Modal Analysis Approach Research Articles

Creating bandgaps in active piezoelectric slender beams through positive position feedback control

Abstract Bandgaps - frequency ranges with reduced vibration transmissibility in elastic structures, are an opportunity for vibration control originating from the research on elastic metamaterials. In this paper, we study the design for bandgap in slender beams with collocated piezoelectric patch transducers. While creating bandgaps using shunted transducers is a well-established research field, using structures with piezoelectric sensors, actuators, and feedback controllers for the same application has not been thoroughly explored. This paper aims to study the use of the tools originating from the active vibration control (AVC) field for bandgap generation in finite beams with collocated piezoelectric sensors and actuators. Lightly damped second-order low-pass filters are used as controllers in the same configuration as positive position feedback (PPF), widely used for active damping. To facilitate the understanding of systems behaviour, we propose a simplified model based on the Euler-Bernoulli beam theory. A modal analysis approach and an assumption of an infinite number of transducers of infinitesimal length distributed along the structure are used to predict the frequency range of the locally resonant bandgap in closed form. The experimental part of the work demonstrates the feasibility of the proposed approach for creating bandgaps in practice. Thanks to the insights from AVC, the control system can be designed purely based on experimental frequency response data without the need for a parametric model of the system. We also show that the uniform distribution of actuators is not necessary for creating bandgap, which can be achieved in a structure with a relatively small number of sparsely placed actuators and compare the obtained results with analytical predictions for ideal metastructure. Low-frequency bandgaps placed between 10 and 320 Hz are obtained in experiments.

Full-field Modal Analysis of a Tensegrity Column Using a Three-dimensional Scanning Laser Doppler Vibrometer with a Mirror

Abstract Tensegrity structures become important components of various engineering structures due to their high stiffness, light weight, and deployable capability. Existing studies on their dynamic analyses mainly focus on responses of their nodal points while overlook deformations of their cable and strut members. This study proposes a non-contact approach for experimental modal analysis of a tensegrity structure to identify its three-dimensional (3D) natural frequencies and full-field mode shapes, which include modes with deformations of its cable and strut members. A 3D scanning laser Doppler vibrometer is used with a mirror for extending its field of view to measure full-field vibration of a novel three-strut metal tensegrity column with free boundaries. Tensions and axial stiffnesses of its cable members are determined using natural frequencies of their transverse and longitudinal modes, respectively, to build its theoretical model for dynamic analysis and model validation purposes. Modal assurance criterion (MAC) values between experimental and theoretical mode shapes are used to identify their paired modes. Modal parameters of the first 15 elastic modes of the tensegrity column identified from the experiment, including those of the overall structure and its cable members, can be classified into five mode groups depending on their types. Modes paired between experimental and theoretical results have MAC values larger than 78%. Differences between natural frequencies of paired modes of the tensegrity column are less than 15%. The proposed non-contact 3D vibration measurement approach allows accurate estimation of 3D full-field modal parameters of the tensegrity column.

Temperature-assisted electromagnetic surface modes in graphene-based temperature sensitive metafilms

Graphene has the potential to manipulate surface modes in frequency bands from THz to mid-IR regions. Typically, due to single-atom thickness and low charge-carrier density, the thermal response of graphene is ineffective. Temperature-sensitive materials (TSMs) can play an active role in enhancing the thermal response of graphene-based devices. In the present work, graphene-based temperature-sensitive metafilms have been proposed for thermally tunable propagation of electromagnetic surface modes. A detailed analytical and numerical solution for temperature-dependent electromagnetic surface (even and odd) modes supported by the graphene-based temperature-sensitive metafilm has been studied. The Kubo’s formulation has been used to model optical conductivity (σ g ) while the hybrid Drude’s model is implemented to realize the indium antimonide (InSb) as temperature-sensitive material. To simulate the metafilm, the waveguide modal analysis approach was implemented, while the realization of the graphene sheets was achieved by the use of impedance boundary conditions (IBCs). The propagation characteristics for even/odd surface modes were analyzed under different values of temperature (T), chemical potential (µ c ), and thickness of metafilm (d). Further, the numerical results for even and odd surface modes under two phases of InSb [Insulator phase (T = 200 K) and metallic phase (T = 300 K)] were compared under different values of chemical potential (µ c ) and TSM film thickness (d). It is concluded that the propagation characteristics of surface modes are sensitive to the external temperature and can be tailored by tuning the temperature, chemical potential (µ c ), and TSM film thickness (d). Moreover, the degeneracy of the even and odd modes can be controlled by varying the temperature and TSM film thickness. The work is suitable for designing temperature-assisted dual channel waveguides, THz optical switches, THz optical logic designs, and flexible thermal-optical sensors.

