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Patterns and instability of grannular flow

Description: Dense granular flows are often observed to become unstable and form inhomogeneous structures in nature or industry. Although recently significant advances have been made in understanding simple flows, instabilities are often not understood in detail. We present experimental and numerical results that show the formation of longitudinal stripes. These arise from instability of the uniform flowing state of granular media on a rough inclined plane. The form of the stripes depends critically on the mean density of the flow with a robust form of stripes at high density that consists of fast sliding plug-like regions (stripes) on top of highly agitated boiling material -- a configuration reminiscent of the Leidenfrost effect when a droplet of liquid lifted by its vapor is hovering above a hot surface.
Date: January 1, 2009
Creator: Ecke, Robert E; Borzsonyi, Tamas & Mcelwaine, Jim N
Partner: UNT Libraries Government Documents Department

An exploratory investigation of some types of aeroelastic instability of open and closed bodies of revolution mounted on slender struts

Description: Report presenting an investigation of aeroelastic instability phenomena of isolated open and closed rigid bodies of revolution free to move under elastic restraint at low speeds. Results regarding speeds at which flutter or divergence occurs, the flutter-speed coefficient, and speed coefficients are provided.
Date: November 1954
Creator: Clevenson, S. A.; Widmayer, E., Jr. & Diederich, Franklin W.
Partner: UNT Libraries Government Documents Department

Combustion instability in an acid-heptane rocket with a pressurized-gas propellant pumping system

Description: Report presenting results of experimental measurements of low-frequency combustion instability of a 300-pound-thrust acid-heptane rocket engine as compared with the trends predicted by an analysis of combustion instability in a rocket engine. Results regarding the chugging frequency, combustion time delay, effect of rocket combustion-chamber characteristic length, effect of throttling, effect of injection velocity, effect of oxidant-fuel ratio, variation of chugging frequency with amplitude of chamber pressure fluctuations, and evaluation of the analysis are provided.
Date: May 1953
Creator: Tischler, Adelbert O. & Bellman, Donald R.
Partner: UNT Libraries Government Documents Department

An Exploratory Investigation of Some Types of Aeroelastic Instability of Open and Closed Bodies of Revolution Mounted on Slender Struts

Description: Aeroelastic instability phenomena of isolated open and closed rigid bodies of revolution free to move under elastic restraint have been investigated experimentally at low speeds by means of models suspended at zero angles of attack and yaw on slender flexible struts from a wind tunnel ceiling. Three types of instability were observed - flutter similar to classical bending-torsion flutter, divergence, and an uncoupled oscillatory instability which consists in nonviolent continuous or intermittent small-amplitude oscillations involving only angular deformations. The speeds at which this oscillatory instability starts were found to be as low as about one-third of the speed at flutter or divergence and to depend on the shape of the body, particularly that of the afterbody, and on the relative location of the elastic axis. An attempt has been made to calculate the airspeeds and, in the case of the oscillatory phenomena, the frequencies at which these instabilities occur by using slender-body theory for the aerodynamic forces on the bodies and neglecting the aerodynamic forces on the struts. However, the agreement between the speeds and frequencies calculated in this manner and those actually observed has been found to be generally unsatisfactory; with the exception of the frequencies of the uncoupled oscillations which could be predicted with fair accuracy. The nature of the observed phenomena and of the forces on bodies of revolution suggests that a significant improvement in the accuracy of analytical predictions of these aeroelastic instabilities can be had only by taking into account the effects of boundary-layer separation on the aerodynamic forces.
Date: June 30, 1953
Creator: Clevenson, S. A.; Wildmayer, E., Jr. & Diederich, Franklin W.
Partner: UNT Libraries Government Documents Department

