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In recent years, the frequency of forced oscillation events due to control system malfunctions or improper parameter settings has increased. Tuning the parameters of exciters and governor models is crucial for maintaining power system stability. Traditional simulation studies typically involve small transient disturbances or step changes to find optimal parameter sets, but existing optimization algorithms often fall short in fine-tuning for forced oscillations. Identifying the sensitive parameters within these control models is essential for ensuring stability during large, sustained disturbances. This study focuses on identifying these critical exciter and governor model parameters by analyzing their influence on sustained forced oscillations. Using Kundur's two-area system, we analyze common exciter models such as SCRX, ESST1A, and AC7B, along with governor models like GAST, HYGOV, and GGOV1, utilizing PSS®E software version 34. Sustained forced oscillations are injected at generator-1 of area-1, with individual parameter changes dynamically simulated. By considering a local oscillation frequency of 1.4 Hz and an inter-area oscillation mode of 0.25 Hz, we analyze the impact of each parameter change on the magnitude and frequency of forced oscillations as well as on active and reactive power outputs. This novel approach highlights the most influential parameters of each tested model—such as exciter, governor, and turbine gains, as well as time constant parameters—on the impact of forced oscillations. Based on our findings, the sensitive parameters of each tested model are ranked. These would provide valuable insights for industry operators to fine-tune control settings during oscillation events, ultimately enhancing system stability. [ABSTRACT FROM AUTHOR]

The synchronous reference frame phase-locked loop (SRF-PLL), also called dqPLL, is an electric circuit commonly used in power electronics to estimate the phase angle of a three-phase AC grid. If the voltage is unbalanced, the PLL is modeled as a periodically forced nonlinear oscillator and is known in practice to converge to a steady oscillation. In the existing literature, the oscillation has been studied via linearization assuming a low level of unbalance. Aiming for stronger nonlocal statements, we present nonlinear analysis. We apply the method of autonomous comparison systems and incremental stability to show that the steady oscillation is unique and attractive in a wider neighborhood. Its lock-in domain is estimated using numerical phase portrait analysis. The oscillation is further approximated up to the terms of the second order in the unbalance factor—it yields an estimation of the time average of the PLL's phase error which is not visible by linearization only. The results provide stability guarantees and can guide the tuning of SRF-PLL. [ABSTRACT FROM AUTHOR]

Natural gas mixed with hydrogen combustion is one of the important measures to reduce carbon emissions of gas turbine. However, the composition change of fuel will lead to changes in the flame structure and combustion stability of the combustor. In order to analyze the combustion instability of natural gas mixed with hydrogen combustion in the central staged burner, the effects of different hydrogen doping ratios on the transient flame structure, pressure and heat release response of the central staged combustion are experimentally studied. The proper orthogonal decomposition (POD) method is used to extract the characteristic modes of flame pulsation. It is found that the flame pulsation mainly includes two modes: a strong pulsation in the interference zone of flame and an axial disturbance. The experimental results show that as the volume ratio of hydrogen doped increases from 0% to 30%, the flame front moves upstream, the spacing between two staged flames is shortened, the energy proportion of pulsation mode corresponding to the flame interference increases, and the coupling of pressure and heat release is strengthened, which ultimately results in a 9% increase in pressure response and a 37% increase in heat release response in the combustor.

Copyright of Journal of Shanghai Jiao Tong University (1006-2467) is the property of Journal of Shanghai Jiao Tong University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Time series motifs are used for discovering higher-order structures of time series data. Based on time series motifs, the motif embedding correlation field (MECF) is proposed to characterize higher-order temporal structures of dynamical system time series. A MECF-based unsupervised learning approach is applied in locating the source of the forced oscillation (FO), a periodic disturbance that detrimentally impacts power grids. Locating the FO source is imperative for system stability. Compared with the Fourier analysis, the MECF-based unsupervised learning is applicable under various FO situations, including the single FO, FO with resonance, and multiple concurrent FOs. The MECF-based unsupervised learning is a data-driven approach without any prior knowledge requirement of system models or typologies. Tests on the UK high-voltage transmission grid are conducted to validate the effectiveness of MECF-based unsupervised learning. In addition, the impacts of coupling strength and measurement noise on locating the FO source by the MECF-based unsupervised learning are investigated. Simulation results show that within typical ranges of coupling strength and measurement noise standard deviation of power systems, the MECF-based unsupervised learning is completely correct in locating the single FO, FO with resonance, and multiple concurrent FOs. [ABSTRACT FROM AUTHOR]

The unsteady characteristics of the second throat of a transonic wind tunnel have an important influence on the design and test of the wind tunnel. Therefore, the forced oscillation characteristics were studied by a numerical simulation method. The governing equation was the viscous compressible unsteady Navier–Stokes equation. Under the sinusoidal pressure disturbance of the computational domain exit, the shock wave presents a clear forced oscillation state, and the shock wave periodically changes its position. Under a pressure disturbance of 1%, the shock wave displacement reaches 150 mm. Additionally, overshoot occurs when the shock moves upstream or downstream. The shock-boundary layer interference is very sensitive to the motion characteristics of the shock wave, resulting in a transformation of the flow field symmetry. The flow field downstream of the shock wave exhibits periodic structural changes. Compared with the pressure change at the outlet, the pressure change near the shock wave has a phase delay. The increasing disturbance near the shock wave shows a clear amplification effect. The pressure disturbance near the shock wave had an obvious amplification effect, and its fluctuation amount reached 16% under the pressure disturbance of 1%. The variation trend of the second throat wall force, wavefront Mach number, and Mach number in the test section with time is similar to that of the downstream disturbance, but it does not have a complete follow-up effect, which indicates that the pressure disturbance can propagate into the test section through the boundary layer or the shock gap. Nevertheless, the second throat choking can still control the Mach number stability of the test section. The dynamic characteristics of shock oscillation are related to the amplitude and frequency of the applied pressure disturbance. The shock displacement decreases with the increase in the excitation frequency. When the excitation frequency is higher than 125 Hz, the flow field basically does not change. [ABSTRACT FROM AUTHOR]

