Self-sustained Oscillations Research Articles

Observation of a phase transition from a continuous to a discrete time crystal

Discrete (DTCs) and continuous time crystals (CTCs) are novel dynamical many-body states, that are characterized by robust self-sustained oscillations, emerging via spontaneous breaking of discrete or continuous time translation symmetry. DTCs are periodically driven systems that oscillate with a subharmonic of the external drive, while CTCs are continuously driven and oscillate with a frequency intrinsic to the system. Here, we explore a phase transition from a continuous time crystal to a discrete time crystal. A CTC with a characteristic oscillation frequency ωCTC is prepared in a continuously pumped atom-cavity system. Modulating the pump intensity of the CTC with a frequency ωdr close to 2ωCTC leads to robust locking of ωCTC to ωdr/2 , and hence a DTC arises. This phase transition in a quantum many-body system is related to subharmonic injection locking of non-linear mechanical and electronic oscillators or lasers.

Experiment research on the characteristics and mechanism of noise caused by a car door sealing cavity at wind excitation

Due to the various geometric shapes of cavities within car door sealing systems, the resulting cavity noise induced by wind excitation exhibits complex characteristics, leading to unclear noise mechanisms. To address this, a study was conducted to examine the acoustic properties of door sealing cavities located in the B-pillar, C-pillar, and backdoor of an SUV within a full-scale aeroacoustic wind tunnel. The main objective was to explore the relationship between cavity noise and interior noise. The results showed that peak components in the sound pressure level spectrum of the cavities significantly contribute to interior noise, particularly for cavities located close to the car’s interior. To gain further insight, the geometric characteristics of the cavities were extracted and transformed into equivalent regular cavities. These equivalent cavities were subsequently tested for their noise performance in a small-scale aeroacoustic wind tunnel, and the occurrence mechanisms were thoroughly investigated. The results demonstrated that the noise spectra of different cavities, whether they had sealing strips or leakage gaps, exhibited typical multipeak characteristics, with some cases even leading to whistling (e.g. backdoor cavity). The reason for the whistling was a resonance when the self-sustained oscillation frequencies of the cavities coincided with or approached the Helmholtz resonance frequencies or modal frequencies of the cavities. Interestingly, the self-sustained oscillation frequency and cavity modal resonance still persisted even when the two frequencies were somewhat separated, albeit with peaks of lower magnitude in the sound pressure spectrum (e.g. in the sealed cavities of the B-pillar and C-pillar).

Dynamical states associated with the shift in whistling frequency in aeroacoustic system

Self-sustained aeroacoustic oscillations arising from the interactions between the hydrodynamic and acoustic fields are perceived as a whistle. Such whistling can lead to large amplitude acoustic oscillations that have disastrous consequences for engineering systems such as large segmented solid rocket motors and large gas pipelines. The whistling corresponds to the state of limit cycle oscillations (LCO) in dynamical systems theory. An aeroacoustic system exhibits different dynamical states when the bulk flow velocity is varied as a control parameter. Understanding the dynamical states and the transitions between them, as the control parameter is varied, is crucial in designing control strategies for such aeroacoustic oscillations. Previous studies have shown that as the control parameter varies, in an aeroacoustic system that has a flow through orifices, the whistling frequency shifts. We show that such a change in frequency occurs via three different scenarios— (1) direct transition between the two LCOs as an abrupt transition, (2) via a state of intermittency, and (3) via a state of aperiodicity. In the current aeroacoustic system, the abrupt transition between the LCOs is manifested as a bursting behaviour where the amplitude of the acoustic pressure fluctuations abruptly switches between the high and low-amplitude LCOs. Further, we show that the dynamical state and the transition between them during the frequency shift have a correlation with the magnitude of the frequency shift. Using recurrence theory we show that there is a change in the dynamical state of the system during the frequency shift. Further, we use synchronisation theory to investigate the coupled behaviour of the velocity (u′) and the acoustic pressure (p′) fluctuations during the different dynamical states. Our findings imply that u′ and p′ exhibit phase synchronisation (PS) during the state of LCO, corresponding to whistling. In contrast, u′ and p′ are desynchronised during the state of aperiodicity, corresponding to stable operation. Furthermore, the bursts of periodic oscillations during intermittency correspond to the phase-synchronised epochs of periodic u′ and p′, and the aperiodic epochs correspond to the desynchronised aperiodic u′ and p′.

