Longitudinal Instability Research Articles

Spontaneous and Triggered Longitudinal Combustion Instability in a Single-Injector Liquid-Rocket Combustor

Spontaneous and triggered longitudinal combustion instability is simulated numerically in a single-injector liquid-rocket engine using a recently developed axisymmetric compressible flow solver. Turbulence is treated using delayed detached-eddy simulation, whereas chemical reactions are modeled using a compressible flamelet progress variable method. The baseline case is an unstable case that exhibits spontaneous instability and simulates well the experimental evidence. Heat loss is then introduced by imposing a isothermal boundary condition on the chamber wall. Various temperature values are used, with spontaneous longitudinal-mode instability still occurring at the higher wall temperature. Stable but inefficient combustion occurs for the lowest wall temperature. Subsequently, triggered instability of the chamber with low wall temperatures is simulated by perturbing the propellant mass flow rates. Unsteady oscillation can be triggered to higher-amplitude limit cycles. The effectiveness of various disturbances as triggers for instability is quantified through the definition of “triggered energy” and an unsteady Rayleigh-index analysis.

Longitudinal wave instability due to rotating beam-plasma interaction in weakly turbulent astrophysical plasmas

ABSTRACTThe aim of this study is to highlight the temporal evolution of the longitudinal wave instability due to the interaction between a rotating electron beam and the magnetoactive plasma region in space plasma structures. The plasma structure which could be either in the solar atmosphere or any active plasma region in space is considered weakly turbulent, where the quasi-linear theory is implemented to enable analytic insight on the wave–particle interaction in the course of the event. It is found that in a weakly turbulent plasma, quasi-linear saturation of the longitudinal wave is accompanied by a significant alteration in the distribution function in the resonant region. In case of a pure electrostatic wave, the wave amplitude experiences elevation due to the energy transfer from the plasma particles. This causes flattening of the bump on tail (BOT) in the electron distribution function. If the gradient of the distribution function is positive, the chance that the beam would excite the wave is probable. In such a situation a plateau on the distribution function (∂f/∂v ≈ 0) is formed that will stop the diffusion of beam particles in the velocity space. Evolution of the electron distribution function experiences a decreases of the instability of the longitudinal wave. It is deduced that the growth rate of the wave instability is inversely proportional to the wave energy. Regarding the Sun, in addition to creating micro-turbulence due to wave–particle interaction, as the wave elevates to higher altitudes it enters a saturated energy state before releasing energy that may be a candidate for the generation of radio bursts.

Behavior of the Fusion Hole in Tungsten Inert Gas Thin-Plate Welding

One-side welding with back formation technique is widely used in pressure vessels and the pressure piping industry. Welders provide high importance to the role of the fusion hole in this type of welding. Therefore, the formation and instability of the fusion hole should be studied to realize the welding automation of the one-side welding with back formation process. A visual sensing system is established to capture the image of the fusion hole, and the behavior of the fusion hole is investigated. Results show that the typical process of the fusion hole includes formation, growth, and macroquasisteady state. The welding parameter scoop to obtain the stable fusion hole is narrow. After deviating from the appropriate parameter scoop, the fusion hole will enter unstable states, thereby resulting in the “fusion hole effect” failure of the welding process. Depending on the change in the shape of the fusion hole, instability types of the fusion hole can be divided into longitudinal, lateral, and gradual shrinking instability. Different instability types correspond to different changing laws of fusion hole morphology. The instability characteristics of the fusion hole can provide effective early warning information before the “fusion hole effect” failure.

Investigating ride quality of the machine-tractor unit with a variable mass during transport work

The disclosure of issues related to solving problems of transport work has always been relevant. Today, in the study of types of transport works used in agriculture, there is a problem in the study of the movement of machine-tractor unit with trailed or semi-trailer tanks, which are filled with liquid. Since the transportation of liquid cargo causes significant internal oscillations, the impact on the operation of the wheeled tractor and tank is significant. This process causes longitudinal and transverse instability, which contributes to the increase of both energy costs and emergency situations (there is an effect on vibrations of motion, controllability and stability of the vehicle, increasing the dynamic loading of the running system, etc.). The goal of the work is to study the smoothness of the machine-transport unit with variable weight (semitrailer-tank and tank-trailer) by simulating the vibrational system of the vehicle. Namely, to study the effect of forced oscillations on the movement of a semi-trailer tank and a trailer tank, as well as when varying the rigidity of the tires of the wheeled tractor. In solving the set goal, a methodology was used which included the analysis of the results of mathematical modeling of the vibration of the wheeled tractor and the tank with a liquid containing a free surface in the dynamic solution of the problem. The model used takes into account the redistribution of liquid in the tank, which is caused by fluctuations of the shell, using the characteristics of the Rayleigh surface waves. As a result, the data of theoretical research, which take into account the variable weight of the semi-trailer and trailer unit during the performance of the machine-tractor unit of transport work with varying rigidity of tires, is obtained. The originality is in determining the dependence of the rigidity index of the wheels of the wheeled tractor on the smoothness of the machine-tractor unit with a semi-trailer and trailer tanker. The practical value of the work is in the recommendations on the use of one or another pressure in the tires of the wheeled tractor during operation.Key words: wheeled tractor; transport work; tank, vibrations; mass redistribution.