Copyright of Knowledge: International Journal is the property of Institute for Knowledge Management and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Oscillation source location is the key to suppressing the forced oscillation and guaranteeing power system stability. For forced oscillation events with clear disturbance sources, locating the source of the oscillation quickly and accurately is the primary task to achieve forced oscillation suppression. The existing methods cannot make full use of the spatial topology information of the power grid and the temporal characteristics of oscillations, which limits the location accuracy. Therefore, a forced oscillation location method based on temporal graph convolution network is proposed. Firstly, the graph data is constructed according to node features and topology information of the power grid, and the oscillation space features are extracted by graph convolution neural network. At the same time, the temporal correlation of oscillation data of multiple nodes is extracted by the gated recurrent unit neural network. The spatial and temporal characteristics are fused by the spatiotemporal graph convolution unit. Then, the forced oscillation location is modeled as a classification problem, and the location model based on temporal graph convolution neural network is trained. The case analysis shows that the method proposed has higher accuracy and better generalization. The method proposed still has good performance in the case of data noise and missing.

The planar inverted pendulum with a vibrating pivot point in the presence of an additional horizontal force field is studied. The horizontal force is not assumed to be small or rapidly oscillating. We assume that the pivot point of the pendulum rapidly oscillates in the vertical direction and the period of these oscillations is commensurable with the period of horizontal force. This system can be considered as a strong generalization of the Kapitza pendulum. Previously it was shown that for any horizontal force there always exists a non-falling periodic solution in the considered system. In particular, when there is no horizontal force, this periodic solution is the vertical upward position. In the paper we present analytical and numerical results concerning the existence of asymptotically stable non-falling periodic solutions in the system. [ABSTRACT FROM AUTHOR]

In this paper, we study the forced oscillation of solutions of a fractional partial differential system with damping terms by using the Riemann-Liouville derivative and integral. We obtained some new oscillation results by using the integral averaging technique. The obtained results are illustrated by using some examples.

The paper deals with the second order nonhomogeneous linear differential equation \[(p(t) y'(t))' + q(t) y(t) = f(t),\] which is oscillatory under the assumption that \(p(t)\) and \(q(t)\) are positive, continuously differentiable and monotone functions on \([0,\infty)\). Throughout this paper we shall use pairs of quadratic forms, which obtained by different methods than Kusano and Yoshida. This form will lead to a property of qualitative behavior, including amplitudes and slopes, of oscillatory solutions of the above equation. In addition, we will discuss the existence of three types (moderately bounded, small, large) of oscillatory solutions, which are based on results due to Kusano and Yoshida.

There exists the subsynchronous oscillation (SSO) instability risk in large-scale photovoltaic(PV) grid-connected systems with series compensation, which is generally explained by the negative damped oscillation theory. In this paper, the inter-photovoltaic harmonics due to maximum power point tracking (MPPT) control are used as the disturbance source and the large-scale PV grid-connected system with series compensation as the forced system. The forced oscillation theory is used to reveal the SSO mechanism of PV power generation based on the interaction between the perturbed MPPT and the series compensation grid-connected system, and verified in the PSCAD/EMTDC simulation platform. The results show that the perturbed MPPT-based PV inverter outputs interharmonic currents to the system due to the modulation coupling on the AC-DC side, which may lead to serious forced SSO problems when the interharmonic frequency is close to the frequency of inherent weakly damped mode of the system, causing a shock to the system stability. The simulation results verify the correctness of the proposed theory.

An approach is presented to investigate the 1:2 internal resonance of the sling and beam of a suspension sling–beam system. The beam was taken as the geometrically linear Euler beam, and the sling was considered to be geometrically nonlinear. The dynamic equilibrium equation of the structures was derived using the modal superposition method, the D'Alembert principle and the Hamilton principle. The nonlinear dynamic equilibrium equations of free vibration and forced oscillation were solved by the multiple-scales method. We derived the first approximation solutions for the single-modal motion of the system. Numerical examples are provided to verify the correctness of formula derivation and obtain the amplitude–time response of free vibration, the primary resonance response, the amplitude–frequency response, and the amplitude–force response of forced oscillation. According to the analysis, it is evident that the combination system exhibits robust nonlinear coupling properties due to the presence of internal resonance, which are useful for engineering design. [ABSTRACT FROM AUTHOR]

The tug of war is a sport known for strength, however a weaker team can also win the game by a suitable team co-operation. A mathematical model is established, showing that the team co-operation or rhythmical frequency plays an important role in victory. A team can win even the rope is pulled to the opposite direction depending upon the rhythmical frequency. A criterion for rhythmical frequency is obtain to guarantee victory when the strength is almost same for both teams. Additionally the rope pulling can be also used for moving a heavy weight object, the principle might be used for building the great pyramids in ancient Egypt. Finally magic ramie ropes with special thermoplastic properties and controllable frequency are discussed. [ABSTRACT FROM AUTHOR]