Double-kink Fishbone Instability Caused by Circulating Energetic Ions

Description: The destabilization of double kink modes by the circulating energetic ions in tokamaks with the plasma current having an off-axis maximum is studied. It is shown that the high-frequency fishbone instability [Energetic Particle Mode (EPM)] and the low-frequency (diamagnetic) fishbones are possible for such an equilibrium, their poloidal and toroidal mode numbers being not necessarily equal to unity. A new kind of the EPM instability, ''doublet fishbones,'' is predicted. This instability is characterized by two frequencies; it can occur in a plasma with a non-monotonic radial profile of the energetic ions when the particle orbit width is less than the width of the region where the mode is localized. It is found that the diamagnetic fishbone branch exists even when the orbit width exceeds the mode width; in this case, however, the instability growth rate is relatively small.
Date: January 12, 2004
Creator: Kolesnichenko, Ya.I.; Lutsenko, V.V.; Marchenko, V.S. & White, R.B.
Partner: UNT Libraries Government Documents Department

The Longitudinal Stability of Intense Non-Relativistic Particle Bunches in Resistive Structures

Description: The longitudinal stability of intense particle bunches is investigated theoretically in the limit of small wall resistivity compared to total reactance. It is shown that both in the absence of resistivity and to lowest order in the resistance that an intense bunch is stable against longitudinal collective modes. An expression is derived for the lowest order instability rate. Application of these results are made to drivers for heavy ion inertial fusion.
Date: February 1, 1981
Creator: Channell, P. J.; Sessler, A. M. & Wurtele, J. S.
Partner: UNT Libraries Government Documents Department

Observations on an aileron-flutter instability encountered on a 45 degree swept-back wing in transonic and supersonic flight

Description: Report presenting a flight test of a supersonic research pilotless aircraft in which large-amplitude aileron oscillations, most likely aileron compressibility flutter, were encountered in the transonic and supersonic speed ranges. Results regarding power-on flight and coasting flight are provided.
Date: April 11, 1947
Creator: Pitkin, Marvin; Gardner, William N. & Curfman, Howard J., Jr.
Partner: UNT Libraries Government Documents Department

Strong stabilization of the Rayleigh-Taylor instability by material strength at Mbar pressures

Description: Experimental results showing significant reductions from classical in the Rayleigh-Taylor (RT) instability growth rate due to high pressure effective lattice viscosity are presented. Using a laser created ramped drive, vanadium samples are compressed and accelerated quasi-isentropically at {approx}1 Mbar pressures, while maintaining the sample in the solid-state. Comparisons with simulations and theory indicate that the high pressure, high strain rate conditions trigger a phonon drag mechanism, resulting in the observed high effective lattice viscosity and strong stabilization of the RT instability.
Date: November 19, 2009
Creator: Park, H S; Lorenz, K T; Cavallo, R M; Pollaine, S M; Prisbrey, S T; Rudd, R E et al.
Partner: UNT Libraries Government Documents Department

Study of the microbunching instability in single-pass systemsusing a direct 2D Vlasov solver

Description: We apply a recently developed Vlasov solver to the study ofthemicrobunching instability generated by shot noise in the beamdeliverysystems of x-ray Free Electron Lasers (FELs). We discusstwo latticespresently under consideration for the FEL FERMI project at Elettra andshow that at least one of the two lattices appears capable of deliveringa beam with the desired quality in the longitudinal phasespace.
Date: June 30, 2007
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

Simple model for linear and nonlinear mixing at unstable fluid interfaces with variable acceleration

Description: A simple model is described for predicting the time evolution of the half-width h of a planar mixing layer between two immiscible incompressible fluids driven by an arbitrary time-dependent variable acceleration history <i>a(l)</i>a (t): The model is based on a heuristic expression for the kinetic energy per unit area of the mixing layer. This expression is based on that for the kinetic energy of a linearly perturbed interface, but with a dynamically renormalized wavelength which becomes proportional to h in the nonlinear regime. An equation of motion for h is then derived by means of Lagrange�s equations. This model reproduces the known linear growth rates of the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities, as well as the quadratic RT and power-law RM growth laws in the nonlinear regime. The time exponent in the RM power law depends on the rate of kinetic energy dissipation. In the case of zero dissipation, this exponent reduces to 2/3 in agreement with elementary scaling arguments. A conservative numerical scheme is proposed to solve the model equations, and is used to perform calculations that agree well with published mixing data from linear electric motor experiments. Considerations involved in implementing the model in hydrodynamics codes are briefly discussed.
Date: December 23, 1998
Creator: Ramshaw, J. D. & Rathkopf, J.
Partner: UNT Libraries Government Documents Department

