High Frequency Instability Research Articles

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

Systematic design of hybrid high power microwave amplifiers using large gate periphery GaN HEMTs

In this paper, a design methodology for realization of hybrid microwave power amplifiers (PAs) using discrete GaN HEMTs is systematically explained. High power solid-state amplifiers usually suffer from self-heating, high frequency instability, unbalanced feeding effects and suchlike that should be carefully taken into account in a reliable and robust design. Though most of these issues are discussed in some of the previously published literatures, a single self-contained document that systematically describes the hybrid microwave PA design is limited. In this work, a comprehensive design methodology for implementation of high frequency PA is presented while the important challenges of high power amplifier design are discussed in more detail and several useful practical hints are given. Especially, we focus on hybrid PAs that are realized using large gate periphery GaN HEMTs. As a real example, a PA is fabricated using a 0.25 μm GaN on SiC HEMT transistor with a gate width of 12.5 mm. The fabricated PA achieves a 12 dB small-signal gain, 30 W output power, and 52% PAE across the frequency range of 9–9.8 GHz. The thermal analyses of the power transistor are performed using a 3D model, which is developed in ANSYS software and calibrated by measured data. The model can be used to calculate the transistor channel temperature and consequently predict the device lifetime.

The Effect of Wall Inertia on High-Frequency Instabilities of Flow Through an Elastic-Walled Tube

We examine the effect of wall inertia on the onset of high-frequency self-excited oscillations in flow through an elastic-walled tube. The previous asymptotic model of Whittaker et al. (Proc. Roy. Soc. A466, 2010), for a long-wavelength high-frequency instability in a Starling-resistor set-up, neglected inertia in the tube wall. Here, we extend this model by modifying the ‘tube-law’ for the wall mechanics to include inertial effects. The resulting coupled model for the fluid and solid mechanics is solved to find the normal modes of oscillation for the system, together with their frequencies and growth rates. In the system and parameter regime considered, the addition of wall inertia reduces the oscillation frequency of each mode, however its effect on the stability of the system is not as straightforward. Increasing wall inertia lowers the mean flow rate required for the onset of instability, and is therefore destabilising. However, at higher flow rates the instability growth rate is decreased, and so wall inertia is stabilising here. Overall, the addition of wall inertia decreases the sensitivity of the system to the mean axial flow rate. The theoretical results show good qualitative and reasonable quantitative agreement with direct numerical simulations performed using the oomph-lib framework.

High-frequency instabilities of small-amplitude solutions of Hamiltonian PDEs

Generalizing ideas of MacKay, and MacKay and Saffman, a necessary condition for the presence of high-frequency (i.e., not modulational) instabilities of small-amplitude periodic solutions of Hamiltonian partial differential equations is presented, entirely in terms of the Hamiltonian of the linearized problem. With the exception of a Krein signature calculation, the theory is completely phrased in terms of the dispersion relation of the linear problem. The general theory changes as the Poisson structure of the Hamiltonian partial differential equation is changed. Two important cases of such Poisson structures are worked out in full generality. An example not fitting these two important cases is presented as well, using a candidate Boussinesq-Whitham equation.

High-frequency instabilities of stationary crossflow vortices in a hypersonic boundary layer

Complex interrelated mechanisms mediate the stability of hypersonic boundary layers. Simulations are used to investigate key mechanisms of primary and secondary instabilities, including nonparallel effects and intermode phase synchronization.

A Rocket Engine under a Magnifying Glass

Even though the technology of cryogenic rocket engines is well mastered today, and has been applied successfully in many launchers all over the world, research activities on the various elementary or coupled processes involved in these complex systems are still relevant and useful for future developments, cost reduction, and knowledge improvement. The Mascotte test facility was designed and built with this in mind twenty years ago. Since then, numerous configurations have been tested, enabling almost all of the phenomena involved in the operation of a rocket engine to be investigated, in a research lab environment, but nevertheless under representative conditions. Research items addressed on the Mascotte test rig include: injector concepts ; liquid oxygen jet atomization and combustion, new propellant combinations (methane instead of hydrogen), ignition, heat transfer at the chamber walls, high frequency instabilities, flow-separation in an over-expanded nozzle, plume and infrared signature.

