Effect Of Density Gradient Research Articles

Theoretical models for unstable IAWs and nonlinear structures in the upper ionosphere

Physical mechanisms are discussed for the excitation of ion-acoustic waves (IAWs) by field-aligned shear flow of ions and parallel current produced by electrons in the upper ionospheric oxygen hydrogen plasma within auroral latitudes. Theoretical models are presented for the formation of solitary structures by nonlinear IAWs. It is pointed out that the small concentration of hydrogen ions in the oxygen plasma should not be ignored, because it plays important role in the linear instability of IAWs and in determining the size of the nonlinear electrostatic structures. The growth rates of IAWs and size of nonlinear structures vary with altitude, because both depend upon the density ratio of oxygen-to-hydrogen ions along with other parameters. Current-driven electrostatic ion-acoustic waves are studied using kinetic theory which shows that parallel current produces these waves if the concentration of protons is very small about 4% or lesser in the presence of field-aligned shear flow of both kind of ions. Fluid theory is used to look for shear flow-driven instabilities and formation of nonlinear structures ignoring ion temperature effects in this plasma where Freja observations indicate $$T_{i}\approx $$ (0.3–0.1) $$T_{e}$$ . Effects of nonthermal electrons and density gradient on the instabilities and size of the structures are also pointed out.

Laser and electron deflection from transverse asymmetries in laser-plasma accelerators.

We report on the deflection of laser pulses and accelerated electrons in a laser-plasma accelerator (LPA) by the effects of laser pulse front tilt and transverse density gradients. Asymmetry in the plasma index of refraction leads to laser steering, which can be due to a density gradient or spatiotemporal coupling of the laser pulse. The transverse forces from the skewed plasma wave can also lead to electron deflection relative to the laser. Quantitative models are proposed for both the laser and electron steering, which are confirmed by particle-in-cell simulations. Experiments with the BELLA Petawatt Laser are presented which show controllable 0.1-1 mrad laser and electron beam deflection from laser pulse front tilt. This has potential applications for electron beam pointing control, which is of paramount importance for LPA applications.

Mass of dusty clumps with temperature and density structure

We consider a dusty clump in two cases of spherical and cylindrical symmetry to investigate the effect of temperature and density gradients on the observed flux density. Conversely, we evaluate how the presence of these gradients affects the calculation of the clump mass from the observed flux. We provide approximate expressions relating flux density and mass in the optically thick and thin limits and in the Rayleigh-Jeans regime, and we discuss the reliability of these expressions by comparing them to the outcome of a numerical code. Finally, we present the application of our calculations to three examples taken from the literature, which shows how the correction introduced after taking into account temperature and density gradients may affect our conclusions on the stability of the clumps.

Modelling H3+ in planetary atmospheres: effects of vertical gradients on observed quantities.

Since its detection in the aurorae of Jupiter approximately 30 years ago, the H3+ ion has served as an invaluable probe of giant planet upper atmospheres. However, the vast majority of monitoring of planetary H3+ radiation has followed from observations that rely on deriving parameters from column-integrated paths through the emitting layer. Here, we investigate the effects of density and temperature gradients along such paths on the measured H3+ spectrum and its resulting interpretation. In a non-isothermal atmosphere, H3+ column densities retrieved from such observations are found to represent a lower limit, reduced by 20% or more from the true atmospheric value. Global simulations of Uranus' ionosphere reveal that measured H3+ temperature variations are often attributable to well-understood solar zenith angle effects rather than indications of real atmospheric variability. Finally, based on these insights, a preliminary method of deriving vertical temperature structure is demonstrated at Jupiter using model reproductions of electron density and H3+ measurements. The sheer diversity and uncertainty of conditions in planetary atmospheres prohibits this work from providing blanket quantitative correction factors; nonetheless, we illustrate a few simple ways in which the already formidable utility of H3+ observations in understanding planetary atmospheres can be enhanced. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Impurity effects on ion temperature gradient driven multiple modes in transport barriers

