Finite Pressure Research Articles

The Effect of Trapped Particles on Gradient Drift Instabilities in Finite Pressure Plasma with a Longitudinally Nonuniform Magnetic Field

Gradient drift instabilities are considered on the basis of local dispersion relation developed for the case of longitudinally nonuniform magnetic field. Such a field allows simulating the effect of trapped particles in tokamak geometry separately from other effects. Completely electromagnetic approach is developed to take into account finite pressure. Trapped particles and magnetic field nonuniformity are taken into account by the frequency of the magnetic drift of the particle. Modes propagating perpendicularly to the magnetic field lines are typical from the viewpoint of effect of trapped electrons. In finite pressure plasma (ratio of the plasma pressure to the magnetic pressure β ~ 0.1), growth rate decrease is significant, as compared with electrostatic limit (β → 0).

Modeling fluid–structure interaction by the particle finite element method in OpenSees

The OpenSees finite element software framework is extended for simulating fluid–structure interaction (FSI) by the particle finite element method (PFEM). At high levels of the framework, new classes handle meshing and interface detection of the fluid and structure domains and implement the fractional step method in order to solve the governing equations of linear momentum and mass conservation. At lower levels of the framework, new finite element and pressure constraint classes assemble fluid contributions to the global system of equations. Verification and validation examples are presented along with a demonstrative example of wave loading on a coastal structure modeled using geometrically nonlinear frame elements with material nonlinear fiber sections. The extension of OpenSees for FSI allows analysts to simulate the complex phenomena of wave loading on structural models as well as the response of these models to sequential natural hazards such as earthquake induced tsunamis.

The role of pressure flattening in calculating tearing mode stability

Calculations of tearing mode stability in tokamaks split conveniently into one in an external region, where marginally stable ideal magnetohydrodynamics (MHD) is applicable, and one in a resonant layer around the rational surface where sophisticated kinetic physics is needed. These two regions are coupled by the stability parameter Δ′. Axisymmetric pressure and current perturbations localized around the rational surface significantly alter Δ′. Equations governing the changes in the external solution and Δ′ are derived for arbitrary perturbations in axisymmetric toroidal geometry. These equations can be used in two ways: (i) the Δ′ can be calculated for a physically occurring perturbation to the pressure or current; (ii) alternatively we can use these equations to calculate Δ′ for profiles with a pressure gradient at the rational surface in terms of the value when the perturbation removes this gradient. It is the second application we focus on here since resistive magnetohydrodynamics (MHD) codes do not contain the appropriate layer physics and therefore cannot predict stability for realistic hot plasma directly. They can, however, be used to calculate Δ′. Existing methods (Ham et al 2012 Plasma Phys. Control. Fusion 54 025009) for extracting Δ′ from resistive codes are unsatisfactory when there is a finite pressure gradient at the rational surface and favourable average curvature because of the Glasser stabilizing effect (Glasser et al 1975 Phys. Fluids 18 875). To overcome this difficulty we introduce a specific artificial pressure flattening function that allows the earlier approach to be used. The technique is first tested numerically in cylindrical geometry with an artificial favourable curvature. Its application to toroidal geometry is then demonstrated using the toroidal tokamak tearing mode stability code T7 (Fitzpatrick et al 1993 Nucl. Fusion 33 1533) which employs an approximate analytic equilibrium. The prospects for applying this approach to resistive MHD codes such as MARS-F (Liu et al 2000 Phys. Plasmas 7 3681) which utilize a fully toroidal equilibrium are discussed.

