Zero Pressure Research Articles

CSF formation rate—a potential glymphatic flow parameter in hydrocephalus?

BackgroundStudies indicate that brain clearance via the glymphatic system is impaired in idiopathic normal pressure hydrocephalus (INPH). This has been suggested to result from reduced cerebrospinal fluid (CSF) turnover, which could be caused by a reduced CSF formation rate. The aim of this study was to determine the formation rate of CSF in a cohort of patients investigated for INPH and compare this to a historical control cohort.MethodsCSF formation rate was estimated in 135 (75 ± 6 years old, 64/71 men/women) patients undergoing investigation for INPH. A semiautomatic CSF infusion investigation (via lumbar puncture) was performed. CSF formation rate was assessed by downregulating and steadily maintaining CSF pressure at a zero level. During the last 10 min, the required outflow to maintain zero pressure, i.e., CSF formation rate, was continuously measured. The values were compared to those of a historical reference cohort from a study by Ekstedt in 1978.ResultsMean CSF formation rate was 0.45 ± 0.15 ml/min (N = 135), equivalent to 27 ± 9 ml/hour. There was no difference in the mean (p = 0.362) or variance (p = 0.498) of CSF formation rate between the subjects that were diagnosed as INPH (N = 86) and those who were not (N = 43). The CSF formation rate in INPH was statistically higher than in the reference cohort (0.46 ± 0.15 vs. 0.40 ± 0.08 ml/min, p = 0.005), but the small difference was probably not physiologically relevant. There was no correlation between CSF formation rate and baseline CSF pressure (r = 0.136, p = 0.115, N = 135) or age (-0.02, p = 0.803, N = 135).ConclusionsThe average CSF formation rate in INPH was not decreased compared to the healthy reference cohort, which does not support reduced CSF turnover. This emphasizes the need to further investigate the source and routes of the flow in the glymphatic system and the cause of the suggested impaired glymphatic clearance in INPH.

SunbYte: an autonomous pointing framework for low-cost robotic solar telescopes on high altitude balloons

The design and usability of a fully autonomous robotic control system (SunbYte - Sheffield University Balloon “lYfted” TElescope) for solar tracking and observational applications onboard high-altitude balloons are addressed here. The design is based on a six-step development plan balancing scientific objectives and practical engineering requirements. The high-altitude solar observational system includes low-cost components such as a Cassegrain-type telescope, stepper motors, harmonic drives, USB cameras and microprocessors. OpenCV installed from ROS (Robotic Operating System), python and C facilitated the collection, compression, and processing of housekeeping and scientific data. This processed data was then transmitted to the ground station through the launch vehicle’s telecommunication link. The SunbYte system allows the brightest spot in the sky, the sun, to be identified, and a telescope pointed towards it with high enough accuracy that a scientific camera can capture images. This paper gathers and presents the results from primarily two missions with the High-Altitude Student Platform (HASP, NASA Balloon Program office and LaSpace). Additionally, a discussion will be made comparing these with an earlier iteration flown with the German-Swedish “REXUS/BEXUS” programme coordinated by the European Space Agency. By capturing and analysing a series of tracking images with the location of the Sun at the calibrated centre, the system demonstrated the tracking capabilities on an unstable balloon during ascent. Housekeeping sensor data was collected to further analyse the thermal and mechanical performance. The low temperature increased friction in the drive train and reduced the responsiveness of the harmonic drive actuation system. This caused some issues which require further work in future missions, for example, with SunbYte 4 and its work when flying with the HEMERA ZPB (Zero Pressure Balloon) program.

Large and Pressure-Dependent c-Axis Piezoresistivity of Highly Oriented Pyrolytic Graphite near Zero Pressure.

