•·The vibration frequency of the core support barrel (CSB) of pressurized water reactor (PWR) nuclear power plants was found to drop during the operation of the reactor.•·Numerical model of the PWR ...reactor internals was proposed with consideration of the frictional contact and fluid-structure interaction for the modal analysis of the CSB.•·The drop of the vibration frequency of the CSB was found to be caused by the stiffness degradation of the fuel assembly hold-down spring (FAHDS).
As a key component of pressurized water reactor (PWR) nuclear power plants, the motion of the core support barrel (CSB) has a great impact on the safety of the nuclear reactor. However, the vibration characteristics of the CSB have not yet been fully investigated and may change due to the high doses of radiation and high temperature in the reactor. In this paper, the ex-core neutron noise signals of two PWR units throughout the multiple fuel cycles were analyzed and the vibration frequency of the CSB beam mode was found, for the first time, to drop with time during the operation of PWRs. Then, the statics analysis of the CSB was conducted to preliminarily identify the potential causes. For the further clarification of such vibration frequency drop phenomenon, a numerical assembly model of PWR reactor internals, including the CSB, hold-down ring (HDR), fuel assembly hold-down spring (FAHDS), etc., was proposed with consideration of the frictional contact and fluid-structure interaction for the modal analysis of the CSB. The results showed that the decrease of the FAHDS stiffness is the main cause for the drop of the vibration frequency of the CSB beam mode in real conditions.
Modal testing is used to experimentally determine the dynamic behavior of mechanical structures. The planning of the positions for exciting the structure, the so-called driving points, is essential ...for efficient experimental modal testing. Driving points are identified either by an initial assumption of possible excitation points and the experimental evaluation of their quality, or with the help of numerical models and indicators for optimal driving point selection. However, for practical applications, several driving points are usually required to excite all modes in the frequency range of interest, which is not covered in state-of-the-art indicators for driving point selection. This paper therefore presents a method for driving point identification with consideration of multiple driving points. Additionally, a criterion was developed that considers the excitability of modes, risks of double hits, and the excitation orientation simultaneously in order to increase the accuracy of the driving point identification. This criterion enables the evaluation of the excitation quality of each surface node and any mode combination using an automated selection method for determining the optimum set of driving points. The presented method assumes that natural frequencies and eigenvectors from preliminary numerical models are available for planning. By applying the method on application examples, the reduced effort required for a full modal analysis is demonstrated.
Violin varnishes influence the vibrational properties of tonewood. However, the frequency dependence of the varnish influence and mechanical properties of typical varnishes has received little ...attention. The viscoelastic properties of various violin varnish materials over the audible frequency range were characterized by dynamic mechanical analysis. The properties of the studied varnishes showed comparable frequency dependencies. For all varnishes, E increased and tan(δ) decreased with increasing frequency. The results were in good agreement with an analytical mechanical model, which was used for additional numerical FEM calculations. The approach of numerically determining varnish-induced changes in the vibrational properties on basis of the individual wood and varnish properties was confirmed through comparison with experimental results obtained in an earlier study. The latter procedure was subsequently used to analyse varnish-induced changes in the eigenfrequencies of a violin soundboard. The results revealed that the frequency dependence of the varnish properties determined the specific influence of varnishes on the vibrational properties of tonewood, which should be taken into account when assessing the impact of varnishes.
•Viscoelastic characterization of violin varnishes over the audible frequency range.•Properties of the studied varnishes showed a strong dependence on frequency.•For all varnishes, E increased and tan(δ) decreased with increasing frequency.•The frequency dependence was most pronounced for tan(δ) for frequencies < 1000 Hz.•FEM calculations of violin top plates revealed the expected shifts in eigenfrequencies.
•Modal properties of biological systems.•The stochastic finite element method in the determination of natural frequencies.•Determination of operating frequency used in the design of machines for ...harvesting.
