Three-dimensional Computational Fluid Dynamics Simulations Research Articles

Using three turbulence simulation methods for modeling the flying of high-speed railway tunnel lining blocks under piston airflow

The detached tunnel lining concrete blocks poses a serious threat to the safety of moving high-speed train. Understanding the flight characteristics of detached blocks from the tunnel soffit under train piston airflow is essential for devising appropriate measures to mitigate such risks during train operation. Computational Fluid Dynamics (CFD) simulation methods, known for their high efficiency and good repeatability, are effective approaches to study this subject. Large Eddy Simulation (LES), Improved Delayed Detached Eddy Simulation (IDDES), and Unsteady Reynolds-Averaged Navier-Stokes (URANS) are three commonly used turbulence simulation methods in the CFD simulation of train/tunnel aerodynamics. In this study, a three-dimensional CFD simulation model based on the three turbulence simulation methods is established for the airflow-tunnel-train-detached block system, and an experiment is conducted for validating the CFD model. Using the established models, the influence of different turbulence simulation methods on the flight trajectory and aerodynamic coefficients of detached lining blocks under train-induced airflows is compared. By visualizing the macroscopic flow field inside the tunnel and the local flow field near the detached block, the flow mechanisms of detached blocks under different turbulence simulation method conditions are revealed. The results showed that in the 1st stage (BTT stage), the flight of the detached block is influenced by the airflow near the train body, while in the 2nd stage (ATT stage), it is mainly affected by the train wake vortex. Compared with experimental results, the errors of LES and IDDES in simulating the longitudinal direction displacement of the block are only 2.3 % and 5.5 %, respectively, while the error of URANS is 10.6 %. The simulation error of URANS for the longitudinal direction flight speed reached 8.8 % in the ATT stage. In the BTT stage, LES and IDDES predicates more and larger leeward side vortex structures of the detached block, while URANS predicates fewer and smaller vortex structures. Compared with URANS method, the dissipation rate of the vortex structures simulated by LES and IDDES in the ATT stage is slower. These are the main reasons explaining why the flight characteristics of detached lining blocks obtained by URANS method are different from the LES and IDDES methods.

Flow and depositional response of turbidity currents to complex canyon topographies: A numerical simulation perspective

Submarine canyons are prominent features on continental margins, acting as major conduits for sediment transport primarily through turbidity currents. Understanding how these currents interact with complex canyon topographies is crucial for deciphering the canyon system evolution, yet challenging due to limited field observations. Focusing on a group of slope-confined canyons within the Pearl River Mouth Basin, northern South China Sea, this study integrates multibeam bathymetry, core analysis, and process-based Computational Fluid Dynamics (CFD) modeling to investigate the influence of realistic canyon topography on turbidity current dynamics and resulting sediment deposition. Analysis of core samples revealed a potential turbidite layer characterized by silts and sandy silts. Using these sediment information and bathymetric data, we conducted a series of three-dimensional CFD simulations. Our findings highlight the significant variability in turbidity current characteristics, particularly flow velocity and sediment concentration, within the canyon groups. This variability is primarily controlled by canyon head depth, the width-to-relief ratio, and slope gradients. Notably, the simulations revealed unique flow structures not typically observed in experimental settings, including unidirectional flows, small-scale helical flows, stacked and mixed flow cells, and flow separation and convergence across inter-canyon ridges. Our CFD simulations also revealed a distinct near-wall pattern of linear deposition of coarse-grained sediments, which is similar to the observed bathymetric changes between 2009 and 2017. Based on the consistent results, we propose a conceptual model wherein differential erosion, driven by oceanographic processes, plays a key role in shaping the observed linear depositional patterns.

Computational study of energy separation in convergent-divergent vortex tube for different throat diameters, throat locations, inlet pressures, and working gas

This paper aims to improve the energy separation performance of vortex tube with a converging–diverging design of the hot tube. Three-dimensional computational fluid dynamic simulations are conducted to explore energy separation in a straight vortex tube (SVT) and in convergent-divergent vortex tubes (CDVT) with variations in throat diameter, throat position, inlet pressure, and working gas. Detail analysis of flow and temperature distributions reveal that flow is accelerated in the converging part of CDVT, which increases the transfer of shear work from the axial region to the peripheral region of CDVT compared to SVT, thereby producing significantly higher cold temperature separation in CDVT compared to SVT. CDVT with non-dimensional throat diameter of 0.40 is observed to produce the maximum value of cold temperature separation, which is up to 9 K (∼35 %) higher than the same obtained with SVT. Coefficient of performance of cooling obtained with this CDVT is found to be up to ∼ 30 % higher compared to SVT. When CDVT is operated with helium, ΔTc is found to have mean increment of 20 K (56 %) compared to SVT being operated with air. Overall, CDVT having non-dimensional throat diameter of 0.40, throat position in the middle of the hot tube, and operating with helium is recommended for obtaining maximum temperature separation performance in a wide range of cold mass fraction.

