Forced Oscillation Research Articles

THE EFFECT OF SURFACE ROUGHNESS AND ELECTROSTATIC BONDS OF RESIDUAL GASES ON THE SETTLING OF THE GYROSCOPE ROTOR

Consider the rotor of an electrostatic gyroscope (ESG) for contactless operation. The rotors of the ESG are made in the shape of a ball and placed in an electric field. The gyroscope operates contactless between the rotor and the mutual oscillation force of the electrodes. Air is sucked out of the chamber in which the rotor is located. Due to the lack of air resistance and the lack of friction force, the rotor can rotate for a long time. But the rotor, located inside the vacuum, also slows down. The reason for this is the roughness and residual deposits of the rotor surface. This is the notorious effect of the rotor of these residual gases on braking. When calculating the effect of residual gases on the rotor, it is considered as separate gases, and not as a homogeneous medium. For this reason, the viscosity of the gas can be set without consideration. The effect of the action of individual particles on the rotor is determined by the roughness of its surface. That is, the momentum by which the particles somehow hit the "peaks" or "troughs" creates a torque that acts on the rotor.

On the nonlinear dynamic characteristics of forced acoustic oscillations in a heat-driven thermoacoustic engine

On the nonlinear dynamic characteristics of forced acoustic oscillations in a heat-driven thermoacoustic engine

Directional sensitivity of the cerebral pressure-flow relationship during forced oscillations induced by oscillatory lower body negative pressure

A directional sensitivity of the cerebral pressure-flow relationship has been described using repeated squat-stands. Oscillatory lower body negative pressure (OLBNP) is a reproducible method to characterize dynamic cerebral autoregulation (dCA). It could represent a safer method to examine the directional sensitivity of the cerebral pressure-flow relationship within clinical populations and/or during pharmaceutical administration. Therefore, examining the cerebral pressure-flow directional sensitivity during an OLBNP-induced cyclic physiological stress is crucial. We calculated changes in middle cerebral artery mean blood velocity (MCAv) per alterations to mean arterial pressure (MAP) to compute ratios adjusted for time intervals (ΔMCAvT/ΔMAPT) with respect to the minimum-to-maximum MCAv and MAP, for each OLBNP transition (0 to −90 Torr), during 0.05 Hz and 0.10 Hz OLBNP. We then compared averaged ΔMCAvT/ΔMAPT during OLBNP-induced MAP increases (INC) (ΔMCAvT/ Δ MAP T INC ) and decreases (DEC) (ΔMCAvT/ Δ MAP T DEC ). Nineteen healthy participants [9 females; 30 ± 6 years] were included. There were no differences in ΔMCAvT/ΔMAPT between INC and DEC at 0.05 Hz. ΔMCAvT/ Δ MAP T INC (1.06 ± 0.35 vs. 1.33 ± 0.60 cm⋅s−1/mmHg; p = 0.0076) was lower than ΔMCAvT/ Δ MAP T DEC at 0.10 Hz. These results support OLBNP as a model to evaluate the directional sensitivity of the cerebral pressure-flow relationship.

Study of high power steam turbine forced oscilla-tions near operating frequency

Problem. Forced oscillations of the turbine unit-foundation-base system near the operating frequency are considered. Goal. The purpose of the work is to study forced vibrations of the turbine unit-foundation-base system near the operating frequency range to assess the vibration state of the system and determine the causes of increased vibrations. The object of the study is the turbine unit-foundation-base system. The system consists of a single reinforced concrete foundation on which a steam turbine and generator are installed. The subject of the study is the amplitude-frequency dependences of forced oscillations of the turbine unit-foundation-base system. Methodology. The study was carried out using vibration methods and the finite element method. Also worth noting is the use of original methods developed directly by the author for constructing models of complex mechanical engineering systems. Results. As a result of the research, three-dimensional finite element models of low-pressure cylinder housings, the foundation and the entire turbine unit-foundation-base system were built. The conducted research allows us to draw a conclusion regarding the reasons for the increased level of vibration near the operating frequency and directions for their prevention. Originality. Regarding the methods for constructing a model of the turbine unit-foundation-base system, it should be noted that they are unique. The features of the applied modeling method make it possible to take into account the variable interaction between the low-pressure cylinder bodies and the foundation. Fixpoints are also taken into account. Previous studies of the turbine unit-foundation-base system did not allow us to determine the causes of increased vibrations of the shaft line supports. Practical significance. The results of the work relate to direct practical application. As a result of the work, conclusions were drawn about the ways to increase the vibration reliability of the turbine unit-foundation-base system and further directions for research.