ObjectiveIn order to achieve the accurate prediction of the amplitude response of a vortex-induced vibrating cylinder under the sub-critical Reynolds number, a method for establishing a Cl-A/D (lift coefficient-amplitude ratio)model of the forced vibration of the cylinder by numerical simulation is proposed.MethodsBased on the Realizable k-ε model, a two-dimensional numerical simulation of the forced vibration of a cylinder is carried out using the finite volume method. The calculated lift coefficient curves under different amplitude ratios A/D in the range of excitation frequency ratio fe/fn=1 are obtained. The lift coefficient corresponding to the maximum vibration velocity of the cylinder is then selected to establish the Cl-A/D model. ResultsThe results show that the overall trend of the Cl-A/D fitting curve is in good agreement with the predicted results of SHEAR7. At the same time, it is found that the "zero lift coefficient" points under each excitation frequency ratio fe/fn are all located near the amplitude ratio A/D=0.8, and the wake shedding mode changes around A/D=0.8 from "P+S" to "2P" (P represents a pair of vortex shedding with opposite rotation directions, and S represents a single vortex shedding). In the vortex-induced vibration experiment of a single cylinder, the maximum amplitude when "lock-in" occurs is around 0.8D. ConclusionsThe amplitude ratio corresponding to the "zero lift coefficient" of the Cl-A/D model of forced vibrating cylinder under sub-critical Reynolds number is consistent with the maximum response amplitude ratio of the cylinder under vortex-induced vibration, and the shedding mode of the wake vortex changes under this amplitude ratio.

Abstract The most common explanation for the depletion of high frequency waves that defines low‐frequency earthquakes (LFEs) and very low‐frequency earthquakes (VLFEs) is that fault rupture and slip are slower than typical earthquakes. However, it is difficult to rule out the possibility that the high frequency waves are produced during slip, but attenuated near the LFE source. One reason this hypothesis has been poorly tested is that there are no measurements of attenuation on the relevant rocks. We present the results of forced oscillation experiments that measure the frequency‐dependent attenuation of a chlorite‐rich metapelitic schist, a lithology found along the subduction plate boundary where LFEs and VLFEs have been documented. Experiments were run on dry and water‐saturated schist at effective pressures of 2–10 MPa and at frequencies of 2 × 10−5–30 Hz. We find that pore fluids and low effective pressure result in the attenuation of high frequencies. The frequency‐dependent attenuation is consistent with the concomitant operation of two wave‐induced fluid flow mechanisms, squirt flow, and patchy saturation. When the effects of these mechanisms are extrapolated to geologic conditions using rock physics models, our results predict that attenuation is capable of completely diminishing the frequencies depleted in LFEs and VLFEs. Therefore, LFEs and VLFEs may not necessarily record slow fault slip, but possibly the presence of high fluid pressure.

In this paper, we study the forced oscillation of solutions of a fractional partial differential system with damping terms by using the Riemann-Liouville derivative and integral. We obtained some new oscillation results by using the integral averaging technique. The obtained results are illustrated by using some examples. [ABSTRACT FROM AUTHOR]

The unsteady characteristics of the second throat of a transonic wind tunnel have an important influence on the design and test of the wind tunnel. Therefore, the forced oscillation characteristics were studied by a numerical simulation method. The governing equation was the viscous compressible unsteady Navier–Stokes equation. Under the sinusoidal pressure disturbance of the computational domain exit, the shock wave presents a clear forced oscillation state, and the shock wave periodically changes its position. Under a pressure disturbance of 1%, the shock wave displacement reaches 150 mm. Additionally, overshoot occurs when the shock moves upstream or downstream. The shock-boundary layer interference is very sensitive to the motion characteristics of the shock wave, resulting in a transformation of the flow field symmetry. The flow field downstream of the shock wave exhibits periodic structural changes. Compared with the pressure change at the outlet, the pressure change near the shock wave has a phase delay. The increasing disturbance near the shock wave shows a clear amplification effect. The pressure disturbance near the shock wave had an obvious amplification effect, and its fluctuation amount reached 16% under the pressure disturbance of 1%. The variation trend of the second throat wall force, wavefront Mach number, and Mach number in the test section with time is similar to that of the downstream disturbance, but it does not have a complete follow-up effect, which indicates that the pressure disturbance can propagate into the test section through the boundary layer or the shock gap. Nevertheless, the second throat choking can still control the Mach number stability of the test section. The dynamic characteristics of shock oscillation are related to the amplitude and frequency of the applied pressure disturbance. The shock displacement decreases with the increase in the excitation frequency. When the excitation frequency is higher than 125 Hz, the flow field basically does not change.

In this paper, an authentic near-real-time methodology is proposed for locating the sources of FOs using the fundamental frequency of the periodic external driving force based on the direction of the potential energies exchanged between the synchronous generators. First, A directed graph of the network reduced to the generator buses is defined in which the weight and direction of its branches are assigned based on a well-defined energy-driven function. Next, based on the cosine part appearing in the edge weights, subsequent decisions are made, depending on the generators' angular velocity and active power phases. The proposed algorithm can make a correct and swift diagnosis even under the most challenging operational circumstances, such as resonances and limit cycles. The methodological procedure has been validated on the WECC 3-machine 9-bus and New England 10-machine 39-bus benchmark power systems modeled in the Real-Time Digital Simulator and then analyzed in the MATLAB environment. The simulation results revealed that the presented approach reliably releases all different FOs sources with various kinds of origin. [ABSTRACT FROM AUTHOR]