In the last few years, the trend towards the use of fossil fuels has been enormously discouraged, driving research and development into new energy sources, joining efforts from both the scientific community and industry. The global trend to increase the use of renewable energy sources is essential to develop appropriate technologies for the cultivation and management of crops in order to obtain production on an industrial scale, associated with the least environmental impact. The Macaw palm (Acrocomia aculeata), a palm tree native to Tropical America, has great potential as a raw material for the biodiesel production. Macaw palm fruits can be harvested by mechanical vibrations and the modal properties identification, such as natural frequencies and modes of vibration, of the fruit-rachilla system is fundamental for the development of harvesting machines and devices that use this principle. This paper presents a systematic approach to identify the modal properties of macaw palm fruit-rachilla system, when the inherent variability of the biological systems are involved. The inherent variability of the specific mass and elasticity modulus of the fruit-rachilla systems of macaw palm in different plantation sites, as well as the influence of the fruit maturation stage, were considered in the analysis, as random variables. Using the stochastic finite element model it was possible to determine the spectral response of the natural frequencies of interest, as well as its statistical parameters in order to serve as base of knowledge for design harvesting machines. The analysis and discussions had contributed to establish three possible operating frequency ranges to design macaw palm harvesting machines.
In this study, a mode combination approach is proposed to place actuator/sensor pairs according to the experimental modal analysis results. The method is carried out by combining modes using the ...weighting coefficients with a finite element package program. First, this approach is applied to four different structures presented in the previous works in literature to compare and verify. Our results demonstrate that the determined best locations overlap well with the published results. Then, this combination approach is applied to the steel plate with bolt connections on four sides to determine actuator/sensor (a/s) pairs’ placement accuracy, which is directly related to experimental/numerical modal correlation. For this purpose, three different cases varying by weighting coefficients are determined to show the modal correlation's effectiveness. Finally, the possible best a/s positions for three cases are compared. In conclusion, the presented mode combination approach is a fast and practical solution for determining a/s pairs’ placement, and the experimental/numerical modal analysis correlation directly affects the placement's success.
This paper aims at identifying the dynamic characteristics of a structure, by Experimental Modal Analysis (EMA), necessary for predicting the response of a structure due to forces acting on it in the ...working environment. The three modal parameters namely- natural frequency, damping and mode-shapes are obtained using the aforementioned technique. Numerical validation is carried out using Hypermesh 13.0, a finite element package, to validate the results and check rationality of the assumptions. While the natural-frequency and mode-shape prediction using numerical methods is known to be fairly accurate, damping is very difficult to predict analytically due to the complex nature of physics involved. The damping obtained using this test is inserted as a material property in the FEA model to obtain FRF numerically, which is compared with the FRF obtained experimentally. Two mathematical indicators, the Modal Assurance Criteria and Stability Diagram are used to correlate mode shapes and assess the stability of experimentally obtained modes.
Abstract
This paper presents an experimental modal test of a Kaplan turbine model and provides the corresponding analysis of the results. The modal test of the rotor including the runner, the shaft ...and the generator was performed using, as exciters, an impact hammer and a shaker and, as sensors, several accelerometers. Additionally, numerical models of the rotor with the runner surrounded by air (dry condition) or submerged in water (wet condition) were also built. By comparing the numerical and experimental results, the main modes of vibration of a runner blade in dry conditions and of the rotor shaft both in dry and wet conditions have been identified and discussed. The most significant deviations between experimental and numerical natural frequencies were found around 10% for the rotor shaft and around 6% for the runner blade. Moreover, it was observed that the fourth mode of vibration presents the highest added mass effect with a Frequency Reduction Ratio of about 6.7% and the second mode of vibration shows the lowest damping ratios in both dry and wet conditions with values of about 1.5 and 2.1%, respectively.