Theoretical Study of Forced Van Der Pol Oscillator Equation Using Multiple Two-timing Regular Parameter Perturbation and Asymptotic Expansion Techniques

This paper presents the theoretical study of forced Van der Pol oscillator equation. Oscillatory systems are studied to know measures that can reduce the amplitude of oscillation of the oscillatory system. Here, multiple two-timing regular parameter perturbation is applied since it is a kind of perturbation among other perturbation techniques that enables the study of the behaviour of a system under certain conditions. Asymptotic expansion technique was also applied. Excel Microsoft was used to analyse the uniformly valid asymptotic solution of the Van der Pol oscillator equation obtained. The uniformly valid asymptotic solution in the independent variable obtained, showed that damping alters the amplitude of the oscillatory system thereby affecting its motion. Increase in damping decreases the amplitude of oscillation of the system. With damping incorporated in the system though very small damping, the amplitude of oscillation reduces with time.

Exploring alveolar recruitability using positive end-expiratory pressure in mice overexpressing TGF-β1: a structure–function analysis

Pre-injured lungs are prone to injury progression in response to mechanical ventilation. Heterogeneous ventilation due to (micro)atelectases imparts injurious strains on open alveoli (known as volutrauma). Hence, recruitment of (micro)atelectases by positive end-expiratory pressure (PEEP) is necessary to interrupt this vicious circle of injury but needs to be balanced against acinar overdistension. In this study, the lung-protective potential of alveolar recruitment was investigated and balanced against overdistension in pre-injured lungs. Mice, treated with empty vector (AdCl) or adenoviral active TGF-β1 (AdTGF-β1) were subjected to lung mechanical measurements during descending PEEP ventilation from 12 to 0 cmH2O. At each PEEP level, recruitability tests consisting of two recruitment maneuvers followed by repetitive forced oscillation perturbations to determine tissue elastance (H) and damping (G) were performed. Finally, lungs were fixed by vascular perfusion at end-expiratory airway opening pressures (Pao) of 20, 10, 5 and 2 cmH2O after a recruitment maneuver, and processed for design-based stereology to quantify derecruitment and distension. H and G were significantly elevated in AdTGF-β1 compared to AdCl across PEEP levels. H was minimized at PEEP = 5–8 cmH2O and increased at lower and higher PEEP in both groups. These findings correlated with increasing septal wall folding (= derecruitment) and reduced density of alveolar number and surface area (= distension), respectively. In AdTGF-β1 exposed mice, 27% of alveoli remained derecruited at Pao = 20 cmH2O. A further decrease in Pao down to 2 cmH2O showed derecruitment of an additional 1.1 million alveoli (48%), which was linked with an increase in alveolar size heterogeneity at Pao = 2–5 cmH2O. In AdCl, decreased Pao resulted in septal folding with virtually no alveolar collapse. In essence, in healthy mice alveoli do not derecruit at low PEEP ventilation. The potential of alveolar recruitability in AdTGF-β1 exposed mice is high. H is optimized at PEEP 5–8 cmH2O. Lower PEEP folds and larger PEEP stretches septa which results in higher H and is more pronounced in AdTGF-β1 than in AdCl. The increased alveolar size heterogeneity at Pao = 5 cmH2O argues for the use of PEEP = 8 cmH2O for lung protective mechanical ventilation in this animal model.

