Self-induced Oscillations Research Articles

The Prediction of Self-Induced Oscillations in Horizontal Cylindrical Vessel using CFD and Elucidation of Their Generation Mechanism

The Prediction of Self-Induced Oscillations in Horizontal Cylindrical Vessel using CFD and Elucidation of Their Generation Mechanism

Numerical parametric study of a sweeping-vortex low-frequency fluidic oscillator

Fluidic oscillators are garnering increasing attention in various fields due to their self-induced oscillation characteristics. This study proposes a novel fluidic oscillator, the sweeping-vortex fluidic oscillator (SVFO), characterized by unique feedback properties. These properties exploit fluid feedback from the side opposite the attached wall and the spin delay of the captive vortex to yield low-frequency oscillating jets. The oscillation characteristics and the influence of main geometric parameters were investigated using computational fluid dynamics approach. Results show that the evolution of the flow field within the SVFO demonstrates an intricately dynamic interacting. The deflection switching of the main jet is primarily controlled by a pair of counter-rotating moving vortexes. Geometric parameters influence the stability of the dynamic evolution of both captive and moving vortexes, thereby altering the signal components of the output performance transmitted to the inlet. The overall width of the oscillator has a pronounced impact on the oscillation frequency, and with an increase in the overall width, a maximum frequency increase of 87 % can be observed. Furthermore, the height of the sweeping chamber and the diameter of the vortex chamber outlet have a noticeable effect on the pressure pulse amplitude. A pressure fluctuation coefficient greater than 1.1 was recorded. The results also suggest that the geometric parameters of the fluidic oscillator need to be designed within a reasonable range; otherwise, the jet oscillator will not function. A more refined optimization of geometric parameters could be obtained with the aim of achieving a stable main jet, feedback flow of appropriate intensity, and optimal attachment conditions. This work reveals the oscillation mechanism and the influence of geometric parameters on SVFOs, providing a theoretical basis for designing related oscillators.

Spiking ruby revisited: self-induced periodic spiking oscillations leading to chaotic state in a Cr:Al2O3 laser with cw 532-nm pumping

This paper reexamines a 60-year-old mystery of spiking behavior in ruby lasers with a cw 532-nm pump, paying special attention to mode matching between the pump and lasing beam within a ruby crystal placed in a semi-confocal laser cavity. Periodic spiking oscillations were observed in a limited pump power regime, where spikes obeying the generic asymmetric hyperbolic function appeared at a repetition rate around 50 kHz and with a 130-150 ns width and 0.1-0.6 µJ energy depending on the pump power. The physics of the spiking behavior based on Kleinman’s mechanical approach and a plausible interpretation for the periodic spiking oscillation in terms of self-induced mode matching between the pump and laser beams through the self-induced Kerr-lens effect are addressed. The statistical nature inherent to spiking and the associated self-organized critical behavior in the quasi-periodic spiking oscillations as well as chaotic states occurring outside the periodic spiking regime are clarified from a nonlinear dynamics point of view and intriguing statistical properties inherent to chaotic oscillations in usual solid-state lasers subjected to external modulations are shown to be present in the self-induced instabilities of the ruby laser

Instabilities near Ultrastrong Coupling in a Microwave Optomechanical Cavity.

With artificially engineered systems, it is now possible to realize the coherent interaction rate, which can become comparable to the mode frequencies, a regime known as ultrastrong coupling (USC). We experimentally realize a cavity-electromechanical device using a superconducting waveguide cavity and a mechanical resonator. In the presence of a strong pump, the mechanical-polaritons splitting can nearly reach 81% of the mechanical frequency, overwhelming all the dissipation rates. Approaching the USC limit, the steady-state response becomes unstable. We systematically measure the boundary of the unstable response while varying the pump parameters. The unstable dynamics display rich phases, such as self-induced oscillations, period-doubling bifurcation, and period-tripling oscillations, ultimately leading to the chaotic behavior. The experimental results and their theoretical modeling suggest the importance of residual nonlinear interaction terms in the weak-dissipative regime.