The current paper presents an investigation into novel modal testing methods applied to a disk–shaft structure at different rotating speeds in air and water. The structure was excited using three ...different methods: an instrumented hammer, a piezoelectric PZT patch glued on the disk and a transient ramp-up. The structural response was measured using an accelerometer and strain gauges mounted on board as well as accelerometers and displacement lasers mounted off board. The potential to excite the natural frequencies using each excitation method and to detect natural frequencies with each sensor was analyzed and compared. Numerical structural and acoustic–structural modal and harmonic analyses of the non-rotating disk in air and water were also performed, taking into consideration the PZT patch. The numerical results showed a close agreement with the experimental ones in both air and water. It was found that the rotating speed of the disk modified the detected natural frequencies, depending on the frame of reference of the sensor. Finally, the PZT patch and the transient ramp-up were proven to be reliable methods to excite the natural frequencies of the current test rig and to be potentially applicable in full-scale hydraulic turbines under operating conditions.
Aircraft experience various phases during each flight. Optimal performance, without compromise, during various phases can be achieved through adaptability in the wing design. Morphing wing design ...encompasses most, if not all, the flight conditions variations, and can respond interactively. In the present work, the dynamic characteristics of a reconfigurable modular morphing wing of two topological architectures, developed in-house by a research group at Toronto Metropolitan University (formerly Ryerso University), were investigated. This modular morphing wing, developed based on the idea of a parallel robot, consists of a number of structural elements connected to each other and to the wing ribs through eye-bolt joints. Euler–Bernoulli and Timoshenko bending beam theories, in conjunction with Finite Element Analysis, were exploited. Free vibration of unmorphed (Original) and morphed configurations subjected to spanwise extensions were studied. The results of systems’ free vibration analyses were validated against those obtained from Ansys and Dynamic Stiffness Matrix (DSM) method. The effect of various spanwise extensions, as well as topology on system’s natural frequencies, was also studied and reported on.
Accurate assessments of the internal structure and boundary conditions of unstable rock slopes are imperative for evaluating landslide hazard scenarios. However, instability characterization at depth ...remains challenging and is often limited by costly or invasive subsurface investigations. Here, we develop a new approach coupling array‐based ambient vibration modal analysis and numerical modeling to improve structural characterization of rock slope instabilities at depth. We used ambient noise cross‐correlation on 4 hr of seismic data recorded by an array of 30 nodal geophones at a 500‐m‐long toppling rock slab in Utah, USA to identify modal frequencies between 0.8 and 3.5 Hz and derive modal displacements. We show that transverse and longitudinal bending modes span the length of the instability, indicating an interconnected slab. Statistical comparison of field results with outputs from >1,000 finite element models with varying boundary conditions showed that the instability depth varies between 40–70 and 10–20 m in the middle and lateral regions, respectively. Our approach yields new information on the structural conditions of rock cliff and column instabilities at depth, which is not easily obtained by other means but is imperative for change detection monitoring and improved hazard assessments.
Plain Language Summary
Rock slope failures constitute a major hazard in areas with steep terrain and can endanger human lives and damage infrastructure. An important step in minimizing the hazard from unstable rock slopes is obtaining a detailed picture of the structure and geometry of an instability, including size, material properties, and the manner in which failure is likely. Characterizing the structure of landslides below the surface, however, remains challenging and is often costly or requires intrusive measurements. We developed a new approach that combines surface‐based measurements of the natural vibrations of an unstable rock cliff with numerical modeling to improve estimates of the instability geometry. We measured the vibrational properties of a large unstable rock slab in southern Utah, USA and compared the results of our field data analysis with >1,000 model iterations to narrow down the possible geometries of the instability below the surface. This technique improved our assessment of the instability size and structure, and has the potential to be a valuable tool for similar evaluations at other unstable rock slopes.
Key Points
We combine ambient vibration modal analysis and numerical modeling to assess structural conditions of a toppling rock slab in Utah, USA
Mode shapes derived from geophone array data are used to calibrate numerical eigenfrequency models and invert for boundary conditions
Our approach can be applied in other settings to enhance characterization of unstable rock slopes in support of improved hazard assessments
PDF