Non-conventional Fishbone Instabilities

Description: New instabilities of fishbone type are predicted. First, a trapped-particle-induced m = n = 1 instability with the mode structure having nothing to do with the conventional rigid kink displacement. This instability takes place when the magnetic field is weak, so that the precession frequency of the energetic ions is not small as compared to the frequency of the corresponding Alfven continuum at r = 0 and the magnetic shear is small inside the q = 1 radius [the case relevant to spherical tori]. Second, an Energetic Particle Mode fishbone instability driven by circulating particles. Third, a double-kink-mode instability driven by the circulating energetic ions. In particular, the latter can have two frequencies simultaneously: we refer to it as ''doublet'' fishbones. This instability can occur when the radial profile of the energetic ions has an off-axis maximum inside the region of the mode localization.
Date: November 10, 2004
Creator: Kolesnichenko, Ya.I.; Lutsenko, V.V.; Marchenko, V.S. & White, R.B.
Partner: UNT Libraries Government Documents Department

Annual progress report

Description: The research topics covered include a study of the nonlinear behavior of the drift cyclotron loss-cone instability in mirror machines and a study of the longer time behavior of the nonlinear O'Neil trapped particle saturation. The saturation amplitude for a single dominant mode near the stability onset boundary was determined for the drift cyclotron loss-cone instability for a variety of mirror-like distributions. The results are in good agreement with computer simulation studies of Cohen and Maron.
Date: August 31, 1979
Creator: Simon, A.
Partner: UNT Libraries Government Documents Department

Investigations of the Rayleigh-Taylor and Richtmyer-Meshkov Instabilities

Description: The present program is centered on the experimental study of shock-induced interfacial fluid instabilities. Both 2-D (near-sinusoids) and 3-D (spheres) initial conditions are studied in a large, vertical square shock tube facility. The evolution of the interface shape, its distortion, the modal growth rates and the mixing of the fluids at the interface are all objectives of the investigation. In parallel to the experiments, calculations are performed using the Raptor code, on platforms made available by LLNL. These flows are of great relevance to both ICF and stockpile stewardship. The involvement of four graduate students is in line with the national laboratories' interest in the education of scientists and engineers in disciplines and technologies consistent with the labs' missions and activities.
Date: March 14, 2008
Creator: Bonazza, Riccardo; Anderson, Mark & Oakley, Jason
Partner: UNT Libraries Government Documents Department

Microtearing Instability In The ITER Pedestal

Description: Unstable microtearing modes are discovered by the GS2 gyrokinetic siimulation code, in the pedestal region of a simulated ITER H-mode plasma with approximately 400 WM DT fusion power. Existing nonlinear theory indicates that these instabilities should produce stochastic magnetic fields and broaden the pedestal. The resulted electron thermal conductivity is estimated and the implications of these findings are discussed.
Date: December 1, 2010
Creator: Wong, K. L.; Mikkelsen, D. R.; Rewoldt, G. M. & Budny, R.
Partner: UNT Libraries Government Documents Department

Noise-Sustained Convective Instability in a Magnetized Taylor-Couette Flow

Description: The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.
Date: February 20, 2009
Creator: Liu, W.
Partner: UNT Libraries Government Documents Department

Ballooning instability precursors to high {beta} disruptions

Description: Strongly ballooning modes have been found as precursors to high {beta} disruptions on TFTR. The modes are typically localized to a region spanning about 60{degree} in the toroidal direction. The toroidal localization is associated with lower frequency, global Magneto-Hydro-Dynamic (MHD) activity, typically an ideal n = 1 kink mode. They have moderate to high frequency (f = 10--20 f{sub rot}), implying toroidal mode numbers in the range n = 10--20. The growth rates for the modes are large, of order 10{sup 4}/sec.
Date: December 1, 1995
Creator: Fredrickson, E.D.; McGuire, K.M. & Chang, Z.Y.
Partner: UNT Libraries Government Documents Department