Optimum parametric performance characterization of an irreversible gas turbine Brayton cycle

Abstract A general mathematical model is developed to specify the performance of an irreversible gas turbine Brayton cycle incorporating two-stage compressor, two-stage gas turbine, intercooler, reheater, and regenerator with irreversibilities due to finite heat transfer rates and pressure drops. Ranges of operating parameters resulting in optimum performance (i.e., η I ≥ 38 ≥ η II ≥ 60%, ECOP ≥ 1.65, x loss ≤ 0.150 MJ/kg, BWR ≤ 0.525, w net ≥ 0.300 MJ/kg, and q add ≤ 0.470 MJ/kg) are determined and discussed using the Monte Carlo method. These operating ranges are minimum cycle temperature ranges between 302 and 315 K, maximum cycle temperature ranges between 1,320 and 1,360 K, maximum cycle pressure ranges between 1.449 and 2.830 MPa, and conductance of the heat exchanger ranges between 20.7 and 29.6 kW/K. Exclusive effect of each of the operating parameters on each of the performance parameters is mathematically given in a general formulation that is applicable regardless of the values of the rest of the operating parameters and under any condition of operation of the cycle.

Origin of pressure induced second superconducting dome in Ay Fe2−xSe2 [A = K, (Tl,Rb)

Recent observation of a pressure induced second superconducting phase in AyFe2−xSe2 [A = K, (Tl,Rb)] calls for the models of superconductivity that are rich enough to allow for multiple superconducting phases. We propose the model where pressure induces renormalization of band parameters in such a way that it leads to changes in Fermi surface topology even for a fixed electron number. We develop a low-energy effective model, derived from first-principles band-structure calculation at finite pressure, to suggest the phase assignment where a low pressure superconducting state with no hole pocket at the Γ point is a nodeless d-wave state. It evolves into a s± state at higher pressure when the Fermi surface topology changes and the hole pocket appears. We analyze the pairing interactions using a five band tight binding fitted band structure and find that a strong pairing strength is dependent on pressure. We also evaluate the energy and momentum dependence of neutron spin resonances in each of the phases as verifiable predictions of our proposal.

Seepage Stability Analysis for Surrounding Rock Mass of Diversion Tunnel by FEM

Based on the actual conditions of the project and related experience, the seepage model of diversion tunnel and its surrounding rock mass was built to analyze its seepage stability. The seepage field was calculated by using finite element method (FEM) and cut-off negative pressure method. Two typical cross-sections and working conditions were chosen to evaluate the seepage stability of the surrounding rock mass. According to the results, the seepage gradient is larger when one tunnel is filled with water while the other is empty. The maximum seepage gradient is less than the allowable seepage gradient, so the surrounding rock mass of the diversion tunnel can meet the demand of seepage stability.

Constructal entropy generation rate minimization of line-to-line vascular networks with convective heat transfer

The constructal optimization of line-to-line vascular networks (proposed by Lorente and Bejan) are carried out by taking the minimization of dimensionless overall entropy generation rate (combining entropy generation rates caused by heat transfer with finite temperature difference and by fluid flow with finite pressure difference) per unit load of heat transfer as the optimization objective, with the constraints of fixed total area occupied by the vasculature and fixed total volume of channels. The correlations of vasculature performance versus dimensionless mass flow rate and dimensionless pumping power are investigated. The analytical correlations and numerical examples of the dimensionless overall entropy generation rate and the dimensionless overall entropy generation rate per unit load of heat transfer versus the number of assembly levels, the dimensionless mass flow rate and the dimensionless pumping power are presented. The results based on the minimization of flow resistance are special cases of this paper. The results show that the comprehensive performance of the vasculature is improved with the increase of the number of assembly levels, but the increase of the number of assembly levels to improve the comprehensive performance of vasculature has a threshold value. The effect of the change of load on comprehensive performance of vasculature with larger number of assembly levels is a little.

Minimally Constrained Model of Self-Organized Helical States in Reversed-Field Pinches

We show that the self-organized single-helical-axis (SHAx) and double-axis (DAx) states in reversed field pinches can be reproduced in a minimally constrained equilibrium model using only five parameters. This is a significant reduction on previous representations of the SHAx which have required an infinite number of constraints. The DAx state, which has a nontrivial topology, has not previously been reproduced using an equilibrium model that preserves this topological structure. We show that both states are a consequence of transport barrier formation in the plasma core, in agreement with experimental results. We take the limit of zero pressure in this work, although the model is also valid for finite pressure.