Extraction and characteristic analysis of the nonlinear acoustic impedance of circular orifice in the presence of bias flow

The acoustic behavior of the circular orifice is influenced by the bias flow and high amplitude sound excitation. The present study proposes an approach based on the three-dimensional (3D) time-domain computational fluid dynamics (CFD) simulation to extract the nonlinear acoustic impedance of circular orifice. Impact coefficients of the bias flow on the acoustic impedance of circular orifice are defined as the ratio of the acoustic impedance difference between the cases in the presence and absence of bias flow under high amplitude sound excitation to that under low amplitude sound excitation. The effects of bias flow Mach number, orifice diameter, plate thickness, and porosity on the impact coefficients under different amplitude sound excitations are studied, and the impact coefficients could collapse all the predictions by using the non-dimensional sound particle velocity. The fitting formulas of nonlinear acoustic impedance of circular orifice are presented by using nonlinear regression analysis.

Evolution law of flow characteristics for straight pipeline after leaking

Aiming at the leakage problem of the fluid loop system for the thermal control system of spacecraft caused by the impact of micro-meteoroid and orbital debris, we investigate the dependence of the leak rate, pressure drop and flow characteristics before and after leaking on leak position, inlet pressure, and leaking aperture, calculated in a straight pipeline with single leakage, using stationary and transient three-dimensional computational fluid dynamics simulations with commercial software (FLUENT). The working fluid adopts single-phase water without gravity. It is found that the pressure increases obviously near the leak point, which is caused by the local peak of pressure. As the length between the leak point and the inlet becomes larger, the local peak of pressure and the dimensionless leak rate are smaller. When the inlet pressure increases, the leaked mass flow rate increases; however, the ratio of the leaked flow rate to inlet flow rate decreases. Furthermore, it is observed that the dependence of dimensionless leaking rate on dimensionless leaking aperture has a scaling relation with the scaling exponent approximating to 2, which may be related to the proportion of the vortex formed by the backward flow at the leak hole, the amplitude, and the affected length along the pipe of the local peak of pressure. This study is instructive and meaningful to the leak detection and plugging of fluid loop in space.

Three-dimensional computational fluid-dynamic simulation of polypropylene steam gasification

Among the recycling techniques, gasification allows to obtain a hydrogen-rich gas from polypropylene (PP), a widely used thermoplastic material. In this work, devolatilization and steam gasification tests of PP were conducted in fluidized bed reactors. The results show a gas yield of 1.3 and 3.0 Nm3/kgPP, and a tar content of 117.3 and 20.9 g/Nm3 from devolatilization and gasification, respectively. Experimental results provide an overall overview of this process, able to produce a syngas with an H2-content of 50 vol%dry.Experimental data coupled with kinetic analysis were used to develop a computational fluid dynamic model (CFD) capable of describing both thermal degradation and subsequent gasification reactions. Although some discrepancies on water conversion and H2/CO ratio probably related to the influence of steam in radical chain-scission the model allows to describe gas production and hydrocarbons evolution providing important support for modeling and sizing of fluidized bed reactors on an industrial scale.

Investigating Slurry Flow Patterns in Chemical Mechanical Polishing (CMP) Using Computational Fluid Dynamic (CFD) Simulations