Stabilization of global MHD instabilities by toroidal plasma rotation

Description: Theoretical study and experimental observations suggest that rotation can play a crucial role in determining plasma stability. Since conventional magnetohydrodynamic (MHD) analysis ignores rotation, more advanced computational tools are being developed to confirm the theoretical understanding and to perform comparison between theory and experiment. In a previous work, the authors reported on the formulation and computation of MHD modes in plasmas with a small (subsonic) toroidal rotation. R.otation is found to have a substantial stabilizing effect under many circumstances. In this work, they extend the formulation in Ref. 4 to include an arbitrary (large) toroidal plasma rotation. It is the purpose of this work to examine the difference between these two formulations and report on results from computations using these formulations.
Date: July 1, 1995
Creator: Chu, M.S.; Miller, R.L.; Bondeson, A.; Luetjens, H.; DeRidder, G. & Sauter, O.
Partner: UNT Libraries Government Documents Department

THE USE OF RF-KNOCKOUT FOR DETERMINATION OF THE CHARACTERISTICS OF THE TRANSVERSE COHERENT INSTABILITY OF AN INTENSE BEAM

Description: The response of an intense beam of interacting particles to a deflecting rf-signal is computed theoretically and shown to be closely related to transverse coherent beam stability. It is shown that the beam response to sinusoidal excitation provides a direct measure of the stability of beam modes for given machine conditions (beam intensity, octupole current, sextupole current, momentum spread, etc.). This measurement includes the properties of the beam surroundings as well as the frequency spread effective for Landau damping. Since it is generally difficult to evaluate theoretically the wall and beam properties that enter into stability calculations, the information which can be obtained from rf excitation experiments should be very valuable; especially in devising practical procedures for reducing the severity of coherent transverse instabilities.
Date: August 1, 1971
Creator: Muhl, Dieter & Sessler, Andrew M.
Partner: UNT Libraries Government Documents Department

LONGITUDINAL RESISTIVE INSTABILITIES OF INTENSE COASTING BEAMS IN PARTICLE ACCELERATORS

Description: The effect of finite resistance in the vacuum-tank walls on the longitudinal stability of an intense beam of particles in an accelerator is investigated theoretically. We show that even if the particle frequency is an increasing function of particle energy, the wall resistance can render the beam unstable against longitudinal bunching. In the absence of frequency spread in the unperturbed beam, the instability occurs with a growth rate that is proportional to (N/{sigma}){sup 1/2}, where N is the number of particles in the beam and {sigma} is the conductivity of the surface material. By means of the Vlasov equation a criterion for beam stability is obtained. In the limit of highly conducting walls the criterion involves the frequency spread in the unperturbed beam, the number of particles N, the beam energy, geometrical properties of the accelerator, but not the conductivity {sigma}. A numerical example presented indicates that certain observations of beam behavior in the MURA 40-Mev-electron accelerator may be related to the phenomenon we investigated.
Date: September 29, 1964
Creator: Neil, V. Kelvin & Sessler, Andrew M.
Partner: UNT Libraries Government Documents Department

An Analytic Study of the Perpendicularly Propagating Electromagnetic Drift Instabilities in the Magnetic Reconnection Experiment

Description: A local linear theory is proposed for a perpendicularly propagating drift instability driven by relative drifts between electrons and ions. The theory takes into account local cross-field current, pressure gradients and modest collisions as in the Magnetic Reconnection Experiment (MRX) [10]. The unstable waves have very small group velocities in the direction of the pressure gradient, but have a large phase velocity near the relative drift velocity between electrons and ions in the direction of cross-field current. By taking into account the electron-ion collisions and applying the theory in the Harris sheet, we establish that this instability could be excited near the center of the Harris sheet and have enough efoldings to grow to large amplitude before it propagates out of the unstable region. Comparing with the other magnetic reconnection related instabilities (LHDI, MTSI et.) studied previously, we believe the instability we find is a favorable candidate to produce anomalous resistivity because of its unique wave characteristics, such as electromagnetic component, large phase velocity, and small group velocity in the cross current layer direction.
Date: December 3, 2008
Creator: Wang, Y.; Kulsrud, R. & Ji, H.
Partner: UNT Libraries Government Documents Department