The impurity effects on ion temperature gradient (ITG) driven instability in transport barriers (TBs) are numerically investigated with the gyrokinetic integral eigenmode equations in tokamak plasmas. In particular, the effects of temperature and density gradients of the main ions ( and ) are analyzed independently to understand the physical mechanisms better, instead of keeping their ratio as carried out in previous works, when the parameters of impurity ions vary. It is found that the effect of impurity ions with outwardly peaked density profiles on ITG modes depends on the competition between the destabilizing effect of the impurity density gradient and the stabilizing effect induced by the dilution of main ions from impurity ions when is fixed, which is in significant contrast with the results for a fixed . The destabilizing effects include enhancement of ITG modes and coupling to the impurity mode (IM) in weak ITGs (big ) and strong impurity density gradient regimes. In addition, the stability boundaries for ITG modes, including high-order modes, are discussed in detail, and compared with previous works (Fröjdh et al 1992 Nucl. Fusion 32 419). Furthermore, the impurity ions with either inwardly or slightly outwardly peaked density profiles have weaker and stronger stabilizing effects on small and big poloidal wave vector modes, respectively. However, the impurity ions with steeper outwardly peaked density profiles have stronger stabilizing effects on big modes. Moreover, the inwardly peaked impurity ion density profiles are beneficial for main ion confinement and impurity decumulation, due to the main (impurity) ions flowing inwardly (outwardly). Finally, analyses of eigenmode structure and the quasi-linear particle flux are performed in detail. The results show that impurity ions have non-negligible effects, especially on higher-order ITG modes.

A viscous magnetohydrodynamic Kelvin–Helmholtz instability in the interface of two fluid layers: Part I. Basic mechanism

This study investigates the combined effect of density, velocity and magnetic field gradients on the Kelvin–Helmholtz instability of two viscous fluid layers. For the linear phase of instability that refers to the early stage of development of Kelvin–Helmholtz instability, the linear growth rate and frequency are presented. With respect to our selected variables and the Atwood number, the behaviour of growth rate and frequency are analysed. It is found that, the behaviour of frequency is not affected by the magnetic field and viscous term. The velocity gradient with the small Atwood numbers tends to stabilize KHI flows, while the velocity gradient with the large Atwood numbers has destabilizing effect on KHI. The growth rate reduces with the constant magnetic field and viscous term, while it enhances with magnetic field gradient.

Spatial and Temporal Variability of Dense Shelf Water Cascades along the Rottnest Continental Shelf in Southwest Australia

Along the majority of Australian shallow coastal regions, summer evaporation increases the salinity of shallow waters, and subsequently in autumn/winter, the nearshore waters become cooler due to heat loss. This results in the formation of horizontal density gradients with density increasing toward the coast that generates gravity currents known as dense shelf water cascades (DSWCs) flowing offshore along the sea bed. DSWCs play important role in ecological and biogeochemical processes in Australian waters through the transport of dissolved and suspended materials offshore. In this study a numerical ocean circulation model of Rottnest continental shelf, validated using simultaneous ocean glider and mooring data, indicated that the passage of cold fronts associated with winter storms resulted in rapid heat loss through evaporative cooling. These conditions resulted in enhancement of the DSWCs due to modifications of the cross-shelf density gradient and wind effects. Specifically, onshore (offshore) directed winds resulted in an enhancement (inhibition) of DSWCs due to downwelling (vertical mixing). Consequently, the largest DSWC events occurred during the cold fronts when atmospheric temperatures reinforced density gradients and onshore winds promoted downwelling that enhanced DSWCs. Advection of DSWCs was also strongly influenced by the wind conditions, with significantly more transport occurring along-shelf compared to cross-shelf.

Shock loading simulation using density-graded metallic foam projectiles

Density-graded metallic foam projectiles are proposed for shock loading simulation, with special focus placed upon designing pressure pulses having specific shapes. Experiments are performed to explore the potential of density-graded foam projectiles in generating shock loadings of various pulse shapes. Subsequently, three-dimensional Voronoi foam models with varying gradient profiles along the length direction are constructed for finite element (FE) simulations, which are validated against the experimental data. Then, FE simulations of density-graded foam projectiles impacting a stationary rigid wall are conducted to quantify the effect of density gradient on the shape of the pressure pulse generated and explore the physical mechanisms underlying such effect. It is demonstrated that density gradient affects significantly local crushing stress and local density within the shock front when it propagates from the impact end to the other end of the foam projectile. Inspired by the FE simulation results and the classical Taylor impact model, one-dimensional theoretical model for density-graded foam projectiles is developed to predict the contact force between the projectile and the fixed rigid wall. Finally, the theoretical model is employed to determine the geometry, density gradient, and firing velocity of foam projectiles needed to generate shock loadings with prescribed pulse shapes.

