Constant External Pressure Research Articles

Electromechanics and Volume Dynamics in Nonexcitable Tissue Cells

Cell volume regulation is fundamentally important in phenomena such as cell growth, proliferation, tissue homeostasis, and embryogenesis. How the cell size is set, maintained, and changed over a cell’s lifetime is not well understood. In this work we focus on how the volume of nonexcitable tissue cells is coupled to the cell membrane electrical potential and the concentrations of membrane-permeable ions in the cell environment. Specifically, we demonstrate that a sudden cell depolarization using the whole-cell patch clamp results in a 50% increase in cell volume, whereas hyperpolarization results in a slight volume decrease. We find that cell volume can be partially controlled by changing the chloride or the sodium/potassium concentrations in the extracellular environment while maintaining a constant external osmotic pressure. Depletion of external chloride leads to a volume decrease in suspended HN31 cells. Introducing cells to a high-potassium solution causes volume increase up to 50%. Cell volume is also influenced by cortical tension: actin depolymerization leads to cell volume increase. We present an electrophysiology model of water dynamics driven by changes in membrane potential and the concentrations of permeable ions in the cells surrounding. The model quantitatively predicts that the cell volume is directly proportional to the intracellular protein content.

A dynamic-pulse pseudo-pressure method to determine shale matrix permeability at representative reservoir conditions

Matrix permeability is a key factor in determining long term gas production from shale reservoirs – requiring that it is determined under true reservoir conditions. We suggest a variable pressure gradient (VPG) protocol to measure shale matrix permeability using real reservoir fluids in powdered samples. The VPG method is described and a mathematical protocol for its analysis is developed. The first measures gas fractional production rate history under constant external pressure for each production stage and with a designated pressure gradient. The second establishes the mathematical protocol for analysis using pseudo-pressure to accommodate both the effect of gas pressure-dependent PVT parameters and desorption rate coefficient. The matrix permeability is determined by matching the solution of the model with the experimental data. The model fits the experimental data well when the fractional production is <0.75. Shale matrix permeability is calculated in the order of magnitude of 10−7–10−6 md. Methane permeability decreases with a decrease in both average pore pressure and particle size of the individual component grains. Permeability considerably more sensitive to changes in desorption rate coefficient than flow regimes. Compared with current small pressure gradient (SPG) methods, the VPG method is considerably more applicable to actual gas production and reduces to the SPG method under simplified boundary conditions. Although some approximate treatments are used for establishing the VPG method and some flow mechanisms are not considered, this study still provides an information-rich technique to determine shale matrix permeability at conditions close to reality.

Separate-ice frost heave model for one-dimensional soil freezing process

For one-dimensional soil freezing process, a separate-ice frost heave model is established, and the coupled process of heat transfer, fluid flow and stress development is considered in the model. First, a coupled heat–fluid–stress model describing the growth of a single ice lens is developed by extending the coupled heat–fluid model presented by Zhou and Zhou (Can Geotech J 49(6):686–693, 2012). Second, the mechanism for the formation of a new ice lens in the frozen fringe is studied, and we indicate that if the total vertical disjoining pressure at certain place exceeds the sum of the external pressure and the critical pressure, a new ice lens will emerge. By combining the growth model of a single ice lens and the criterion for the formation of a new ice lens, the separate-ice frost heave model is then established. The difference between the separate-ice model and the rigid-ice model is explained, and the relations for different mathematical models which describe the soil freezing process are also discussed. Numerical analysis of the separate-ice model is conducted using the finite volume method. The freezing tests for Devon silt under no external pressure and Xuzhou silty clay under a constant external pressure are applied to verify the computational results. The consistence between the calculation and the observation validates the separate-ice frost heave model.

Nonstationary Oscillations of a Cylindrical Shell Located in a Rigid Pipe and Interacting with Flows of Liquid Under Impulsive Disturbances of Pressure in the Internal Flow

A technique for analyzing the non-stationary oscillations of cylindrical shells interacting with the external and internal flow of liquid under constant external and internal pressure of liquid. The internal pressure is subject to finite harmonic disturbances linearly decreasing along the shell length. This technique is used to numerically analyze the transient processes in the shell–liquid system for different values of disturbance parameters.

