Time-domain Computational Fluid Dynamics Research Articles

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.

Acoustics performance of compact micro-perforated panel absorber with grazing flow

This paper presents numerical and experimental investigations into the acoustic behaviour of a micro-perforated panel (MPP) backed by a shallow cavity under a fully developed grazing flow. A three-dimensional time-domain computational fluid dynamics approach is applied alongside hybrid evaluation methods to accurately capture the acoustic behaviour variations of an MPP orifice. Meanwhile, a hybrid Rayleigh's end correction model for T-shaped resonators is utilized to calculate the acoustic reactance of a very shallow cavity and an acoustics impedance model of an orifice backing with shallow cavity at grazing flow is established. The proposed acoustic impedance model of orifice has been validated by experimental study. The practical significance of this study is that it elucidates the influence of cavities upon the acoustic behaviour of MPPs in the presence of grazing flows; furthermore, it proposes an impedance model that is more suitable for predicting the acoustic performance of booming compact meta-structures.

Extraction and characteristic analysis of the nonlinear acoustic impedance of circular orifices

For investigating the nonlinear acoustic behavior of circular orifices under high amplitude sound excitation, an approach based on the three-dimensional time-domain computational fluid dynamics simulation is proposed to extract the nonlinear acoustic impedance of circular orifices. By solving laminar flow equations, the propagation of the acoustic signal in the vicinity of orifice is simulated, and the effect of the interaction among adjacent orifices is considered by using the lateral periodic boundary condition. The effects of diameter, thickness, and porosity on acoustic impedance under different amplitude sound excitations are studied. By using nonlinear regression analysis of the dimensionless parameters composed of particle velocity amplitude, frequency, and geometric parameters, fitting formulas of nonlinear acoustic impedance of circular orifices are presented. As an application of engineering computation, transmission loss of perforated silencers under low and high amplitude sound excitations is predicted by using the present acoustic impedance fitting formulas and via the finite element computation. By comparing the predicted and measured results, the accuracy and practicability of fitting formulas of nonlinear acoustic impedance of circular orifices are validated.

Acoustic behaviour of micro-perforated panel backed by shallow cavity under fully developed grazing flow

This paper presents numerical and experimental investigations into the acoustic behaviour of a micro-perforated panel (MPP) backed by a shallow cavity under a fully developed grazing flow. A three-dimensional time-domain computational fluid dynamics approach is applied alongside hybrid evaluation methods to accurately capture the acoustic behaviour variations of an MPP orifice, considering the interaction of flow dynamics and the acoustics in the orifice and cavity. Meanwhile, a hybrid Rayleigh's end correction model for T-shaped resonators is utilized to calculate the acoustic reactance of a very shallow cavity. The numerical results show that the vertical shear layer is observed alongside the hole and cavity and impinges the bottom of shallow cavity. Subsequently it alters acoustic and flow interaction significantly that leads to variation of acoustic behaviour of orifice. In this regard, an acoustic impedance model of an orifice backing with shallow cavity at grazing flow is established. The proposed acoustic impedance model of orifice has been validated by experimental study. The transmission loss predictions for a compact MPP absorber with single or double cavities agree well with the experimental results for the grazing flow condition. The practical significance of this study is that it elucidates the influence of cavities upon the acoustic behaviour of MPPs in the presence of grazing flow; furthermore, it proposes an impedance model that is more suitable for predicting the acoustic performance of booming compact meta-structures.

Numerical Modeling and Geometry Enhancement of a Reactive Silencer

Internal combustion engines and blowers frequently utilize silencers to reduce exhaust noise. In the current paper, the transmission loss of reactive silencers is predicted using the plane wave decomposition method and a three-dimensional (3-D) time-domain computational fluid dynamics (CFD) approach. A mass-flow-inlet boundary condition is first used to perform a steady flow computation, which serves as an initial condition for the two subsequent unsteady flow computations. At the model's inlet, an impulse (acoustic excitation) is placed over the constant mass flow to perform the first unstable flow computation. Once the impulse has fully propagated into the silencer, the non-reflecting boundary condition (NRBC) is then added. For the scenario without acoustic excitation at the inlet, a second unsteady flow computation is performed. During the two transient computations, the time histories of the pressure and velocity at the upstream measuring points as well as the history of the pressures at the downstream measuring point are recorded. The related acoustic quantities show variations between the two unsteady flow computational findings. As a result, the transmitted sound pressure signal is just the sound pressure downstream, while the incident sound pressure signal is obtained by utilizing plane wave decomposition upstream. The transmission loss (TL) of the silencer is then calculated after the Fast Fourier Transform (FFT) converts the two sound pressure signals from the time domain to the frequency domain. The numerical calculations and the reported data are in good agreement for the published results, in addition to geometry enhancement by increasing number of holes in the cross section for muffler.