LES of the adverse-pressure gradient turbulent boundary layer

We describe large-eddy simulations (LES) of the flat-plate turbulent boundary layer in the presence of an adverse pressure gradient. The stretched-vortex subgrid-scale model is used in the domain of the flow coupled to a wall model that explicitly accounts for the presence of a finite pressure gradient. The LES are designed to match recent experiments conducted at the University of Melbourne wind tunnel where a plate section with zero pressure gradient is followed by section with constant adverse pressure gradient. First, LES are described at Reynolds numbers based on the local free-stream velocity and the local momentum thickness in the range 6560–13,900 chosen to match the experimental conditions. This is followed by a discussion of further LES at Reynolds numbers at approximately 10 times and 100 times these values, which are well out of range of present day direct numerical simulation and wall-resolved LES. For the lower Reynolds number runs, mean velocity profiles, one-point turbulent statistics of the velocity fluctuations, skin friction and the Clauser and acceleration parameters along the streamwise, adverse pressure-gradient domain are compared to the experimental measurements. For the full range of LES, the relationship of the skin-friction coefficient, in the form of the ratio of the local free-stream velocity to the local friction velocity, to both Reynolds number and the Clauser parameter is explored. At large Reynolds numbers, a region of collapse is found that is well described by a simple log-like empirical relationship over two orders of magnitude. This is expected to be useful for constant adverse-pressure gradient flows. It is concluded that the present adverse pressure gradient boundary layers are far from an equilibrium state.

Reaction cycles and poisoning in catalysis by gold clusters: A thermodynamics approach

In heterogeneous catalysis, a catalytic process takes place at finite temperature and at finite pressure of the atmosphere of the reactant gases. By applying ab initio atomistic thermodynamics to the model case of free Au2 and clusters in an atmosphere of O2 and CO, we derive all the thermodynamically possible reaction paths for the oxidation of CO to CO2. This analysis lets us explain how gold clusters enable oxidation reactions without breaking the spin-conservation rule. Furthermore, we identify special cluster + ligands compositions such as reaction intermediates and poisoned species. In particular, a thermodynamically driven poisoning is identified for the catalytic system containing free Au2, and the experimental (p, T) conditions that avoid its formation are suggested. This implies that for some systems a catalytic cycle can be established, on thermodynamics grounds, only in a defined range of temperatures and pressures. In addition, our predictions for provide the so far most complete interpretation of the available experimental data (Socaciu et al, J. Am. Chem. Soc. 2003). © 2013 Wiley Periodicals, Inc.

Nonlinear fluid-structure interaction of an elastic panel in an acoustically excited two-dimensional inviscid compressible fluid

The focus of this paper is on the asymptotic investigation of the nonlinear fluid-structure interaction of an acoustically excited clamped panel immersed in an inviscid compressible fluid. A multiple-scales analysis of the corresponding two-dimensional unsteady potential flow initial-boundary-value-problem is employed to investigate both primary resonance and a 3:1 internal resonance between the panel fifth and ninth modes. Validation of the asymptotic structural response and the fluid pressure shows good agreement with numerical solution of a weakly nonlinear panel in a quadratic Euler field. The results shed light on the intricate acoustic interaction bifurcation structure which exhibits coexisting bi-stable periodic solutions, and quasiperiodic response reflecting spatially periodic modal energy transfer for both panel and fluid. This behavior is found to occur for panel excitation by finite level acoustic pressure waves that can be a crucial factor for design of high integrity structural systems required for aviation or space where light structures are exposed to intensive acoustic pressure fluctuations.

Calculation Method of Lubricant Film Pressure Distribution of Axial Piston Pump Slippers

A method is implemented to get the pressure distribution of the axial piston pump slipper. Slipper was seen as translating thrust bearing, taking slipper tilt and spin in account, based on finite volume method, hydrodynamic and hydrostatic pressure has been calculated by using the mass conservation principle. For a representative element volume, the difference flow was averaged by the difference flow between the tilting planes, and the shearing flow by slipper translating was averaged by the shearing flow between the tilting planes. The numerical calculating result based two liquid resistance assume was compared, the results showed that two methods have got the same pressure distribution schematics, and the high pressure area locates at the slipper titling direction, but for the pressure values at high pressure area, the second method is slightly higher than the first method, and that the higher pump speed were, the higher the pressure values, and at the same pump speed, the slipper spin speed affects slightly the pressure, and at the lower pump speed, the lubricant pressure tends to the hydrostatic lubrication.