Large-scale periodic velocity oscillation in the filamentary cloud G350.54+0.69

We use APEX mapping observations of 13CO, and C18O (2-1) to investigate the internal gas kinematics of the filamentary cloud G350.54+0.69, composed of the two distinct filaments G350.5-N and G350.5-S. G350.54+0.69 as a whole is supersonic and gravitationally bound. We find a large-scale periodic velocity oscillation along the entire G350.5-N filament with a wavelength of ~1.3 pc and an amplitude of ~0.12 km/s. Comparing with gravitational-instability induced core formation models, we conjecture that this periodic velocity oscillation could be driven by a combination of longitudinal gravitational instability and a large-scale periodic physical oscillation along the filament. The latter may be an example of an MHD transverse wave. This hypothesis can be tested with Zeeman and dust polarization measurements.

Instability in longitudinal childhood IQ scores of Guatemalan high SES individuals born between 1941-1953.

Childhood IQ has been used to predict later life outcomes across disciplines in epidemiology, education, and psychology. Most often only a single childhood IQ test is available or is used for these purposes in the belief that IQ is stable across the life course. The primary aim of this study was to examine the longitudinal stability of individuals’ IQ test scores derived from school-age tests. The secondary aim was to investigate the association of the pre-adult scores with later life intelligence scores. The longitudinal pre-adult IQ scores of 42 high socioeconomic status Guatemalans born 1941–1953 were analysed and showed low stability of longitudinal test scores. Fluctuations of >1SD were found for 59.5% of the sample. The same participants, aged 64–76 years, were re-assessed and average pre-adult IQ explained 12% of variance in the older age intelligence score. The reasons behind the longitudinal instability in test scores reported in this study remains unknown but the results suggest single point measurements of intelligence before adulthood should be regarded with some caution.

On the observation of high-order, multi-mode, thermo-acoustic combustion instability in a model gas turbine combustor firing hydrogen containing syngases

This paper explicates the characteristics of high-order multi-mode combustion instability generated in a model GE7EA combustor that has a partially premixed flame, which can prohibit flashback when firing fast-burning hydrogen-containing fuels. Five test cases varying both the fuel composition and heat input were selected, which generates a high multi-mode without a fundamental mode. Multi-mode instability phenomena were observed by 11 dynamic pressure sensors in both the time and frequency domains and by OH planar laser induced fluorescence (PLIF) images, which were time-synchronized with a dynamic pressure signal. A distortion in pressure waves, which resulted from the superposition of several harmonic modes of combustion instability, was observed, whereas distortion in heat release was not clearly observed. PLIF images and their statistical analysis showed scientific clues that proved the dependency of the flame location and movement on the combustion instability. The driving source of the combustion instability of the longitudinal mode was not only the longitudinal variations but also the radial variations in the center of intensity (COI) of the flame images so the Pythagorean addition of the interquartile ranges of both longitudinal and radial directions was suggested as a good indicator of the size of the longitudinal mode's combustion instability. The time series trajectory of the COI also provided meaningful information on the combustion instability mechanism. A linear and longer COI trajectory was likely to generate more intense instability because the one-way movement of the flame can be more effective in driving instability by transferring consistent force in the same direction.

Experimental study on hydrodynamic performance of SWATH vessels in calm water and in head waves

An extensive experimental investigation on four SWATH hull forms has been conducted in calm water and in regular waves at University of Naples Federico II. Calm water tests have been analyzed in the range of Froude number FrT from 0.1 to 0.6. For all four SWATH configurations at the speed, corresponding to FrT 0.32, the behaviour in regular waves has been tested. The results of heave, pitch and vertical accelerations are presented in nondimensional form as RAO. For the “most promising” SWATH #4 configuration, a set of stabilizing fins have been designed and an active stabilization system has been developed. The developed SWATH#5 has been tested in calm water on three displacements in the range of FrT from 0.1 to 0.65. The dynamic wetted surface has been identified and the residual resistance coefficient CR as well as RT/Δ are reported. Seakeeping tests have been performed in regular head sea and in head and following irregular sea at FrT = 0.50. The conditions for the occurrence of dynamic longitudinal instabilities have been identified. The results allows to comment the effect of slenderness of struts and SWATH’s immersed bodies on resistance and seakeeping and concerns the applicability of SWATH concept to small craft.