On the stability of piston-driven planar shocks

We present a theoretical and numerical stability analysis for a piston-driven planar shock against two-dimensional perturbations. The results agree with the well-established theory for isolated planar shocks: in the range of $h_c< h<1+2 {\mathcal {M}}_2$ , where $h$ is the D'yakov–Kontorovich (DK) parameter related to the slope of the Rankine–Hugoniot curve, $h_c$ is its critical value corresponding to the onset of the spontaneous acoustic emission (SAE) and ${\mathcal {M}}_2$ is the downstream Mach number, non-decaying oscillations of shock-front ripples occur. The effect of the piston is manifested in the presence of additional frequencies occurring by the reflection of the sonic waves on the piston surface that can reach the shock. An unstable behaviour of the shock perturbation is found to be possible when there is an external excitation source affecting the shock, whose frequency coincides with the self-induced oscillation frequency in the SAE regime, thereby being limited to the range $h_c< h<1+2 {\mathcal {M}}_2$ . An unstable evolution of the shock is also observed for planar shocks restricted to one-dimensional perturbations within the range $1< h<1+2 {\mathcal {M}}_2$ . Both numerical integration of the Euler equations via the method of characteristics and theoretical analysis via Laplace transform are employed to cross-validate the results.

Flow instability control of an ultra-highly loaded transonic compressor rotor using self-excited casing bleed

Flow instability is a common issue encountered by high-speed compressors when they operate outside of their optimal range, especially in highly loaded compressors. This study investigates the potential of an unsteady passive flow control technique, self-excited bleed (SEB), which involves casing modification, to improve the base flow and stability characteristics of an ultra-highly loaded low reaction transonic compressor rotor. Through transient computational fluid dynamics simulations, we demonstrate that SEB can extend the rotor's operating range by up to 14.07%. The physical mechanism underlying this stability enhancement is the suppression of the shock-induced breakdown of the tip leakage vortex (TLV) near the blade leading edge and the attenuation of the double leakage flow by SEB. The unsteady excitation of the bleed effect dominates the tip flow and eliminates the spontaneous closed-loop feedback process based on the dynamic interaction between the TLV breakdown, the tip secondary vortex, and the blade loading. Time-resolved tip-region flow patterns elucidate the self-organization and reconstruction of this feedback mechanism. Frequency spectral analysis further reveals that the self-induced oscillation frequency of the tip leakage flow formed during the feedback process disappears, and the bleed excitation frequency replaces it as the main frequency of the tip flow field. However, increasing the bleed flow rate causes the boundary layer on the suction surface to migrate radially outward, resulting in increased flow blockage at the rear of the tip passage. These two influences of SEB are quantified by a blockage factor, and determining the optimal bleed flow rate requires a trade-off between beneficial and detrimental impacts.

Wing rock mode and its mechanism of a flying-wing aircraft

The flying wing is an aerodynamic configuration with high efficiency, but the lack of lateral-directional stability has always been an obstacle that limits its application. In this study, the wing rock motion of a 65° swept flying-wing aircraft is studied via wind tunnel experiments and numerical simulations at a low speed, and various unsteady motion phenomena are focused on. Both the experimental and numerical results show that the flying wing has a bicyclic ${C_l}$ – $\phi $ hysteresis loop during its wing rock, different from the slender delta wing, rectangular wing, generic aircraft configuration, etc., which have a tricyclic hysteresis loop. This form of hysteresis loop implies a different energy exchange manner of the flying wing in the wing rock oscillation. Further analysis shows that the flying wing forms a unilateral leading-edge vortex (LEV) under a high roll angle, with its wing rock oscillation driven by the ‘vortex–shear-layer’ structure, which is different from that of slender and non-slender delta wings. Moreover, the quantitative dynamic hysteresis characteristics of the LEV's strength and location for the flying wing and the slender delta wing are also different. These results have proven the existence of a wing rock mode which is different from previous investigations, which enriches the understanding of self-induced oscillation. Present discoveries are also conducive to the aerodynamic shape design and flight manipulation of a flying-wing aircraft, which is significant for its wider application.

Light-Induced Self-Oscillations and Spoiling of the Bragg Resonance Due to Nonlinear Optical Propagation in Heliconical Cholesteric Liquid Crystals

In a recent paper, we have reported the results of a study of the nonlinear light propagation of a beam traveling along the helix direction of a heliconical cholesteric liquid crystal, showing that optical reorientation leads to instabilities in the optical transmission when the light wavelength is close to the Bragg resonance. Here we report a detailed study of this phenomenon, using Ambartsumian’s layer addition modified method to take into account the continuous modification of the wave field during propagation. We show that the whole transmission spectrum is modified by increasing the light intensity and point out that self-induced oscillations take place at lower intensities on the red side edge of the Bragg resonance while stable values of transmittivity are still observed on the blue side edge. A further increase in the intensity leads to oscillations of lower amplitude on the blue side while an irregular behavior of the transmission is achieved on the red side. At higher intensities, the Bragg resonance disappears and transmission becomes unstable for any light wavelength. A simple phenomenological model is proposed to account for the onset of the oscillations and the asymmetry of the behavior at the opposite side of the Bragg resonance. We also point out that the static electric field is a driving parameter to switch from stable to oscillatory to irregular behavior in the transmittivity at a given light wavelength.