The most challenging circumstance of forced oscillations (FOs) is when the power system is forced to oscillate at its natural frequencies. This paper uses a novel PMU data-driven mechanism to pinpoint the source of such phenomena under resonance. Following the detection of FOs, the instantaneous changes in the output power and angular velocity of the rotors are calculated. Accordingly, an energy-driven multilateral interaction pattern is obtained for all synchronous generators. Next, an appropriate positive weighted undirected graph is constructed through these functional patterns based on the spectral graph theory. These quantitative indicators are then analyzed through the eigenvalue spectrum of the normalized Laplacian matrix of the system graph reduced to the internal generator buses. Finally, the smallest value in eigenvectors corresponding to the two largest eigenvalues reveals the location of the source. The proposed methodology's validation and verification studies have been performed on the WECC 3-machine 9-bus and New England 10-machine 39-bus benchmark power systems modeled in the Real-Time Digital Simulator (RTDS) and then analyzed in the MATLAB environment. The proposed methodology revealed to be fast and accurate in locating the source of FOs under challenging resonance situations with promising results while addressing the generator side origins. [ABSTRACT FROM AUTHOR]

Abstract With the continuous expansion of power systems and the application of power electronic equipment, forced oscillation has become one of the key problems in terms of system safety and stability. In this paper, an interline power flow controller (IPFC) is used as a power suppression carrier and its mechanism is analyzed using the linearized state-space method to improve the system damping ratio. It is shown that although the IPFC can suppress forced oscillation with well-designed parameters, its capability of improving the system damping ratio is limited. Thus, combined with the repetitive control method, an additional repetitive controller (ARC) is proposed to further dampen the forced power oscillation. The ARC control scheme is characterized by outstanding tracking performance to a system steady reference value, and the main IPFC controller with the ARC can provide higher damping, and further reduce the amplitude of oscillations to zero compared with a supplementary damping controller (SDC). Simulation results show that the IPFC with an ARC can not only greatly reduce the oscillation amplitude, but also actively output the compensation power according to the reference value of the ARC tracking system.

An approach is presented to investigate the 1:2 internal resonance of the sling and beam of a suspension sling–beam system. The beam was taken as the geometrically linear Euler beam, and the sling was considered to be geometrically nonlinear. The dynamic equilibrium equation of the structures was derived using the modal superposition method, the D’Alembert principle and the Hamilton principle. The nonlinear dynamic equilibrium equations of free vibration and forced oscillation were solved by the multiple-scales method. We derived the first approximation solutions for the single-modal motion of the system. Numerical examples are provided to verify the correctness of formula derivation and obtain the amplitude–time response of free vibration, the primary resonance response, the amplitude–frequency response, and the amplitude–force response of forced oscillation. According to the analysis, it is evident that the combination system exhibits robust nonlinear coupling properties due to the presence of internal resonance, which are useful for engineering design.

This article introduces an innovative approach for online detection and localization of forced oscillation (FO) in power systems with significant integration of wind turbine generation (WTG). The synergistic combination of Multichannel Prony analysis with Periodogram and dissipating energy flow (DEF) approach is proposed in this paper to overcome the challenges inherent in detecting FO amidst the complexities introduced by WTG integration. An adaptive version of multichannel Prony analysis is proposed in this paper that dynamically estimates Prony's system order via the recursive process. It estimates the frequency and damping ratio of all oscillatory modes, which is used to identify low damping modes that are potentially attributed to FO. Leveraging this information, the proposed method enhances both FO detection and source localization capabilities. The proposed method is verified on measurements taken from IEEE benchmark 4-machine, 11-bus system and 10-machine, 39-bus system with high wind energy penetration. Comprehensive testing demonstrates the proposed approach's effectiveness across various FO scenarios, including governor valve malfunction, exciter malfunction, and cyclic loads. Furthermore, the methodology is evaluated under resonant conditions and scenarios involving the simultaneous identification of multiple disturbances. • Detection & source location in WTG integrated power system. • MDMMO for dynamic estimation of Prony model order. • OFDM using Periodogram technique. • OFSLM using dissipating energy flow method. [ABSTRACT FROM AUTHOR]

Forced oscillation events have become a challenging problem with the increasing penetration of renewable and other inverter-based resources (IBRs), especially when the forced oscillation frequency coincides with the dominant natural oscillation frequency. A severe forced oscillation event can deteriorate power system dynamic stability, damage equipment, and limit power transfer capability. This paper proposes a two-dimension scanning forced oscillation grid vulnerability analysis method to identify areas/zones in the system that are critical to forced oscillation. These critical areas/zones can be further considered as effective actuator locations for the deployment of forced oscillation damping controllers. Additionally, active power modulation control through IBRs is also proposed to reduce the forced oscillation impact on the entire grid. The proposed methods are demonstrated through a case study on a synthetic Texas power system model. The simulation results demonstrate that the critical areas/zones of forced oscillation are related to the areas that highly participate in the natural oscillations and the proposed oscillation damping controller through IBRs can effectively reduce the forced oscillation impact in the entire system. [ABSTRACT FROM AUTHOR]

In this study, a new algorithm dealing with time-varying modes for determining and tracing multiple frequency oscillations in power systems is proposed. Multiple modes or resonance of forced and natural oscillations can have severe effects on a power system. Therefore, it is crucial to recognize the dominant natural and forced oscillations because the oscillation mode possesses time-varying features that depend on the system operating conditions or changes in the parameters. The salient features of the proposed algorithm include the use of a time-series-based approach to recognize undesired modes (including multiple oscillations over a wide frequency range), tracing time-varying modes as the power system operating condition changes, and effectively determining and applying the oscillation features before implementing the corresponding control measures in the power system. In this study, multiple frequency oscillation scenarios for the test system and practical measurement data for a recent incident that occurred in Korea Electric Power Corporation (KEPCO) system are discussed. Therefore, the proposed algorithm can be practically applied in wide-area monitoring systems, not only for a single forced oscillation or local mode detection but also for system-wide inter-area mode recognition.

