a compact footprint [3]. Besides, to overcome the disadvantages of conventional food monitoring methods, our study focuses on measuring the increase in air pressure inside the food package caused by gas emission during storage. To confirm the effectiveness of the proposed system, we performed experiments on different sorts of food such as chicken, pork and fish. Results and Conclusions As a result, the sensor module is designed using a high accuracy pressure sensor and other ultra-low-power components, allowed it to consume low average power of only 1mW. The designed Yagi antenna has a high gain of 5.23dBi in the direction of the maximum radiation and voltage standing wave ratio (VSWR) better than 1.3 in approximately 60Mhz band (890-950Mhz). Finally, the increase in air pressure during food spoilage was used to categorize food freshness, demonstrating the feasibility of our proposed system.

Different noise generation mechanisms of a controlled diffusion aerofoil and their dependence on Mach number

The self-noise of an isolated controlled-diffusion aerofoil is investigated using direct noise computations. The high-order finite difference solver HiPSTAR is used to conduct four large eddy simulations and one direct numerical simulation. The motivation is to investigate the multiple sources of aerofoil self-noise on a realistic compressor blade geometry, the physical mechanisms leading to the generation of sound, and the influence of compressibility effects. The angle of attack is 8°, the chord-based Reynolds number is 105, and results obtained for four values of the free-stream Mach number [0.2, 0.3, 0.4, 0.5] are compared. For those flow parameters, the pressure side is fully laminar, whereas a separation bubble is present on the suction side close to the leading edge that promotes transition to turbulence. The size of the separation bubble is found to increase with the Mach number. Two noise sources are observed, one at the trailing edge and one in the leading edge transition/reattachment region. The first has a broadband, low frequency spectrum, while the second displays a tone whose frequency depends on the local Mach number. Because the leading edge separation bubble is very small, the associated tone frequency is high and requires a significantly finer grid to faithfully resolve the acoustic propagation than what is typically deemed sufficient. Finally, cross correlations between the surface pressure and the far-field pressure reveal that the pressure fluctuations reaching the trailing edge are initially generated in the transition/reattachment region, which indicates that the trailing edge noise is a consequence of the pressure fluctuations generated by the separation bubble.

Wall-Pressure Fluctuations Induced by a Compressible Jet in Installed Configuration

Wall-pressure fluctuations induced by a subsonic jet over a wing/flap scaled model have been experimentally investigated in a large-scale wind-tunnel facility. The pressure field on the wing was measured by flush-mounted pressure transducers installed on the pressure side, whereas synchronized near- and far-field pressures were measured using microphone arrays. Experiments were carried out with the coaxial jet operating in single-stream-like jet conditions (that is, with the primary and secondary jets at the same flow conditions) for two different jet Mach numbers of and . Flight effects were explored as well for the jet flow condition of and the flight-stream Mach number set equal to 0.18. Data were analyzed in the time and frequency domains in order to characterize the wall-pressure statistics as well as the cross statistics between the wall-pressure fluctuations and the near-/far-field pressure. Particular attention was devoted to determining universal scalings of the spectral quantities using both static and flight parameters. Semiempirical models commonly used in canonical turbulent boundary layers to reproduce the wall-pressure coherence were tested and verified for both the static and flight conditions.

Analysis of the factors contributing to the skin friction coefficient in adverse pressure gradient turbulent boundary layers and their variation with the pressure gradient

This paper reports on a study of the various factors contributing to the skin friction in incompressible adverse pressure gradient turbulent boundary layer (APG-TBL) flows. Specifically, it deals with the contributions to the skin friction coefficient from the Reynolds stresses and the viscous effects and the role of the pressure gradient. The skin friction coefficient is calculated based on the theoretical decomposition for mean skin friction generation introduced by Renard and Deck (2016). This decomposition is compatible with spatially developing flows as it is applicable to every local streamwise position. The turbulent flows are generated through the direct numerical simulation of a TBL on a smooth flat plate with the desired farfield pressure gradient. It is observed that the Reynolds shear stress provides the dominant positive contribution to the skin friction coefficient for all the pressure gradient cases. However, with increasing adverse pressure gradient, the skin friction coefficient continues to decrease and approaches zero as the positive contribution from the Reynolds shear stress is diminished by the negative contribution of the pressure gradient. When the flow reaches the verge of separation, the predominant Reynolds shear stress contribution to the skin friction coefficient is from a spatially localized outer peak at an approximate height of the displacement thickness (y=δ1) which coincides with the inflection point of the mean streamwise velocity. Even though, the decompositions in Renard and Deck (2016) and Fukagata et al. (2002) give a different distribution for the skin friction coefficient in the zero pressure gradient (ZPG) and the mild APG cases, both of the identities capture the dominant outer peak of the Reynolds shear stress contribution when the flow reaches the verge of separation. This emphasizes the growing importance of the outer layer dynamics with increasing pressure gradient as it pertains to skin friction generation.

Numerical and Experimental Analysis of the Effect of Acoustic Loads on the Source Characterization for an Internal Combustion Engine Exhaust System

The identification of source characteristics, commonly characterized as the source strength and source impedance, is essential for predicting the acoustic performance of an internal combustion (IC) engine exhaust system. This study contributes to a theoretical analysis of the effect of the acoustic parameters of loads (i.e., four-pole parameters, load impedance, and radiation impedance of the tailpipe) on the identification error of the source characteristics for an IC engine. A model based on the linear time-invariant hypothesis was constructed. A dispersion estimation function of the source strength and a deviation estimation function of the source impedance were established as indicators to test the identification accuracy. A three-dimensional (3D) multifield coupling numerical simulation method, which can thoroughly consider the influences of airflow and temperature, was applied to obtain the acoustic parameters of loads and compare them with those obtained by a one-dimensional (1D) analytical method. Then, the acoustic parameters of loads and the radiated sound pressure level of the tailpipe obtained in the measurement were substituted into the source characteristics identification model. Based on the analysis and calculation, the estimated error of the source characteristics acquired by using the 3D numerical simulation method is significantly lower than that by using the 1D analytical method. Moreover, the experiment and the 3D multifield coupling numerical simulation method were used to verify the identification results of the engine source characteristics. The results show that the far-field sound pressure level of the tailpipe predicted via the 3D numerical simulation method agrees well with the experimental results, indicating that accurate acoustic parameters of loads can effectively improve the source identification accuracy for an IC engine.