Modal instabilities in high power fiber laser oscillators.

Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.

Effect of laser noise on the propagation of laser radiation in dispersive and nonlinear media

The effect of laser noise on the atmospheric propagation of high-power CW lasers and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. High-power CW laser systems have relatively large fractional levels of intensity noise and frequency noise. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. A paraxial wave equation, containing dispersion and nonlinear effects, is expanded in terms of fluctuations in the intensity and phase. Longitudinal and transverse intensity noise and frequency noise are considered. The laser propagation model includes group velocity dispersion, Kerr, delayed Raman response, and optical self-steepening effects. A set of coupled linearized equations are derived for the evolution of the laser intensity and frequency fluctuations. In certain limits, these equations can be solved analytically. For example, we find that, in a dispersive medium, frequency noise can couple to and induce intensity noise (fluctuations) and vice versa. At high intensities, the Kerr effect can reduce this intensity noise. In addition, significant spectral modification can occur if the initial intensity noise level is sufficiently high. Finally, our model is used to study the transverse and longitudinal modulational instabilities. We present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm, 1 μm, and 10.6 μm.

Calculation of longitudinal collective instabilities with mbtrack-cuda

Macroparticle tracking is a prominent method for studying the collective beam instabilities in accelerators. However, the heavy computationalload often limits the capabilities of tracking codes. One widely used macroparticle tracking code for simulating collective instabilities in storage rings is mbtrack. The Message Passing Interface (MPI) has already been implemented in mbtrack to accelerate the simulations. However, many Central Processing Unit (CPU) threads are requested in mbtrack for the analyses of the coupled-bunch instabilities.Therefore, computer clusters or desktops with many CPU cores are needed. Since these are not always available, we employ a Graphics Processing Unit (GPU) with a CUDA programming interface as an alternative to run such simulations in a stand-alone workstation. All the heavy computations have been moved to the GPU. The benchmarks confirm that mbtrack-cuda can be used to analyze coupled-bunch instabilities of 484 bunches. Compared to mbtrack on an 8-core CPU, a 36-core CPU and a cluster, mbtrack-cuda is faster for simulations of up to 3 bunches. For 363 bunches, mbtrack-cuda requires approximately six times the execution time of the cluster and twice that of the 36-core CPU. The multibunch instability analysis demonstrates that the length of the ion-cleaning gap does not have a substantial influence, at least at 34 filling.

High-frequency transition characteristics of synthetic natural gas combustion in gas turbine

AbstractIn this study, the combustion instability and emission characteristics of flames of different H2/CH4compositions were investigated in a partially premixed model gas turbine combustor. A mode shift in the frequency of instability occurred under varying experimental conditions from the first to the seventh mode of longitudinal frequency in the combustor, and a parametric study was conducted to determine the reasons for this shift by using the length of the combustor, a factor that determines the mode frequency of longitudinal instability, as the main parameter. Furthermore, heat load and fuel composition (H2ratio) were considered as parameters to compare the phenomenon under different conditions. The GRI-3.0 CANTERA code, OH chemiluminescence and the Abel inversion process were applied to analyse the frequency mode shift. NOxemissions, which occurred through the thermal NOxmechanism, increased with increasing heat load and H2ratio. The instability frequency shifted from the first to the seventh mode as the H2ratio increased in the H2/CH4mixture. However, 100% H2as fuel did not cause combustion instability because it has a higher burning velocity and extinction stretch rate than CH4. Furthermore, the laminar flame speed influenced the frequency mode shift. These phenomena were confirmed by the flame shapes. The Abel inversion process was applied to obtain the cross section of the flames from averaged OH chemiluminescence images. Stable and unstable flames were identified from the radial profile of OH concentration. The combustor length was found to not influence frequency mode shift, whereas the H2ratio significantly influenced it as well as the flame shape. The results of this experimental study can help in the reliable operation of gas turbine systems in SNG plants.