In future power systems, conventional synchronous generators will be replaced by converter controlled-based generations (CCGs), i.e., wind and solar generations, and battery energy storage systems. Thus, the paradigm shift in power systems will lead to the inferior system strength and inertia scarcity. Therefore, the problems of forced oscillation (FO) will emerge with new features of the CCGs. The state-of-the-art review in this paper emphasizes previous strategies for FO detection, source identification, and mitigation. Moreover, the effect of FO is investigated in a power system with CCGs. In its conclusion, this paper also highlights important findings and provides suggestions for subsequent research in this important topic of future power systems.

Forced oscillation is challenging power system stability. Most previous research was based on linear models, so analysis of nonlinear characteristics of forced oscillation in power system is missing. Thus, this letter aims to theoretically reveal the effect of the quadratic nonlinearity on forced oscillation. A multi-scale technique is employed, by which the amplitude-frequency function of the steady response and the approximated first-order analytical expression of the dynamic response are derived for nonlinear forced oscillation. Interesting phenomena of frequency deviation and jumping caused by nonlinearity are found and explained. Above expressions and phenomena are verified by numerical simulation. Although revealing all characteristics of nonlinear forced oscillation in power system is difficult, the analysis results in this letter could offer insights. [ABSTRACT FROM AUTHOR]

A generation unit that is poorly damped or has periodical mechanical power outputs from its prime mover can become a source of oscillation in a bulk power system. Choosing the proper electric quantity to extract the oscillation characteristics is crucial for the location and mechanism analysis of the oscillation source. This paper proposes the concept of oscillation phasor to characterize a type of sustained power system oscillations that originate from power plants. On this basis, a two-stage scheme is presented to locate one type of the sustained oscillation source and discover its mechanism: first, Prony analysis and the first wave algorithm are applied to extract the oscillation phasors on active power and angular frequency; second, the phase relationships between the oscillation-source generator and other generators are derived to guide the location of the source; finally, damping estimation on these oscillation phasors help distinguish a forced oscillation from a poorly damped oscillation. The proposed scheme is first demonstrated in detail on a two-area four-machine system and then tested using historical oscillation event data.

The motion of a large gas bubble in a liquid-filled cell was experimentally and theoretically investigated under microgravity and forced vibration conditions. The experiment was conducted aboard the Space Shuttle during the STS-85 mission using two shallow, cylindrical fluid cells (97.3 mm I.D. × 16.8 mm thickness), each containing a mixture of air/mineral oil (cell #1) and air/water-surfactant solution (cell #2). In microgravity, a large air bubble formed in the middle of the cell, and the cell was oscillated in the direction normal to the cell axis at a specified frequency and amplitude. The bubble responded to periodic cell vibration by translating within at the same frequency but with a different amplitude, and deforming its shape from a circular cross-section to an oval shape. The bubble translation amplitude was found to vary linearly with the cell translation amplitude for both fluid systems. For the low viscosity system (cell #2), the bubble translation amplitude was weakly dependent on the cell vibration frequency and acceleration level. Resonance phenomena were observed at the lowest cell vibration frequencies tested (0.1–0.2 Hz) at which the shape deformation became periodic and the bubble translation amplitude was larger than the values expected from a linear response. A theoretical model was also developed to predict the bubble translation amplitude for both infinite and finite cell sizes, and an inviscid or viscous liquid. The linear two-dimensional model predictions compared well with the measured bubble translation data. [ABSTRACT FROM AUTHOR]

Spirometry is considered the gold standard method for monitoring lung function of patients with cystic fibrosis (CF) but it requires patients’ cooperation and therefore it is not useful for the majority of preschool-aged children. Oscillometry is an alternative modality for lung function monitoring that requires minimal cooperation and can be applied in children as young as 3 years of age. Furthermore, it generates lesser aerosol compared to spirometry, an issue that is of considerable importance in the COVID-19 era. The aim of this review was to present the existing clinical data regarding the application of oscillometry in children and adolescents with CF. The method seems to have acceptable feasibility and repeatability. However, there is conflicting data regarding the correlation of oscillometry values with the clinical symptoms of CF patients either in clinically stable or in exacerbation periods. Furthermore, it is not clear to what extent oscillometry measurements correlate with the spirometry indices. Based on current evidence, spirometry cannot be substituted by oscillometry in the monitoring of the respiratory status of children and adolescents with CF. [ABSTRACT FROM AUTHOR]

In this study, experimental measurements are performed on a swirl-stabilized combustor to study the nonlinear combustor dynamic characteristics under acoustic excitations. The imposed acoustic disturbances with different frequencies and amplitudes generated by a loudspeaker are applied to the combustion system. The acoustic pressure oscillations and heat release rate fluctuations are measured using acoustic pressure transducers and a photomultiplier tube equipped with a bandpass OH* filter, respectively. Various parametric studies are conducted to evaluate the effects of the equivalent ratio ϕ , amplitude Af, and frequency ff of the acoustic excitations on the combustor dynamics. The obtained results indicate that the combustor experiences frequency locking-in phenomenon accompanied by high amplitude acoustic oscillations when ff is close to the natural frequency fa of the combustor quarter-wave mode. Such physical phenomena are observed even for a low forcing wave amplitude. However, heat release rate fluctuations reach saturation at high amplitude excitations. Moreover, the combustor experiences super-harmonic resonances when ff is close to fa/2 or fa/3. Under such conditions, high amplitude excitations are required to stimulate the natural resonant mode, while the heat release rate fluctuations rarely reach saturation. Cross-correlation and cross-power spectral density analyses on the OH* chemiluminescence and acoustic pressure signals reveal that super-harmonic resonances are caused by "quasi-couplings" between the acoustic pressure and unsteady heat release. Further, the results demonstrate that the excited natural mode oscillations affect the acoustic wave characteristics at the inlet section, while they hardly influence the heat release rate fluctuations. The obtained results indicate that the equivalence ratio has a negligible effect on the combustor dynamics under forced conditions. Finally, discussions are provided to address the influences of the excited natural mode oscillations on the experimental measurements of the flame describing function (FDF). The achieved acoustic velocity disturbances at the injector outlet are suppressed by excited natural mode oscillations, which confine the velocity disturbance amplitude within a narrow range when ff matches fa. Hence, excited natural mode oscillations should be avoided in practice if we need to measure the FDF under a wide range of acoustic velocity amplitudes. The nonlinear characteristics of forced oscillations studied in this paper are crucial to the development of control strategy and FDF measurement. [ABSTRACT FROM AUTHOR]