Magnetic Force and Laser Excitation of Transverse Oscillations in Optical Microfibers

Magnetic Force and Laser Excitation of Transverse Oscillations in Optical Microfibers

A statistical analysis of causal decomposition methods applied to Earth system time series

Causal Decomposition (CD) constitutes a novel and widely accepted method for discovering and quantifying the internal causal relationships inherent to complex systems. This new causality bases not on time nor state. Instead, it relies on instantaneous phase coherence between the corresponding Intrinsic Mode Functions (IMFs) of the signals, obtained via Empirical Mode Decomposition (EMD). In this paper we compare the results obtained with two noise-assisted methods: Ensemble EMD (EEMD) and Noise Assisted Multivariate EMD (NA-MEMD). Given the inherent stochastic nature of noise-assisted methods, CD was treated as a randomized experiment. Hence, we repeated procedures to perform a robust statistical analysis. Confidence intervals, adequate normality conditions and differential causality hypothesis testing were then evaluated. The methodology was adjusted through the paradigmatic case of a forced mechanical oscillator, since the causal implications are known in advance. CD was then applied to a couple of series from the Earth system: insolation and oxygen isotope rate. Differential causality was established in favor of insolation at the frequency corresponding to the obliquity cycle. The apparent causality detected with EEMD on other time scales was discarded and attributed to a mode mixing problem. Therefore, NA-MEMD outperformed EEMD with less mode mixing and sharper mode alignment.

Study of vibration shock processes of non-linear mechanical systems with distributed parameters

In practice, under the conditions of perfection and constructive development of modern equipment and machines, nonlinear mechanical systems with distributed parameters are often encountered, which, depending on the principles of operation, are affected by vibration shock. Therefore, the study of vibration shock processes of the mentioned systems has great theoretical and practical importance and as a result to determine the optimal parameters of vibration protection devices to ensure their safe operation. In our case, the displacement field of two interacting non-linear mechanical systems with distributed parameters is considered, when their interaction is of vibration shock nature. Obviously, the mentioned events are more pronounced when the self-oscillation frequency of one or both systems momentarily approaches the frequency of forced vibration shock processes. In addition, critical moments are fixed during the phase shifts of forced oscillations of oscillatory systems, in this case, the frequencies of forced oscillations approach mutually opposing phase moments. By choosing the optimal parameters of hysteresis losses, it is possible to almost exclude sub-harmonic modes superimposed on the main resonance modes in vibration shock processes. During hysteresis losses of the parabolic type, the value of µ changes automatically in connection with impulsive loads, which will allow us to transfer the vibration shock processes to automatic modes and, accordingly, the practically safe operation of the mentioned systems.

Impact force of non-Boussinesq gravity-current head against a circular cylinder above the seabed

We study the impact force of the gravity-current head on a circular cylinder above the seabed for the non-Boussinesq effect using a large-eddy simulation model well validated against laboratory data. Most existing studies of the gravity current were for the slight density difference between the current and the ambient fluid, assuming the density is constant using the Boussinesq approximation. The aim of the present study, however, is about gravity current of significant density difference. We detail the coherent turbulence structure of the gravity current and the transient oscillation of the impact force on the cylinder over a range of density differences varying from (ρs−ρa)/ρa=0.02 to 2 for the elevation relative to the diameter of the cylinder E/D=0.05, 0.4 and 1.0. The velocity and vorticity field in the current show a gravity-current head separating from the primary current driven by excess pressure at the back, with a structural similarity almost independent of the density difference. The gravity-current head’s frontal and wake velocities are determined from the numerical simulations and related to the buoyancy speed. The impact force on the cylinder rises to a peak immediately after the arrival of the gravity-current head. We propose a set of new drag and lift coefficients using a gravity-driven pressure as a reference for the impact force of the non-Boussinesq current. The peak impact force, made dimensionless by the gravity-driven pressure, is found to be relatively independent of the density difference, regardless of how large the current density may be.

Nested mixed-mode oscillations in the forced van der Pol oscillator

The forced van der Pol oscillator has played a fundamental role in the development of nonlinear science. It is notable that the van der Pol oscillator in the absence of an AC forcing term explains the underlying mechanism that induces limit cycles and relaxation oscillations; the forced van der Pol oscillator was the first electric oscillator that, via measurements of the emitted sound, was inferred to exhibit chaotic behavior (van der Pol and van der Mark, 1927). This oscillator thus had a significant influence on the development of chaos theory. It was subsequently demonstrated, via a nonstandard analysis undertaken in the 1980s, that a canard explosion was present in the dynamics exhibited by this oscillator. In previous works (Inaba and Kousaka, (2020); Inaba et al., (2023)), we established the existence of bifurcation structures that are referred to as nested mixed-mode oscillations (MMOs); these structures are generated by a forced Bonhoeffer–van der Pol (BVP) oscillator. Nested MMOs, despite their complexity, are robust, repeatable, and composed of higher dimensional oscillations. In this study, we show that nested MMOs can also be produced by the forced van der Pol oscillator. This paper is of extreme importance as it demonstrates that nested MMOs are not a phenomenon that can only be observed in specific BVP systems. Based on the fact that nested MMO bifurcations also occur in the forced van der Pol oscillator, this study indicates that nested MMOs could represent a phenomenon that is more widely observed than previously believed.

