A Method for Computation of Wave Propagation in Heterogeneous Solids: Implementation and Performance

Computer implementation of the new algorithm developed in [1, 2, 3] and its numerical performance is presented, with detailed discussions of the element-by-element decomposition of the extensional and shear components and step-by-step algorithmic procedures. Numerical results as applied to wave propagating through cracked plane stress problems, three-dimensional problems and elasto-plastic problems illustrate high-fidelity of the present algorithm compared with existing ones, and the new algorithm is implemented into an open source research code TAHOE[4] code along with the further computational performance.

A Method for FE Model Updating of Coupled Vibro-Acoustic Systems Using Constrained Optimization

Finite element (FE) model updating is now recognized as an effective approach to reduce modeling inaccuracies present in an FE model. FE model updating has been researched and studied well for updating FE models of purely structural dynamic systems. However there exists another class of systems known as vibro-acoustics in which acoustic response is generated in a medium due to the vibration of enclosing structure. Such systems are commonly found in aerospace, automotive and other transportation applications. Vibro-acoustic FE modeling is essential for sound acoustic design of these systems. Vibro-acoustic system, in contrast to purely structural system, has not received sufficient attention from FE model updating perspective and hence forms the topic of present paper. In the present paper, a method for finite element model updating of coupled structural acoustic model, constituted as a problem of constrained optimization, is proposed. An objective function quantifying error in the coupled natural frequencies and mode shapes is minimized to estimate the chosen uncertain parameters of the system. The effectiveness of the proposed method is validated through a numerical study on a 3D rectangular cavity attached to a flexible panel. The material property and the stiffness of joints between the panel and rectangular cavity are used as updating parameters. Robustness of the proposed method under presence of noise is investigated. It is seen that the method is not only able to obtain a close match between FE model and corresponding ‘measured’ vibro-acoustic characteristics but is also able to estimate the correction factors to the updating parameters with reasonable accuracy.

Direct Transmission Loss Calculation of Simplified Muffler Configurations Using a Lattice-Boltzmann Method

Lattice-Boltzmann Method (LBM) is broadly used for the simulation of aeroacoustics problems. This time-domain CFD/CAA approach is transient, explicit and compressible and offers an accurate and efficient solution to simultaneously resolve turbulent flows and their corresponding flow-induced noise radiation. Some examples of applications are ground transportation wind-noise problems, buffeting, Heating, Ventilation, and Air Conditioning (HVAC), fan noise, etc. As shown in previous studies, LBM can also be used to accurately handle linear acoustics problems if the source of noise is not a flow but a simple acoustic source. This set of capabilities makes LBM a suitable candidate for evaluating the acoustics performances of exhaust systems and mufflers. Compared to other traditional acoustics methods, LBM presents the advantage to skip tedious volume meshing operations since the mesh generation is fully automatic. Furthermore, considering that all geometrical details are included in the simulation domain and that LBM is explicit, high frequencies mechanisms up to 10–20 kHz can be captured. The upper frequency limit is indeed solely driven by the spatial resolution used to discretize the system. In this paper, three academic 3-D geometries representative of production muffler systems are studied. Transmission Loss (TL) measurements are performed on three configurations and these experiments are reproduced numerically with LBM. The experimental setup is described in a first part and the numerical details are given in a second part and third part. In particular, the method used to calculate the TL in the simulation and the convergence of the results with respect to the spatial resolution are shown. In a third part, the simulations are compared to the TL measurements and a numerical investigation of the effect of geometry details on the simulated results is proposed. This study highlights the sensitivity of acoustics measurements to geometry details.