Chemical Mechanical Polishing has emerged as a leading area approach for planarizing wafer surfaces during semiconductor manufacturing due to its ability to provide defect-free and uniform surfaces. Achieving nanoscale planarity on a wafer and die is one of the most significant steps in the process [1]. As part of the CMP process, it is essential to understand the interactions between consumables to facilitate the production of smooth and mirror-like surfaces [2], [3]. Simulation and numerical modeling have become critical means for elucidating intricate flow patterns to probe the underlying fundamentals of CMP processes. The computational fluid dynamics (CFD) simulations provide a detailed understanding of fluid flow behavior for various conditions [4], [5]. This study uses a three-dimensional CFD simulation to provide an understanding of the slurry flow pattern and its effects on CMP performance. Several CFD simulations were performed to study the effect of different parameters on slurry flow patterns. The simulations provided information on the interaction between the slurry, the wafer surface, and the polishing pad, in addition to the abrasive distribution, slurry velocity, and pressure variations within the CMP tool. It was observed that changing the pad and wafer rotation speeds, the wafer-pad gap size and the center-to-center distance affect the slurry flow conditions. For the same rotational speeds, as the center-to-center distance decreases, the flow velocity increases toward the wafer edge. Moreover, with the same rotation speed, the pad rotation dominates the wafer rotation speed.[1] B. Suryadevara, Advances in Chemical Mechanical Planarization (CMP). Woodhead Publishing, 2021.[2] J. Seo, Journal of Materials Research, 36, 235–257, 2021.[3] A. R. Mazaheri and G. Ahmadi, J Electrochem Soc, 150, G233, 2003.[4] J. Tu, K. Inthavong, and G. Ahmadi, Springer Science & Business Media, 2012.[5] G. Ahmadi, “Chapter 2 -, R. Kohli and K. L. Mittal, Eds., Oxford: William Andrew Publishing, 2012, pp. 81–105.

Capture of the Morphology Transfer Process in a High-Momentum Centrally Aerated Bubble Column with a Hybrid Multifield Two-Fluid Model

The role of pool scrubbing in attenuating radioactivity release after severe accidents has been explored extensively. It is known that scrubbing efficiency is largely determined by the hydrodynamic phenomenology in pools, which is still insufficiently understood. Aerosol gas forms large globules at the nozzle exit, which subsequently break up into a swarm of stable bubbles, where the change of bubble size can reach more than two orders depending on the injection rate. Furthermore, with the increase of flow rates, the injection regime changes from globule to jet characterized by a continuous gas structure. The flow field in the pool can be divided into two zones, injection and rise (swarm), according to the gas-liquid interface morphology. In different zones, scrubbing is governed by different mechanisms such as inertial impact, diffusion, and gravity, whereby bubble rise velocity is one major influential parameter. So far, numerical analysis of pool scrubbing is routinely based on system codes, which rely on empirical correlations for the determination of hydrodynamic parameters as well as scrubbing zones. More recently, owing to the increasing availability of computational resources, knowledge is improved through three-dimensional computational fluid dynamics simulations. Nevertheless, morphology and regime change still present a challenge for both two-fluid and interface-tracking models. Because of the limitation of closures, the conventional two-fluid model (TFM) is generally effective for bubble size smaller than cell size while interface-tracking (capturing) methods demand dozens of cells per bubble size. Combining the two kinds of approaches into one simulation is not straightforward. The present work aims to present a hybrid multifield TFM with extensions for capturing the transfer between different morphologies and for considering the effect of mesh resolution in momentum exchange. The developments are validated by simulating the complex hydrodynamic process in a pool-scrubbing column, which operates in the globule regime with an injection Weber number between 103 and 105. By comparison with experimental data, the results are shown to be promising, which provides a versatile framework for investigation of particle scrubbing in the future. The most attractive feature of the model is that compared to one-fluid interface-tracking models, it reliably predicts the bubble rise velocity. Furthermore, it is able to capture the lateral gas distribution without the need of applying a population balance model since most large bubbles are resolved.

Electro-thermal co-design of a variable pole toroidal induction motor

The United States Department of Energy targets an 88 % reduction in motor and power electronics volume by the year 2025 while minimizing or eliminating magnet usage. Induction machines are a magnet-free alternative with advantages such as low cost, high reliability, and capability of withstanding high short-term overload. To meet these societal goals, we develop an induction machine that uses an electro-thermal co-designed converter-integrated thermal management approach. The machine utilizes toroidal windings in the stator, and an 18-phase power converter to achieve 40 kW/L power density. An optimized jacket design was developed to enable the effective thermal management of the stator and integrated power electronics. The toroidal winding configuration provides advantages for thermal performance over conventional distributed windings due to an increased winding copper surface area without reducing slot-fill factor, thereby allowing for a higher stator density. The improved thermal performance of the toroidal winding design was verified using three-dimensional conjugate computational fluid dynamic simulations. A reduced-order transient model was then developed to generate torque-speed-temperature plots at various load conditions. The transient thermal analysis was extended to verify safe operation of the motor during a typical drive cycle for the desired application. The thermal design models developed in this work are robust and scalable to other motor sizes providing a framework for co-designing and developing thermal management systems for electric motors.