The most challenging circumstance of forced oscillations (FOs) is when the power system is forced to oscillate at its natural frequencies. This paper uses a novel PMU data-driven mechanism to pinpoint the source of such phenomena under resonance. Following the detection of FOs, the instantaneous changes in the output power and angular velocity of the rotors are calculated. Accordingly, an energy-driven multilateral interaction pattern is obtained for all synchronous generators. Next, an appropriate positive weighted undirected graph is constructed through these functional patterns based on the spectral graph theory. These quantitative indicators are then analyzed through the eigenvalue spectrum of the normalized Laplacian matrix of the system graph reduced to the internal generator buses. Finally, the smallest value in eigenvectors corresponding to the two largest eigenvalues reveals the location of the source. The proposed methodology’s validation and verification studies have been performed on the WECC 3-machine 9-bus and New England 10-machine 39-bus benchmark power systems modeled in the Real-Time Digital Simulator (RTDS) and then analyzed in the MATLAB environment. The proposed methodology revealed to be fast and accurate in locating the source of FOs under challenging resonance situations with promising results while addressing the generator side origins.

This paper presents the viscous flow predictions for fixed, slender and cylindrical Oscillating Water Columns (OWCs). Comparisons with linear potential solutions indicate nonlinearities in the scattered wave and the internal water surface elevations. The predictions also confirm high-frequency contents and asymmetrical shapes in the forces acting on the water columns. To identify the physical causes of the nonlinearities, various geometries, excitation conditions and mechanisms experienced by the OWCs are considered. This study confirms that a nonlinear wave scattering by the cylindrical structures occurs yet has little implication for the nonlinearities in the internal water surface elevations and the forces. In contrast, the vortex shedding at the lower edges of the OWCs and the nonlinear inertia forcing instigate the nonlinear characteristics of the elevations. The vortex shedding also provokes the nonlinearities in the acting forces. While the influence of the vortex shedding on the second-harmonic air force diminishes, the role of the water column oscillation on the harmonics becomes increasingly important at large amplitudes of internal surface elevation. Given the improved physical understanding, practical implications concerning the guidance on developing an appropriate time-domain modelling and improving the hydrodynamic efficiency of an OWC arise in this study. • Fixed OWCs undergoing forced oscillations and wave excitations are studied. • Viscous flow and linear potential solutions are compared to investigate the OWCs. • Nonlinear inertia forces and vortex shedding cause nonlinear internal elevations. • Vortex shedding generates zeroth and second-harmonics in inline pressures. • Vortex shedding and water column oscillation drive nonlinearities in air forces. [ABSTRACT FROM AUTHOR]

Abstract Based on the properties of nonlocal fractional calculus generated by conformable derivatives, we establish some sufficient conditions for oscillation of all solutions for fractional differential equations with damping term. Forced oscillation of conformable differential equations in the frame of Riemann, as well as of Caputo type, is established. Examples are provided to demonstrate the effectiveness of the main results.

The forced shock train oscillation is a common phenomenon in a practical working scramjet isolator. To evaluate the similarities and differences between the forced normal shock train oscillation and the forced oblique shock train oscillation, experiments were conducted in a rectangular duct at Mach 2 and 3. The normal/oblique shock trains were forced to oscillate for different back-pressure levels by changing the mechanical throttling ratio. High-frequency pressure measurements were mainly utilized for data acquisition and analysis. The frequency contents, the intermittency characteristics and the perturbation propagation of forced shock train oscillations were investigated in this work. Experimental results illustrate that the forced oscillation at Mach 3 is a composite pattern, which is dominated by the excited frequency and coupled with the inherent unsteadiness. This phenomenon is not obvious for the forced oscillation at Mach 2. For the forced oscillations at Mach 2 and 3, the maximum zero-crossing frequency is close to the downstream excitation frequency, and the whole intermittent region almost holds this value, which is very different from the self-excited oscillation pattern. By using the cross-spectral analysis, the frequency-dependent correlation was evaluated. It shows that the excitations and its harmonic contents can propagate upstream to affect the intermittent region and its downstream. Besides the excitation frequency and its harmonic contents, low-frequency contents (tens to more than a hundred Hz) can propagate nearby the separation region, and the propagation to downstream the separation bubble is very weak. Considering the time-lag between different transducers, the propagation speed of perturbations can be obtained. It implies that the propagation speed is independent of the magnitude of the back-pressure, but affected by the velocity of the incoming flow. Specifically, the higher the freestream Mach number is, the greater the propagation speed will be. • The forced oscillation phenomenon of normal/oblique shock trains were experimentally studied. • The maximum zero-crossing frequency is very close to the excited frequency content. • Strong correlations are found at the excited frequency and its harmonic contents. • The propagation speed of perturbations is affected by the velocity of incoming flow. [ABSTRACT FROM AUTHOR]