The c-axis piezoresistivity is a fundamental and important parameter of graphite, but its value near zero pressure has not been well determined. Herein, a new method for studying the c-axis piezoresistivity of van der Waals materials near zero pressure is developed on the basis of in situ scanning electron microscopy and finite element simulation. The c-axis piezoresistivity of microscale highly oriented pyrolytic graphite (HOPG) is found to show a large value of 5.68 × 10-5 kPa-1 near zero pressure and decreases by 2 orders of magnitude to the established value of ∼10-7 kPa-1 when the pressure increases to 200 MPa. By modulating the serial tunneling barrier model on the basis of the stacking faults, we describe the c-axis electrical transport of HOPG under compression. The large c-axis piezoresistivity near zero pressure and its large decrease in magnitude with pressure are attributed to the rapid stiffening of the electromechanical properties under compression.

Deterministic engines extending Helmholtz thermodynamics

Helmholtz formalism provides a tantalizing interpretation of classical thermodynamics, based on time integrals of purely mechanical quantities and without need of statistical description. Here we extend this approach to include heat flux and pressure at the walls to enable it to describe actual thermodynamic transformations, such as isothermal compressions and expansions. The presence of hard walls, which gives rise to non zero pressure, is justified by means of the virial theorem, while the heat fluxes are introduced as quasi-static limits of suitably thermostatted Hamiltonians. Particular attention is paid to generalizing the minimalist cases of the harmonic oscillator and elastic bouncer, which afford clear physical interpretations. With such extensions, a complete picture of thermodynamics emerges, amenable to phase transitions and cyclic deterministic transformations, which produce mechanical work from heat, like the Carnot cycle.

Pressure-induced phase transformation and mechanical stability of HfAl2

Utilizing the particle swarm optimization algorithm in conjunction with first-principles calculations, we systematically investigated the structural evolution and mechanical properties of HfAl2 up to 300 GPa at 0 K. Our findings indicate that HfAl2 adopts the MgZn2-type structure (hP12-HfAl2) with space group P63/mmc under zero pressure, and a phase transition to oC12-HfAl2 occurs at approximately 260 GPa. This transition, characterized by a volume collapse, is identified as a first-order phase transition. Phonon dispersion curves provide supporting evidence for the structural stability of these two phases. Simultaneously, we conducted a comprehensive assessment of the elastic constants and modulus of these two phases under both zero and high pressure. The research findings substantially will enhance our understanding of the structural evolution in HfAl2 under high pressure.

Theoretical and experimental study on improving diaphragm compressor design for hydrogen refueling stations through use of a free moving oil piston concept

The diaphragm compressor is the essential component of a hydrogen refueling station, but the traditional design method results in low cavity volumetric efficiency and high refilling oil pressure. This study proposes a design method using a free moving oil piston to optimize the cavity volume. This method allows the cavity volume to be utilized to the maximum, and a “zero pressure” stage occurs at the end of the suction process, which greatly improves the cavity volumetric efficiency and reduces the refilling oil pressure. To further examine and verify the superiority of this method, 90 MPa and 200 MPa diaphragm compressors were designed and produced for comparison, and their simulation study was conducted simultaneously. The results demonstrate that this proposed design method can raise the cavity volumetric efficiency of the 90 MPa diaphragm compressor in the experimental case from 0.37 to 0.66. Additionally, the benefit increases with the compressor pressure. For the case of a 200 MPa diaphragm compressor, the cavity volumetric efficiency increases from 0.19 to 0.64.

High D2/H2 selectivity performance in MOF-303 under ambient pressure for potential industrial applications

The commercial demand for D2 is poised to increase significantly; however, the low natural abundance and the energy- and capital-intensive industrial separation (i.e., 24 K cryogenic distillation) will hamper future scientific and industrial growth in isotopologue separation. Alternatively, kinetic quantum sieving (KQS)-based adsorptive D2 separation has been proposed recently, but the separation performance is reported mostly at near zero pressure or in the sub-few ten mbar range. Herein, an Al-based Metal-Organic Framework, MOF-303, with 1-D narrow-micro pores is studied for D2/H2 adsorptive separation at ambient pressure. Cryogenic thermal desorption spectroscopic analysis of MOF-303 confirmed that the synergetic effect of binding affinity & enhanced KQS (owing to molecular rearrangement of D2 adsorbed phase at high pressure induced by strong D2 confinement), along with D2 partial condensation, leads to a significant increase in the D2 uptake with increasing exposure pressure up to 1,000 mbar. Consequently, a remarkable selectivity of 21.6 at 25 K has been achieved even at an operating pressure of 1000 mbar, which is an industry-friendly condition. The observed D2/H2 separation selectivity is about ten times higher than that of the industrial cryogenic method (best selectivity of below 2.5 at 24 K), and comparable to the performance of the adsorbent materials already reported with low operating pressure, making adsorptive D2/H2 separation through MOF-303 an alternative for cryogenic industrial isotopologue separation.