Application of Machine Learning to Forced Response Predictions of an Industrial Axial Compressor Rotor Blade

Abstract Machine learning has gone way beyond a ground-breaking technology a few decades ago to now taken for granted in many day-to-day activities. It is now providing new ways for manufacturing, assembling, operating, monitoring, and maintaining products. Typical application areas include performance optimization, quality improvements, fault detection and predictive maintenance. In this paper application of machine learning algorithms to forced response prediction during the design and analysis of a typical gas turbine compressor blade is reported. The forced response prediction process typically involves utilizing harmonic or time domain computational fluid dynamics (CFD) analyses to compute the forcing and the aero damping, to calculate reserve factors that represent the high cycle fatigue life of the blade. This time-consuming process is generally limited to the later phases of the design cycle and can lead to hundreds of calculations if one must address all the resonances in a typical twin shaft running range. A neural network trained using historical data is used to directly predict the reserve factor with high confidence without the need for costlier high fidelity CFD by using just the finite elements (FE) predicted parameters. This allows to perform high-fidelity aero-mechanical assessment at an early stage in the design process. Further, application of image recognition using a convoluted neural network to aid in the identification of FE predicted Modeshapes is also demonstrated, which can also improve the quality of the reserve factor predictions.

Aeroelastic Flutter Prediction Using Multifidelity Modeling of the Generalized Aerodynamic Influence Coefficients

This work proposes a multifidelity modeling approach for predicting aeroelastic flutter of airfoils and wings. Using aerodynamic models based on the doublet-lattice method and time-accurate Euler equations, cokriging-based surrogates of the generalized aerodynamic influence coefficients are generated as functions of Mach number and reduced frequency. The surrogate-based matrix terms are then used in the method to determine flow conditions at flutter onset. To demonstrate the multifidelity process, a widely used pitching and plunging airfoil case is considered. Verification of the approach is done by comparing with results from a mode-based time-domain aeroelastic solver, as well as data from the literature. The approach draws inspiration from Timme et al., but focuses more on widely used industry tools (namely, the method, panel-based aerodynamics, and time-domain computational fluid dynamics). The benefit of using multiple aerodynamic fidelities, rather than high-fidelity kriging models, is also quantified by examining a flutter speed error metric as the number of high-fidelity samples is varied.

Acoustic impedance of perforated plates in the presence of fully developed grazing flow

The perforated components are widely used in the intake and exhaust mufflers. The coupling between instable shear layer and acoustic fluctuation has a significant influence on the acoustic impedance of perforated components. To obtain practical and precise acoustic impedance of perforated plates in the presence of grazing flow, the three-dimensional (3D) time-domain computational fluid dynamics (CFD) approach was employed in the present study, and the extraction method of acoustic impedance was presented. The predicted non-dimensional acoustic impedance of the rectangular slot is consistent with the previous published measurement, which validates the accuracy of the present approach. Characterization of the turbulent boundary layer of flat-plate flow was studied and the relation between the convection velocity of vorticity and mean grazing flow velocity was provided. Numerical evaluations were carried out for different mean grazing flow Mach number Mmean (0.05–0.20), thickness to diameter ratio of orifice t/dh (0.2–0.5), porosity ϕ (4.51%–24.93%), and Reynolds number Re resulted from the orifice diameter dh (2 mm–6 mm), and their effects on the non-dimensional acoustic impedance of circular orifice were investigated in detail. The acoustic impedance formulae are obtained using the piecewise numerical fitting method based on the parameter analyses. As an application of engineering computation, the transmission loss of straight-perforated tube mufflers was predicted by using the present acoustic impedance model and the empirical impedance model of Lee and Ih via finite element computations. Comparison of the predicted and measured transmission loss (TL) results demonstrates that the present model yields highly accurate prediction.

Acoustic impedance of perforated plates in the presence of bias flow

The acoustic impedance of perforated plates is influenced by fluid flow through the orifice. The present study employs the three-dimensional (3-D) time-domain computational fluid dynamics (CFD) approach to determine the acoustic impedance of perforated plates with bias flow, and the extraction method of acoustic impedance of perforated plates is presented. The non-dimensional acoustic impedance of orifices is predicted, and the good agreement between the numerical predictions and the measurements in published literature validates the present CFD approach. The numerical evaluations are carried out for different bias flow Mach number Mo (0.05–0.20) through the orifice, distribution form of the orifices, the ratio t/dh (0.2–0.5) of orifice thickness to diameter, Reynolds number Re of flow through the orifice and porosity φ (4.51%–24.93%) of the perforated plates. The effects of related parameters on the non-dimensional acoustic impedance of orifices are analyzed in detail. The expressions of acoustic impedance are obtained by using nonlinear regression analysis of the various results of the parametric studies. As an application of engineering calculation, the transmission loss (TL) of cross-flow perforated tube mufflers with flow is predicted by using Jing and Sun model and the present model via finite element computations. The comparison between measured and predicted values showed the accuracy of the present model.