Numerical Simulations of Linearly Stratified Flow Past Submerged Bodies

Abstract In this study, a methodology was presented to predict density stratified flows in the near-field of submerged bodies. The energy equation in temperature form was solved coupled with momentum and mass conservation equations. Linear stratification was achieved by the definition of the density as a function of temperature. At first, verifications were performed for the stratified flows passing a submerged horizontal circular cylinder, showing excellent agreement with available experimental data. The ability of the method to cope with variable density was demonstrated. Different turbulence models were used for different Re numbers and flow states. Based on the numerical methods proposed in this paper, the stratified flow was studied for the real scale benchmark DAPRA Suboff submarine. The approach used the VOF method for tracing the free surface. Turbulence was implemented with a k − ω based Detached Eddy Simulation (DES) approach. The effects of submarine speed, depth and density gradient on the free surface wave pattern were quantitatively analyzed. It was shown that, with the increasing of the speed of the submarine, the wavelength and wave height of the free surface wave were gradually increasing. The wave height of the free surface wave was gradually reduced as the submarine’s depth increased. Relative to the speed and submarine depth, the changes of the gradient density gradient have negligible effects on the free surface wave field.

When do dendrometric rules fail? Insights from 20 years of experimental thinnings on sessile oak in the GIS Coop network

ContextLowering stand density has been suggested to adapt forests to warmer and drier conditions. Whether common dendrometric rules used to guide growth models and support silviculture are still valid at these densities lower than usual needs be tested. This includes (1) estimating the stand growth-density relationship over wider density gradients (‘Langsaeter’s rule’), (2) testing the stability of site index, and (3) ‘Eichhorn’s rule’ (relating site index to stand growth) at lower densities. Additionally, environmental changes call for a more direct inclusion of environmental drivers in growth models. Material and methodsWe investigated the effects of relative stand density (RDI) on stand growth of even-aged stands of sessile oak. We also analyzed effects of climatic conditions to assess whether sessile oak is already at a critical stage that requires adapting forest management. We used a network of silvicultural trials designed to test the effects of maximum density gradients on stand dynamics along climatic gradients. ResultsAs predicted by Langsaeter’s rule, gross stand growth increased with RDI then saturated for RDI > 0.4–0.5, mortality began after crown closure, and diameter increment decreased non-linearly with RDI. Dominant height growth first increased with RDI then saturated for RDI > 0.8. Positive effects of RDI on both stand and height growth caused Eichhorn’s rule to be stable over a large range of RDI (0.25–1). Effects of climate predictors were weak and only visible for radial growth indices. They however suggested antagonistic effects of water and temperature constraints in summer and autumn. DiscussionLangsaeter’s and Eichhorn’s rules were found valid over a large range of RDI values. We suggest using them to guide growth models and management. However, assumptions behind the use of site index as a proxy for stand productivity were invalidated. Weak and opposed effect of temperature in summer and autumn makes the outcome of climate warming on sessile oak growth uncertain.

Kelvin–Helmholtz instability of two finite-thickness fluid layers with continuous density and velocity profiles

The effect of density and velocity gradients on the Kelvin–Helmholtz instability (KHI) of two superimposed finite-thickness fluid layers are analytically investigated. The linear normalized frequency and normalized growth rate are presented. Then, their behavior as a function of the density ratio of the light fluid to the heavy one (r) was analyzed and compared to the case of two semi-infinite fluid layers. The results showed that the values of normalized frequency of KHI for two finite-thickness fluid layers are less than their counterparts for two semi-infinite fluid layers. The behavior of normalized growth rate as a function of the velocity and density gradients capitulates to the effect of velocity gradient at the large values of (r).

Numerical modeling of laser-driven ion acceleration from near-critical gas targets

In the past two decades, laser-accelerated ion sources and their applications have been intensely researched. Recently, it has been shown through experiments that proton beams with characteristics comparable to those obtained with solid targets can be obtained from gaseous targets. By means of particle-in-cell simulations, this paper studies in detail the effects of a near-critical density gradient on ion and electron acceleration after the interaction with ultra high intensity lasers. We can observe that the peak density of the gas jet has a significant influence on the spectrum features. As the gas jet density increases, so does the peak energy of the central quasi-monoenergetic ion bunch due to the increase in laser absorption while at the same time having a broadening effect on the electron angular distribution.