Data-driven modal decomposition of R134a refrigerant cavitating flow in Venturi tube

This study utilized high-speed camera and large eddy simulation methods to explore the cavitating flow mechanisms and turbulence structures of R134a refrigerant inside a Venturi tube under varying cavitation numbers (CNs). Data-driven modal analysis approaches, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were introduced to identify and extract the energy hierarchy and transient characteristics within the cavitating flow. The analysis of grayscale images indicated that the cavitating flow gradually transitioned from quasi-periodic to unsteady flow as the CN decreased, and the severity of cavitation correlates with lower peak frequencies. The POD analysis facilitated the extraction of coherent structures in the cavity's temporal evolution, and the results indicate that the quasi-ordering shedding and collapse of large-scale cavity clouds predominantly occur under low cavitation intensity conditions. As the CN increases, the influence of small-scale cavity shedding becomes more significant. The first 30 most energetic modes occupied over 75% of the entire energy, and they were used to reconstruct the cavitating flow, achieving good consistency with transient flow snapshots. Additionally, the DMD results of the cavitating flow yield three frequency spans, including several prominent characteristic frequencies. These spans are closely linked to the cavity cloud structures of varying scales, unveiling the structural characteristics of unsteady cavitating flow.

Effectiveness of Discrete Piezo-patch Locations in Clamped-simply Supported Plates

Present study investigates the problem of determining the control effectiveness of piezoelectric patches for controlling vibration in higher modes of clamped-simply supported plates. A modal analysis approach is used to identify high average strain (or curvature) locations, as desired patch locations and represented in the form of geometrical parameters. The 9-patch configuration is used and modal analysis of integrated structure is carried out, which clearly shows that the initial patch configuration yields better effectiveness for square plates. In case of rectangular plates, the above strategy provides mixed results for the patch distribution effectiveness, for the modes considered here. Next, a concept of ‘stretching’ the patch distribution is applied for improving the effectiveness of the patch distribution and a Fig. of Merit (FOM), based on modal strains, is defined for quantifying the effectiveness. The results show that particular patch distribution geometry is suitable only for a specific group of modes. In view of this, a combined Fig. of Merit, based on the concept of modal weights, is also introduced, which has the potential to help in determining a suitable patch distribution through a formal optimization methodology. The control effectiveness comparison study of the initial and stretched patch configuration of plates also supports the quantification of control effectiveness of patch configuration through the FOM.

Development of the equivalent Great Britain 36-zone power system for frequency control studies

This paper presents a dynamic model of the equivalent Great Britain (GB) 36-zone power system, which can be used for reliable and realistic assessment of emerging load frequency control mechanisms. Flexible architecture of the presented dynamic test system permits a broad range of security of supply and small-signal stability studies for design of future power grids. It can be particularly useful for academic research, but also for undertaking feasibility studies in power industries. The proposed dynamic test system, which is obtained through network reduction of the original full-scale GB transmission power system developed by National Grid Electricity System Operator (NGESO) Company, provides detailed information about the GB power system. In this regard, the required data and modelling approaches to develop the 36-zone system are provided in detail. The presented dynamic test system represents the system topology, impedance characteristics and electromechanical oscillations of the original GB power system however, it is not an exact equivalent of the master GB system. Illustrative dynamic models of the key system components, including synchronous generators, automatic voltage regulators, power system stabilizers, hydro and steam turbines models along with speed governing systems are presented. Dynamic behavior of 36-zone test system in response to infeed loss contingencies is investigated. Particularly, the impact of changes in the system inertia on the system electromechanical modes is examined using the modal analysis approach. In this context, the mode shape concept is employed to determine dominant generators and contribution of different zones in the low frequency oscillations. Moreover, time-domain simulations are undertaken to validate the modal analysis results. Additionally, the condition of different zones from the viewpoint of frequency nadir and maximum rate of change of frequency for various contingencies and extreme cases are examined.