Time of Consolidation and Humidity Influence on Properties of Food Powders

The effect of short-term storage at 75% and 90% ambient humidity on the mechanical properties of selected materials was determined using a new device for measuring the strength of food powders. A series of tests were conducted on wheat flour and potato starch subjected to various consolidation loads. The high accuracy and repeatability of the measurements confirmed the suitability of the pull-based tester for assessing the degree of caking in food powders. The pull-based tester allows for the measurement of strength parameters of agglomerates under various consolidation loads while simultaneously wetting the powder, introducing a novel approach to assessing the mechanical properties of powders. The analysis of force oscillation during the withdrawal of the measuring rod from the powder facilitates the identification of the slip-stick effect in these materials and the determination of parameters characterizing that phenomenon. The outcomes of this study may be of interest to farmers, manufacturers, and companies processing raw materials.

Standard technical specifications for methacholine chloride (Methacholine) bronchial challenge test (2023)

Standard technical specifications for methacholine chloride (Methacholine) bronchial challenge test (2023)

Review of: "Periodic second-order systems and coupled forced Van der Pol oscillators"

Review of: "Periodic second-order systems and coupled forced Van der Pol oscillators"

Fourier series-based approximation of time-varying parameters in ordinary differential equations

Many real-world systems modeled using differential equations involve unknown or uncertain parameters. Standard approaches to address parameter estimation inverse problems in this setting typically focus on estimating constants; yet some unobservable system parameters may vary with time without known evolution models. In this work, we propose a novel approximation method inspired by the Fourier series to estimate time-varying parameters (TVPs) in deterministic dynamical systems modeled with ordinary differential equations. Using ensemble Kalman filtering in conjunction with Fourier series-based approximation models, we detail two possible implementation schemes for sequentially updating the time-varying parameter estimates given noisy observations of the system states. We demonstrate the capabilities of the proposed approach in estimating periodic parameters, both when the period is known and unknown, as well as non-periodic TVPs of different forms with several computed examples using a forced harmonic oscillator. Results emphasize the importance of the frequencies and number of approximation model terms on the time-varying parameter estimates and corresponding dynamical system predictions.

Localization method of subsynchronous oscillation source based on high-resolution time-frequency distribution image and CNN

The penetration of new energy sources such as wind power is increasing, which consequently increases the occurrence rate of subsynchronous oscillation events. However, existing subsynchronous oscillation source-identification methods primarily analyze fixed-mode oscillations and rarely consider time-varying features, such as frequency drift, caused by the random volatility of wind farms when oscillations occur. This paper proposes a subsynchronous oscillation source-localization method that involves an enhanced short-time Fourier transform and a convolutional neural network (CNN). First, an enhanced STFT is performed to secure high-resolution time-frequency distribution (TFD) images from the measured data of the generation unit ports. Next, these TFD images are amalgamated to form a subsynchronous oscillation feature map that serves as input to the CNN to train the localization model. Ultimately, the trained CNN model realizes the online localization of subsynchronous oscillation sources. The effectiveness and accuracy of the proposed method are validated via multimachine system models simulating forced and natural oscillation events using the Power Systems Computer Aided Design platform. Test results show that the proposed method can localize subsynchronous oscillation sources online while considering unpredictable fluctuations in wind farms, thus providing a foundation for oscillation suppression in practical engineering scenarios.

Oscillatory Force Autocorrelations in Equilibrium Odd-Diffusive Systems.