The Nonlinear Flow Characteristics within Two-Dimensional and Three-Dimensional Counterflow Models within Symmetrical Structures

In this paper, we investigate the nonlinear characteristics of the flow in a two-dimensional and a three-dimensional counterflow model with symmetrical structures. Through numerical simulations, we obtain the velocity fields of the fluid flow in these models for different Re. The numerical results are analyzed to understand the nonlinear characteristics and differences between the two-dimensional and three-dimensional models. The findings indicate that, when Re varies, both the two-dimensional and three-dimensional models exhibit solution bifurcations and nonlinear phenomena such as symmetry breaking, self-sustained oscillations, and chaos. As Re increases, the two-dimensional counterflow model displays a unique solution, an asymmetric solution, and an oscillating solution. Specifically, when Re < 4320, both the laminar and turbulent models show a unique, symmetric, and steady-state velocity distribution. For 4652 < Re < 8639, the two-dimensional model solutions are not unique, presenting a pair of antisymmetric, asymmetric solutions that nevertheless remain steady-state. When Re > 8639, the solution becomes oscillatory and unsteady. The three-dimensional counterflow model exhibits a two-dimensional solution independent of the Z-axis. At Re = 4652, both the three-dimensional and two-dimensional models produce the same unique, symmetric, and steady-state velocity distribution with no three-dimensional flow (W = 0). At Re = 8639, the three-dimensional model solutions are not unique, showing a pair of antisymmetric, asymmetric solutions, while still being steady and time-independent. At Re = 87,627, the three-dimensional model solution becomes oscillatory and unsteady. By elucidating the flow characteristics and nonlinear features of both models, this study compares the differences between the two-dimensional and three-dimensional flows, thereby laying the groundwork for simplification of the problem and further theoretical research.

New type of self-sustained divertor oscillation driven by magnetic island dynamics in Large Helical Device

A new self-sustained divertor oscillation is discovered in magnetic island induced detached plasmas in the Large Helical Device. The divertor oscillation is found to be a self-regulation of the width of an edge magnetic island accompanied by detachment-attachment transitions. The modified Rutherford equation combined with an ad-hoc bootstrap current equation is introduced to describe the divertor oscillation as a predator–prey model between the magnetic island width and a remnant X-point bootstrap current. The model successfully reproduces the experimental observations in terms of the oscillation frequency, the phase relation between variables, and the oscillation amplitude.

Biomechanics of sound production in high-pitched classical singing

Voice production of humans and most mammals is governed by the MyoElastic-AeroDynamic (MEAD) principle, where an air stream is modulated by self-sustained vocal fold oscillation to generate audible air pressure fluctuations. An alternative mechanism is found in ultrasonic vocalizations of rodents, which are established by an aeroacoustic (AA) phenomenon without vibration of laryngeal tissue. Previously, some authors argued that high-pitched human vocalization is also produced by the AA principle. Here, we investigate the so-called “whistle register” voice production in nine professional female operatic sopranos singing a scale from C6 (≈ 1047 Hz) to G6 (≈ 1568 Hz). Super-high-speed videolaryngoscopy revealed vocal fold collision in all participants, with closed quotients from 30 to 73%. Computational modeling showed that the biomechanical requirements to produce such high-pitched voice would be an increased contraction of the cricothyroid muscle, vocal fold strain of about 50%, and high subglottal pressure. Our data suggest that high-pitched operatic soprano singing uses the MEAD mechanism. Consequently, the commonly used term “whistle register” does not reflect the physical principle of a whistle with regard to voice generation in high pitched classical singing.

Driven generalized quantum Rayleigh-van der Pol oscillators: Phase localization and spectral response.