Dynamic Response Analysis of Wind Turbine Structure to Turbulent Wind Load: Comparative Assessment in Time and Frequency Domains

This study investigates wind turbine structural dynamics using stochastic analysis and computational methods in both the time and frequency domains. Simulations and experiments are utilized to evaluate the dynamic response of a wind turbine structure to turbulent wind loads, with the aim of validating the results based on real wind farm conditions. Two approaches are employed to analyze the dynamic responses: the frequency domain modal analysis approach, which incorporates von Kármán spectra to represent the turbulent wind loads, and the time domain Monte Carlo simulation and Newmark methods, which generate wind loads and determine dynamic responses, respectively. The results indicate that, for a larger number of samples, both methods consistently yield simulated turbulent wind loads, dynamic responses and peak frequencies. These findings are further validated through experimental data. However, when dealing with a smaller number of samples, the time domain analysis produces distorted results, necessitating a larger number of samples to achieve accurate findings, while the frequency domain method maintains accuracy. Therefore, the accurate analysis of wind turbine structural dynamics can be achieved using simulations in both the time and frequency domains, considering the importance of the number of samples when choosing between time domain and frequency domain analyses. Taking these considerations into account allows for a more comprehensive and robust analysis, ultimately leading to more effective outcomes.

A machine learning-based stochastic subspace approach for operational modal analysis of civil structures

Based on two machine learning algorithms and eight evaluation indexes, this paper proposes a novel stochastic subspace identification (SSI) approach for the operational modal analysis of civil structures. The detailed procedure of the proposed approach is first demonstrated via a numerical example of a five-degree-of-freedom (5DOF) simulation model, and the results show that the proposed approach can automatedly and effectively distinguish the physical modes from non-physical ones for the SSI. The modal parameters of the 5DOF model estimated by the proposed approach agree well with their theoretical values, verifying the accuracy and effectiveness of the proposed approach. Furthermore, the proposed approach is applied to field measurements on a 195-m-tall building under its operating condition, and the modal parameters of the building are well identified in an automated manner. Through the statistical analysis, the probability distributions and amplitude-dependent features of the natural frequencies and damping ratios of the 195-m-tall building are further revealed. The objective of this study is to provide an efficient tool for the operational modal analysis of civil structures.

Uncertainty transmission of fluid data upon proper orthogonal decompositions

Proper orthogonal decomposition (POD) serves as a principal approach for modal analysis and reduced-order modeling of complex flows. The method works robustly with most types of fluid data and is fundamentally trusted. While, in reality, one has to discern the input spatiotemporal data as passively contaminated globally or locally. To understand this problem, we formulate the relation for uncertainty transmission from input data to individual POD modes. We incorporate a statistical model of data contamination, which can be independently established based on experimental measurements or credible experiences. The contamination is not necessarily a Gaussian white noise, but a structural or gusty modification of the data. Through case studies, we observe a general decaying trend of uncertainty toward higher modes. The uncertainty originates from twofold: self-correlation and cross correlation of the contamination terms, where the latter could become less influential, given the narrow correlation width measured in experiments. Mathematically, the self-correlation is determined by the inner product of the data snapshot and the mode itself. Therefore, the similarity between the input data and the resulting POD modes becomes a critical and intuitive indicator when quantifying the uncertainty. A scaling law is shown to be applicable for self-correlation that promotes fast quantification on sparse grids.

