High Instabilities Research Articles

High geo-stress distribution and high geo-stress concentration area models for eastern margin of Qinghai-Tibet plateau

High geo-stress and its engineering problems have severely affected the development of civil infrastructures in western China. The problems include high rock slope instabilities, rock burst, gas explosion and large-scale soft rock deformation in deep tunnels. This paper investigates the distribution of the high geo-stresses and the models of the stress concentration areas in the eastern margin of Qinghai-Tibet plateau so that a solid foundation can be formed to address the problems. The investigation is based on a comprehensive analysis of the previous research data of the eastern margin and uses remote sensing techniques, geophysics, geochemistry, and large scale geological surveying methods. The investigation has found that some special tectonic zones have high geo-stresses. The high geo-stresses are located at (1) the convergent boundary areas between two fault blocks with large strength differences, (2) the tectonic necks in front of active fault blocks, and (3) the intersection and/or termination areas of faults within the fault blocks. An example for (1) is the north Qilian high geo-stress area. Another example for (2) is the Minshan high geo-stress area in the northwest Sichuan. Furthermore, the investigation has summarized six basic models to characterize the high geo-stress concentration areas. The first one is the convergent stress concentration model at the boundary of two fault blocks. The other five stress concentration modes are oblique fissures or intersecting areas, areas without lower velocity layer in the crust, areas of compression induced tensile cracking, tectonic wedge areas, and tectonic neck areas, respectively.

Flame response to acoustic excitation in a rectangular rocket combustor with LOx/H2 propellants

Research efforts are currently underway at the German Aerospace Center (DLR) Lampoldshausen, which aim to understand the mechanisms by which self-sustaining oscillations in combustion chamber pressure, known as high frequency combustion instabilities, are driven. Testing has been conducted in the rectangular combustor ‘BKH’, running cryogenic oxygen and hydrogen propellants under pressure and injection conditions which are representative of real rocket engines and with acoustic forcing. For the first time, such tests with LOx/H2 propellants and acoustic forcing have been conducted at combustion chamber pressures above 10 bar, the reported results herein from a test at 42 bar. Optical access to the combustor allowed the application of high speed hydroxyl radical (OH*) chemiluminescence imaging of the flame during periods of forced excitation of acoustic resonance modes of the combustion chamber. This paper reports the investigation of flame response to acoustic excitation. Both fluctuation in OH* emission intensity and deflection of the flame at frequencies corresponding to the excitation frequency have been observed. These responses are then discussed as potential indicators of driving mechanisms for combustion instabilities.

Challenges in process integration of catalytic DC plasma synthesis of vertically aligned carbon nanofibres

The ability to synthesize free-standing, individual carbon nanofibres (CNFs) aligned perpendicularly to a substrate has enabled fabrication of a large array of devices with nanoscale functional elements, including electron field emission sources, electrochemical probes, neural interface arrays, scanning probes, gene delivery arrays and many others. This was made possible by development of a catalytic plasma process, with DC bias directing the alignment of nanofibres. Successful implementation of prototypical devices has uncovered numerous challenges in the integration of this synthesis process as one of the steps in device fabrication. This paper is dedicated to these engineering and fundamental difficulties that hinder further device development. Relatively high temperature for catalytic synthesis, electrical conductivity of the substrate to maintain DC discharge and other difficulties place restrictions on substrate material. Balancing non-catalytic carbon film deposition and substrate etching, non-uniformity of plasma due to growth of the high aspect ratio structures, plasma instabilities and other factors lead to challenges in controlling the plasma. Ultimately, controlling the atomistic processes at the catalyst nanoparticle (NP) and the behaviour of the NP is the central challenge of plasma nanosynthesis of vertically aligned CNFs.

High order instabilities of the Poincaré solution for precessionally driven flow

Sloudsky in 1895 and Poincaré in 1910 were the first to derive solutions for the flow driven in the Earth's fluid core by the luni-solar precession. In 1993, Kerswell investigated the stability of this so-called “Poincaré flow” by applying a method devised in 1992 by Ponomarev and Gledzer to study the instability of flows with elliptical streamlines. They represented the components of the perturbed flow by sums of polynomials. Kerswell restricted attention to the linear and quadratic cases. Here cubic, quartic, quintic, and sextic generalizations are developed. Instabilities are located in new areas of parameter space, including some that verge on the small oblateness of the Earth's core