Time-domain CFD computation and analysis of acoustic attenuation performance of water-filled silencers

The multi-dimensional time-domain computational fluid dynamics (CFD) approach is extended to calculate the acoustic attenuation performance of water-filled piping silencers. Transmission loss predictions from the time-domain CFD approach and the frequency-domain finite element method (FEM) agree well with each other for the dual expansion chamber silencer, straight-through and cross-flow perforated tube silencers without flow. Then, the time-domain CFD approach is used to investigate the effect of flow on the acoustic attenuation characteristics of perforated tube silencers. The numerical predictions demonstrate that the mean flow increases the transmission loss, especially at higher frequencies, and shifts the transmission loss curve to lower frequencies.

Transmission loss prediction of reactive silencers using 3-D time-domain CFD approach and plane wave decomposition technique

Abstract A predictive method is proposed to determine the transmission loss of reactive silencers using the three-dimensional (3-D) time-domain computational fluid dynamics (CFD) approach and the plane wave decomposition technique. Firstly, a steady flow computation is performed with a mass-flow-inlet boundary condition, which provides an initial condition for the following two unsteady flow computations. The first unsteady flow computation is conducted by imposing an impulse (acoustic excitation) superimposed on the constant mass flow at the inlet of the model and then adding the non-reflecting boundary condition (NRBC) when the impulse completely propagates into the silencer. The second unsteady flow computation is conducted for the case without acoustic excitation at the inlet. The time histories of pressure and velocity at the upstream monitoring point as well as history of pressure at the downstream monitoring point are recorded during the two transient computations. The differences between the two unsteady flow computational results are the corresponding acoustic quantities. Therefore, the incident sound pressure signal is obtained by using plane wave decomposition at upstream, while the transmitted sound pressure signal is just the sound pressure at downstream. Finally, those two sound pressure signals in the time-domain are transformed into the frequency-domain by Fast Fourier Transform (FFT) and then the transmission loss (TL) of silencer is determined. For the straight-through perforated tube silencers with and without flow, the numerical results agree well with the published measurements.

Sound attenuation analysis of water-filled perforated pipe silencers using three-dimensional time-domain computational fluid dynamics approach

The three-dimensional time-domain computational fluid dynamics approach is employed to calculate and analyze the sound attenuation behavior of water-filled perforated pipe silencers. Transmission loss predictions from the time-domain computational fluid dynamics approach and the frequency-domain finite element method agree well with each other for the straight-through and cross-flow perforated pipe silencers without flow. Then, the time-domain computational fluid dynamics approach is used to investigate the effects of flow velocity, diameter, and porosity of orifices on the sound attenuation behavior of the silencers. The numerical predictions demonstrate that the flow increases the transmission loss, especially at high frequencies. Based on the above analysis, partially plugged straight-through perforated pipe silencer is proposed to improve the sound attenuation performance by increasing the flow velocity through the orifices. In order to eliminate the pass frequency of the perforated pipe silencers and improve the sound attenuation performance in mid- to high-frequency range, a folded straight-through perforated pipe silencer is designed and its sound attenuation behavior is analyzed numerically using the time-domain computational fluid dynamics approach.

Influence of a tapered cylinder nozzle on pressure pulsations in a variable high-speed reciprocating compressor piping system

This paper investigates the influence of a tapered cylinder nozzle (TCN) on pressure pulsations in a high-speed reciprocating compressor piping system. Numerical methods, the 3D frequency-domain finite element method (FEM) and the 3D time-domain computational fluid dynamics (CFD) were used to separately calculate gas column natural frequencies and pressure pulsations. With favorable agreement between model predictions and experimental data, it is concluded from predictions that an installed TCN changes both the values and distribution of frequencies because installation of a TCN filter can alter gas column structure in the piping system. The order of frequency dominated by cylinder nozzle response increases sharply as the TCN diameter ratio increases and the TCN length to diameter ratio decreases. Pressure fluctuations can be effectively attenuated in piping system under different functioning conditions, e.g. variable compressor speeds and variable discharge pressures.