Automated Bayesian operational modal analysis of the long-span bridge using machine-learning algorithms

The Bayesian Operational Modal Analysis (OMA) approach can both estimate the most probable value (MPV) of modal parameters and swiftly give identification uncertainty. However, the determination of certain parameters shall still require the expertise of professionals and process is labor-intensive. In light of the massively available data from the in-situ structural health monitoring (SHM) system of the long-span bridge, the proposal and development of an automated strategy for identifying and tracking varying bridge modal parameters are much desired. A novel machine-learning-based framework is proposed to automatically estimate the parameters required for the Bayesian OMA. The major contributions of the proposed method are: (i) The cross-modal assurance criterion (CMAC) matrix is proposed and adopted to obtain the correlation between the target modes from a global perspective. It is reconstructed using the convolutional autoencoder (CAE) to remove the embedded noise information and the modal resonant frequency band can be detected, and (ii) The Kohonen network is used to automatically determine the number of target modes by clustering the normalized singular value (SV) sequence. Collected data from numerical simulation and a full-scale long-span suspension bridge are adopted to validate the proposed framework. Results show that the proposed framework can successfully estimate the required parameters from the raw data in an automated manner. This feature transforms the conventional Bayesian OMA into an automated strategy for identifying and tracking the modal parameters of the long-span bridge.

Provenance changes revealed by a multi-proxy approach to sandstone analysis and its implications on palaeogeography: Mesozoic Kutch Basin, India

Sedimentary rocks preserve the geological history of a basin and play a key role in constraining the palaeogeography. The Mesozoic Kutch Basin (India) comprises a number of sub-basins representing a pericratonic rift formed during the break-up of Gondwanaland. It preserves sediment deposited during the rifting and drifting of the Indian subcontinent. The current study identifies evidence of provenance changes recorded in Mesozoic sandstone units using a multi-proxy approach of quantitative modal analysis, quantitative heavy-mineral analysis and garnet chemistry. The Mesozoic sedimentary successions in the sub-basins are classified as the Kutch Mainland Group (KMG), the Pachchham Island Group (PIG) and the Eastern Kutch Group (EKG). Although the sub-basins are thought to share sediment from common source areas and the sandstone units are highly mature quartz arenites, the heavy-mineral assemblages (especially the content of zircon, tourmaline, rutile, apatite and staurolite) indicate previously unreported variation in the sediment source for the PIG when compared to the KMG and EKG. Moreover, an increase in the content of staurolite and kyanite in combination with rising trends of the ratios of staurolite–kyanite and zircon–tourmaline–rutile over garnet (St + Ky/Grt and ZTR/Grt) for the Callovian and younger strata is observed in the KMG and EKG. The above indicates a change in source from the older to the younger strata in each of the two sub-basins. The absence of their coeval strata in the PIG gives credence to the previously postulated rise of a subsurface basement high, the Median High in this basin. However, the timing of the rise remained so far unspecified, mentioned as “later in Jurassic” (Biswas, 2016b). The observations made in the current study constrain the rise of the Median High in the Kutch Basin to the Callovian time. The similarities in trends of the ratio of heavy minerals and garnet chemistry as well as variations in igneous garnet suggest recycling within the Kutch Basin from the Callovian to Oxfordian–Kimmeridgian strata in the EKG to the Kimmeridgian to Albian strata in the KMG.Besides revealing previously unknown details in the evolutionary and palaeogeographic history of the different sub-basins constituting the Kutch Basin, this multi-proxy study demonstrates the high potential of using recently developed techniques in provenance analysis i.e., the semi-automated heavy-mineral analysis by Raman spectroscopy and the state-of-the-art machine-learning aided garnet discrimination scheme. This study also provides a globally applicable workflow for other sedimentary basins, especially rift basins characterised by overall highly mature quartz arenites.

An experimental procedure for the identification of the dynamic parameters for the rigid-ring tyre model

Several approaches have been developed over the years for the modelling of the tyre behaviour in vehicle-dynamic applications. The so-called ‘rigid-ring’ models are among the classics for the modelling of the belt dynamics. Although there are several works dealing with the vibrating properties of tyres, the problem of the identification of the related rigid-ring model parameters has not been described other than qualitatively or partially. The aim of this work is thus to fill this gap and to devise a procedure for the experimental characterisation of such parameters, namely the frequency and damping of the in-plane and out-of-plane belt vibration modes as well as the associated masses and inertias. An experimental modal analysis (EMA) approach is employed, which involves an instrumented hammer combined with three-axial accelerometers roving on 16 stations equally spaced along the tyre circumference. The method is numerically demonstrated on the finite-element models of a motorcycle tyre and a car tyre. The approach is also experimentally validated on a real tyre. The rigid-ring vibration modes of the motorcycle tyre are in the range 70–220 Hz, while those of the car tyre are in the range 51–85 Hz. The ratios of the mass/inertia of the rigid ring to the mass/inertia of the tyre are in the range 40–87% and 68–74% for the motorcycle and car respectively.