Controlling relative polymorph stability in soft porous crystals with a barostat.

We use Monte Carlo simulations to investigate the thermodynamic behavior of soft porous crystal (SPC) adsorbents under the influence of an external barostat. We consider SPCs that naturally exhibit polymorphism between crystal forms of two distinct pore sizes. In the absence of barostatting, these crystals may be naturally divided into two categories depending on their response to stress applied by the adsorbate fluid: those which macroscopically deform and change the volume of their unit cell ("breathing") and those which instead undergo internal rearrangements that change the adsorbate-accessible volume without modifying the unit cell volume ("gate-opening"). When breathing SPCs have a constant external pressure applied, in addition to the thermodynamic pressure of the adsorbate fluid, we find that the free energy difference between the crystal polymorphs is shifted by a constant amount over the entire course of adsorption. Thus, their relative stability may be easily controlled by the barostat. However, when the crystal is held at a fixed overall pressure, changes to the relative stability of the polymorphs tend to be more complex. We demonstrate a thermodynamic analogy between breathing SPCs held at a fixed pressure and macroscopically rigid gate-opening ones which explains this behavior. Furthermore, we illustrate how this implies that external mechanical forces may be employed to tune the effective free energy profile of an empty SPC, which may open new avenues to engineer the thermodynamic properties of these polymorphic adsorbents, such as selectivity.

Measurement and evaluation of the stress sensitivity in tight reservoirs

Abstract The revised details of SY/T5358—2010 of National Energy Administration Standard from five aspects are put forward, it is aimed at existing flaws of the standard, including measurement procedures, evaluation method and discrimination criteria of the rock stress sensitivity. Terzaghi's effective stress was derived according to stress equation of Bane and Maximov, which indicates Terzaghi formula, is not only suitable for soil mechanics, but also for rock mechanics. An exponential model of Terzaghi effective stress and rock permeability was built from former researcher's formula of permeability modulus. The exponential model was used to fit experimental data, with a good accuracy under two different experiment conditions, constant confining pressure (constant-external pressure) and changing-pore-fluid pressure (changing-internal pressure), and constant-pore-fluid pressure (constant-internal pressure) and changing-confining pressure (changing-external pressure). Since constant-external pressure and changing-internal pressure accords with the actual oilfield development, it is recommended to be used in the laboratory operation. It is concluded through the study that the stress sensitivity of reservoir rock can be evaluated with stress sensitivity coefficient α of the exponential model, the stress sensitivity coefficient, 0.005 MPa−1

Plasma heating via adiabatic magnetic compression-expansion cycle

Heating of collisionless plasmas in closed adiabatic magnetic cycle comprising of a quasi static compression followed by a non quasi static constrained expansion against a constant external pressure is proposed. Thermodynamic constraints are derived to show that the plasma always gains heat in cycles having at least one non quasi static process. The turbulent relaxation of the plasma to the equilibrium state at the end of the non quasi static expansion is discussed and verified via 1D Particle in Cell (PIC) simulations. Applications of this scheme to heating plasmas in open configurations (mirror machines) and closed configurations (tokamak, reverse field pinche) are discussed.

On elastic-plastic collapse of subsea pipelines under external hydrostatic pressure and denting force

This paper presents analytical and numerical researches on the buckling or collapse of offshore pipelines under external hydrostatic pressure. Firstly the case of homogeneous ring model is investigated followed by a detailed study on corroded rings. The elastic-plastic collapse pressure could be treated as the least root of an elementary function. We prove that collapse pressure is a strictly increasing function of mode number in this paper and present some interesting structures of the roots. Partially corroded ring is parametrized by corrosion depth and angle extent. A comprehensive comparison shows that plasticity should not be neglected when the ring is thick-walled. Moreover, a study on large deflection deformation of 3D cylindrical shells quasi-statically dented under constant external pressure is carried out theoretically and numerically. The buckle propagation pressure is shown to be a meaningful value to normalize external pressure. This paper serves to enhance the understanding of destabilizing effect of external pressure mainly applicable and relevant to subsea offshore industry.