Bench Assessment of Work of Breathing During a Spontaneous Breathing Trial on Zero Pressure Support and Zero PEEP Compared to T-Piece.

Analysis of observational data suggests that both a T-piece and zero pressure support ventilation (PSV) and zero PEEP impose work of breathing (WOB) during a spontaneous breathing trial (SBT) similar to what a patient experiences after extubation. The aim of our study was to compare the WOB imposed by the T-piece with zero PSV and zero PEEP. We also compared the difference in WOB when using zero PSV and zero PEEP on 3 different ventilators. This study was conducted by using a breathing simulator that simulated 3 lung models (ie, normal, moderate ARDS, and COPD). Three ventilators were used and set to zero PSV and zero PEEP. The outcome variable was WOB expressed as mJ/L of tidal volume. An analysis of variance showed that WOB was statistically different between the T-piece and zero PSV and zero PEEP on all the ventilators (Servo-i, Servo-u, and Carescape R860). The absolute difference was lowest for the Carescape R860, which increased WOB by 5-6%, whereas the highest for Servo-u, which reduced the WOB by 15-21%. Work may be imposed or reduced during spontaneous breathing on zero PSV and zero PEEP when compared to T-piece. The unpredictable nature of how zero PSV and zero PEEP behaves on different ventilators makes it an imprecise SBT modality in the context of assessing extubation readiness.

Velocity spectra and scale decomposition of adverse pressure gradient turbulent boundary layers considering history effects

Experiments at relatively large Reynolds number were conducted to better understand the influence of an adverse pressure gradient on the turbulence in two-dimensional boundary layer flows. One focus is on documenting the scale contributions to the Reynolds normal and shear stress profiles under the influence of a positive streamwise pressure gradient. Toward these aims, velocity spectra at friction Reynolds numbers (7100≤Reτ≤8600) are presented, analyzed, and compared to a zero pressure gradient case from the same facility at nominally matching Reynolds number. Distance-from-the-wall scaling is central to numerous theoretical and modeling considerations of the canonical flat plate flow. Consistently, the present spectral measurements reveal clear evidence for its preservation on the inertial sublayer under modest adverse pressure gradients. Increases are seen in the viscous-scaled premultiplied spectra in the outer region as the pressure gradient increases — commensurate with the increases observed in the outer peaks of the streamwise and wall-normal velocity variances, as well as the Reynolds shear stress. Effects of varying Reτ on the streamwise and wall-normal velocity variances and the Reynolds shear stress are studied by comparing the present data at varying Clauser pressure gradient parameter, β, with previous experiments having similarly varying β at significantly lower Reτ (≈1900). This allows to study the effects of changing Reynolds number at matched β. Cases at similar β and Reynolds number, but with varying flow history are also examined, wherein the same β value is arrived either by starting from a smaller or larger initial β. The effects of β>0 on the large and small scale motions are investigated by segregating the signal contributions according to a cut-off wavelength established in previous zero pressure gradient flows. Under a normalization that uses a hybrid velocity scale this analysis reveals similarity between the outer regions of the spectrally-decomposed variances and Reynolds shear stress. These analyses also reveal a more dramatic reduction in the near-wall large scale contribution to the Reynolds shear stress gradient in the β>0 flow when compared to the β=0 flow.