Strong flame interaction-induced collective dynamics of multi-element lean-premixed hydrogen flames

Multi-element injector configurations, typically comprising numerous small-scale jet nozzles, will be required to enable reliable lean-premixed gas turbine combustion operating with pure hydrogen or high hydrogen content fuels. The integration of large numbers of millimeter-scale injectors in tightly clustered arrays is highly likely to produce peculiar topological states determined by the collective dynamics of strongly interacting premixed hydrogen flames. To understand the influence of inter-element flame interactions in a multi-element nozzle environment, here we experimentally investigate the combustion dynamics of two different pure hydrogen flame ensembles, one relatively coarse array of 293 round jet nozzles, and the other a dense array of 421 nozzles. Measurements of self-induced instability were conducted for both configurations under 60–130 thermal power conditions, in conjunction with phase-synchronized OH PLIF and high-speed OH∗ chemiluminescence emission measurements. We demonstrate that for both cases the characteristic dimension of a single injector element is the main determinant of the fundamental frequency of self-induced pressure oscillations, and that the oscillations are primarily driven by the combination of coherent structure-related flame surface rollup and local extinction/pinch-off. Whereas the coarse injector arrangement exhibits well-organized parallel propagation and deformation of isolated vortical structures, the dense array case manifests strong repulsive interactions between adjacent coherent structures, naturally creating lateral flame surface modulations as well as limited streamwise oscillations. Importantly, collision-induced lateral flame dynamics play a mechanistic role in the substantial growth of higher harmonics in the frequency range of 1–2 kHz, leading to the creation of multiple frequency excitation states of commensurate amplitude, which uniquely define the thermoacoustic state of clustered lean-premixed hydrogen flames.

Liquid jet injection in supersonic crossflow: Self-induced jet oscillations and its effects

Liquid jet injection into supersonic crossflow is one of the conceptually simple ways of fuel atomization in Scramjet engines. In the present work, we experimentally investigate the flow and spray formed due to the injection of a liquid jet in supersonic crossflow with free stream Mach number of M∞ = 2.5. The main focus of the work is on the unsteady aspects of the flow and spray, which are found to be self-induced and occur naturally in the flow. High speed shadowgraphy shows large variations in the windward spray edge and penetration with time. Unsteady pressure measurements in the liquid injection line show that in the presence of a supersonic crossflow, there exist significant self-generated mass flow fluctuations that occur in the injection line. The frequency of these oscillations is seen to be close to a Strouhal number, St = fδ /U∞ ≈ 0.007, close to the typical low frequencies seen in Shock-Wave Boundary Layer Interaction studies (δ = boundary layer thickness). The variations in mass flow rate within the injection line leads to significant fluctuations of the spray penetration and shocks. In particular, the variations in spray penetration with time is observed to lead to significant variations in both the droplet numbers and size at a given location in the generated spray, which can have significant implications for applications where such configurations are used.

Synergistic Numerical and Experimental Investigations of Dynamic Stability of a Lifting Capsule

The dynamic stability of a reentry lifting capsule with a large aftbody was studied by synergistic experimental and numerical investigations. Free-to-rotate wind-tunnel tests showed that self-induced pitch oscillations depended on the Mach number, Reynolds number, and surface roughness. The experiment provided the amplitudes and frequencies of oscillations for numerical simulations in prescribed motions. Both data reduction methods for the damping coefficient are presented, and the time-series results of both methods are compared. The validated numerical simulations enabled detailed visualization of the separated flow on the side surface of the aftbody to investigate the effects of flow separation and attachment on the dynamic stability and the effect of sting interference. The dynamic characteristics observed in a postflight test with a scaled model of the recovered capsule agreed with those of the reconstructed flight in spite of deceleration effects. Wind-tunnel testing and numerical simulations as well as their mutually validated investigations provided sufficient information to fly the small lifting capsule.

A Lattice-Boltzmann study of premixed flames thermo-acoustic instabilities

We present possibly for the first time Lattice-Boltzmann numerical simulations of thermo-acoustic instabilities of premixed flames. We study flames interacting with an imposed acoustic field where flames submitted to a parametric instability can be observed, as well as plane flames re-stabilized by the acoustic forcing. Self-induced thermo-acoustic oscillations of flames propagating in narrow channels are also studied, indicating an unexpected dependency with the channel width. For both excited and self-excited flames, results confirm that Lattice-Boltzmann method can capture the complex coupling between flame dynamics and acoustics.