The application of magnetic field at low pressure for optimal laser-induced plasma spectroscopy

The application of magnetic field in controlling the charged particles of the laser induced plasma is considered. The chara\\cteristics of the plasma depend on the electron density and its recombination. The magnetic field effects as well as the ambient pressure are the key factors for determining the electron density of the plasma. Two different laser energies (Low: 30mJ/pulse, High: 140mJ/pulse) and the pressure range between 760Torr and 10Torr are used when focusing a Nd:YAG laser beam (1064nm, 6ns) onto the metal samples where the applied magnetic field varied between 0.1 and 0.5Tesla. At 760Torr, the plasma signal enhancement was obtained in the presence of magnetic field when the laser energy was low. With a lowered pressure however, the signal enhancement due to a magnetic field was achieved regardless of the laser energy. The enhancement of the signal in the presence of magnetic field is related to the electron density and the high frequency instability which are controlled by the ambient pressure and magnetic field intensity. In this paper, the optimal conditions for the laser-induced plasma spectroscopy in the magnetic field at various low pressures are discussed.

High frequency instabilities induced by energetic ions inside magnetic islands

Energetic ions, which are used for plasma heating in modern tokamaks, as well as fusion alphas, which will sustain fusion burn in future reactors, have a gyro-radius comparable with the half-width of magnetic islands, which are often present in discharges with high plasma pressure. When such ions are injected inside the island, they encircle the magnetic axis of the island, which is formed by magnetic field lines with the island O-points. As a result, a population of particles with strong azimuthal flow around the axis of the island is formed. In the present work it is shown that such population excites high-frequency modes, which are localized inside and move together with the island. The mode frequency (growth rate) can considerably exceed the frequency of the lower hybrid resonance (fast ion gyro-frequency). The possible relation of these modes with observed anomalous scattering of the gyrotron beams off magnetic islands is discussed.

High-frequency instabilities in an explosion-generated relativistic plasma

A realistic problem of an explosion-generated relativistic plasma is talked about with respect to the instabilities developed in such systems. For this, the dispersion equation is derived analytically and solved numerically for typical values of physical quantities. Our calculations reveal that two types of instabilities occur in the said plasma if the dust particles and relativistic effects of ions and electrons are considered. Both types of the instabilities are high-frequency instabilities, which carry growth rates of different magnitudes. In view of the magnitudes, the instability having higher/lower growth rate is called as high-frequency higher/lower growth rate instability. The relativistic effects of ions support the growth of these instabilities, whereas those of electrons suppress the growth of the instabilities. The waves propagating with larger phase velocity are found to grow at higher rates. There exists a critical value of the drift velocity of dust particles, above which another instability starts growing but with significantly lower growth rate.

Modulating action of low frequency oscillations on high frequency instabilities in Hall thrusters

It is found that the low frequency oscillations have modulating action on high frequency instabilities in Hall thrusters. The physical mechanism of this modulation is discussed and verified by numerical simulations. Theoretical analyses indicate that the wide-range fluctuations of plasma density and electric field associated with the low frequency oscillations affect the electron drift velocity and anomalous electron transport across the magnetic field. The amplitude and frequency of high frequency oscillations are modulated by low frequency oscillations, which show the periodic variation in the time scale of low frequency oscillations.

Low and high frequency instabilities in an explosion-generated-plasma and possibility of wave triplet

An explosion-generated-plasma is explored for low and high frequency instabilities by taking into account the drift of all the plasma species together with the dust particles which are charged. The possibility of wave triplet is also discussed based on the solution of dispersion equation and synchronism conditions. High frequency instability (HFI) and low frequency instability (LFI) are found to occur in this system. LFI grows faster with the higher concentration of dust particles, whereas its growth rate goes down if the mass of the dust is higher. The ion and electron temperatures affect its growth in opposite manner and the electron temperature causes this instability to grow. In addition to the instabilities, a simple wave is also observed to propagate, whose velocity is larger for larger wave number, smaller mass of the dust and higher ion temperature.