Regarding the evaluation of the solid rocket propellant response function to pressure coupling

High frequency combustion instabilities imply a major risk for the solid rocket motor stable working and they are directly linked to the propellant response to chamber pressure coupling. This article discusses a laboratory testing method for the measurement and evaluation of the pressure coupled response for non-metalized propellants in a first stage. Experimental researches were done with an adequate setup, built and improved in our lab, able to evaluate the propellant response by interpreting the pressure oscillations damping in terms of propellant response. Our paper aims at defining a linearized one-dimensional flow study model to analyze the disturbed operation of the solid propellant rocket motors. Based on the applied model we can assert that the real part of propellant response is a function of the oscillations damping, acoustic energy in the motor chamber and various losses in the burning chamber. The imaginary part of propellant response mainly depends on the normalized pulsation, on the burning chamber gas column and on the pressure oscillations frequency. Our research purpose was obviously to minimize the risk of the combustion instabilities effects on the rocket motors working, by experimental investigations using jet modulating techniques and sustained by an interesting study model based on the perturbation method.

Positively Biased Probes in Magnetized Plasmas

AbstractPositively biased probes in magnetized plasmas can create several phenomena which are not described by standard probe theories. These include transient currents for pulsed electrodes in the regime of Electron MHD, relaxation oscillations induced by large probes and high frequency instabilities at the sheath‐plasma frequency. Although they may limit the use of probes for diagnostic purposes they also offer new applications and belong to the general knowledge of probes in plasmas (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A new method preserving the positive definiteness of a second order tensor variable in flow simulations with application to viscoelastic turbulence

We present a new numerical method for the simulation of the flow of complex fluids with internal microstructure that is described by a second order, positive definite, internal structural parameter, so that its positive definiteness is preserved in the simulations. An example of such an internal structural parameter is the conformation tensor, used in the modeling of viscoelastic flows, which characterizes the flow-induced molecular deformation. Numerical methods that guarantee the preservation of the positive definiteness of the conformation tensor help both the physical interpretation of the results and the numerical stability of the simulations. The new method presented here is based on the log-conformation representation of a second order tensor. It is implemented through the application of the Cayley–Hamilton theorem for second order tensors. When necessary, we also use an additional mapping that ensures the boundness of the conformation tensor. This approach has been applied in Direct Numerical Simulations (DNS) of viscoelastic turbulent channel flow, which has received great attention since the mid 1990s with ultimate goal to understand the phenomenon of maximum drag reduction. The algorithm uses a full 3D spectral representation of the spatial dependence for the flow and conformation variables and a second order accurate backward differentiation formula for the integration of the governing equations in time. The key issue for the successful implementation of the proposed scheme is a second order finite difference multigrid diffusion applied to the conformation field. Numerical diffusion has been always added to the hyperbolic evolution equations for the conformation tensor in spectral DNS of viscoelastic turbulent flows in order to remove a-physical high wavenumber instabilities induced due to the chaotic nature of the flow. The main advantage of the finite-difference implementation (as opposed to a spectral implementation as done before) is that it allows for the preservation of the positive definiteness of the conformation tensor.

Passive Control of the Inlet Acoustic Boundary of a Swirled Burner at High Amplitude Combustion Instabilities

Perforated panels placed upstream of the premixing tube of a turbulent swirled burner are investigated as a passive control solution for combustion instabilities. Perforated panels backed by a cavity are widely used as acoustic liners, mostly in the hot gas region of combustion chambers to reduce pure tone noises. This paper focuses on the use of this technology in the fresh reactants zone to control the inlet acoustic reflection coefficient of the burner and to stabilize the combustion. This method is shown to be particularly efficient because high acoustic fluxes issued from the combustion region are concentrated on a small surface area inside the premixer. Theoretical results are used to design two types of perforated plates featuring similar acoustic damping properties when submitted to low amplitude pressure fluctuations (linear regime). Their behaviors nonetheless largely differ when facing large pressure fluctuation levels (nonlinear regime) typical of those encountered during self-sustained combustion oscillations. Conjectures are given to explain these differences. These two plates are then used to clamp thermoacoustic oscillations. Significant damping is only observed for the plate featuring a robust response to increasing sound levels. While developed on a laboratory scale swirled combustor, this method is more general and may be adapted to more practical configurations.