Self-induced oscillations in Si and other semiconductors

Abstract Some metals share an elusive property with silicon (and other semiconductors): they may exhibit strong self-induced current oscillations during anodic dissolution in electrochemical experiments. While this feature, as well as related features concerning self-organization at reactive solid-liquid interfaces, is still not well understood, the so-called “current-burst model” of the authors succeeded in reproducing many effects quantitatively that have been observed at the Si electrode. The current-burst model assumes that current flow through the electrode on a nm scale is inhomogeneous in both time and space; a single current-burst is a stochastic event. Current oscillations in time and space result from interactions in space or time of single current-bursts. The paper outlines the basics of the model and gives results of Monte Carlo simulations concerning stable and damped oscillations for the current and, as a new feature, for the voltage. With the current-burst model a kind of “nano”-electrochemistry is introduced; its strengths, weaknesses, and possible implications for other electrochemical phenomena and for other materials are briefly discussed.

Amplitude and phase locking of mechanical oscillation driven by radiation pressure

Cavity optomechanical systems have been widely studied for many of their interesting properties, which can be interpreted with their linearized dynamics. Previously, the genuine nonlinear dynamics of optomechanical systems was only considered for very few phenomena such as self-induced oscillation and optomechanical chaos. Beyond the scope of those previous studies, a phenomenon of frozen mechanical oscillation irrespective of the intensity variation on the driving fields was recently discovered for an optomechanical system under two pump fields having their frequencies properly matched [He et al., Phys. Rev. A 102, 011503(R) (2020)]. Here we significantly advance the study by showing that similar phenomena can manifest under more general conditions for the external driving fields. The mechanical oscillation in such doubly driven optomechanical systems can be locked in either its amplitude or both amplitude and phase. Simultaneous amplitude and phase locking is possible under a specific condition for the difference of the drive frequencies, whenever the drive intensity is sufficiently high. The variations of the phenomena also exist in two coupled cavities containing one mechanical breathing mode.

Coupling analysis of thin cropped delta wing at transonic speed based on partitioned approach

The oscillation of a thin cropped delta wing at transonic speed was investigated by fluid-structure interaction (FSI) analysis based on a coupling approach. The present analysis model was composed of open-source software fluid solver SU2, structure solver CalculiX, and coupling library preCICE. It was found that self-induced oscillation of the cropped delta wing with elastic deformation occurs at the transonic speed. The self-induced oscillation grew with the time elapsed, and then, the behavior reached limit-cycle oscillation (LCO). The times which reach LCO varied according to the freestream dynamic pressure. The primary frequency of the oscillation was approximately 55 Hz for all conditions considered herein. However, the eigenfrequencies of the present condition of the delta wing, which are 26.66, 89.12, 133.23, 204.60, and 312.34 Hz, etc., did not correspond to the oscillation frequency. It was indicated that the strong aerodynamic force induced the oscillation. Through the comparison with the experimental data, it was found that improvement of the present FSI analysis model, i.e., turbulence model and finer computational mesh, is required.

Development and Integration of the Dual Fuel Combustion System for the MGT Gas Turbine Family

Abstract Industrial gas turbines like the MGT6000 are often operated as power supply or as mechanical drives. In these applications, liquid fuels like “diesel fuel no. 2” can be used either as main fuel or as backup fuel if natural gas is not reliably available. The MAN gas turbines (MGT) operate with the advanced can combustion (ACC) system, which is capable of ultralow NOx emissions for gaseous fuels. This system has been further developed to provide dry dual fuel capability. In this paper, we describe the design and detailed experimental validation process of the liquid fuel injection, and its integration into the gas turbine package. A central lance with an integrated two-stage nozzle is employed as a liquid pilot stage, enabling ignition and startup of the engine on liquid fuel only. The pilot stage is continuously operated, whereas the bulk of the liquid fuel is injected through the premixed combustor stage. The premixed stage comprises a set of four decentralized nozzles based on fluidic oscillator atomizers, wherein atomization of the liquid fuel is achieved through self-induced oscillations. We present results illustrating the spray, hydrodynamic, and emission performance of the injectors. Extensive testing of the burner at atmospheric and full load high-pressure conditions has been performed, before verification within full engine tests. We show the design of the fuel supply and distribution system. Finally, we discuss the integration of the dual fuel system into the standard gas turbine package of the MGT6000.