Giant pulsations as modes of a transverse Alfvénic resonator on the plasmapause

The paper assumes that the giant pulsations are oscillations trapped within a resonator resulting from finite plasma pressure on the outer edge of the plasmapause. This resonator is bounded, across the L-shells, by two turning points allowing the wave energy to be channeled azimuthally. This assumption can explain the basic properties of the giant pulsations: strong localization across magnetic shells, poloidal polarization, presence of a significant compressional component in the Pg magnetic field, the fact that their frequency does not depend on the radial coordinate. The wave field structure both across the L-shells and along the field lines is studied. In order to explain the amplitude modulation it is sufficient to suppose that the resonator is excited by some non-stationary process. Generation by a moving source comprised of substorm-injected particles is considered.

Circuit model for microfluidic bubble generation under controlled pressure

We explore the microfluidic generation of bubbles in a flow-focusing junction using a pressure-controlled device rather than the more common flow rate-controlled devices. This device is a prototype for extending microfluidic drop generation methods to molten polymers. We show that the bubble generation process is highly sensitive to pressure: small changes in pressure induce large changes in bubble size and bubble formation frequency. A simple resistance circuit model can explain this pressure dependence. Briefly, we show that bubble generation is possible only within a finite pressure range. Near the ends of this pressure range, the ratio of the flow rates of the dispersed to continuous phase is highly sensitive to pressure, and therefore so also is the bubble generation process. The circuit model offers a way to use existing models of drop generation (which are based on flow rate-controlled operation) to predict pressure-controlled operation. We also examine drop formation using a highly viscous polymer as the dispersed phase. Drops are formed far downstream of the flow-focusing junction, and they are far smaller than the microfluidic channel dimensions. These results suggest that existing microfluidic drop generation methods may be exploited to make complex particles from thermoplastic polymers.

Jovian plasma torus interaction with Europa. Plasma wake structure and effect of inductive magnetic field: 3D hybrid kinetic simulation

The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon–magnetosphere system with respect to a variable upstream magnetic field and flux or density distributions of plasma and energetic ions, electrons, and neutral atoms. This capability is critical for improving the interpretation of the existing Europa flyby measurements from the Galileo Orbiter mission, and for planning flyby and orbital measurements (including the surface and atmospheric compositions) for future missions. The simulations are based on recent models of the atmosphere of Europa (Cassidy et al., 2007; Shematovich et al., 2005). In contrast to previous approaches with MHD simulations, the hybrid model allows us to fully take into account the finite gyroradius effect and electron pressure, and to correctly estimate the ion velocity distribution and the fluxes along the magnetic field (assuming an initial Maxwellian velocity distribution for upstream background ions). Photoionization, electron-impact ionization, charge exchange and collisions between the ions and neutrals are also included in our model. We consider the models with O++ and S++ background plasma, and various betas for background ions and electrons, and pickup electrons. The majority of O2 atmosphere is thermal with an extended non-thermal population (Cassidy et al., 2007). In this paper, we discuss two tasks: (1) the plasma wake structure dependence on the parameters of the upstream plasma and Europa's atmosphere (model I, cases (a) and (b) with a homogeneous Jovian magnetosphere field, an inductive magnetic dipole and high oceanic shell conductivity); and (2) estimation of the possible effect of an induced magnetic field arising from oceanic shell conductivity. This effect was estimated based on the difference between the observed and modeled magnetic fields (model II, case (c) with an inhomogeneous Jovian magnetosphere field, an inductive magnetic dipole and low oceanic shell conductivity).