Investigation of Wave-Structure Interaction Using State of the Art CFD Techniques

The suitability of computational fluid dynamics (CFD) for marine renewable energy research and development and in particular for simulating extreme wave interaction with a wave energy converter (WEC) is considered. Fully nonlinear time domain CFD is often considered to be an expensive and computationally intensive option for marine hydrodynamics and frequency-based methods are traditionally preferred by the industry. However, CFD models capture more of the physics of wave-structure interaction, and whereas traditional frequency domain approaches are restricted to linear motions, fully nonlinear CFD can simulate wave breaking and overtopping. Furthermore, with continuing advances in computing power and speed and the development of new algorithms for CFD, it is becoming a more popular option for design applications in the marine environment. In this work, different CFD approaches of increasing novelty are assessed: two commercial CFD packages incorporating recent advances in high resolution free surface flow simulation; a finite volume based Euler equation model with a shock capturing technique for the free surface; and meshless Smoothed Particle Hydrodynamics (SPH) method. These different approaches to fully nonlinear time domain simulation of free surface flow and wave structure interaction are applied to test cases of increasing complexity and the results compared with experimental data. Results are presented for regular wave interaction with a fixed horizontal cylinder, wave generation by a cone in driven vertical motion at the free surface and extreme wave interaction with a bobbing float (The Manchester Bobber WEC). The numerical results generally show good agreement with the physical experiments and simulate the wave-structure interaction and wave loading satisfactorily. The grid-based methods are shown to be generally less able than the meshless SPH to capture jet formation at the face of the cone, the resolution of the jet being grid dependent.

Computational Fluid Dynamics-Based Numerical Analysis of Acoustic Attenuation and Flow Resistance Characteristics of Perforated Tube Silencers

The 3D time-domain computational fluid dynamics (CFD) approach is used to calculate the acoustic attenuation performance of perforated tube silencers without and with flow. For the crossflow perforated tube silencer and straight-through perforated tube silencers, the transmission loss predictions agree well with the experimental measurements available in the literature. Then, the 3D time-domain CFD approach is employed to investigate the effects of flow velocity and temperature on the acoustic attenuation performance of perforated tube silencers. The numerical results demonstrated that the transmission loss is increased at most frequencies for the crossflow perforated tube silencer as the air flow increases, while the air flow has little influence on the acoustic attenuation in the plane wave range and increases the acoustic attenuation at higher frequencies for the straight-through perforated tube silencers. Increasing the air temperature shifts the transmission loss curve to higher frequency and lowers the resonance peaks somewhat. The pressure drops of perforated tube silencers are predicted by performing the 3D steady flow computation using CFD. The pressure drop of the crossflow perforated tube silencer is much higher than those of the straight-through perforated tube silencer at the same flow conditions, and the pressure drop of the straight-through perforated tube silencer increases gradually as the porosity increases.

Determination of sound reflection coefficient of circular duct using time-domain computational fluid dynamics method

In this paper, the software FLUENT is used as simulation tool, and the two-dimensional time-domain Computational Fluid Dynamics (CFD) approach is employed to compute the sound reflection coefficient of circular duct without and with gas flow. In the absence of mean flow, the pressure far-field boundary condition could be used as non-reflecting boundary condition in Fluent, and good agreement between the CFD prediction and experiment measurement available in the literature is observed. For the case with gas flow, the general non-reflecting boundary condition is available only with the density-based solver (high-speed compressible flow or Strong coupling flow) in FLUENT, and it is difficult to acquire the convergent solution for the calculation that the density-based solver is used to compute the reflection coefficient of circular pipe. Therefore, the non-reflecting boundary condition is not applied in the model. The computational results from time-domain CFD approach basically agree with experimental resul...

Improved transonic flutter analysis in the frequency domain using computational fluid dynamics

<Abstract>The flutter boundary of the AGARD 445.6 wing was calculated using time domain and frequency domain methods. A frequency domain correction procedure was developed to account for nonlinear aerodynamics in the transonic regime. Nonlinear lifting pressures obtained from unsteady rigid body pitching motion time domain simulations were used to correct the linearized aerodynamic matrices in the frequency domain. It was found that time domain computational fluid dynamics adequately predicted the flutter boundary of a wing. Although uncorrected frequency domain flutter analysis predicted a nonconservative flutter boundary, the corrected frequency domain results also adequately predicted the flutter boundary of the AGARD 445.6 wing. Additionally, nonlinear aerodynamic effects and their contributions to flutter can be analyzed with the corrected frequency domain technique within a few hours, representing significant time savings compared with aeroelastic time domain techniques.

Gigaflop (billion floating point operations per second) performance for computational electromagnetics

Accurate and rapid evaluation of radar signature for alternative aircraft/store configurations would be of substantial benefit in the evolution of integrated designs that meet radar cross-section (RCS) requirements across the threat spectrum. Finite-volume time domain methods offer the possibility of modeling the whole aircraft, including penetrable regions and stores, at longer wavelengths on today's gigaflop supercomputers and at typical airborne radar wavelengths on the teraflop computers of tomorrow. A structured-grid finite-volume time domain computational fluid dynamics (CFD)-based RCS code has been developed at the Rockwell Science Center, and this code incorporates modeling techniques for general radar absorbing materials and structures. Using this work as a base, the goal of the CFD-based CEM effort is to define, implement and evalute various code development issues suitable for rapid prototype signature prediction.