Problem of direct martensite transformation in a thick-walled cylinder made of shape memory alloy

The axisymmetric problem on the stress-strain state of a long thick-walled circular cylinder made of shape memory alloy experiencing the direct thermoelastic martensite phase transition under the action of constant internal and external pressures and a constant axial force is solved. The problem of limit loads in such a process is also considered.

An analytical model for the evolution of starless cores – I. The constant-mass case

We propose an analytical model for the quasistatic evolution of starless cores confined by a constant external pressure, assuming that cores are isothermal and obey a spherically-symmetric density distribution. We model core evolution for Plummer-like and Gaussian density distributions in the adiabatic and isothermal limits, assuming Larson-like dissipation of turbulence. We model the variation in the terms in the virial equation as a function of core characteristic radius, and determine whether cores are evolving toward virial equilibrium or gravitational collapse. We ignore accretion onto cores in the current study. We discuss the different behaviours predicted by the isothermal and adiabatic cases, and by our choice of index for the size-linewidth relation, and suggest a means of parameterising the magnetic energy term in the virial equation. We model the evolution of the set of cores observed by Pattle et al. (2015) in the L1688 region of Ophiuchus in the 'virial plane'. We find that not all virially-bound and pressure-confined cores will evolve to become gravitationally bound, with many instead contracting to virial equilibrium with their surroundings, and find an absence of gravitationally-dominated and virially-unbound cores. We hypothesise a 'starless core desert' in this quadrant of the virial plane, which may result from cores initially forming as pressure-confined objects. We conclude that a virially-bound and pressure-confined core will not necessarily evolve to become gravitationally bound, and thus cannot be considered prestellar. A core can only be definitively considered prestellar (collapsing to form an individual stellar system) if it is gravitationally unstable.

Dynamics and thermodynamics of decay in charged clusters

We propose a method for quantifying charge-driven instabilities in clusters, based on equilibrium simulations under confinement at constant external pressure. This approach makes no assumptions about the mode of decay and allows different clusters to be compared on an equal footing. A comprehensive survey of stability in model clusters of 309 Lennard-Jones particles augmented with Coulomb interactions is presented. We proceed to examine dynamic signatures of instability, finding that rate constants for ejection of charged particles increase smoothly as a function of total charge with no sudden changes. For clusters where many particles carry charge, ejection of individual charges competes with a fission process that leads to more symmetric division of the cluster into large fragments. The rate constants for fission depend much more sensitively on total charge than those for ejection of individual particles.

Two Level Model of Hydrogen Diffusion in Nanoporous Solids with Strongly Bound Adsorption States

A detailed understanding of hydrogen adsorption and diffusion in nanoporous solids is essential to design high-density storage materials. A two level lattice model with a dynamics of thermally activated molecular jumps, inspired by experimental evidence on cubic nitroprussides, is proposed for solids with strongly and weakly bound states coexisting in nanopores. The model provides a simple, unified theoretical framework to study both the thermodynamics and kinetics of hydrogen adsorption–desorption and to calculate, with minimum computational effort, quantities which can be directly compared to experimental results. In the linear and continuous limit, retarded diffusion is described by an integro-differential equation and a frequency-dependent diffusion coefficient. The transport diffusion coefficient shows a very strong loading dependence, with a high coverage behavior determined by intermolecular interactions. Desorption kinetics at constant external pressure and temperature is obtained from the analytical solution of the diffusion equation and compared to the results obtained from the numerical solution of nonlinear balance equations and from Monte Carlo simulations. The relevance of nonlinear, grain-size, and fluctuations effects is discussed. Fluctuations inside pores lead to fast, subdiffusive growth of desorption times with grain size in the nanometric range.