Gravitational Waves from Accreting Neutron Stars Undergoing Common-envelope Inspiral

Abstract The common-envelope phase is a likely formation channel for close binary systems containing compact objects. Neutron stars in common envelopes accrete at a fraction of the Bondi–Hoyle–Lyttleton accretion rate, since the stellar envelope is inhomogeneous, but they may still be able to accrete at hypercritical rates (though not enough to become black holes). We show that common-envelope systems consisting of a neutron star with a massive primary may be gravitational-wave (GW) sources detectable in the Advanced LIGO band as far away as the Magellanic Clouds. To characterize their evolution, we perform orbital integrations using 1D models of 12 M ⊙ and 20 M ⊙ primaries, considering the effects of density gradient on the accretion onto the NS and spin evolution. From the range of possible accretion rates relevant to common-envelope evolution, we find that these systems may be louder GW sources than low-mass X-ray binaries like Sco X-1, which are currently the target of directed searches for continuous GWs. We also find that their strain amplitude signal may allow for novel constraints on the orbital separation and inspiral timescale in common envelopes when combined with pre-common-envelope electromagnetic observations.

Effects of density gradients and fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade

Electron cyclotron emission imaging (ECEI) provides measurements of electron temperature (Te) and its fluctuations (δTe). However, when measuring at the plasma edge, in the steep gradient region, radiation transport effects must be taken into account. It is shown that due to these effects, the scrape-off layer region is not accessible to the ECEI measurements in steady state conditions and that the signal is dominated by the shine-through emission. Transient effects, such as filaments, can change the radiation transport locally, but cannot be distinguished from the shine-through. Local density measurements are essential for the correct interpretation of the electron cyclotron emission, since the density fluctuations influence the temperature measurements at the plasma edge. As an example, a low frequency 8 kHz mode, which causes 10%–15% fluctuations in the signal level of the ECEI, is analysed. The same mode has been measured with the lithium beam emission spectroscopy density diagnostic, and is very well correlated in time with high frequency magnetic fluctuations. With radiation transport modelling of the electron cyclotron radiation in the ECEI geometry, it is shown that the density contributes significantly to the radiation temperature (Trad) and the experimental observations have shown the amplitude modulation in both density and temperature measurements. The poloidal velocity of the low frequency mode measured by the ECEI is 3 km s–1. The calculated velocity of the high frequency mode measured with the magnetic pick-up coils is about 25 km s–1. Velocities are compared with the E × B background flow velocity and possible explanations for the origin of the low frequency mode are discussed.

Dynamics of axially functionally graded cantilevered pipes conveying fluid

The linear dynamics of axially functionally graded (AFG) cantilevered pipes conveying fluid is studied, aiming at enhancing the dynamic stability of such fluid-interaction systems. Either the elastic modulus or the density of the AFG cantilevered pipe is assumed to be varied from the clamped to the free ends. The governing equation of the AFG pipe is derived first and then discretized by using the differential quadrature method (DQM). The effects of elastic modulus gradient and density gradient on the critical flow velocity for flutter instability are analyzed. It is found that, compared with uniform pipes, the decrease of density along the pipe length leads to a more stable system, while the opposite result may be obtained for the decrease of elastic modulus for small values of mass ratio. From the boundary curves of critical flow velocity versus density gradient and of critical flow velocity versus elastic modulus gradient, it is shown that the occurrence of Z-shape segments is possible when the mass ratio becomes large. Furthermore, the phenomenon of mode exchange has also been detected with increasing density gradient or elastic modulus gradient within a certain range of mass ratio.

The effect of shear flow and the density gradient on the Weibel instability growth rate in the dense plasma

Shear stress effect has been often neglected in calculation of the Weibel instability growth rate in laser-plasma interactions. In the present work, the role of the shear stress in the Weibel instability growth rate in the dense plasma with density gradient is explored. By increasing the density gradient, the shear stress threshold is increasing and the range of the propagation angles of growing modes is limited. Therefore, by increasing steps of the density gradient plasma near the relativistic electron beam-emitting region, the Weibel instability occurs at a higher stress flow. Calculations show that the minimum value of the stress rate threshold for linear polarization is greater than that of circular polarization. The Wiebel instability growth rate for linear polarization is 18.3 times circular polarization. One sees that for increasing stress and density gradient effects, there are smaller maximal growth rates for the range of the propagation angles of growing modes π2<θmin<π and 3π2<θmin<2π in circular polarized plasma and for kcωp<4 in linear polarized plasma. Therefore, the shear stress and density gradient tend to stabilize the Weibel instability for kcωp<4 in linear polarized plasma. Also, the shear stress and density gradient tend to stabilize the Weibel instability for the range of the propagation angles of growing modes π2<θmin<π and 3π2<θmin<2π in circular polarized plasma.