An experimental and analytical study of wind turbine wakes under pressure gradient

This work is dedicated to the systematic investigation of wind turbine wakes under the effect of pressure gradients. Wind tunnel experiments are carried out with a wind turbine positioned on straight ramps of increasing angle such that it experiences an approximately linear flow speed-up/slow-down from the induction region into the far wake. Fifteen ramp angles are studied: 7 favorable (FPG), 7 adverse (APG), and 1 zero pressure gradient. The wake center is shown to follow the base flow streamline originating from a virtual turbine hub height. A quasi-linear relationship between the pressure gradient and near wake length is demonstrated. Far wake characteristics, such as the recovery of the wake center velocity deficit and wake growth rate, are observed to systematically vary with the pressure gradient. The wake recovery rate increases (decreases) with the increase in the FPG (APG), and the wake growth rate shows a linear increase from most favorable to most adverse pressure gradient. The turbine power coefficient decreases significantly with increasing APG to a greater degree than the increase in power coefficient under FPG. The engineering approach of superposing the wake deficit predicted by the standard Gaussian model on the modified base flow is shown to work for very moderate pressure gradients. In light of this, a threshold in terms of flow speed-up/slow-down along the wake trajectory is established, below which the engineering approach can be reasonably employed. Finally, a physics-based model for wakes under the pressure gradient is tested. A new theoretical relation for near wake length under the pressure gradient is proposed. Using the theoretical near wake length, the pressure gradient model predicts the turbine wakes for all cases with good accuracy and shows a significant improvement from the engineering approach.

Comparison of smooth- and rough-wall non-equilibrium boundary layers with favourable and adverse pressure gradients

Measurements were made in rough-wall boundary layers subject to favourable, zero and adverse pressure gradients. Profiles of mean velocity and turbulence quantities were acquired and velocity fields were measured in multiple planes to document flow structure. Comparisons were made to equivalent smooth-wall cases with the same free stream velocity distributions. Outer layer similarity was observed between the rough- and smooth-wall cases in all quantities in the favourable and zero pressure gradient regions, but large differences were observed with adverse pressure gradients. In both the smooth- and rough-wall cases, the favourable pressure gradient reduced the turbulence in the boundary layer, and increased the size of turbulence structures relative to the boundary layer thickness in both the streamwise and spanwise directions, while lowering their inclination angle with respect to the wall. When the boundary layer was returned to a zero pressure gradient following the favourable pressure gradient region, the turbulence level and the size and inclination of the structures returned to their canonical zero pressure gradient condition. The response of the boundary layer was somewhat faster in the rough-wall case, causing it to reach equilibrium in a shorter streamwise distance after the changes in pressure gradient than in the smooth-wall case. The adverse pressure gradient increased turbulence levels relative to the wall friction velocity, reduced the size of turbulence structures relative to the boundary layer thickness and increased their inclination angle. The changes with the adverse pressure gradient were significantly larger with the rough wall than the smooth. The results suggest that similarity might be achieved with adverse pressure gradients if smooth- and rough-wall cases with the same Clauser pressure gradient parameter history are compared.

Is isochoric vitrification feasible?

This study investigates the feasibility of ice-free isochoric vitrification for cryopreservation applications using mathematical modeling, computation tools, and the underlying principles of thermo-mechanics. This study is triggered by an increasing interest in the possibility of isochoric vitrification, following promising experimental results of isochoric cryopreservation. In general, isochoric cryopreservation is the preservation of biological materials in subzero temperatures in a rigid-sealed container, where some ice crystallization creates favorable pressure elevation due to the anomaly of water expansion upon ice Ih formation. Vitrification on the other hand is the transformation of liquid into an amorphous solid in the absence of any crystals, which is typically achieved by rapid cooling of a highly viscous solution. The current study presents a mathematical model for vitrification under variable pressure conditions, building upon a recently published thermo-mechanics modeling approach for isochoric cryopreservation. Using the physical properties of dimethyl sulfoxide (DMSO) as a representative cryoprotective agent (CPA), this study suggests that vitrification under isochoric conditions is not feasible, essentially since the CPA solution contracts more than the isochoric chamber by an order of magnitude. This differential contraction can lead to absolute zero pressure in the isochoric chamber, counteracting the premise of the isochoric cryopreservation process. It is concluded that the only alternative to prevent ice formation while benefiting from the potential advantages of higher pressures is to create the required pressures by external means, and not merely by passively enclosing the specimen in an isochoric chamber.