Injection coupling with high amplitude transverse modes: Experimentation and simulation

High frequency combustion instabilities have technical importance in the design of liquid rocket engines. These phenomena involve a strong coupling between transverse acoustic modes and combustion. They are currently being investigated by combining experimentation and numerical simulations. On the experimental level, the coupling is examined in a model scale system featuring a multiple injector combustor (MIC) comprising five coaxial injectors fed with liquid oxygen and gaseous methane. This system is equipped with a novel VHAM actuator (Very High Amplitude Modulator) which comprises two nozzles and a rotating toothed wheel blocking the nozzles in an alternate fashion. This device was designed to obtain the highest possible levels of transverse oscillation in the MIC. After a brief review of the VHAM, this article reports cold flow experiments using this modulator. Velocity maps obtained under resonant conditions using the VHAM are examined at different instants during a cycle of oscillation. Experimental data are compared with numerical pressure and velocity fields obtained from an acoustic solver. The good agreement observed in the nozzle vicinity indicates that numerical simulations can be used to analyze the complex flow field generated by the VHAM. To cite this article: Y. Mery et al., C. R. Mecanique 337 (2009).

Regular and chaotic Bose-Einstein condensate in an accelerated Wannier-Stark lattice

We investigate a Bose-Einstein condensate (BEC) held in a quasi-one-dimensional Wannier-Stark lattice which is a combination of linear potential with an accelerating optical lattice. It is demonstrated that the system can be reduced to a periodically driven Gross-Pitaevskii one, in which we find the exact analytical solution and the regular and chaotic numerical solutions with accelerated atomic flow densities. The higher instabilities in the transport of BEC are predicted for the positive interatomic interaction. The results suggest an experimental scheme to generate and control the accelerated regular and chaotic matter waves.

Stability of operator splitting methods for systems with indefinite operators: Advection–diffusion–reaction systems

This brief paper presents an A-stability result for operator splitting type time integration methods applied to advection–diffusion–reaction equations with possibly indefinite source terms. These results extend our earlier work on diffusion–reaction systems [D.L. Ropp, J.N. Shadid, Stability of operator splitting methods for systems with indefinite operators: reaction–diffusion systems, J. Comput. Phys. 203 (2) (2005) 449–466]. The A-stability result presents sufficient conditions that control both low and high wave number instabilities. A corollary shows that if L-stable methods are used for the diffusion term the high wave number instability will be controlled more easily. Numerical results are presented that verify second-order convergence for the operator splitting methods and demonstrate control of instabilities on a chemotaxis problem by use of an L-stable diffusion integrator.

Effect of temperature fluctuations on high frequency acoustic coupling

This article considers the influence of fluctuations in temperature on acoustic resonance characteristics of a combustion chamber. This analysis is concerned with acoustic coupling experienced in high frequency instabilities. Experiments carried out on a multiple injector combustor (MIC) operating with liquid oxygen and gaseous methane are used to simulate on a model scale the situation prevailing in a liquid rocket thrust chamber. External acoustic modulations are generated by periodically blocking an auxiliary nozzle with a rotating toothed wheel. By continuously changing the rotation velocity, it is possible to apply a linear frequency sweep and obtain the response of the system to an external excitation. It is found that the chamber quality factor under hot fire conditions is reduced with respect to the one measured under cold flow conditions. This is shown to be correlated with the level and spatial extension of temperature fluctuations inside the chamber. Temperature fluctuations induce variations in the speed of sound which in turn perturb the eigenfrequencies of the system and modify its response. This effect is modeled in a simplified way by considering a second order dynamical system featuring an eigenfrequency which fluctuates around its mean value. The dynamics is analyzed by making use of simulations and by the method of averaging. It is shown that when the eigenfrequency fluctuates, the system behaves as if it had an augmented level of dissipation and that this induces a reduction in the quality factor. It is concluded that this mechanism could have an important impact on the development of oscillations in liquid rocket engines.