Hyperpolarized helium-3 MRI (3He MRI) provides detailed visualization of low-(hypo- and non-) ventilated lungs. Physiological measures of gas mixing may be assessed by multiple breath nitrogen washout (MBNW) and of airway closure by a forced oscillation technique (FOT). We hypothesize that in patients with asthma, areas of low-ventilated lung on 3He MRI are the result of airway closure. Ten control subjects, ten asthma subjects with normal spirometry (non-obstructed), and ten asthmatic subjects with reduced baseline lung function (obstructed) attended two testing sessions. On visit one, baseline plethysmography was performed followed by spirometry, MBNW, and FOT assessment pre and post methacholine challenge. On visit two, 3He MRI scans were conducted pre and post methacholine challenge. Post methacholine the volume of low-ventilated lung increased from 8.3% to 13.8% in the non-obstructed group (P = 0.012) and from 13.0% to 23.1% in the obstructed group (P = 0.001). For all subjects, the volume of low ventilation from 3He MRI correlated with a marker of airway closure in obstructive subjects, Xrs (6 Hz) and the marker of ventilation heterogeneity Scond with r² values of 0.61 (P < 0.001) and 0.56 (P < 0.001), respectively. The change in Xrs (6 Hz) correlated well (r² = 0.45, p < 0.001), whereas the change in Scond was largely independent of the change in low ventilation volume (r² = 0.13, P < 0.01). The only significant predictor of low ventilation volume from the multi-variate analysis was Xrs (6 Hz). This is consistent with the concept that regions of poor or absent ventilation seen on 3He MRI are primarily the result of airway closure. [ABSTRACT FROM AUTHOR]

Forced oscillation events have become a challenging problem with the increasing penetration of renewable and other inverter-based resources (IBRs), especially when the forced oscillation frequency coincides with the dominant natural oscillation frequency. A severe forced oscillation event can deteriorate power system dynamic stability, damage equipment, and limit power transfer capability. This paper proposes a two-dimension scanning forced oscillation grid vulnerability analysis method to identify areas/zones in the system that are critical to forced oscillation. These critical areas/zones can be further considered as effective actuator locations for the deployment of forced oscillation damping controllers. Additionally, active power modulation control through IBRs is also proposed to reduce the forced oscillation impact on the entire grid. The proposed methods are demonstrated through a case study on a synthetic Texas power system model. The simulation results demonstrate that the critical areas/zones of forced oscillation are related to the areas that highly participate in the natural oscillations and the proposed oscillation damping controller through IBRs can effectively reduce the forced oscillation impact in the entire system.

The nature of flow field inside the scramjet engine consists of shock-shock interaction and the shock boundary layer interaction. For a sustained combustion, a device called isolator plays a critical role in providing adequate pressure to the combustion chamber by a series of bifurcated shock waves called "shock train". A small downstream pressure perturbation can cause upstream movement of this shock train and results in engine unstart condition. The propagation speed of downstream disturbance can influence the shock train oscillation. In the present study, computational fluid dynamics analysis is conducted to understand the oscillatory characteristic of shock train in a rectangular duct at M =1.75. The impact of downstream perturbation frequency (f b = 10 – 50 Hz) and the amplitude (A = 0.01 – 0.1) are simulated and discussed. The presence of low-frequency oscillation is observed without back pressure perturbation. The downstream pressure perturbation has a noticeable effect on the shock train excursion length i.e., the maximum upstream to downstream distance moved by shock train. The increase in perturbation frequency leads to decrease in disturbance propagation speed. Similarly, the propagation speed increases with the increase in the perturbation amplitude. • Simulation of shock train under forced back pressure perturbation. • Self-excited oscillation depends on local flow properties. • Changing frequency and amplitude in determining forced oscillation characteristics. • Propagation of downstream disturbance influenced more by increasing forcing amplitude. • Downstream perturbations behave as a 2nd order non-homogeneous differential equation. [ABSTRACT FROM AUTHOR]

A series of model tests was carried out to investigate the vortex-induced vibration (VIV) characteristics of a free hanging PVC pipe under forced oscillation conditions. The prescribed displacement with a period and amplitude was forced at the top of the riser. The motion of the riser along its length was measured with underwater cameras in three dimensions. The top-excited responses in the inline direction and vortex-induced vibration in the cross-flow direction were examined in the time and frequency domains. Multi-peak frequencies in the VIV were demonstrated to be strongly dependent upon the Keulegan-Carpenter number, corresponding with the results of Blevin. It was found that the Reynolds numbers (excitation period) was a critical parameter for the dominant VIV characteristics, even under the condition of using the same Keulegan-Carpenter number, under the top-excited condition. In the resonance at the nth natural frequency by the forced-motion induced VIV frequency, the riser responded with a large amplitude and forced frequency, dominantly in the VIV CF direction.

In this article, we establish new oscillation criteria for forced second-order damped differential equations with nonlinearities that include Riemann-Stieltjes integrals. The results obtained here extend related results reported in the literature, and can easily be extended to more general equations of the type considered here. Two examples illustrate the results obtained here.