A Bioinspired Capillary Force‐Induced Driving Strategy for Constructing Ultra‐Low‐Pressure Separation Membranes

AbstractThe development of low‐pressure or pressureless self‐driven membranes is important for saving energy and overcoming the critical trade‐off effect in membrane separation processes. However, conventional self‐driven membranes rely on gravity, which is effective in the separation of large‐sized materials but is still ineffective in the fine separation of small molecules. Herein, inspired by the capillary effect that exists in nature, a capillary force‐induced membrane‐driving strategy for fine separation at ultra‐low pressures is demonstrated. Hydrophilic nanoparticles are prepared by a cross‐linking reaction between tannic acid and 3‐aminopropyltriethoxysilane and then introduce them into membrane pores to simulate sand accumulation with an aim to generate the capillary force. The membrane is then used in ultra‐low pressure membrane separation. Interestingly, it is found that the membrane has excellent performance in the separation of dye/salt mixtures (dye rejection > 99%, salt rejection < 10%) and a high permeate flux (160 L m−2 h−1) under near “zero pressure” conditions. Moreover, the structural stability of the membrane is verified. Introducing capillary forces into membranes as an autonomous driving force can be a promising universal approach that can be added up to the toolbox for the efficient preparation of separation membranes.

Weak asymptotic solution of one dimensional zero pressure dynamics system in the quarter plane

AbstractIn this paper we study a system of equations which appear in the modelling of many physical phenomena. Initially this system appeared in description of the large scale structure formation. Recently it is derived as a second order queueing model. We construct weakly asymptotic solutions of the initial boundary value problem for the system and interaction of waves in the quarter plane with boundary Riemann solution centered at (0,0) and Riemann solution centered at a point .

The mechanical, dynamical, thermodynamical properties and elastic anisotropies of cubic YbAu compound under pressure

The structural, elastic, vibrational, electronic and thermodynamic properties of thermodynamically important YbAu in B2 structure under pressure have been investigated by performing first principles method using DFT. The calculated lattice parameter and equilibrium volume are in good agreement with the experimental values. Zero pressure elastic constants and pressure-dependent elastic constants are positive up to 30 GPa. The resulting elastic constants are stable mechanically as they meet the Born stability condition, which makes the YbAu compound stable in nature. B/G ratio shows that the YbAu has a brittle nature while Poisson’s ratio (ν) values show that an ionic bond nature. The presented phonon dispersion curves and one-phonon DOS also confirms that this compound is stable dynamically. The electronic structure and metallic nature have been analysed quantitatively from band structure and density of states. To better illustrate the nature of bonding and charge transfer, we have studied the charge density plots and mulliken charge analysis. Also, the thermodynamic properties have been investigated under temperature and high pressure. The pressure effects on the volume of YbAu are found to be greater than the temperature effects. • Pressure effect on the structural, elastic, vibrational and electronic properties of YbAu compound was investigated using DFT. • Electronic band structure and density of state calculations revealed that YbAu is metallic in nature. • The elastic anisotropy was visualized in detail by plotting the directional dependence. • The thermodynamic properties were investigated under temperature and high pressure.

Concentration in vanishing adiabatic exponent limit of solutions to the Aw–Rascle traffic model

In this paper, we study the phenomenon of concentration and the formation of delta shock wave in vanishing adiabatic exponent limit of Riemann solutions to the Aw–Rascle traffic model. It is proved that as the adiabatic exponent vanishes, the limit of solutions tends to a special delta-shock rather than the classical one to the zero pressure gas dynamics. In order to further study this problem, we consider a perturbed Aw–Rascle model and proceed to investigate the limits of solutions. We rigorously proved that, as the adiabatic exponent tends to one, any Riemann solution containing two shock waves tends to a delta-shock to the zero pressure gas dynamics in the distribution sense. Moreover, some representative numerical simulations are exhibited to confirm the theoretical analysis.