Development and validation of an intuitive biomechanics-based method for intraocular pressure measurement: a modal analysis approach

BackgroundCurrent intraocular pressure (IOP) measurements based on non-contact tonometry are derived from statistics-driven equations and lack biomechanical significance, which often leads to under-estimation in post-refractive surgery cornea. This study aims to introduce and validate modal analysis-derived intraocular pressure (mIOP) as a novel method generated through Legendre-based modal decomposition of the anterior corneal contour; it provides an accurate and intuitive IOP measurement from an energy-based perspective.MethodsThis retrospective study included 680 patients. Healthy participants were divided into reference (n = 385) and validation (n = 142) datasets, and the others underwent either femtosecond-assisted laser in situ keratomileusis (FS-LASIK, n = 58) or transepithelial photorefractive keratectomy (TPRK, n = 55). Corneal curvature of the right eyes was extracted from raw serial cross-sectional images of the cornea generated by Corvis ST, a noncontact tonometer with a high-speed Scheimpflug-camera. Legendre expansion was then applied to the corneal curvature to obtain the modal profiles (i.e., temporal changes of the coefficient for each basis polynomial [modes]). Using the reference dataset, feature selection on the modal profiles generated a final mIOP model consisting of a single parameter: total area under curve (frames 1–140) divided by the area under curve of the rising phase (frames 24–40) in the fourth mode, i.e. the M4 ratio. Validation was performed in both the healthy validation and postoperative datasets. IOP-Corvis, pachymetry-corrected IOP, biomechanically corrected IOP, and mIOP values were compared. For the FS-LASIK and TPRK groups, pairwise postoperative IOP changes were analyzed through repeated measures analysis of variance, and agreement was examined through Bland–Altman analysis. Using a finite element analysis based three-dimensional model of the human cornea, we further compared the M4 ratio with the true intraocular pressure within the physiological range.ResultsThe M4 ratio-based mIOP demonstrated weak to negligible association with age, radius of corneal curvature, and central corneal thickness (CCT) in all validation analyses, and performed comparably with biomechanically corrected IOP (bIOP) in the refractive surgery groups. Both remained nearly constant postoperatively and were not influenced by CCT changes. Additionally, M4 ratio accurately represented true intraocular pressure in the in silico model.ConclusionsmIOP is a reliable IOP measurement in healthy and postrefractive surgery populations. This energy-based, ratio-derived approach effectively filters out pathological, rotational, misaligned movements and serves as an interpatient self-calibration index. Modal analysis of corneal deformation dynamics provides novel insights into regional corneal responses against pressure loading.

Dynamics of axially moving beams: A finite difference approach

In this paper, a finite difference approach is being presented as the most reliable technique to investigate the non-linear transverse vibration of an axially moving beam. Firstly, Hamilton’s principle is used to derive the governing hyperbolic partial differential equation (HPDE) of free transverse vibration. Then finite difference along with state space method is used to convert the partial differential equation into a system of coupled first-order ordinary differential equations (ODE’s). Comparison of the proposed finite difference model is made with both the numerical and analytical models. It is noted that the finite difference model is in excellent agreement with the modal analysis approach (analytical solutions) as compared to the generalized integral transform technique GITT (numerical method). A parametric study was also presented to examine the impact of system parameters, such as axial translation speed and flexure rigidities of the beam on the transverse response amplitude, frequencies and instabilities. The transverse vibration of the beam at selected points along its length demonstrates higher-frequency oscillations that corresponds to a lower axial speed. With the increase of beam flexural rigidity, the divergence critical speed increases. Coupled effect of axially moving beam and rotating rolls is also considered.

