Applying Incompatible Meshes for Modeling Structural-Acoustic Domains in Energy Finite Element Analysis

The Energy Finite Element Analysis (EFEA) has been developed for modeling coupled structural-acoustic systems at mid-to-high frequencies when conventional finite element methods are no longer computationally efficient because they require very fine meshes. In standard Finite Element Analysis (FEA) approach, governing differential equations are formulated in terms of displacements which vary harmonically with space. This requires larger numbers of elements at higher frequencies when wavelengths become smaller. In the EFEA, governing differential equations are formulated in terms of energy density that is spatially averaged over a wavelength and time averaged over a period. The resulting solutions vary exponentially with space which makes them smooth and allows for using much coarser meshes. However, current EFEA formulations require exact matching between the meshes at the boundaries between structural and acoustic domains. This creates practical inconveniences in applying the method as well as limits its use to only fully compatible meshes. In this paper, a new formulation is presented that allows for using incompatible meshes in EFEA modeling, when shapes and/or sizes of elements at structural-acoustic interfaces do not match. In the main EFEA procedure, joints formulations between structural and acoustic domains have been changed in order to deal with non-matching elements. In addition, the new Pre-EFEA procedure which allows for automatic searching and formation of the new types of joints is developed for models with incompatible meshes. The new method is tested using a spherical shaped structural-acoustic interface. Results for incompatible meshes are validated by comparing to solutions obtained using regular compatible meshes. The effects of mesh incompatibility on the accuracy of results are discussed.

Cavity Resonance Biomedical Sensor

We report on first steps towards a phononic crystal sensor for biomedical applications. Phononic crystals and metamaterials allow for unprecedented control of sound propagation. The classical ultrasonic sensors, acoustic microsensors and MEMS resonator sensors face severe limitations when applying them to small volume liquid analytes. Phononic crystal sensors are a new concept following the route of photonic crystal sensors. Basically, the material of interest, here a liquid analyte confined in a cavity of a phononic crystal having a solid matrix constitutes one component of the phononic crystal. In an application as chemical sensor the value of interest, let’s say the concentration of a toxic compound in liquid, is related to acoustic properties of the liquid in the cavity. A change in the concentration causes measurable changes in the properties of the phononic crystal. Transmission or reflection coefficients are appropriate parameters for measurement. Specifically, a resonance induced well separated transmission peak within the band gap is the most favorable feature. The sensor scheme therefore relies on the determination of the frequency of maximum transmission as measure of concentration. Promising applications like biomedical sensors, point-of-care diagnostics or fast screening introduce further engineering challenges, specifically when considering a disposable element containing the analyte. The three key challenges are the strong restriction coming from limitations to approved materials for the analyte container, geometric dimensions in the mm-range common in hospital or point-of-care environment and acoustic coupling between sensor platform and analyte container.

Silicon Pillars as Resonators in an Acoustical Metamaterial

We have investigated the propagation of Lamb waves in structures made of either an isolated resonant pillar or a set of pillars arranged in a line on a thin plate. The resonators as well as the plate are made of silicon. FEM computations show that two bending modes and one compressional mode are unambiguously identified in the frequency range of interest (0–10 MHz). We used a laser ultrasonic technique to map both the amplitude and the phase of the normal displacements on top of the pillars and at the surface of the sample. When the frequency is tuned to a resonant mode, either compressional or bending, the pillars vibrate 180° out-of-phase with respect to the Lamb waves, resulting in a negative modulus or negative mass density respectively.

Localization Approach of Damage in Welded Joint Based on Acoustic Emission Beamforming

A novel approach of locating damage in welded joints is proposed based on acoustic emission (AE) beamforming, which is particularly applicable to complex plate-like structures. First, five AE sensors used to obtain AE signals generated from damage are distributed on the surface of the structure in a uniform line array. Then the beamforming method is adopted to detect the weld joints in the area of interest rather than all the points of the whole structure, and to determine the location and obtain information of AE sources. In order to study the ability of the proposed method more comprehensively, a rectangular steel tube with welded joints is taken for the pencil-lead-broken test. The localization results indicate that the proposed localization approach can effectively localize the failure welded joints. This improvement greatly reduces the cost of computation and also improves the efficiency of localization work compared with the traditional beamforming.

Power Transformer Fault Diagnosis Based on Vibration Correlation Analysis

In this paper, a condition assessment model using vibration method is presented to diagnose winding structure conditions. The principle of the model is based on the vibration correlation. In the model, the fundamental frequency vibration analysis is used to separate the winding vibration from the tank vibration. Then, a health parameter is proposed through the vibration correlation analysis. During the laboratory tests, the model is validated on a test transformer, and manmade deformations are provoked in a special winding to compare the vibrations under different conditions. The results show that the proposed model has the ability to assess winding conditions.

Coupled FSI Simulations of the Interaction of a Flexible Hydrofoil With Large Scale Unsteady Flows

Fluid-structure interaction (FSI) effects must be considered when flexible structures are subjected to unsteady flows. Large-scale unsteady flows can excite structural vibrations significantly and cause the fluid flow to be altered by the large amplitude vibrations. In this work, an in-house finite-element structural code FEANL is tightly coupled with the open-source computational-fluid dynamics (CFD) library package OpenFOAM to simulate the interaction of a backward-skewed, flexible hydrofoil with vortical flow structures shed from a large upstream rigid cylinder in the Penn State-ARL 12” water tunnel. To simulate the turbulent flow at a moderate computational cost, hybrid LES-RANS approaches, i.e. Delayed-Detached-Eddy-Simulation (DDES) and k–ω SST-SAS, are used. The hybrid approaches have been widely employed to simulate massively-separated flows at moderately high Reynolds numbers. Both of the turbulence models are used for a coarse mesh CFD-only case (no FSI effects by assuming a rigid structure) to test their capabilities, and the results of the two models are compared. DDES is chosen to simulate a fine mesh CFD-only case to conduct a mesh convergence study, and it is then used for final FSI simulations. The purpose of this work is focused on obtaining computational results; detailed comparisons against experimental data will be made in future work.