Molecular Dynamics at Constant Pressure: Allowing the System to Control Volume Fluctuations via a “Shell” Particle

Since most experimental observations are performed at constant temperature and pressure, the isothermal-isobaric (NPT) ensemble has been widely used in molecular simulations. Nevertheless, the NPT ensemble has only recently been placed on a rigorous foundation. The proper formulation of the NPT ensemble requires a “shell” particle to uniquely identify the volume of the system, thereby avoiding the redundant counting of configurations. Here, we review our recent work in incorporating a shell particle into molecular dynamics simulation algorithms to generate the correct NPT ensemble averages. Unlike previous methods, a piston of unknown mass is no longer needed to control the response time of the volume fluctuations. As the volume of the system is attached to the shell particle, the system itself now sets the time scales for volume and pressure fluctuations. Finally, we discuss a number of tests that ensure the equations of motion sample phase space correctly and consider the response time of the system to pressure changes with and without the shell particle. Overall, the shell particle algorithm is an effective simulation method for studying systems exposed to a constant external pressure and may provide an advantage over other existing constant pressure approaches when developing nonequilibrium molecular dynamics methods.

Proposition of an outflow boundary approach for carotid artery stenosis CFD simulation

The purpose of this study was to propose an innovative approach of setting outlet boundary conditions for the computational fluid dynamics (CFD) simulation of human common carotid arteries (CCAs) bifurcation based on the concept of energy loss minimisation at flow bifurcation. Comparisons between this new approach and previously reported boundary conditions were also made. The results showed that CFD simulation based on the proposed boundary conditions gave an accurate prediction of the critical stenosis ratio of carotid arteries (at around 65%). Other boundary conditions, such as the constant external pressure (P = 0) and constant outflow ratio, either overestimated or underestimated the critical stenosis ratio of carotid arteries. The patient-specific simulation results furthermore indicated that the calculated internal carotid artery flow ratio at CCA bifurcation (61%) coincided with the result obtained by clinical measurements through the use of Colour Doppler ultrasound.

Stability of high-Reynolds-number flow in a collapsible channel

AbstractWe study high-Reynolds-number flow in a two-dimensional collapsible channel in the asymptotic limit of wall deformations confined to the viscous boundary layer. The system is modelled using interactive boundary-layer equations for a Newtonian incompressible fluid coupled to the freely moving elastic wall under constant tension and external pressure. The deformation of the membrane is assumed to have small amplitude and long wavelength, whereas the flow comprises the inviscid core and the viscous boundary layers on both walls coupled to each other and to the membrane deformation. Firstly, by linking the interactive boundary-layer model to the small-amplitude, long-wavelength inviscid analysis, we conclude that the model is valid only when the pressure perturbations are fixed downstream from the wall indentation, contrary to the common assumption of classical boundary-layer theory. Next we explore possible steady states of the system, showing that a unique steady solution exists when the pressure is fixed precisely at the downstream end of the membrane, but there are multiple states possible if the pressure is specified further downstream. We examine the stability of these states by solving the generalized eigenvalue problem for perturbations to the nonlinear steady solutions and also by performing time integration of the full boundary-layer equations. Surprisingly, we find that no self-excited oscillations develop in the collapsible channel systems with finite-amplitude deformations. Instead, for each point in the parameter space, with the exception of points subject to numerical instabilities associated with the boundary-layer equations, exactly one of the steady states is predicted to be stable. We discuss these findings in relation to the results reported previously in the literature.

GRAVITATIONAL INSTABILITY OF ROTATING, PRESSURE-CONFINED, POLYTROPIC GAS DISKS WITH VERTICAL STRATIFICATION

We investigate gravitational instability (GI) of rotating, vertically-stratified, pressure-confined, polytropic gas disks using linear stability analysis as well as analytic approximations. The disks are initially in vertical hydrostatic equilibrium and bounded by a constant external pressure. We find that GI of a pressure-confined disk is in general a mixed mode of the conventional Jeans and distortional instabilities, and is thus an unstable version of acoustic-surface-gravity waves. The Jeans mode dominates in weakly confined disks or disks with rigid boundaries. When the disk has free boundaries and is strongly pressure-confined, on the other hand, the mixed GI is dominated by the distortional mode that is surface-gravity waves driven unstable under own gravity and thus incompressible. We demonstrate that the Jeans mode is gravity-modified acoustic waves rather than inertial waves and that inertial waves are almost unaffected by self-gravity. We derive an analytic expression for the effective sound speed c_eff of acoustic-surface-gravity waves. We also find expressions for the gravity reduction factors relative to a razor-thin counterpart, appropriate for the Jeans and distortional modes. The usual razor-thin dispersion relation after correcting for c_eff and the reduction factors closely matches the numerical results obtained by solving a full set of linearized equations. The effective sound speed generalizes the Toomre stability parameter of the Jeans mode to allow for the mixed GI of vertically-stratified, pressure-confined disks.