Low frequency sound absorption performance of large sized active micro-perforated panel absorber in free field

The active micro-perforated panel absorber has excellent low frequency sound absorption performance, which is expected to realize low-frequency noise reduction in large space of the cabin. Since its active sound absorption performance depends on the incident sound field environment, on the basis of the existing research conclusions in the duct, the active sound absorption performance of the large-sized active micro-perforated panel absorber under the excitation of a normal incident plane wave in typical free field environment is in depth investigated. First, the theoretical model of the active micro-perforated panel absorber is established by using the modal analysis approach, in which a reflection coefficient varying with position is introduced to represent the reflected sound wave on the surface of the active micro-perforated panel absorber in free field. Then, the physical mechanism of active control is thoroughly analyzed and the error sensing strategy is established. Finally, an experiment is carried out to validate the theoretical model and findings. Results obtained demonstrate that the greater the amplitude of the (0, 0, 0) cavity mode excited by the incident plane wave, the stronger the reflection effect on the incident sound wave is, and vice versa. The control source suppresses the (0, 0, 0) mode so that this mode will not reflect and absorb the incident plane wave substantially when its amplitude is reduced to an optimal value. This is main mechanism of the sound absorption improvement in the low frequency range. However, the excited high order cavity modes (except for (0, 0, 0) mode) greatly reflect the incident sound energy in free field and exert a negative effect on sound absorption improvement. Thus, the control performance of the active micro-perforated panel absorber weakens in free field in comparison with that in the duct. The pressure-release and impedance-matching strategies are still applicable to free field as long as such a situation holds, i.e. the (0, 0, 0) cavity mode can be substantially suppressed by the control source and at the same time the high order cavity modes cannot be highly excited.

First hypothesis for Optimized Monitoring Strategy through Ambient Vibrations in historic buildings

Dynamic identification strategies and, in particular, Operational Modal Analysis (OMA) approaches demonstrated to be a significant source of information about the condition of an investigated building, as well as, repeated data acquisitions and processing methods, developed in the field of Structural Health Monitoring (SHM), have been successfully used to track the evolution of this condition over time. Nonetheless, planning a cost-effective ambient vibration monitoring campaign is still an open challenge as several uncertainties must be considered to ensure a beneficial trade-off between number of sensors or set-ups and quality of the information collected. This is particularly important when dealing with historical masonry buildings. The present work discusses the preliminary results of a project, currently under development, whose aim is the definition of optimised protocols for data acquisition and processing for built cultural heritage dynamic identification and monitoring, with specific focus on the Venetian palace typology.

Passive and active low frequency sound absorption performance of the finite and large sized micro-perforated panel absorber on oblique incidence condition

This paper presents a comprehensive investigation on the passive and active low frequency sound absorption performance of the finite and large sized micro-perforated panel absorber (FLS-MPPA) for oblique incidence excitation (OIE), which is of great significance for the engineering implementation of this technic. The theoretical model of the FLS-MPPA is firstly established by applying the modal analysis approach. It is also validated by experimental test. Then, the passive sound absorption performance is explored and the underlying physical mechanisms of these peculiar findings are investigated thoroughly. Finally, the active control performance (applying point source to actively control the sound field) and physical mechanism of the sound absorption improvement of the FLS-MPPA are analyzed. Results obtained demonstrate some key findings. The high order cavity modes excited by OIE reflect most of the incident sound power and have little contribution to the sound absorption in the low frequency range owing to their strong mutual coupling effects and being greatly excited with high modal amplitudes. They also cannot produce additional sound absorption peaks due to their wide resonant frequency band. Weakening such mutual coupling effects can improve the sound absorption and produce additional absorption peaks. The low frequency sound absorption of the FLS-MPPA is significantly improved with control and the achievable maximum sound absorption coefficient is slightly greater than the difference between 1 and the sound power reflection coefficient of the MPP, which validates the feasibility of the active control method for OIE. The reflected sound power induced by the cavity modes is highly reduced and part of the reflected sound power produced by the MPP can even be absorbed by these modes when their amplitudes are reduced to the optimal value, which is the physical nature of the active control. Sensing and controlling the cavity sound field in the corner achieves almost equal sound absorption improvement as that of the optimal control state, based on which the error sensing strategy is constructed.