Phased Array Ultrasonic Technique Parametric Evaluation for Composite Materials

A series of ultrasonic elements arranged in a phased array transducer can provide the capability to activate each element separately but in a programmed sequence. This will help the acoustic signal to be generated at desired focusing distances and anticipated angles for specific materials and structures. In case of composite material inspection, this characteristic of the phased array method can improve the undesirable effects of the high attenuation and anisotropic structure of composite materials on response signals. In this study different phased array probes and wedges which are commercially available were evaluated for their response signals’ characteristics. First, the capability and resolution of bulk wave generation were studied for each set of probe and wedge, and the response signals were compared to that of the conventional single element ultrasonic transducers for different thicknesses composite plates. Then the resolution of the response signals and their sensitivity to defect size were evaluated and compared to the single element transducers as well. Next, each phased array probe and wedge set was used to generate plate waves in aluminum plates based on plate wave propagation theory, probe and wedge physical properties and the definition of delay law. Results show a general improvement in response signals’ strength and resolution for phased array method in comparison to the single element transducers. Also some plate wave modes could be generated with optimized signal generation parameters in phased array system.

A Combined Perfectly Matching Layer and Infinite Element Formulation for Unbounded Wave Problems in the Frequency Domain

In this paper, Berenger’s Perfectly Matching Layer (PML) and Bettess’ Infinite Element (IE) scheme are combined to create a new type of element for unbounded acoustic wave problems. An assessment of this new element formulation is made through its use in the calculation of the acoustic modal response of a spherical radiator in the frequency domain. The performance of the PML+IE approach is contrasted with the IE only methodology by comparing them to the exact solution of this test problem in terms of the surface inertia and resistance in the near field. The results are encouraging and the PML+IE approach shows a marked improvement in performance, particularly at lower frequencies.

Vibration Vulnerability of Rod Baffle Type Heat Exchanger: Case Study Badak LNG MCR Aftercooler

Badak LNG plant utilizes Multi Component Refrigerant (MCR) as cooling medium for natural gas liquefaction. In the process system, a heat exchanger is serving as compressor aftercooler that cools down this MCR (gas with mostly Methane and Ethane) by means of sea cooling water. This particular heat exchanger is constructed with Rod-Baffle design with Titanium tubes. The design also incorporates a vapour belt at shell inlet/outlet that is functioned as flow distributor. Badak LNG has 8 plants called process trains; with each process train has one MCR aftercooler. The aftercooler at each train has difference in terms of vapour belt design. During plant operations, vibration problem has repetitively occurred in all MCR aftercooler and caused tube leaks. However, a computational fluid dynamics study revealed that the fluid velocity was low enough to generate tube vibrations. However, Rod-Baffle design uses single support direction per baffle. With 4 axis of support (X+, X-, Y+, Y-), one direction support will repeat at the 5th support, 10th support, and so on. Calculation was then made to confirm the effect if one of the baffle failed to support. This condition will increase the length between baffle by 2 times. Consequently, the fluid critical velocity will decrease by 4 times. This approach was then compared with the fluid velocity pattern inside the exchanger shell. It concludes that the fluid velocity is above its critical velocity. Thus, the exchanger is vulnerable to vibrations. Each MCR aftercooler has different vulnerability to vibrations due to vapour belt different designs. The study has concluded the comparison of vulnerability of the exchangers. Exchanger A has 25.9 % tubes having tubes vulnerable to vibrations, while Exchanger F (with different vapour belt design) has only 1.92 % tubes vulnerable to vibrations. With 2000 tubes quantity, the effect to process conditions is significantly different between exchangers. This paper concludes that design of vapour belt can significantly improve the reliability of Rod-Baffle type heat exchanger.

Characterization of the Damping Behavior of Thin Films With Dynamic Mechanic Analysis in Bending Mode

Usually damped structures, consisting of a constrained layer damping (CLD) and free layer damping (FLD) design, are characterized via dynamic mechanic analysis (DMA) in bending mode. Since laminates with thicknesses from 10 to 100 μm exhibit a very low bending stiffness it isn’t possible to determine their damping properties in bending mode with standard DMA setups. Therefore in the present work the main objective was to introduce a new method to overcome this drawback. Two main geometries were used, such as a variation of the bending mode where the laminates were clamped at the outer supports and on the other hand a set-up where the geometry of a support of loudspeakers was replicated, which was called “speaker” mode. The damping behavior of the laminates then was characterized via the mechanical loss factor tan δ and subsequently compared to results in DMA shear mode. The second objective was to characterize the influence of the design, with a 2-layer laminate consisting of a free layer damping design and a 3-layer laminate with a constrained layer damping design. A method in DMA “speaker” mode was successfully set up. The test parameters were chosen in order to resemble the support of loudspeakers. Therefore with the laminates two beads with a height of approximately 1 mm were formed symmetrically in gaps of 3 mm between the outer fixtures and the drive shaft. Furthermore in the test the laminates were loaded with a dynamic displacement of 600 μm. Due to the low bending stiffness of the laminates the highest test frequency was limited to 10 Hz. In accordance with literature for the 2-layer laminates significant lower damping levels were found than for the 3-layer laminates. Whereas the constrained layer damping laminate (3-layer) showed a good correlation between measurements in “speaker” and in shear mode, the 2-layer laminate showed a significant loss factor increase at high temperatures in shear mode, which was related to entropy elastic effects.