Theory of coupled-bunch longitudinal instabilities in a storage ring for arbitrary rf potentials

We present a theory of coupled-bunch longitudinal instabilities that is based on the coupled set of Vlasov equations governing the particle distribution function, and can be applied to arbitrary longitudinal potentials. We find that the coupled-bunch growth rate is given by a dispersion relation that is parametrized by the eigenvalues of the linear (harmonic) coupled-bunch matrix problem. Our theory therefore treats the wakefield-driven source of the instability and the effect of Landau damping together and on equal footing, and also indicates that the stabilizing effects of Landau damping can approximately be compared to the instability growth rates in a harmonic potential that has the same bunch length. We then apply the theory to a weakly nonlinear oscillator and to a quartic potential that is relevant for ultralow emittance storage rings that employ bunch-lengthening systems. In the latter case we find that the theory compares quite favorably with particle tracking simulations for the parameters of the planned upgrade of the Advanced Photon Source.

Passive higher-harmonic rf cavities with general settings and multibunch instabilities in electron storage rings

© 2018 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. We introduce a perturbation-theory, mode-analysis method for longitudinal multibunch instabilities driven by the higher harmonic cavity (HHC) fundamental mode. The method, based on the exact solution of the unperturbed particle motion in the rf bucket and suitable for modeling the effect of cavities with general settings, is applied to study the feasibility of reutilizing the existing Advanced Light Source (ALS) HHCs in the ALS Upgrade (ALS-U). We find that with ALS cavities the ALS-U would be susceptible to a fast =1 mode instability. Interestingly, the instability is driven by the imaginary rather than the real part of the cavity fundamental-mode impedance.

WKB solutions to the quasi 1-D acoustic wave equation in ducts with non-uniform cross-section and inhomogeneous mean flow properties – Acoustic field and combustion instability

Linear modal analysis based on the spatio-temporal harmonic representation of acoustic fluctuations is a well-known reduced order method to predict combustion instability frequencies. However, a limitation of this method is that it is only applicable to domains with homogeneous mean properties. The Wentzel-Kramers-Brillouin (WKB) method facilitates the development of approximate analytical solutions to the one-dimensional (1-D) acoustic wave equation in domains with inhomogeneous mean properties. The “classical” form of the WKB solution to the wave equation is based on the assumption of high frequency (or, short wavelength), and is applicable to uniform cross-section ducts with a mean temperature gradient, but no mean flow. Subsequently, the WKB method was extended to include uniform mean velocity as well. In the current study, we derive a WKB-type solution to the generalized Helmholtz equation in quasi 1-D ducts with non-uniform cross-sectional areas and inhomogeneities in mean flow properties such as the velocity, temperature, density, and pressure. The WKB solution is applicable under the assumptions of high frequency and slowly varying mean properties. In deriving this solution, a novel approach has been developed to relate the first and second spatial derivatives of the axial velocity fluctuations to the pressure fluctuations (and its derivatives).A second WKB-type solution (referred to as the WKB2 solution) is also obtained by relaxing the slowly-varying mean property assumption, but retaining the high frequency approximation. Thirdly, the Helmholtz equation for pressure is solved numerically without invoking the two WKB assumptions. The WKB, WKB2 and numerical solutions are compared for a number of cases involving combinations of non-uniform duct areas and axially varying mean properties. Finally, we apply the WKB solution to predict the longitudinal combustion instabilities in a 1-D combustor characterized by a sudden expansion, with uniform mean flow in each of the two ducts comprising the combustor, and a linear temperature profile downstream of a compact, planar flame.

Self-excited transverse combustion instabilities in a high pressure lean premixed jet flame

An experimental investigation of self-excited combustion instabilities in a high pressure, lean premixed natural gas jet flame is presented. The combustor is designed with optical access and is instrumented with high frequency pressure transducers at multiple axial and circumferential locations. OH*-chemiluminescence measurements performed at a frequency of 50 kHz were temporally synchronized with the acoustic measurements recorded from the pressure transducer array during the test. Two representative test conditions are analyzed in detail: Flame 1 (F1) that presents longitudinal mode dynamics (p′/pc=3%) and Flame 2 (F2) that presents high amplitude transverse instabilities (p′/pc=15%). Singular Spectrum Analysis (SSA) and Dynamic Mode Decomposition (DMD) analysis indicate a strong correlation of both instabilities to flame-vortex interactions. Longitudinal mode instabilities are correlated with axisymmetric vortex shedding about the combustor axis that result in periodic axial variations in heat release at the 1L frequency. Transverse mode instabilities correspond to asymmetric vortex shedding pattern that drive transverse variations in heat release at the fundamental 1T frequency of the combustion chamber. The phase relationship of the flame emission intensity and the chamber head-end pressure measurement at the 1T frequency indicates presence of a non-stationary transverse mode that rotates about the chamber axis at 55 Hz.