1-hour t-piece spontaneous breathing trial vs 1-hour zero pressure support spontaneous breathing trial and reintubation at day 7: A non-inferiority approach

Physiological data suggest that T-piece and zero pressure support (PS0) ventilation both accurately reflect spontaneous breathing conditions after extubation. These two types of spontaneous breathing trials (SBTs) are used in our Intensive Care Unit to evaluate patients for extubation readiness and success but have rarely been compared in clinical studies. We performed a prospective observational study to confirm the hypothesis that 1-hour T-piece SBT and 1-h PS0 zero PEEP (ZEEP) SBT are associated with similar rates of reintubation at day 7 after extubation. A non-inferiority approach was used for sample size calculation. The cohort consisted of 529 subjects invasively ventilated for more than 24h and extubated after successful 1-hour T-piece SBT (n=303, 57%) or 1-h PS0 ZEEP SBT (n=226, 43%). The reintubation rate at day 7 was 14.6% with PS0 ZEEP and 17.5% with T-piece (difference-2.6% [95% confidence interval, -8.3% to 4.3%]; p=0.40). The reasons for reintubation did not differ significantly when compared between patients with 1-h PS0 ZEEP SBT and patients with 1-hour T-piece SBT. Our results suggest that successful 1-hour T-piece and 1-h PSO ZEEP SBTs are associated with similar reintubation rates at day 7.

A universal velocity profile for turbulent wall flows including adverse pressure gradient boundary layers

A recently developed mixing length model of the turbulent shear stress in pipe flow is used to solve the streamwise momentum equation for fully developed channel flow. The solution for the velocity profile takes the form of an integral that is uniformly valid from the wall to the channel centreline at all Reynolds numbers from zero to infinity. The universal velocity profile accurately approximates channel flow direct numerical simulation (DNS) data taken from several sources. The universal velocity profile also provides a remarkably accurate fit to simulated and experimental flat plate turbulent boundary layer data including zero and adverse pressure gradient data. The mixing length model has five free parameters that are selected through an optimization process to provide an accurate fit to data in the range $R_\tau = 550$ to $R_\tau = 17\,207$ . Because the velocity profile is directly related to the Reynolds shear stress, certain statistical properties of the flow can be studied such as turbulent kinetic energy production. The examples presented here include numerically simulated channel flow data from $R_\tau = 550$ to $R_\tau =8016$ , zero pressure gradient (ZPG) boundary layer simulations from $R_\tau =1343$ to $R_\tau = 2571$ , zero pressure gradient turbulent boundary layer experimental data between $R_\tau = 2109$ and $R_\tau = 17\,207$ , and adverse pressure gradient boundary layer data in the range $R_\tau = 912$ to $R_\tau = 3587$ . An important finding is that the model parameters that characterize the near-wall flow do not depend on the pressure gradient. It is suggested that the new velocity profile provides a useful replacement for the classical wall-wake formulation.

Accurate Assessment of Joule-Thomson Inversion Temperature at Zero Pressure of Imperfect Gases

Background: The aim of this work is to propose an approach for estimating the Joule-Thomson coefficient as an important parameter necessary to the study of changes in fluid temperature at a given change in pressure at constant enthalpy. Objective: The analytical approach presented in this work is very appropriate for detailed studies of the Joule-Thomson inversion temperature at zero pressure for arbitrary temperature values. Methods: A new approach is suggested for the accurate determination of the Joule- Thomson inversion temperature at zero pressure using virial coefficient of the Lennard- Jones (12-6) potential. Results: The usefulness and efficiency of the method are tested by application to various gases Ar, He, Ne, H2, O2, CO2 CO, CH4, Xe, Kr, N2 and Air. The results obtained are in well agreement with other approximation and experimental data. Conclusion: The suggested formula enables correct and rapid calculation of the JT inversion temperature at zero pressure.

Singular Nonlinear Problems for Self-Similar Solutions of Boundary-Layer Equations with Zero Pressure Gradient: Analysis and Numerical Solution

Singular Nonlinear Problems for Self-Similar Solutions of Boundary-Layer Equations with Zero Pressure Gradient: Analysis and Numerical Solution