How to Achieve Flexible Green Ammonia Production: Insights via Three-Dimensional Computational Fluid Dynamics Simulation

How to Achieve Flexible Green Ammonia Production: Insights via Three-Dimensional Computational Fluid Dynamics Simulation

Prediction of three-dimensional flow field inside realistic fibrous filter obtained from x-ray computed tomography images using deep convolutional neural networks

Deep-learning models garnered considerable attention in the field of fluid mechanics for physics discovery and approximation-model generation. This study aims to develop an approximation model to predict the flow field inside realistic fibrous filters based on an image-to-image approach to replace three-dimensional (3D) computational fluid dynamics (CFD) simulations, which are computationally expensive and difficult to apply to realistic fibrous filters. A data-driven framework is proposed using deep convolutional neural networks (CNNs) to provide a per-pixel prediction of the flow field. The model inputs are two-dimensional x-ray computed tomography images, whereas the outputs are the 3D distributions of the velocity vectors and pressure. High-resolution 3D CFD simulations are performed to create a database to train and test the CNN model. The model is applied to surgical and N95 face masks. The relative error of the CNN model over the test dataset is approximately 10% in regions with high velocity and pressure, and the model can provide a detailed high-resolution prediction of the flow field with a speedup of about three orders of magnitudes. A strict generalization test is conducted for completely unseen 3D segments with complex microstructures. The model generalizability still needs more improvements; however, the model can provide a low-resolution 3D flow field for those segments that can be used as the initial condition for CFD simulation to reduce the CFD computational time. This framework can be utilized for other types of filters and provides a basis for the design and optimization of fibrous filters.

Experimental and numerical modeling of photovoltaic modules temperature under varying ambient conditions

In this work, comprehensive three-dimensional computational fluid dynamics simulation, of fluid flow and heat transfer phenomena around a free-standing polycrystalline silicon photovoltaic module is carried out. The objective is to provide accurate calculation of module’s temperature as a key parameter to estimate its power output. Therefore, experiments were conducted at the university of El Oued, south-east Algeria, to collect the necessary dataset for simulations. Considering different heat transfer mechanisms, modeling absorbed solar energy within the cells, and after mesh refinement study and model validation, simulations were performed and different parameters have been investigated. Results show that more accurate module temperature (Tback) estimation can be achieved based on numerical simulations. It was also found that numerical simulation overcome other models from literature and provides better results, achieving an R2 of 0.995 and a mean absolute error (MAE) of 0.822. Results also indicate that, solar radiation (G), ambient temperature (Ta) and wind speed (Ws) tend to have the major impact on Tback, an increase of 100W/m2 in G can produce an increase of 4°C in Tback at low wind speeds, and about 2.4°C for relatively higher Ws. Ta also tends to yield linear increase in Tback, expecting 5.8°C rise, for 6°C increase in Ta at 700W/m2 and 1m/s of solar radiation and wind speed, respectively. Additionally, a regression-based model was proposed for engineering applications, providing accurate results with an R2 of 0.989, a MAE of 1.009, which is 10% more accurate than the best model from literature.

Temperature distribution analysis and mechanical property prediction of wire loops in the Stelmor air-cooling system

During the Stelmor air-cooling process, the temperature distribution has a significant impact on the wire loops' final mechanical properties. The temperature distribution during air-cooling process is accurately solved by establishing three-dimensional model and Computational Fluid Dynamics (CFD) simulation. It is also found that the heat transfer coefficient on the cross section of the wire loop is very different, with a difference of 70–100 Wm−2K−1. The average value of the surface heat transfer coefficient of the wire loops section is used to replace the surface heat transfer coefficient of the wire loops, which improves the temperature distribution calculation accuracy, which is not considered by previous scholars. The finite difference method is used to calculate the mathematical model of the temperature distribution during the Stelmor air-cooling process. It is found that the maximum temperature difference of the wire during the cooling process is 90–110 K. The prediction model of mechanical property of wire after air-cooling was established, and distribution of mechanical property of the wire loops was obtained, and the reason for the uneven distribution of mechanical property of the wire loops was found.