Experimental Studies of the Localized Disturbances and their Secondary High-Frequency Instability in the Flat Plate, Unswept and Swept Wing Boundary Layer (Review)

Results of experimental studies of the streaky structures instability at nonlinear stage of this process in the shear flows are shown in this work. Flow patterns of the streaky structures with secondary high-frequency disturbances generated on them during its spatial evolution are discussed. Different scenarios of the coherent vortical structures occurrence and downstream evolution in the flat plate, unswept and swept wing boundary layer are considered. Features of the sinusoidal and varicose instability of the longitudinal stationary streaky structure at nonlinear stage of its downstream development are shown. To these features concern: modulation of streaky structure in transverse and streamwise direction by frequency of secondary disturbances, occurrence of new streaky structures downstream and occurrence and development of the nonstationary localized vortices such as Λ-structures in both cases. Development of a nonlinear stage of flow instability is considered both in the region of the adverse and zero pressure gradient

Time‐domain augmented electric field integral equation for a robust marching on in time solver

A time domain augmented electric field integral equation (TD-AEFIE) is proposed to render a more stable marching on in time (MOT) solver. Owing to the good operator spectral property in low frequencies, TD-AEFIE is an effective remedy for the low frequency instabilities of the MOT solutions. To obtain a robust MOT solver, the filtering/averaging scheme is also adopted to alleviate the high frequency instabilities. Compared with the traditional TD-EFIE MOT solver, the unknowns of charge density are introduced based on the current continuity equation. Numerical results of the transient scattering problems of perfectly electrical conducting scatterers demonstrate the stability and accuracy of the proposed technique.

Instability in shocked granular gases

Shocks in granular media, such as vertically oscillated beds, have been shown to develop instabilities. Similar jet formation has been observed in explosively dispersed granular media. Our previous work addressed this instability by performing discrete-particle simulations of inelastic media undergoing shock compression. By allowing finite dissipation within the shock wave, instability manifests itself as distinctive high density non-uniformities and convective rolls within the shock structure. In the present study we have extended this work to investigate this instability at the continuum level. We modeled the Euler equations for granular gases with a modified cooling rate to include an impact velocity threshold necessary for inelastic collisions. Our results showed a fair agreement between the continuum and discrete-particle models. Discrepancies, such as higher frequency instabilities in our continuum results may be attributed to the absence of higher order effects.

High and low frequency instabilities driven by counter-streaming electron beams in space plasmas

A four-component plasma composed of a drifting (parallel to ambient magnetic field) population of warm electrons, drifting (anti-parallel to ambient magnetic field) cool electrons, stationary hot electrons, and thermal ions is studied in an attempt to further our understanding of the excitation mechanisms of broadband electrostatic noise (BEN) in the Earth's magnetospheric regions such as the magnetosheath, plasmasphere, and plasma sheet boundary layer (PSBL). Using kinetic theory, beam-driven electrostatic instabilities such as the ion-acoustic, electron-acoustic instabilities are found to be supported in our multi-component model. The dependence of the instability growth rates and real frequencies on various plasma parameters such as beam speed, number density, temperature, and temperature anisotropy of the counter-streaming (relative to ambient magnetic field) cool electron beam are investigated. It is found that the number density of the anti-field aligned cool electron beam and drift speed play a central role in determining which instability is excited. Using plasma parameters which are closely correlated with the measurements made by the Cluster satellites in the PSBL region, we find that the electron-acoustic and ion-acoustic instabilities could account for the generation of BEN in this region.

Experimental and Numerical Investigation of Thermoacoustic Sources Related to High-Frequency Instabilities

Flame dynamics related to high-frequency instabilities in gas turbine combustors are investigated using experimental observations and numerical simulations. Two different combustor types are studied, a premix swirl combustor (experiment) and a generic reheat combustor (simulation). In both cases, a very similar dynamic behaviour of the reaction zone is observed, with the appearance of transverse displacement and coherent flame wrinkling. From these observations, a model for the thermoacoustic feedback linked to transverse modes is proposed. The model splits heat release rate fluctuations into distinct contributions that are related to flame displacement and variations of the mass burning rate. The decomposition procedure is applied on the numerical data and successfully verified by comparing a reconstructed Rayleigh index with the directly computed value. It thus allows to quantify the relative importance of various feedback mechanisms for a given setup.