Studies of the Micro-Bunching Instability in the Presence of a Damping Wiggler

At the KIT storage ring KARA (KArlsruhe Research Accelerator), the momentum compaction factor can be reduced leading to natural bunch lengths in the ps range. Due to the high degree of longitudinal compression, the micro-bunching instability arises. During this longitudinal instability, the bunches emit bursts of intense coherent synchrotron radiation in the THz frequency range caused by the complex longitudinal dynamics. The temporal pattern of the emitted bursts depends on given machine parameters, like momentum compaction factor, acceleration voltage, and damping time. In this paper, the influence of the damping time is studied by utilizing the CLIC damping wiggler prototype installed in KARA as well as by simulations using the Vlasov-Fokker-Planck solver Inovesa.

Chronic Longitudinal Instability of the Forearm Treated With a Combination of Ulnar Shortening Osteotomy, Pronator Teres Transfer, and Tightrope Technique.

Restoration of longitudinal stability of the forearm continues to pose a difficult challenge for the hand surgeon, and no technique has demonstrated success above others. Longitudinal stability to the forearm is conferred by 3 structures: the radial head, which acts as a primary stabilizer, the interosseous membrane, more specifically, the central band and, the distal radioulnar ligaments which are part of the triangular fibrocartilage complex. A combination of techniques is described in this article to address chronic longitudinal instability of the forearm: (1) ulnar shortening osteotomy to restore ulnar variance, (2) pronator teres transfer was used to reconstruct the central band of the interosseous membrane, and (3) tightrope augmentation was used to prevent elongation during the healing process.

A Novel Algorithmic Approach to Chronic Forearm Longitudinal Instability.

Essex-Lopresti Lesions are rare injuries that are often missed in the acute setting. Delayed intervention may lead to chronic wrist and elbow pain and overall poor outcomes. The literature currently supports treatments that involve shortening of the ulna to reduce the relative degree of ulnar impaction, followed by attempted reduction of the distal radioulnar joint. Although such techniques may help to temporarily reduce wrist pain secondary to ulnar impaction, they do not address the proximal migration of the radius and ipsilateral radial head dislocation at the elbow. Subsequent procedures are often needed to replace or resect the radial head. We present a novel approach to chronic Essex-Lopresti lesions resulting in anatomic restoration of forearm length with return of elbow and wrist flexion/extension as well as improved forearm pronation/supination.

Diagnosis and treatment of acute Essex-Lopresti injury: focus on terminology and review of literature

BackgroundAcute Essex-Lopresti injury is a rare and disabling condition of longitudinal instability of the forearm. When early diagnosed, patients report better outcomes with higher functional recovery. Aim of this study is to focus on the different lesion patterns causing forearm instability, reviewing literature and the cases treated by the Authors and to propose a new terminology for their identification.MethodsFive patients affected by acute Essex-Lopresti injury have been enrolled for this study. ELI was caused in two patients by bike fall, two cases by road traffic accident and one patient by fall while walking. A literature search was performed using Ovid Medline, Ovid Embase, Scopus and Cochrane Library and the Medical Subject Headings vocabulary. The search was limited to English language literature. 42 articles were evaluated, and finally four papers were considered for the review.ResultsAll patients were operated in acute setting with radial head replacement and different combinations of interosseous membrane reconstruction and distal radio-ulnar joint stabilization. Patients were followed for a mean of 15 months: a consistent improvement of clinical results were observed, reporting a mean MEPS of 92 and a mean MMWS of 90.8. One case complained persistent wrist pain associated to DRUJ discrepancy of 3 mm and underwent ulnar shortening osteotomy nine months after surgery, with good results.DiscussionThe clinical studies present in literature reported similar results, highlighting as patients properly diagnosed and treated in acute setting report better results than patients operated after four weeks. In this study, the definitions of “Acute Engaged” and “Undetected at Imminent Evolution” Essex-Lopresti injury are proposed, in order to underline the necessity to carefully investigate the anatomical and radiological features in order to perform an early and proper surgical treatment.ConclusionsFollowing the observations, the definitions of “Acute Engaged” and “Undetected at Imminent Evolution” injuries are proposed to distinguish between evident cases and more insidious settings, with necessity of carefully investigate the anatomical and radiological features in order to address patients to an early and proper surgical treatment.