In this article, we present a new controller for stable and robust walking control of ATRIAS, an underactuated bipedal robot designed based on the spring-loaded inverted pendulum (SLIP) model. We propose a forced-oscillation scheme for control of vertical motion, which we prove to be stable and contractive. Moreover, we prove that, through some mild assumptions, the dynamics of the system can be written in a hierarchical form that decouples the stability analyses of the horizontal and vertical directions. We leverage these properties to find a stabilizing class of functions for foot placement. The torso control is also proved to be decoupled using singular perturbation theory and is stabilized through a feedback linearization controller. We also take advantage of the proposed framework's flexibility and extend it to include a new reflex-based uneven-terrain walking control scheme. We test the controller for various desired walking speeds (0 to 2.5 m/s), for stepping up and down unexpected obstacles (15 cm), and for high-speed walking on a random uneven terrain (up to 10 cm of step-ups and step-downs and up to 1.8 m/s). The results show successful performance of the controller and its stability and robustness against various perturbations. [ABSTRACT FROM AUTHOR]

Copyright of China Petroleum Machinery is the property of China Petroleum Machinery Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The ambient noise in synchrophasor measurements carries core dynamic signatures of a power system and its spectrum can strongly reflect the dynamic properties and operating conditions of the system and its loads. Typically, the ambient noise spectrum is estimated from a window of measurements captured during steady state operating conditions of the system. The presence of any Forced Oscillation (FO) in the measurement window severely biases the estimated ambient noise spectrum. In this article, an algorithm is proposed for separable estimation of the ambient noise spectrum in the presence of periodic FOs in synchrophasor measurements. The algorithm utilizes the Thomson's multitaper spectral estimation and harmonic analysis techniques and is capable of effectively reducing the bias in estimating the ambient noise spectrum due to the presence of FOs in the measurements. The performance of the estimator is analyzed theoretically and the theoretical results are verified experimentally using simulation studies. Application of the algorithm to field measured data illustrates that the algorithm is capable of effectively reducing the bias introduced by FOs in estimating the ambient noise spectrum from the synchrophasor measurements containing both ambient noise and FOs. [ABSTRACT FROM AUTHOR]

Copyright of Archivos de Bronconeumología (English Edition) is the property of Sociedad Espanola de Neumologia y Cirugia Toracica (SEPAR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The most prevalently used method to obtain the nonlinear roll damping coefficient is the free roll decay test. However, this method can only be conducted at the resonance frequency and thus cannot consider the effect of the frequency. This is a certain limitation as the resonance frequency can be changed at any time by the ship's loading conditions. Therefore, it is worth investigating the frequency dependency of the nonlinear roll damping coefficients. In this study, a numerical method was proposed to derive the linear and nonlinear roll damping coefficients of ships at different frequencies. Fully nonlinear CFD simulations of forced harmonic roll motion were conducted and the roll damping coefficients were calculated. Then, the damping coefficients were decomposed into the linear and nonlinear components using the linear regression analysis. The linear roll damping coefficients were compared with potential coefficients and showed a good agreement, while the nonlinear roll damping coefficients were compared with the coefficients calculated using a semi-empirical method. The nonlinear roll damping coefficients calculated from the proposed method showed a strong frequency dependency. Finally, possible rationales for the frequency dependence of the nonlinear roll damping coefficient were investigated. [ABSTRACT FROM AUTHOR]

This paper is concerned with the radiated wave and the rotational flow fields generated by the forced oscillations of the water columns within surface-piercing hollow cylinders. The present study utilised Open Foam as the viscous flow solver to numerically model the forced oscillations. An investigation of the radiated waves in the fields near the water columns confirms two distinct nonlinear characteristics. These are progressively reduced amplitude and progressively reduced phase difference. An evaluation of the rotational flow fields around the mouths of the water columns helps explain their origins. This study confirms that the nonlinear characteristics in the near-field are physically caused by the formation and motion of vortex ring at the bottom edge of the cylinder. Interestingly, the progressively reduced amplitude is not evident in the far-field. All these new findings support the confirmation regarding the appropriateness of linear radiation theory and the implication of the vortex motion on the effective draft of a fixed cylindrical Oscillating Water Column (OWC). • Wave radiations generated by forced oscillations of water columns are studied. • Near-field radiated waves have progressive reductions in amplitude and phase difference. • Vortex formation and motion cause nonlinear characteristics in near field. • Far-field radiated waves exhibit insignificant progressive amplitude reductions. • Location of vortex closest to the mouth affects the effective draft of water column. [ABSTRACT FROM AUTHOR]

In this study, a series of experiments is conducted in which a plenum experiences forced oscillation. The oscillating system is driven by a periodic force arising from regular volumetric fluctuations analogous to fixed-height water waves entering a surface-effect ship (SES) plenum below a heave deck. An analytical study is then conducted in which a MATLAB Simulink model is created to incorporate the same parameters taken into account during the experiments. The results of both the experiments and the simulation demonstrated that as the frequency associated with the plenum volume's forced oscillations approaches the natural frequency of the mass/plenum system, the vibration magnitude of the heave deck increases as predicted. Resonance is observed in vibrations of the heave deck. The mean hover height of the heave deck increases predictably and remains high at higher frequencies when the forced oscillations' frequency approaches the natural frequency of the mass/plenum system. Higher frequency oscillations lead to air accumulation in the plenum. Since there was a significant difference between the experimentally-measured (and simulated) natural frequency and the theoretical natural frequency of a similar, but rigid-walled plenum, it appeared that the natural frequency of the heave deck above the plenum is reduced by the flexibility of the seals and water surface under a SES. • Identifying relevant parameters of a SES air plenum to predict dynamic instabilities. • A more complete analytical model of the air cushion of a SES is developed. • The combined effect of pressure and air density is considered. • The rates of change of air pressure and air density are also considered. • Flexibility of plenum boundaries affects the natural frequency of an air cushion. [ABSTRACT FROM AUTHOR]