Study of the parametric dependence of linear and nonlinear microtearing modes in conventional tokamak discharges

A reduced transport model for microtearing modes is developed for use in integrated predictive modeling studies, employing a unified fluid/kinetic approach to derive the nonlinear dispersion relation. This approach advances the kinetic description and allows the inclusion of nonlinear effects due to magnetic fluctuations. In this numerical study, the dependence of the microtearing mode real frequency and growth rate on plasma parameters and on DIII-D like L-mode and H-mode plasma profiles is examined. The magnetic fluctuation strength as well as electron thermal diffusivity due to microtearing modes is computed. The saturated amplitude of the magnetic fluctuations is calculated utilizing numerically determined microtearing mode eigenvalues in the nonlinear microtearing modes envelope equation. It is found that the electron temperature gradient in the presence of moderate collision frequency is required for the microtearing mode to become unstable. The effects of small and large collisionality and small and large wavenumbers on microtearing modes are found to be stabilizing, while the effects of density gradient, plasma beta, low current density, and large magnetic shear are found to be destabilizing. The microtearing mode growth rate, magnetic fluctuation strength, as well as electron thermal diffusivity is found to be larger in the H-mode plasma than in the L-mode plasma.

Formation of impurity transport barrier in LHD plasmas with hollow density profile

The iron impurity density in the central column of Large Helical Device (LHD) plasmas with a hollow electron density profile is found to be much smaller than those with a peaked density profile based on the radial density profile measurement of iron ions in several ionization stages. The iron impurity transport has been studied with the help of a 1D simulation code. The radial structure of the convective velocity suggests that an impurity transport barrier is formed by a strong density gradient effect on the impurity convection near the edge region of LHD plasmas with a hollow electron density profile.

The effects of lateral density gradients, slopes and buoyancy on channel flow: 1D analytical solutions and applications to the SE Canadian Cordillera

We present 1D analytical solutions for channel flow in orogens driven by various types of pressure gradients. Our calculations demonstrate that lateral density gradients in the upper crust, such as would occur across a suture zone separating arc rocks from pericratonic sediments provide a driving force for Poiseuille flow as large as topographic gradients observed in modern mountain belts. For cases for which the gradients are external (topographic and lateral density gradients) and internal (e.g. partial melting of channel material) to the channel, inclination decreases and increases the Poiseuille component of the average flow-velocity within the channel by the cosine and sine of the slope, respectively. The magnitude of the pressure gradient consequent upon the buoyancy generated by partial melting of metapelites in a channel with a 30° slope, such as would occur above an underthrusting basement ramp, is similar to that of topographic or lateral density gradients. Channel flow up a ramp could thus constitute an important exhumation mechanism in large hot orogens. Our calculations indicate that mid-crustal channel flow was a highly likely process in the Late Cretaceous-Paleocene setting of the southeastern Canadian Cordillera. The flow was first driven by the lateral density contrast between pericratonic sediments and the arc-related Intermontane terrane, then by combined effect of topographic gradient and melt-induced buoyancy of the Lower Selkirk Allochthon (part of the Shuswap Complex). Flow up the underthrusting basement ramp resulted in exhumation from mid- to upper-crustal levels. Channel flow then migrated downward to involve basement and overlying cover sequence rocks. Our results indicate that syn-convergent channel flow was a viable and very likely process in the southeastern Canadian Cordillera.

Effect of Dislocation Density Gradient on Deformation Behavior of Pure Nickel Subjected to Torsion

Gradient hierarchical structures, including grain size gradient, twin density gradient, and texture gradient, are believed to broke up the long-standing dilemma-“strength-ductility trade-off” in materials science. Here we study the effect of dislocation density gradient on deformation behavior of pure nickel subjected to torsion. After applying torsion to cylindrical pure nickel samples, we find that Vickers hardness gradually decreased from surface to centre and the there is no obvious grain size gradient along the radial direction. The compression tests have been conducted on samples with different thickness of gradient layers. The results indicate that the yielding strength of the material can be three-times improved. Especially, the work-hardening rate “up-turn” phenomenon which was usually found in gradient nano-structures is presented in the torsion deformed pure nickel. Finally, the possible reasons and the effect of thickness of gradient structure are carefully discussed.