The force autocorrelation function (FACF), a concept of fundamental interest in statistical mechanics, encodes the effect of interactions on the dynamics of a tagged particle. In equilibrium, the FACF is believed to decay monotonically in time, which is a signature of slowing down of the dynamics of the tagged particle due to interactions. Here, we analytically show that in odd-diffusive systems, which are characterized by a diffusion tensor with antisymmetric elements, the FACF can become negative and even exhibit temporal oscillations. We also demonstrate that, despite the isotropy, the knowledge of FACF alone is not sufficient to describe the dynamics: the full autocorrelation tensor is required and contains an antisymmetric part. These unusual properties translate into enhanced dynamics of the tagged particle quantified via the self-diffusion coefficient that, remarkably, increases due to particle interactions.

A convergence‐enhanced Timoshenko beam element for the stochastic nonlinear analysis of reinforced concrete components

AbstractAn effective and precise physical model is generally required for the performance assessment of reinforced concrete (RC) components. In this study, a convergence‐enhanced Timoshenko beam element considering finite deformation is proposed for the stochastic nonlinear analysis of RC components. The unified framework of the rank 2 correction matrix is obtained by approximating the iterative matrix through the linearly independent basis vectors, and two types of Broyden–Fletcher–Goldfarb–Shanno (BFGS) operators are constructed. Then, a consistent solution scheme with superior numerical stability and efficiency is proposed based on the second kind of BFGS operator and improved quasi‐Newton method. Accurate discretization of the displacement fields and description of nonlinear sectional behavior are achieved by the high‐order Lagrangian interpolation functions, concrete damage‐plasticity model and J2 plasticity model. In an updated Lagrangian formulation, these shape functions and constitutive relationships are used to develop a complete secant stiffness with higher‐order terms in the strain tensor and residual force vector. To validate the proposed Timoshenko beam element, numerical applications involving material nonlinearity and geometrical nonlinearity are conducted. The numerical stability and efficiency of the solution scheme, the shear‐locking free technique, and the oscillation of axial force are thoroughly examined and discussed by solution of RC columns, cantilever beam and progressive collapse of planar RC frame. Finally, the parametric study considering the geometric and material randomness indicates that the RC column designed for flexural failure mode suffered considerable degradation of loading capacity and a larger scatter of residual load.

Comparison of Complex Sadik and KAJ Transforms for Ordinary Differential Equations to the Response of an Uncompressed Forced Oscillator

In this paper we have presented a comparison between two novel integral transformations that are of great importance in the solution of differential equations. These two transformations are the complex Sadik transform and the KAJ transform. An uncompressed forced oscillator, which is an important application, served as the basis for comparison. The application was solved and exact solutions were obtained. Therefore, in this paper, the exact solution was found based on two different integral transforms: the first integral transform complex Sadik and the second integral transform KAJ. And these exact solutions obtained from these two integral transforms were new methods with simple algebraic calculations and applied to different problems. The main purpose of this comparison is the exact solutions, and until we show the importance of the diversity and difference of the kernel of the integral transform by keeping the period t between 0 and infinity.

Dynamic mechanical properties of graphene and carbon fabric‐reinforced epoxy nanocomposites

AbstractThe present work attempts to estimate the impact of graphene and carbon fabric on the dynamic‐mechanical properties of epoxy nanocomposites with different weight percentages. By applying an oscillatory force to a composite, its response is measured, and by calculating the viscosity and the stiffness in relation to the temperature, time, or frequency, valuable information can be realized. It helps to identify “thermal transitions” occurring in polymers. The complex modulus shows the resistance of the composites to deformation caused by viscous and elastic effects. Though dynamic mechanical analysis (DMA) is used for the depiction of temperature‐dependent viscoelastic properties, in this investigation, the objectives were multi‐fold. It is aimed at understanding the role of carbon fabric and the incorporation of 1 wt% of graphene nanoplatelet (GNP) in the epoxy matrix, and the effect of 0.5 to 5 wt% of GNP. The dynamic mechanical properties, including storage modulus, loss modulus, and damping factor, are examined across a temperature range of 25–250°C. Based on the findings, the storage modulus of the composites exhibits a range between 2000 and 9000 MPa. Notably, the epoxy composite containing 1 wt% GNP filler demonstrates the highest storage modulus. The details of mechanisms involved in DMA measurement are schematically summarized.Highlights Epoxy‐GNP and Epoxy‐GNP‐carbon fabric composites were fabricated. Storage modulus of composites does not show much dependence on the GNP. An increase in storage modulus by 05 times is observed in the ECG1 composite. The loss modulus of EC and EG1 composite is much higher.