Driven classical self-sustained oscillators have been studied extensively in the context of synchronization. Using the master equation, this work considers the classically driven generalized quantum Rayleigh-van der Pol oscillator, which is characterized by linear dissipative gain and loss terms as well as three nonlinear dissipative terms. Since two of the nonlinear terms break the rotational phase space symmetry, the Wigner distribution of the quantum mechanical limit cycle state of the undriven system is, in general, not rotationally symmetric. The impact of the symmetry-breaking dissipators on the long-time dynamics of the driven system are analyzed as functions of the drive strength and detuning, covering the deep quantum to near-classical regimes. Phase localization and frequency entrainment, which are required for synchronization, are discussed in detail. We identify a large parameter space where the oscillators exhibit appreciable phase localization but only weak or no entrainment, indicating the absence of synchronization. Several observables are found to exhibit the analog of the celebrated classical Arnold tongue; in some cases, the Arnold tongue is found to be asymmetric with respect to vanishing detuning between the external drive and the natural oscillator frequency.

Autonomous and Self-sustained Circadian Oscillators Displayed in Human Primary Parathyroid Cell Culture Drive Parathormone Secretion

Abstract Background Most cells in our body possess circadian oscillators controlling diurnal rhythmicity of body metabolism. The role of molecular clocks in regulating the function and disfunction of human parathyroid gland (PG), responsible for calcium homeostasis, has not been unraveled. Hyperparathyroidism is a common endocrine pathology, characterized by an unadapted PTH secretion to calcium levels. Aims Here we aimed at characterizing molecular makeup of circadian clocks in human primary parathyroid cell culture (HPPCC), and at establishing differential transcriptional patterns of normal and pathological PGs. Methods RNA extracted from normal, adenomatous, and hyperplastic PG tissues was subjected to RNA sequencing (RNAseq) analysis. The rim of normal PG tissue was dissected from parathyroid adenoma specimens based on the higher autofluorescence intensity assessed by the near infra-red imaging. HPPCCs were established from adenomatous and hyperplastic PG biopsies. Efficient circadian clock disruption was achieved in HPPCCs by small interfering RNA-mediated knockdown of CLOCK. The functionality of circadian clock machinery was assessed by continuous recording of circadian bioluminescence introduced via lentivectors and paralleled with around-the-clock measurement of PTH secretion using perfusion system. Results The RNAseq analyses revealed transcriptional signatures that distinguished between pathological and normal PGs, and among adenomatous and hyperplastic PGs. We report, for the first time, robust anti-phasic circadian rhythmicity of Per2-luciferase and Bmal1-luciferase reporters in HPPCCs synchronized in vitro. Strikingly, we observed circadian rhythmicity of PTH secretion by parathyroid adenoma cells. Moreover, clock disruption in PG adenoma cells has an impact on transcription of functional genes. Conclusion Our data indicate presence of cell-autonomous molecular clock in human PG cells paralleled with circadian rhythmic PTH secretion and provide large-scale transcriptional pattern of parathyroid adenoma and hyperplastic tissues.

Existence of a detachment cliff at ASDEX Upgrade

The detachment cliff is a bifurcative transition to partial detachment recently discovered at the DIII-D tokamak (McLean et al 2015 J. Nucl. Mater. 463 533–6). This work presents a database analysis of target parameters in L-mode and H-mode discharges to search for a detachment cliff at ASDEX Upgrade (AUG). Most of the transitions from attached to partially detached divertor conditions observed in H- and L-mode discharges in AUG show bifurcative-like characteristics that are consistent with the properties of the detachment cliff if the B×∇B drift is directed towards the active X-point. In the operational space of power and density, the bifurcative transitions identified during an L-mode discharge occur at injected power and density higher than a threshold value ( Ptot > 0.7 MW and n e > 1.6 × 10 19 m−3, respectively). Furthermore, the temperatures at which the transitions start are found to be insensitive to the injected impurity, the injected power and the value of the upstream density. Finally, the study of the evolution of the target parameters, of the intensity of the Dα line and of specific manometers and bolometer lines of sights shows that the physical process underlying the detachment cliff and the self-sustained divertor oscillations (Heinrich 2020 Nucl. Fusion 60 076013) might be the same.