Full-scale numerical simulations of standing-wave thermoacoustic engines with circular-pore and pin-array stacks

Thermoacoustic engines (TAEs) can convert thermal energy into acoustic energy with no moving parts. Previous numerical studies normally focused on the TAEs with a parallel-plate stack due to their simple structures and used two-dimensional (2-D) computational fluid dynamics (CFD) models to save computational costs. In this study, we conduct full-scale three-dimensional (3-D) CFD simulations on the standing-wave TAEs with more complicated circular-pore and pin-array stacks. Firstly, the dynamic behavior of the standing-wave TAEs in the start-up process is investigated. It is found that the optimal ratios of hydraulic radius rh to thermal penetration depth δk for the TAEs with circular-pore and pin-array stacks are 2 and 3.2, respectively. Secondly, the acoustic, hydrodynamic, and thermodynamic characteristics of the standing-wave TAEs in the steady-state process are explored. We find that when operating at optimal rh/δk, the TAE with a pin-array stack generates much larger acoustic power than that with a circular-pore stack. Examination of the vortex shedding at the stack ends indicates that the pin arrays exhibit less flow resistance than circular pores. At optimal rh/δk, the time-averaged transversal heat flux at the pin array ends is calculated to be around 1.2 × 105 W/m2, which is larger than 7.5 × 104 W/m2 of circular pores, corresponding to a better thermodynamic performance. The 3-D numerical investigations in this work give deeper insights into the performances of TAEs with circular-pore and pin-array stacks which were investigated in experiments, providing useful guidance for the future design and development of more sophisticated thermoacoustic devices for low-grade heat recovery.

CFD simulation and experimental study of CO2 absorption in a rotating packed bed

The rotating packed bed (RPB), a process intensification equipment, has been widely studied and applied to chemical engineering processes such as gas absorption, nanomaterial preparation, and polymer devolatilization. In this study, we presented a typical RPB model to study the intensification mechanism of high gravity on gas-liquid mass transfer. Based on this, three-dimensional computational fluid dynamics (CFD) simulations and experiments of CO2 physical absorption were conducted to investigate the gas-liquid mass transfer characteristics in the RPB. The predicted and experimental values of CO2 saturation rates of the liquid phase showed good agreement, and the errors were within ± 15 % under various operating conditions. Moreover, detailed flow field and mass transfer information that are difficult to measure experimentally, such as the distribution of CO2 mass fraction in the liquid, liquid holdup, turbulent kinetic energy dissipation rate, and gas-liquid interfacial area, were analyzed by CFD simulation results. These results provide a solid foundation for the further development and application of RPB.

Numerical Simulation of the Interaction between a Planar Shock Wave and a Cylindrical Bubble

Three-dimensional (3D) computational fluid dynamics (CFD) simulations have been carried out to investigate the complex interaction of a planar shock wave (Ma = 1.22) with a cylindrical bubble. The unsteady Reynolds-averaged Navier–Stokes (URANS) approach with a level set coupled with volume of fluid (LSVOF) method has been applied in the present study. The predicted velocities of refracted wave, transmitted wave, upstream interface, downstream interface, jet, and vortex filaments are in very good agreement with the experimental data. The predicted non-dimensional bubble and vortex velocities also have great concordance with the experimental data compared with a simple model of shock-induced Rayleigh–Taylor instability (i.e., Richtmyer–Meshkov instability) and other theoretical models. The simulated changes in the bubble shape and size (length and width) against time agree very well with the experimental results. Comprehensive flow analysis has shown the shock–bubble interaction (SBI) process clearly from the onset of bubble compression up to the formation of vortex filaments, especially elucidating the mechanism on the air–jet formation and its development. It is demonstrated for the first time that turbulence is generated at the early phase of the shock cylindrical bubble interaction process, with the maximum turbulence intensity reaching about 20% around the vortex filament regions at the later phase of the interaction process.