Acoustic Emission Tests in the Monitoring of Cavitation Erosion in Hydraulic Turbines

This work presents an experimental study about the application of acoustic emission (AE) techniques in the monitoring of cavitation erosion mass loss in small Francis turbines. A vertical Francis turbine test bench is specially devised to perform some experiments designed to evaluate the influence of small surface mass losses on turbine blades in the acoustic emission signals. An AE wideband transducer is employed in the test bench instrumentation system. In order to evaluate the AE levels associated with the turbine erosion stages, a small defect is introduced into the turbine runner. This defect is intended to simulate a small mass loss in the turbine runner. The measurements of the AE signals are performed in the Turbine Francis model at two situations: 1) turbine without defect, which means that the runner blades are free of apparent geometric imperfections; 2) turbine with defect, which is represented by a small hole drilled into a runner blade. The AE transducer is installed on the turbine draft tube and the AE measurements are performed at several operating conditions. The preliminary results obtained for the AE amplitude in this investigation show that the small defect introduced into a runner blade causes variations in the AE levels measured in the experiments, confirming that there is a large potential for the application of AE monitoring techniques in the accurate evaluation of cavitation wear on hydraulic turbines in field.

Kinematics, Dynamics and Vibration Models for 3RPR Parallel Kinematics Manipulator

Parallel link manipulators are the type of mechanisms that have closed kinematics chains. Some of their advantages over open kinematics chains (called also serial kinematics manipulators) are their high stiffness and accuracy. This paper carries out forward and inverse kinematic and dynamic analysis on a certain type of parallel kinematic mechanisms. This is needed to conduct vibration analysis on the same platform. The type of mechanism is planar 3 RPR manipulator. This entails identifying the modes of the manipulator. A simplified vibration theoretical model is derived. This derivation helps in the optimization of parallel kinematics machine design for improved/optimized dynamic performance. The implications of dynamic stiffness modeling should reflect on better noise rejection, less chatter during machining, and increasing the bandwidth of such mechanisms to admit running at higher speeds.

A Simple Method for Measuring Muffler Transmission Loss With PU Probes

The conventional approaches for measuring muffler transmission loss based on measurement in impedance tube are mainly decomposition methods and transfer matrix method. The decomposition method needs an anechoic termination, but it is not easy in some cases particularly for low frequency measurement. Two-load method and two-source method based on transfer matrix techniques are considered to be an alternative approach which does not require an anechoic termination. PU probe can measure both sound pressure and particle velocity, which is applied to some acoustic measurement such as absorption coefficient in recent years. A straightforward method for measuring muffler transmission loss by two PU probes measuring particle velocity at the inlet and outlet of muffler is presented. The four-pole parameters of the muffler can be calculated directly. The transmission loss measured by the PU method agrees well with the result measured by conventional four-pole method and FEM result. To instruct the approach, the influence of measurement distance between PU probe and the inlet or outlet of muffler and ambient noise are analyzed, which gives comprehensive suggestions for measurement set up.

Signal Processing Techniques for Ultrasonic Waves to Measure Stresses in Oil Pipelines

Ensuring the structural integrity of oil pipelines is vital to prevent environmental damage and economic losses. In that sense, it is important to know the magnitude of the stress in the pipe, which must be done using non-destructive techniques. Measuring stress using ultrasonic longitudinal critically refracted waves (LCR) has been applied to pipelines with very promising results. The technique is based on the acoustoelastic theory that relates speed variation of elastic waves traveling in the material with its state of strain. Nevertheless, the signals acquired from piezoelectric transducers in such application may show high levels of noise coming mainly from material sources (grain boundaries, irregularities). The noise makes the measurement of wave velocity difficult, resulting in imprecise evaluations of the stress in the pipeline. The aim of this study is to evaluate techniques for filtering digital signals of LCR waves propagating in an oil pipe fabricated with API 5L X70 steel. We analyzed the signal-to-noise ratio (SNR) of digitalized acquired signals in four circumstances: without treatment; signals treated with successive averages; treated with FIR (Finite Impulse Response) and IIR (Infinite Impulse Response) digital filters, and with the Discrete Wavelet Transform (DWT). The results show that the signals treated with DWT present better SNR compared to the other techniques.