Effect of Transition on Self-Sustained Shock Oscillations in Highly Loaded Transonic Rotors

Recent trends in numerical studies suggest the possible need for scale-resolving simulations for resolving shock oscillations on transonic compressors, the sources of which are unclear. The effect of promoting transition on the suction side upstream of self-sustained shock oscillations from a laminar shock/boundary-layer interaction (altitude conditions) in transonic compressors is studied using implicit large-eddy simulation, as well as experiments in a transonic cascade. The experiment is performed at conditions leading to different shock structures, and the shock behavior is captured with high-speed schlieren imaging. The Rolls-Royce HYDRA in-house code is employed for the numerical simulations. Trends in the change of oscillation behavior are discussed. The work addresses the fundamentally different behavior between an idealized quasi-2D LES case and an experimental cascade. This suggests that a simplification of the shock oscillation problem to a more canonical form is needed in order to investigate the oscillation mechanism itself experimentally and validate the CFD of the mechanism before returning to more complex cases. The reader is referred to youtu.be/CNRz7IYl1Pk for a condensed summary of the work with flow visualization.

Sweeping jet impingement heat transfer on concave surface: Influence of trapped vortex ring revealed by generalized k-ω (GEKO) model

Jet impingement-based heat transfer enhancement is widely employed due to the significant mass, momentum, and energy it accompanied. While the steady jet with high energy flux is commonly favored, it still exhibit few defects, including heat transfer deterioration away from the stagnation point and mutual flow interference in the jet arrays, which are particularly critical in demanding applications. Recently, the sweeping jet has been proposed to resolve the above challenges owing to its self-excited and self-sustaining unsteady oscillation. However, current research predominately focuses on impinging on the flat surface, neglecting impinging on the concave surface, such as internal cooling at the gas turbine blade leading-edge as well as the hot gas de-icing at the aircraft wing leading-edge. Understanding the heat transfer characteristics in these scenarios is crucial as they notably differ from those on flat surfaces. Therefore, this paper investigates the heat transfer characteristics and the related essential flow dynamics of a sweeping jet with a Reynolds number of 2577 impinging on a concave surface, and those of a steady jet under the same conditions are compared. The jet-to-wall distance and the curvature of the impingement surface are 4 and 10 times the jet's hydraulic diameter, respectively. To correctly capture the three-dimensional characteristics of the sweeping jet, this research employs the Reynolds stress model for turbulence modeling and the Generalized k-ω (GEKO) model for better calibration of the jet's spreading and dissipation rates, as well as the characteristics of trapped vortex generated due to the confined concave flow channel. The study initially focuses on the average Nusselt number on the impingement surface, revealing a 15.2% increase in heat transfer intensity of the sweeping jet compared to the steady jet, and the sweeping jet covers a larger area with enhanced uniformity. Subsequent analysis on streamlines in two perpendicular planes and Q criterion iso-surfaces in the spatial domain shows the formation of a trapped vortex ring with a constant location around the jet, restricting the effective cooling range to its inner side and thus resulting in a steep temperature rise on the two lateral sides of the vortex ring. The stability of the trapped vortex is enhanced by the curved impingement wall in the oscillation plane, which on the one hand forces the high-temperature fluid that has exchanged heat with the wall to repeatedly wash the same location, and on the other hand significantly reduces the jet oscillation angle compared to a free sweeping jet. Consequently, the heat transfer intensity of the sweeping jet is lower in the main oscillation plane than in the non-oscillation plane. Analysis of the near-wall flow parameters indicates that the sweeping jet generates higher velocity and turbulence kinetic energy in the wall jet zone far away from the stagnation point compared to the steady jet, which results in a higher local heat transfer intensity. The unsteady transverse oscillation of the sweeping jet disturbs the wall jet zone and disrupts the formation of the wall boundary layer, causing a higher velocity gradient at the wall and hence the lower thicknesses of the velocity boundary layer and the thermal boundary layer than those of the steady jet. In conclusion, this study reveals the flow and heat transfer features of the sweeping jet impingement on confined concave surface, with the influence of the trapped vortex ring mattering significantly. The findings provide a reference for designing a more effective jet arrangement. Future work will focus on investigating the effects of jet Reynolds number and impingement distance variations on the trapped vortex ring, and how to mitigate the adverse effects of the trapped vortex ring on impingement heat transfer.