Influence of mesoscale friction interface geometry on the nonlinear dynamic response of large assembled structures

Friction interfaces are unavoidable components of large engineering assemblies since they enable complex designs, ensure alignment, and enable the transfer of mechanical loads between the components. Unfortunately, they are also a major source of nonlinearities and uncertainty in the static and dynamic response of the assembly, due to the complex frictional physics occurring at the interface. One major contributor to the nonlinear dynamic behavior of the interface is the mesoscale geometry of a friction interface. Currently, the effects of the interface geometry on the nonlinear dynamic response is often ignored in the analysis due to the high computational cost of discretizing the interface to such fine levels for classical finite element analysis. In this paper, the influence of mesoscale frictional interface geometries on the nonlinear dynamic response is investigated through an efficient multi-scale modeling framework based on the boundary element method. A highly integrated refined contact analysis, static analysis, and nonlinear modal analysis approach are presented to solve a multi-scale problem where mesoscale frictional interfaces are embedded into the macroscale finite element model. The efficiency of the framework is demonstrated and validated against an existing dovetail dogbone test rig. Finally, the effects of different mesoscale interface geometries such as surface waviness and edge radius, are numerically investigated, further highlighting the influence of mesoscale interface geometries on the nonlinear dynamics of jointed structures and opening a new research direction for the design of friction interfaces in friction involved mechanical systems.

On the study of vortex-induced vibration of a straked pipe in bidirectionally sheared flow

Helical strakes are widely used to suppress vortex-induced vibration (VIV) in ocean engineering. However, the VIV response and suppression efficiency of the straked pipe are unclear under the recently discovered bidirectionally sheared flow field. Experiments on vortex-induced vibration for a flexible pipe with three kinds of helical strakes were carried out in an ocean basin. The bare pipe model was 28.41mm in diameter and 7.64m in length. Three kinds of helical strakes with different pitch/height (17.5D/0.15D, 17.5D/0.25D,and 17.5D/0.35D) were applied. The test was performed on a rotating test rig to simulate bidirectionally sheared flow conditions. Fiber Bragg Grating (FBG) strain sensors were arranged along the test pipe to measure bending strains, and the modal analysis approach was used to determine the VIV response. The reduced velocities based on the tested first natural frequency of the bare pipe in water reached 30.79. The cross-flow and in-line VIV amplitudes, VIV suppression efficiency, and fatigue damage are presented in this article. The results showed that the helical strakes can suppress bidirectionally sheared flow-induced VIV significantly at a high reduced velocity regime with fatigue damage suppression efficiency over 99% in the CF and IL directions, but cannot suppress the in-line initial displacement, which was also found in previous studies as a drawback of helical strakes.

Identification of in-process machine tool dynamics using forced vibrations in milling process

An accurate description of machine tool dynamics is essential for health monitoring, chatter prevention, and improvement of manufacturing accuracy. The standard identification approach of experimental modal analysis at the standstill of the machine does not realize the possible variations in the dynamics due to operational conditions. The in-process identification methods in the literature, on the other hand, are associated with implementation difficulties in industrial environments due to the required complex and specially-designed setup, limited excitation forms, or excessive measurement efforts. This paper proposes an industrial-friendly method to estimate the in-process structural dynamics of machine tools, considering practical demands and implementation limits. Knowing that the operational dependency of the dynamics is associated with the machine and spindle structure, the machine–spindle and holder–tool structures are considered two distinguished subsystems. The cross Frequency Response Function (FRF) of the coupled structure, between the tooltip and spindle flange, is determined by measuring forced vibrations during milling operations. The milling forces are considered as excitation input and the process is designed to be stable and chatter-free so that the simulated forces can be accurately used in the identification. Then, the receptance coupling theory is utilized to develop an optimization algorithm. Given the model of the coupled structure, the evolutionary optimization tunes the modal parameters of the machine–spindle dynamics and joint parameters until the predicted cross FRFs match the experimentally determined FRFs. The direct FRF at the tooltip is predicted using identified in-process machine–spindle dynamics through the receptance coupling method. The identified in-process dynamics are used to predict stability diagrams for different tooling systems and, moreover, to design an augmented Kalman filter to estimate cutting forces. Comparison of estimated SLDs and cutting forces with chatter test results and dynamometer measurements validate the outcomes of the proposed identification method. This paper demonstrates that the system dynamics under operational conditions can be successfully identified without requiring costly setup, specially-designed workpieces or operations, and destructive chatter tests.