Buckling pathways in spherical shells with soft spots

Thin elastic spherical shells are known to exhibit a buckling instability at a finite external pressure. We study how this buckling is influenced by a weak region in an otherwise uniform shell, focusing on the case of a single small circular soft spot that is thinner than the rest of the shell. Using numerical simulations and theoretical arguments, we show that the soft region fundamentally alters the buckling behavior of the shell. The soft spot influences both the pressure at which the shell buckles, and the postbuckling shape of the shell. Depending on the properties of the soft spot, we find either a single buckling transition or two separate transitions as the external pressure is increased. We analyze the dependence of these buckling transitions on the size and thickness of the soft region. Besides contributing to our fundamental understanding of buckling transitions in inhomogeneous shells, our results can be applied to designing capsules with tunable shapes.

High-Frequency Electrostatic SW at the Boundary between Quantum Plasma and Metal

It is shown that high-frequency electrostatic surface waves (SW) could be propagated at right angles to an external magnetic field on the boundary between metal and gaseous plasma due to a finite pressure electron gas in quantum plasma by using the quantum hydrodynamic QHD equations. The dispersion relation for those surface waves in uniform electron plasma is derived under strong external magnetic field. We have shown that the electrostatic surface waves exist also in the frequency for the ranges where electromagnetic SW is impossible. The surface plasma modes are numerically evaluated for the specific case of gold metallic plasma at room temperature. It has been found that dispersion relation of surface modes depends significantly on these quantum effects (Bohm potential and statistical) and should be into account in the case of magnetized or unmagnetized plasma.

Ab initioatomistic thermodynamics study of the early stages of Cu(100) oxidation

Using an ab initio atomistic thermodynamics framework, we identify the stable surface structures during the early stages of Cu(100) oxidation at finite temperature and pressure conditions. We predict the clean surface, the 0.25 monolayer oxygen-covered surface, and the missing-row reconstruction as thermodynamically stable structures in range of 100--1000 K and 10${}^{\ensuremath{-}15}$--10${}^{5}$ atm, consistent with previous experimental and theoretical results. We also investigate the thermodynamic stabilities of possible precursors to Cu${}_{2}$O formation including missing-row reconstruction structures that include extra on- or subsurface oxygen atoms as well as boundary phases formed from two missing-row nanodomains. While these structures are not predicted to be thermodynamically stable for oxygen chemical potentials below the nucleation limit of Cu${}_{2}$O, they are likely to exist due to kinetic hindrance.

Anomalous sound absorption of finite amplitude sound in liquid 4He

We report the nonlinear and unsteady response of the finite amplitude pressure wave in superfluid 4He, using a classical vibrating liquid column standing wave resonator. The frequency was kept at the resonance frequency of the liquid column, so that the maximum of the pressure field always came to the surface of the driver. As a result, in the low driving-amplitude regime, I/O relations were linear. However, in the high driving-amplitude regime, anomalous responses were found; The amplitude of the signals suddenly reduced and gradually recovered; they were repeated with random intervals. Moreover, in the highly suppressed region in the waveform, we found a generation of new type of turbulence. These absorptions are expected to be the result of the vapor bubble ejection from the surface of the driver transducer.

Applications of Nanostructured Materials as Additives in Space Fluid Lubricants

With an extended service life expectancy of space crafts, fluid lubricants attracted intense interests of the fields of tribology, due to their long life under EDH conditions, higher thermal conductance, lower frictional and torque noise, and rapid processing compared to the solid lubricants. However, there are still some disadvantages of fluid lubricants, such as finite vapor pressure, narrow range of operating temperature, degradation of some oils, viscosity temperature-dependence and so on. So far, there have existed some space fluid lubricants, such as PFPE, MAC, PAO and mineral oil based lubricants. It seems that there will be a challenge for a new type of fluid lubricants to come out without any limits above. Recent arisen nanostructured materials assisted in improvement of the research of space fluid lubricants, and the results indicated that some nanostructured materials exactly promoted the tribological properties of fluid lubricants. This paper reviewed recent development of nanostructured materials as additives in space fluid lubricants.