Magnon-photon coupling in an opto-electro-magnonic oscillator

The opto-electronic oscillators (OEOs) hosting self-sustained oscillations by a time-delayed mechanism are of particular interest in long-haul signal transmission and processing. On the other hand, owing to their unique tunability and compatibility, magnons—as elementary excitations of spin waves—are advantageous carriers for coherent signal transduction across different platforms. In this work, we integrated an opto-electronic oscillator with a magnonic oscillator consisting of a microwave waveguide and a yttrium iron garnet sphere. We find that, in the presence of the magnetic sphere, the oscillator power spectrum exhibits sidebands flanking the fundamental OEO modes. The measured waveguide transmission reveals anti-crossing gaps, a hallmark of the coupling between the opto-electronic oscillator modes and the Walker modes of the sphere. Experimental results are well reproduced by a coupled-mode theory that accounts for nonlinear magnetostrictive interactions mediated by the magnetic sphere. Leveraging the advanced fiber-optic technologies in opto-electronics, this work lays out a new, hybrid platform for investigating long-distance coupling and nonlinearity in coherent magnonic phenomena.

Identifying Buffet Oscillations Using Sliding-Window Dynamic Mode Decomposition

Buffet is a self-sustaining oscillation that can endanger an aircraft in transonic flight. Early automatic detection of the onset of buffet from simulation data is a necessary step in optimization algorithms used to design new aircraft geometries. We use the Dynamic Mode Decomposition (DMD) eigenvalues to detect the onset of buffet from transient simulations of the OAT15A airfoil across a range of angles of attack. During transients, different eigenvalues may be expressed at different times, and we process shorter segments of the data (sliding windows) in order to obtain robust, time-varying estimates of frequency of oscillation and decay/growth rates. We demonstrate that the onset of oscillation can be identified by analyzing DMD eigenvalues, and that physically different mechanisms of oscillation can additionally be identified from spatial profiles associated with DMD modes. The dominant buffeting modes can be detected through DMD before they become apparent in the physically relevant quantities, such as the coefficient of lift. We expect that such early detection of the onset of buffet could reduce the computational burden of aircraft design and expand the feasible design space.

Two-dimensional hydrodynamic simulation for synchronized oscillatory flows in two collapsible channels connected in parallel.

We investigated self-sustained oscillation in a collapsible channel, in which a part of one rigid wall is replaced by a thin elastic wall, and synchronization phenomena in the two channels connected in parallel. We performed a two-dimensional hydrodynamic simulation in a pair of collapsible channels which merged into a single channel downstream. The stable synchronization modes depended on the distance between the deformable region and the merging point; only an in-phase mode was stable for the large distance, in-phase and antiphase modes were bistable for the middle distance, and again only an in-phase mode was stable for the small distance. An antiphase mode became stable through the subcritical pitchfork bifurcation by decreasing the distance. Further decreasing the distance, the antiphase mode became unstable through the subcritical Neimark-Sacker bifurcation. We also clarified the distance dependences of the amplitude and frequency for each stable synchronization mode.

Dynamical behaviour of a new model for the UJT relaxation oscillator

We propose a new two dimensional model for the UJT (Unipolar Junction Transistor) relaxation oscillator whose parameters were chosen by fitting the experimental data. The dynamical properties of the model were studied by using nonlinear dynamics methods and it was proved that self-sustained oscillations may occur. Then, the system was forced to study the transition to chaos, reproducing the torus breakdown mechanism observed experimentally. Finally, two identical systems were diffusively coupled and the synchronization properties have been studied.

Autonomous circadian rhythms in the human hepatocyte regulate hepatic drug metabolism and inflammatory responses.

Critical aspects of physiology and cell function exhibit self-sustained ~24-hour variations termed circadian rhythms. In the liver, circadian rhythms play fundamental roles in maintaining organ homeostasis. Here, we established and characterized an in vitro liver experimental system in which primary human hepatocytes display self-sustained oscillations. By generating gene expression profiles of these hepatocytes over time, we demonstrated that their transcriptional state is dynamic across 24 hours and identified a set of cycling genes with functions related to inflammation, drug metabolism, and energy homeostasis. We designed and tested a treatment protocol to minimize atorvastatin- and acetaminophen-induced hepatotoxicity. Last, we documented circadian-dependent induction of pro-inflammatory cytokines when triggered by LPS, IFN-β, or Plasmodium infection in human hepatocytes. Collectively, our findings emphasize that the phase of the circadian cycle has a robust impact on the efficacy and toxicity of drugs, and we provide a test bed to study the timing and magnitude of inflammatory responses over the course of infection in human liver.