A fully coupled fluid–structure–acoustic interaction simulation on reed-type artificial vocal fold

The sound generation mechanisms of vocal folds’ self-induced oscillations have been investigated using vocal fold replicas. In this study, a compressible flow simulation coupled with a one-dimensional beam equation was conducted to study a reed-type artificial vocal fold. The vibrating reed was expressed by the volume penalization method in the aeroacoustic fields. The sound of the artificial vocal fold was measured in a flow experiment and compared with the simulation. The results showed that the predicted acoustic characteristics agreed well with those measured in the experiment. The flow fields during reed oscillation indicated that the rapid change in the flow rate downstream from the reed formed a strong source in the high-frequency harmonics.

Self-Induced Roll–Yaw Oscillations of an Aircraft Model in a Wind Tunnel

Wind tunnel oscillatory testing is a technique conventionally used for the measurement of dynamic aerodynamic loads on subscale aircraft models. Oscillatory testing is commonly used for identification of dynamic stability derivatives for flight mechanics models. Model oscillation is normally driven by a rig drive mechanism, or with the use of its control surfaces. This paper presents a new test technique in which self-induced coupled roll and yaw oscillations are driven purely by the aircraft’s natural modes of motion when constrained in translation and in pitch rotation. This method does not require external mechanisms to drive the motion, nor the use of aircraft control surface inputs to sustain the oscillation. Without these sources of error, it is potentially advantageous for identification of dynamic stability derivatives using parameter estimation techniques. The mechanics of and requirements for this self-induced oscillation are discussed. Computational simulation and dynamic wind tunnel tests of a subscale BAe Hawk model are used to demonstrate the self-induced roll–yaw oscillations and their results are compared. Effects of varying parameters such as wind tunnel speed and the addition of spring stiffness onto the aircraft rotation axes on the frequency and stability of the oscillation are investigated.

Numerical investigation of flow separation control over an airfoil using fluidic oscillator

Leading edge flow separation control over a stalled National Advisory Committee for Aeronautics 0015 airfoil using a fluidic oscillator (FO) is investigated by means of numerical simulation possibly for the first time to elucidate the flow control mechanism and evaluate control authority. The flow is assumed to be two-dimensional and fully turbulent and resolved using unsteady Reynolds-averaged Navier–Stokes calculations with the elaborate Reynolds stress turbulence model employed. Our simulation is proved to be able to successfully resolve the basic characteristics of a FO operating in quiescent air, which include both the qualitative prominent flow structures and quantitative jet oscillation frequency. It is seen that the driving force behind the self-induced and self-sustaining oscillation of jet flow inside the oscillator is Coanda effect induced alternating development of a recirculation bubble on either side of the mixing chamber walls. We provide a comprehensive analysis of the flow control procedure over an airfoil at Reynolds number of Re = 4.8 × 105 and an elucidation of the flow control mechanism. It is found that the most prominent flow feature resulting from the interplay between an oscillating jet and external crossflow over an airfoil is the production of spanwise vortices. The strong entrainment effect of the induced spanwise vortices is the dominant mechanism leading to the mitigation of flow separation. Periodic jet oscillation generates a series of downstream moving vortices over an airfoil surface and results in a greatly delayed flow separation. The recovery of a strong suction pressure peak near the leading edge and significant lift enhancement and drag reduction reflects the improvement of an aerodynamic performance of the airfoil under control. Also observed is the phenomenon of local flow frequency lock-in to forcing frequency near the leading edge region. Moreover, the mass supply rate at the inlet of the oscillator is found to have an appreciable effect on the flow control authority. A higher mass flow usually leads to a better flow control performance.

Evaluation of self-induced oscillation of the flow control valve by fluid structure interaction analysis

In the flow control valve used in the upper or the lower stream of a turbomachinery, when opening is small, the self-induced oscillation resulting from a clearance gap flow may occur. In this study, the mechanism of self-induced oscillation which occurs on the small valve opening condition was examined. And a concept to control the self-induced oscillation was built. The air model test of the flow control valve was carried out, and it was confirmed that self-vibration occurs under specific conditions. Moreover, stability evaluation using CFD was carried out, and it checked that the calculation results become the almost same tendency as a test results. The area where the instability force is generated on a valve was grasped from the simulation results, and it checked that stability was changed with flow field differences. Based on these results, stabilization of the flow control valve was attained by improving clearance gap geometry of the valve sheet area. It checked that the intended flow field was obtained in the geometry in CFD analysis, and it confirmed that fluid additive damping became positive in CFD.