Laser generation of regular wavy patterns by nonlinear instability of a metal nanolayer

We report on an experimental study on the formation of structures caused by surface-tension-driven instability of a molten chromium layer generated by a single KrF laser pulse of nanosecond duration. The molten metal layer is formed in a confined configuration in a 500 nm thick Cr film encapsulated between a glass substrate and a quartz cover plate. The large lateral thermal gradient ∇Ty generates a surface tension gradient, which causes the formation of instabilities on a liquid Cr film with a very high aspect ratio, Γ=13 000, and of a low Prandtl number of Pr∼0.01. Two types of patterns were observed, the low and high wave-number wavylike instabilities, inclined left and right with respect to the direction of ∇Ty, respectively. As the laser energy increases from 38 to 100 mJ, the inclination angle of the waves alternates, indicating transitions between the right and left inclined structures, and vice versa. The analysis shows that the formation of inclined wavy structures occurs because the ratio W of the Rayleigh number Ra and the Marangoni number Ma (W=Ra/Ma) takes values W>1. The oscillation of inclined waves is governed by the Biot number, which decreases from the intermediate values of Bi∼49 to the low values Bi∼3–4, as the energy of the laser pulse is increased.

Investigation of Flow Instability in High Temperature Deformation of INCONEL Alloy 783 Using Processing Map Approach

The high temperature deformation behavior and flow instabilities of Ni-Fe-Co base superalloy, INCONEL alloy 783 during hot working process were investigated with process maps consisting of a power dissipation of dynamic materials model (DMM) and various flow instability criteria. In order to establish the processing map of INCONEL alloy 783, hot compression tests were carried out under different temperature and strain rate conditions, with true strain up to 0.7. On the basis of the comparison between processing maps and microstructural analysis, the reliability of various flow instability criteria was estimated. Finally the useful instability criterion for predicting the forming defects was suggested through the compression test results and experimental observations of actual ring rolling process of INCONEL alloy 783.

Linear and nonlinear shear flow behavior of monodisperse polyisoprene melts with a large range of molecular weights

Shear rheological data for a wide range of nearly monodisperse linear polyisoprene melts are reported herein using the results of linear and nonlinear measurements. The number of entanglements per chain ranges from about 0.5 to 230. In order to extend the range of available frequencies and shear rates the procedure of time-temperature superposition is improved and validated in both the linear and the nonlinear regime. In the linear flow regime a detailed comparison with the tube theory by Likhtman and McLeish [Macromolecules 35, 6332–6343 (2002)] was performed. The overall agreement was found to be satisfactory but several minor disagreements, e.g., at the crossover regime and for the steady-state compliance, were observed and have been discussed. Until now, most nonlinear models for entangled polymers were compared with shear data on solutions. However, the equivalence between solutions and melts is not well established. In this paper the results of a series of nonlinear start-up shear experiments are compared with those predicted by the tube model [Graham et al., J. Rheol. 47, 1117–1200 (2003)] and with several literature datasets on entangled solutions having similar numbers of entanglements. The melts were found to behave in a very similar manner to the previously studied solutions and agree well with the tube theory. Disagreements are observed when the shear rate exceeds the Rouse time of one entanglement and also for chains with very small number of entanglements. For the high molecular weight samples possible instabilities and edge fracture can occur at high strains and the results are discussed in light of these effects. The extensive dataset presented here could serve as a test for future theories and models of entangled melts.

Modeling of dynamic rotors with flexible bearings due to the use of viscoelastic materials

Nowadays rotating machines produce or absorb large amounts of power in relatively small physical packages. The fact that those machines work with large density of energy and flows is associated to the high speeds of rotation of the axis, implying high inertia loads, shaft deformations, vibrations and dynamic instabilities. Viscoelastic materials are broadly employed in vibration and noise control of dynamic rotors to increase the area of stability, due to their high capacity of vibratory energy dissipation. A widespread model, used to describe the real dynamic behavior of this class of materials, is the fractional derivative model. Resorting to the finite element method it is possible to carry out the modeling of dynamic rotors with flexible bearings due to the use of viscoelastic materials. In general, the stiffness matrix is comprised of the stiffnesses of the shaft and bearings. As considered herein, this matrix is complex and frequency dependent because of the characteristics of the viscoelastic material contained in the bearings. Despite of that, a clear and simple numerical methodology is offered to calculate the modal parameters of a simple rotor mounted on viscoelastic bearings. A procedure for generating the Campbell diagram (natural frequency versus rotation frequency) is presented. It requires the embedded use of an auxiliary (internal) Campbell diagram (natural frequency versus variable frequency), in which the stiffness matrix as a frequency function is dealt with. A simplified version of that procedure, applicable to unbalance excitations, is also presented. A numerical example, for two different bearing models, is produced and discussed.