Empirical Modeling of Subcritical Hopf Bifurcation of Thermoacoustic Stirling Engine

This study models the subcritical Hopf bifurcation in thermoacoustic Stirling engines using the Stuart–Landau model, highlighting the role of nonlinear dynamics. By inducing self-sustained oscillations and measuring pressure fluctuations across different temperature gradients imposed on the regenerator, we reveal the engine’s transition to a nonlinear domain, characterized by heightened oscillation amplitudes and unique periodic patterns. Interpreted Landau constants and growth rates illuminate the stabilizing effects of nonlinear dynamics, demonstrating the Stuart–Landau model’s applicability in thermoacoustic engine analysis. Our research confirms that this empirically refined model reliably describes oscillation amplitudes and transient phenomena, contributing valuable perspectives for advancing thermoacoustic engine design and operational understanding.

Transonic buffet simulation using a partially-averaged Navier-Stokes approach

The present article assesses the capability of the partially averaged Navier-Stokes (PANS) method to reproduce accurately the self-sustained shock oscillations, also known as transonic buffet, occurring on airfoils and wings at transonic regime under certain conditions of Mach number and angle of attack. The test case under analysis is an OAT15A unswept wing at Mach number M∞=0.73 and Reynolds number Rec=3×106. The three-dimensional flow is studied by accounting for the wind tunnel walls adopted in the experiments of Jacquin et al. [1] in the simulations. The computations on a large-span, confined configuration reveal a strong three-dimensionality of the flow both before and after the buffet onset. Attention is paid to the comparison with unsteady Reynolds-averaged Navier Stokes (URANS) results, to show the benefits of PANS in resolving flow unsteadiness at different flow resolutions, especially on affordable CFD grids, at limited additional cost. In this context, the role of the mesh metrics and the local turbulence level in the formulation of the model is described, as well as the relation of this latter with the spatiotemporal discretization used for the numerical simulations. The aim is to extend the use of PANS and obtain accurate predictions of flow cases involving shock-wave boundary layer interactions without expensive approaches.

Breaking the left-right symmetry in fluttering artificial cilia that perform nonreciprocal oscillations

Recent investigations on active materials have introduced a new paradigm for soft robotics by showing that a complex response can be obtained from simple stimuli by harnessing dynamic instabilities. In particular, polyelectrolyte hydrogel filaments actuated by a constant electric field have been shown to exhibit self-sustained oscillations as a consequence of flutter instability. Owing to the nonreciprocal nature of the emerging oscillations, these artificial cilia are able to generate flows along the stimulus. Building upon these findings, in this paper we propose a design strategy to break the left-right symmetry in the generated flows, by endowing the filament with a natural curvature at the fabrication stage. We develop a mathematical model based on morphoelastic rod theory to characterize the stability of the equilibrium configurations of the filament, proving the persistence of flutter instability. We show that the emerging oscillations are nonreciprocal and generate thrust at an angle with the stimulus. The results we find at the level of the single cilium open new perspectives on the possible applications of artificial ciliary arrays in soft robotics and microfluidics.

Coherent control of an optical tweezer phonon laser

The creation and manipulation of coherence continues to capture the attention of scientists and engineers. The optical laser is a canonical example of a system that, in principle, exhibits complete coherence. Recent research has focused on the creation of coherent, laser-like states in other physical systems. The phonon laser is one example where it is possible to amplify self-sustained mechanical oscillations. A single mode phonon laser in a levitated optical tweezer has been demonstrated through appropriate balance of active feedback gain and damping. In this work, coherent control of the dynamics of an optical tweezer phonon laser is used to share coherence between its different modes of oscillation, creating a multimode phonon laser. The coupling of the modes is achieved by periodically rotating the asymmetric optical potential in the transverse focal plane of the trapping beam via trap laser polarization rotation. The presented theory and experiment demonstrate that coherence can be transferred across different modes of an optical tweezer phonon laser, and are a step toward using these systems for precision measurement and quantum information processing.