Mechanical Property Analysis of Two Kinds of Steel Wire Reinforces Composite Pips

Recently the steel wire reinforced composite pipelines are increasingly used in many different fields with researches usually focus on their mechanical properties. A constitutive model of the composite pipe's mechanical properties is presented, which can be used to solve the problems of stress, strain and displacements. After that, 3D CAD models of two kinds of steel wire reinforced composite pipes are built, and then the displacements and stress distribution of the pipes under constant internal and external pressure are analyzed by using FEA software, respectively. The differences between the mechanical properties of the two kinds of composite pipes are elaborated by comparing the displacements of the distribution curve along their corresponding paths. The results show that the braided steel wire reinforced composite pipes have better mechanical properties than the welded ones. That may be due to the crisscross that makes both the clockwise and counter-clockwise steel wires bear the same pressure.

Thermohydrodynamics of boiling in a van der Waals fluid

We present a modeling approach that enables numerical simulations of a boiling Van der Waals fluid based on the diffuse interface description. A boundary condition is implemented that allows in and out flux of mass at constant external pressure. In addition, a boundary condition for controlled wetting properties of the boiling surface is also proposed. We present isothermal verification cases for each element of our modeling approach. By using these two boundary conditions we are able to numerically access a system that contains the essential physics of the boiling process at microscopic scales. Evolution of bubbles under film boiling and nucleate boiling conditions are observed by varying boiling surface wettability. We observe flow patters around the three-phase contact line where the phase change is greatest. For a hydrophilic boiling surface, a complex flow pattern consistent with vapor recoil theory is observed.

Heat Transfer Augmentation of Parallel Flows by Means of Electric Conduction Phenomenon in Macro- and Microscales

Electrohydrodynamic conduction phenomenon takes advantage of the electrical Coulomb force exerted on a dielectric liquid generated by externally applied electric field and dissociated charges from electrolytes. The electric conduction phenomenon can be applied to enhance or control mass transport and heat transfer in both terrestrial and microgravity environments with advantages of simplicity and no degradation of fluid properties for isothermal as well as nonisothermal liquids. This paper numerically studies the heat transfer augmentation of externally driven macro- and microscale parallel flows by means of electric conduction phenomenon. The electric conduction is generated via electrode pairs embedded against the channel wall to mainly enhance the heat transfer and not necessarily to pump the liquid. Two cases of Poiseuille and Couette parallel flows are considered where for the former, a constant external pressure gradient is applied along the channel and for the latter, the channel wall moves with a constant velocity. The electric field and electric body force distributions along with the resultant velocity fields are presented. The heat transfer enhancements are illustrated under various operating conditions for both macro- and microscales.

THE EVOLUTION OF EXTRAGALACTIC RADIO SOURCES

A model for the evolution of low-luminosity radio galaxies is presented. In the model, the lobes inflated by low-power jets are assumed to expand in near pressure-balance against the external medium. Both cases of constant external pressure and decreasing external pressure are considered. Evolution of an individual source is described by the power-size track. The source appears as its lobe is inflated and radio luminosity increases to above the detection level; the source then moves along the track and eventually disappears as its luminosity drops below the detection limit. The power-size tracks are calculated including the combined energy losses due to synchrotron radiation, adiabatic expansion, and inverse Compton scattering. It is shown that in general, the constant-pressure model predicts an excess number of luminous, small-size sources while underpredicting large-size sources in the power-size diagram. The predicted spectra are steep for most sources, which is inconsistent with observations. By comparison, the pressure-limiting model fits observations better. In this model, low-luminosity sources undergo substantial expansion losses in the initial phase and as a result, it predicts fewer luminous, small-size sources. The resultant spectra are flat for most sources except for the oldest ones, which seems consistent with observations. The power-size tracks, in contrast to that of high-luminosity radio galaxies, are characterized by a slow increase in luminosity for most of the source's life, followed by a rapid decline when the synchrotron or inverse Compton scattering losses set in.