Measured Variation of High Frequency Sound Absorption in Porous Metals Subject to Changes in Temperature and Temperature Gradient

Porous metal, as a new acoustic material, bears the general metal properties, such as good conductivity, ductibility, heat transfer and high specific stiffness and intensity, and meanwhile exhibits good performance in sound absorption. Thus, it enjoys a growing popularity in industrial and civilian sound-absorbing applications where non-metallic materials are impracticable. Therefore it is of great significance to explore the sound-absorption properties of porous metals. The existing studies mainly focus on the low frequency range and are under uniform temperature assumption. In this paper, an experimental setup was built up to investigate the sound absorption of porous metals subject to temperature gradients, and much concern was paid to that of high frequency range. The setup is composed of five modules: I. heating module; II. cooling module; III. temperature controlling & testing module; IV. spectrum analyzer and V. impedance tube testing module. Based on this setup, the sound absorption of a hard-backed porous metal in a high-frequency range (2000–4000Hz)and under different temperature gradients (+2°C/mm, +4°C/mm and +6°C/mm) are measured, and results show that: 1) The sound absorption of porous metal is significantly influenced by temperature gradients; 2) The peak of sound absorption curve moves to a higher frequency range as the temperature gradient increases in the frequency range 2000∼4000Hz but the peak value decreases slightly; 3) The peak value of sound absorption curve enlarges as the temperature gradient increases but the frequency of peak value is fixed.

Investigation of a Vibration Reduction System for Vehicle Seats by a Vibration Model Consisting of a Vehicle, Seat, and Human Body

This paper describes a vibration reduction system that can minimize the vertical vibrations of the human body in a vehicle. This system can control the mechanical properties of the seat cushions, such as the spring constants and damping coefficients. To examine the feasibility of this vibration reduction system, we design a vibration model of both an occupant–seat–steering wheel–pedals–vehicle system and a calculation system. Further, we carry out a numerical analysis to calculate the magnitude of vibrations transmitted from the road surface to the human body based on ISO7096-EM6. Comparison results of the frequency response between the analysis and the experiment indicate the feasibilities of both the vibration model and the analysis method. Furthermore, vibration of the head was reduced 60.1% by controlling the mechanical properties of the seat from 1/5 to 5 times. In summary, the in-vehicle vibration reduction system successfully reduces vibrations from the seat to the human body.

Finite Element Simulation of Ultrasound Propagation in Trabecular Bone

Quantitative ultrasound is used to identify healthy versus osteoporotic bone. However the physics of ultrasound propagation in trabecular media is still not sufficiently understood. This lack of understanding is reported to be an obstacle in further development of this bone assessment technique. Numerical models of wave propagation stand as a potentially successful tool to explain the various experimental observations. The main issue in the numerical modeling of wave propagation in trabecular bone is the complex geometry of the trabecular structures surrounded by a fluid (bone marrow). So far, the complex geometrical domain of trabecular structures has been approximated by finite difference grids for wave propagation analyses. In this work, numerical simulation of ultrasound propagation into trabecular bone sample is performed using the finite element method (FEM). A new procedure for numerical modeling of trabecular bone tailored for the FEM is introduced. The entire complex trabecular geometries of two cubic bone samples are reconstructed using computed microtomography data. For the first time a three dimensional finite element mesh using tetrahedral elements is generated for the two-phase medium of a trabecular bone. Separate meshes for the bony part and the filling marrow (considered as non-viscous water) are generated and acoustic-structure interaction condition is imposed on their interface. It is shown that the three-dimensional simulation using the FEM can predict ultrasound propagation phenomena observed in experiments: linear dependency of attenuation on frequency, the effect of bone volume on the attenuation and speed of sound, and the propagation of fast and slow waves. Moreover, the broadband ultrasound attenuation (BUA) for two ultrasonic signals propagating into a healthy and an osteoporotic sample are compared. A distinguishable difference in BUA between the two samples is observed expressing lower BUA for osteoporotic bone. Our developed model is the first three-dimensional finite element analysis model to compare the ultrasound propagation in healthy versus osteoporotic bone. The developed model can be further utilized as a tool to explain various experimental observations of quantitative ultrasound of bone.