Enhancing Axial Flow Fan Performance in Air-Cooled Condensers: Tip Vortex Manipulators and Comparative Analysis of Numerical Simulation and Experimental Testing

Abstract Direct dry-cooled power plants typically operate numerous large-diameter axial flow fans in air-cooled condensers (ACCs) to facilitate the condensation of steam in the plant's thermodynamic cycle. Enhanced fan efficiency may, at scale, lead to significant parasitic power reductions and subsequent increases in plant power output. The performance of these fans may be increased by adding blade-tip modifications, which act to control blade-tip leakage flow. Previous studies have shown that manipulating the tip leakage vortex (TLV) increases the blade-to-air momentum transfer and stabilizes the air flow structures as it traverses the fan blade. This paper takes a step toward the development of improved tip vortex manipulators for an ACC fan. Three-dimensional computational fluid dynamics (CFD) simulations, and experimental testing of the modified and unmodified fan within an ISO test facility are performed. The results are then utilized to assess the accuracy of the simulation model, visualize the manipulated flow structures at the blade tip, and ultimately quantify the effect of the endplate designs on fan performance. Excellent correlation between numerical and test results for the unmodified fan is observed, and the benefit of blade-tip modification on fan performance is again confirmed experimentally. The simulation model, however, fails to predict improvements in fan performance under modification, and simulated tip leakage flows are investigated for clarification. It is hypothesized that deflection of tip endplates under operation account for differences between simulation and experiment, and that fluid–structure interaction analyses could potentially resolve discrepancies.

Unveil carbon dioxide recycling potential throughout distributor-type membrane reactor

With the cost of green hydrogen production decreasing year by year, methanation, a carbon dioxide hydrogenation reaction, is becoming a more critical technology for the realization of a carbon-neutral society. Here, we show the procedure of membrane preparation for a distributor-type membrane reactor that simultaneously captures and recycles CO2 together with experimental tests of the proposed reactor design. To decrease the temperature gradient in the reactor and to improve methane recovery from the inlet gas mixture, a three-dimensional computational fluid dynamics simulation of the reactor was conducted. The numerical results suggest that the distributed feed design results in a uniform distribution of carbon dioxide, preventing localized reactions at the reactor inlet. In addition, it reduces the temperature rise in the reactor by up to about 300 K compared to the case where the carbon dioxide and hydrogen gas mixture is fed from a single inlet. The effect of the feed CO2 concentration on reactivity is also discussed. Simulation results show that the membrane reactor with inlet concentration of about 15 %, such as those emitted from industrial boilers, is capable of recovering methane at concentrations about 1.5 times higher in comparison to 100 % CO2 concentration at the inlet in a classical reactor.

The Impact of High-Rise Building Shapes on Wind Flow Characteristics and Energy Potential

In recent years, wind energy has become a potential source of low carbon energy. The shape of a building is a significant factor in aerodynamics, providing an opportunity for wind power control and wind energy proliferation. This research project aims to study the design of high-rise buildings and investigate how wind affects energy absorption by developing an aerodynamic optimization procedure (AOP) and using Computational Fluid Dynamics (CFD) in COMSOL Multiphysics. This study aimed to optimize the building shape for wind energy exploitation. Optimizing the building shape in the early stages of design enables the control of wind-induced loads and responses and reduces the energy demand in high-rise buildings, where resource consumption is higher than that in low-height buildings. This study used a three-dimensional CFD simulation of wind loading on tall buildings to optimize the building shape. This research will provide valuable insights for architects, engineers, and building developers to design and optimize high-rise buildings for wind energy exploitation, reduce the carbon footprint, and improve the energy efficiency of buildings.

Nonlinear Acoustic Behavior Prediction and Analysis of Straight Through Perforated Tube Silencer With Flow

Straight through perforated tube silencers are widely used in intake and exhaust noise control, and their acoustic behaviors are influenced by the gas flow and high amplitude sound remarkably. To investigate the acoustic attenuation performance of straight through perforated tube silencers in the presence of flow and high amplitude sound, an approach based on the three-dimensional (3D) time-domain computational fluid dynamics (CFD) simulation is employed and validated by comparing predictions and measurements. Transmission loss of the straight through perforated tube silencer under pure tone and multi-tone sound excitations is predicted. Results show that, under the pure tone sound excitations, the acoustic attenuations are affected remarkably by the high amplitude sound, and the nonlinear effects turn to weaken as flow velocity increases. With the sound pressure level (SPL) decreasing along the axis of the silencer, the acoustic nonlinearity in the orifice weakens gradually. The shedding vortices are restricted to the region near perforations because of the transport of flow. When the multi-tone sound excitation is enforced on the inlet, transmission loss of the silencer is different from the pure tone excitation at the same SPL of every compositional frequency.