High temperature instabilities of ohmic contacts on p‐GaN

AbstractThis paper describes an investigation of the instabilities of ohmic contacts on p‐type GaN submitted to long term high temperature stress. Transfer Length Method (TLM) was used to separately analyze the contribution of contact resistivity and sheet resistance on contact degradation. Before treatment, contacts showed linear behaviour, indicating good ohmic properties. On the other hand, thermal ageing at 250 °C induced the degradation of electrical characteristics: in particular, (i) an increase of the contact resistivity was detected after stress, as well as (ii) a rectifying behaviour in current‐voltage (IV) characteristics of contact appeared. The degradation process has been ascribed to the diffusion of an atomics species from PECVD‐deposited passivation layer to p‐type ohmic contact: the interaction of this species with acceptor dopant can lower acceptor concentration, thus broadening the Schottky barrier at the interface and inducing the observed non‐linearity of the contacts. The square root time dependence of degradation of the electrical characteristics agreed with the hypothesis of diffusion. After 160 hours of aging, the passivation layer was removed and the structures were submitted to a new thermal treatment. This further treatment was found to induce the recovery of the electrical characteristics of the contacts, due to the fact that passivation acts like (i) a source of atomic species responsible of diffusion and/or (ii) a blocking layer for these species: the removal of passivation stopped the diffusion process and make outgassing process possible. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Combustion Instability and Emission Control by Pulsating Fuel Injection

Open-loop control methodologies were used to suppress symmetric and helical thermoacoustic instabilities in an experimental low-emission swirl-stabilized gas-turbine combustor. The controllers were based on fuel (or equivalence ratio) modulations in the main premixed combustion (premixed fuel injection (PMI)) or, alternatively, in the secondary pilot fuel. PMI included symmetric and asymmetric fuel injection. The symmetric instability mode responded to symmetric excitation only when the two frequencies matched. The helical fuel injection affected the symmetric mode only at frequencies that were much higher than that of the instability mode. The asymmetric excitation required more power to obtain the same amount of reduction as that required by symmetric excitation. Unlike the symmetric excitation, which destabilized the combustion when the modulation amplitude was excessive, the asymmetric excitation yielded additional suppression as the modulation level increased. The NOx emissions were reduced to a greater extent by the asymmetric modulation. The second part of the investigation dealt with the control of low frequency symmetric instability and high frequency helical instability by the secondary fuel injection in a pilot flame. Adding a continuous flow of fuel into the pilot flame controlled both instabilities. However, modulating the fuel injection significantly decreased the amount of necessary fuel. The reduced secondary fuel resulted in a reduced heat generation by the pilot diffusion flame and therefore yielded lower NOx emissions. The secondary fuel pulsation frequency was chosen to match the time scales typical to the central flow recirculation zone, which stabilizes the flame in the burner. Suppression of the symmetric mode pressure oscillations by up to 20dB was recorded. High frequency instabilities were suppressed by 38dB, and CO emissions reduced by using low frequency modulations with 10% duty cycle.

ACOUSTIC CAVITIES DESIGN PROCEDURES

Combustion instability is recognized as one of the major problems frequently faced by engineers during the development of either liquid or solid propellant rocket engines. The performance of the engine can be highly affected by these high frequencies instabilities, possibly leading the rocket to an explosion. The main goal while studying combustion chambers instability, either by means of baffles or acoustic absorbers, is to achieve the stability needed using the simplest possible manner. This paper has the purpose of studying combustion chambers instabilities, as well as the design of acoustic absorbers capable of reducing their eigenfrequencies. Damping systems act on the chamber eigenfrequency, which has to be, therefore, previously known.

A wave propagation algorithm for viscoelastic fluids with spatially and temporally varying properties

An algorithm is presented for simulating the fluid and stress equations that arise in a continuum model of platelet aggregation [A.L. Fogelson, R.D. Guy, Platelet–wall interactions in continuum models of platelet thrombosis: formulation and numerical solution, Math. Med. Biol. 21 (2004) 293–334]. The model is equivalent to models of viscoelastic fluids with spatially and temporally varying material parameters. A subsystem containing the elastic terms is handled using a wave propagation algorithm. Two different methods for propagating waves are compared in computational tests. One method linearizes the equations about cell edges, and the other is based on the propagation of waves in a heterogeneous elastic medium. When the viscoelastic fluid is in contact with a Newtonian fluid, the method based on linearization about the edges gives more accurate results. No high Weissenberg number instabilities were observed. As long as the time step is chosen to obey a CFL condition, the algorithm is stable and no unphysical oscillations appear in the solution.