High and low frequency instabilities driven by a single electron beam in two-electron temperature space plasmas

In an attempt to understand the excitation mechanisms of broadband electrostatic noise, beam-generated electrostatic instabilities are investigated using kinetic theory in a four-component magnetised plasma model composed of beam electrons (magnetic field-aligned), background hot and cool electrons and ions. All species are fully magnetised and considered to be Maxwellian. The dependence of the instability growth rates and real frequencies on various plasma parameters such as beam speed, particle densities and temperatures, magnetic field strength, wave propagation angle, and temperature anisotropy of the beam are examined. In this study we have found that the electron-acoustic, electron beam-resonant and ion-acoustic instabilities are excited. Our studies have focused on three velocity regimes, namely, the low (vdbz<Ch), intermediate (Ch<vdbz<2 Ch), and high velocity (vdbz>2 Ch) regimes, where vdbz (Ch) is the electron beam drift speed (thermal speed of the hot electrons). Plasma parameters from satellite measurements are used where applicable to provide realistic predictions.

High frequency electromagnetic resistive instability in a Hall thruster under the effect of ionization

Two types of high frequency electromagnetic resistive instabilities are found to occur in a Hall thruster plasma in the presence of collisions and ionization, out of which one of lower growth rate (called lower magnitude high frequency instability (LMHFI)) is sensitive to the axial component of the wave vector. The effects of ionization, collisions, and electron drift velocity on the growth rates of these instabilities are studied in greater details. The LMHFI grows faster in the presence of ionization but shows weaker dependence on the electrons' E→×B→ drift, contrary to the case of other instability (called higher magnitude high frequency instability) which is sensitive to the azimuthal wave number and collisions.

Nonlinear dependence of anomalous resistivity on the reconnecting electric field in the Earth’s magnetotail

Based upon the observational data of the fast magnetic reconnection in the nearly collisionless magnetotail and the particle in cell (PIC) simulations on the electron acceleration in the reconnecting current sheet with guide magnetic field, we self consistently solved one dimension Vlasov equation with the magnetotail parameters and realistic mass ratio to explore the relationship between the anomalous resistivity and the induced electric field. As compared with theoretic formula for the current driven ion-acoustic and Buneman anomalous resistivity, the anomalous resistivity may result from the ion acoustic instability for small reconnecting electric field and the Buneman instability for large reconnecting electric field. The discrepancy between the theoretic results and numerical simulations may be caused by the high frequency instability that results from the deviation of electron distribution from Maxwellian one. These results are consistent with the early experimental results and favorable for the fast reconnection to take place.

Suppression of Low- and High-Frequency Instabilities and Grid-Induced Disturbances in Distributed Generation Inverters

This paper presents a robust current control scheme for inductor-capacitor-inductor (LCL)-filtered distributed generation (DG) inverters featuring effective suppression of low- and high-frequency instabilities and grid-induced distortion and disturbances. The conceptual design of the proposed control scheme is to maximize the disturbance rejection performance against grid disturbances and parametric uncertainties, and minimize the coupling among active damping, disturbance rejection, and tracking controllers. First, a simple and robust active damping controller is realized by drooping the inverter control voltage with the capacitor current. Second, the augmented damped dynamics is used to design a robust controller, which is composed of a tracking controller, dynamic grid-disturbance rejection controller, and uncertainty rejection controller. The tracking controller is designed to yield deadbeat control performance to maximize the dynamic performance of the converter. The grid-disturbance rejection controller is designed to provide a base-line of grid-induced dynamics within the closed-loop system as an internal model; therefore, effective mitigation of grid-induced distortion can be achieved without a priori knowledge of the frequency modes to be eliminated. The uncertainty rejection controller is designed to reject voltage disturbances associated with parameter variation and imperfect compensation of the grid-disturbance-rejection controller. Theoretical analysis and comparative test results are presented to demonstrate the effectiveness of the proposed control scheme.