Showing papers on "Pressure gradient published in 2019"
TL;DR: The internal average energy loss caused by entropy generation for steady mixed convective Poiseuille flow of a nanofluid, suspended with titanium dioxide particles in water, and passed through a wavy channel, was investigated.
Abstract: The internal average energy loss caused by entropy generation for steady mixed convective Poiseuille flow of a nanofluid, suspended with titanium dioxide (TiO2) particles in water, and passed through a wavy channel, was investigated. The models of thermal conductivity and viscosity of titanium dioxide of 21 nm size particles with a volume concentration of temperature ranging from 15 °C to 35 °C were utilized. The characteristics of the working fluid were dependent on electro-magnetohydrodynamics (EMHD) and thermal radiation. The governing equations were first modified by taking long wavelength approximations, which were then solved by a homotopy technique, whereas for numerical computation, the software package BVPh 2.0 was utilized. The results for the leading parameters, such as the electric field, the volume fraction of nanoparticles and radiation parameters for three different temperatures scenarios were examined graphically. The minimum energy loss at the center of the wavy channel due to the increase in the electric field parameter was noted. However, a rise in entropy was observed due to the change in the pressure gradient from low to high.
93 citations
TL;DR: In this paper, the authors use streaming instability simulations that include particle self-gravity to study how the planetesimal initial mass function depends on the radial pressure gradient, and they find a single value $p \approx 1.6$ describes simulations in which the pressure gradient varies by approximately 1.3.
Abstract: The streaming instability concentrates solid particles in protoplanetary disks, leading to gravitational collapse into planetesimals. Despite its key role in producing particle clumping and determining critical length scales in the instability's linear regime, the influence of the disk's radial pressure gradient on planetesimal properties has not been examined in detail. Here, we use streaming instability simulations that include particle self-gravity to study how the planetesimal initial mass function depends on the radial pressure gradient. Fitting our results to a power-law, ${\rm d}N / {\rm d}M_p \propto M_p^{-p}$, we find a single value $p \approx 1.6$ describes simulations in which the pressure gradient varies by $\gtrsim 2$. An exponentially truncated power-law provides a significantly better fit, with a low mass slope of $p^\prime \approx 1.3$ that weakly depends on the pressure gradient. The characteristic truncation mass is found to be $\sim M_G = 4 \pi^5 G^2 \Sigma_p^3 / \Omega^4$. We exclude the cubic dependence of the characteristic mass with pressure gradient suggested by linear considerations, finding instead a linear scaling. These results strengthen the case for a streaming-derived initial mass function that depends at most weakly on the aerodynamic properties of the disk and participating solids. A simulation initialized with zero pressure gradient---which is {\em not} subject to the streaming instability---also yields a top-heavy mass function but with modest evidence for a different shape. We discuss the consistency of the theoretically predicted mass function with observations of Kuiper Belt planetesimals, and describe implications for models of early stage planet formation..
75 citations
TL;DR: Pressure gradients underlying CSF flow in the cerebral aqueduct are dominated by cardiac pulsations, but induce CSFflow volumes dominated by respiration, which leads to significant differences in pressure gradient pulsations in the sleeping versus awake state.
Abstract: Current theories suggest that waste solutes are cleared from the brain via cerebrospinal fluid (CSF) flow, driven by pressure pulsations of possibly both cardiac and respiratory origin. In this study, we explored the importance of respiratory versus cardiac pressure gradients for CSF flow within one of the main conduits of the brain, the cerebral aqueduct. We obtained overnight intracranial pressure measurements from two different locations in 10 idiopathic normal pressure hydrocephalus (iNPH) patients. The resulting pressure gradients were analyzed with respect to cardiac and respiratory frequencies and amplitudes (182,000 cardiac and 48,000 respiratory cycles). Pressure gradients were used to compute CSF flow in simplified and patient-specific models of the aqueduct. The average ratio between cardiac over respiratory flow volume was 0.21 ± 0.09, even though the corresponding ratio between the pressure gradient amplitudes was 2.85 ± 1.06. The cardiac cycle was 0.25 ± 0.04 times the length of the respiratory cycle, allowing the respiratory pressure gradient to build considerable momentum despite its small magnitude. No significant differences in pressure gradient pulsations were found in the sleeping versus awake state. Pressure gradients underlying CSF flow in the cerebral aqueduct are dominated by cardiac pulsations, but induce CSF flow volumes dominated by respiration.
71 citations
TL;DR: In this article, a series of water flow tests through both intact and fractured samples were conducted in a triaxial cell under different confining pressures (10-30 MPa) and varying inlet hydraulic pressures (0.4-6 MPa).
Abstract: This study focuses on the transport properties and permeability evolution characteristics of fluid flow through thermally treated rock samples containing single fractures. First, splitting fractures were generated in cylindrical granite samples after high-temperature exposure (25–800 °C). Then a series of water flow tests through both intact and fractured samples were conducted in a triaxial cell under different confining pressures (10–30 MPa) and varying inlet hydraulic pressures (0.4–6 MPa). The results show that as the temperature increases from 25 to 800 °C, the standard deviations of the 3D spatial distribution parameters, including the asperity height, slope angle, and aspect direction of the fracture surface mesh element planes, all increase, indicating gradually increasing fracture surface roughness. The relationships between the pressure gradient and flow rate of intact samples, fractured rock samples, and the fractures themselves can all be well fitted using the Forchheimer’s law. Both linear and nonlinear coefficients in the Forchheimer’s law increase with increasing confining pressure. An exponential function is used to evaluate the equivalent permeability of intact samples based on temperature levels. The permeability undergoes an increasing trend as the temperature increases due to thermally induced defects, but undergoes a decreasing trend as the confining pressure increases due to defect closure. Two representative types of flow characteristics through the fractured rock samples, dominated by either the rock matrix or fracture flow, are identified. In the temperature range of 25–800 °C, the critical Reynolds number of the fractures declines, which first remains generally constant for temperatures of 25–400 °C and then experiences a dramatic decrease for temperatures of 400–800 °C. The nonlinear coefficient bf in Forchheimer’s law versus the hydraulic aperture eh curves displays a decreasing trend following a power-law relationship. The Forchheimer’s law results are evaluated by plotting the normalized transmissivity against the pressure gradient. An increase in the confining pressure shifts the fitted curves downward. As the temperature increases, the contribution of the matrix to the overall discharge capacity of the fractured rock samples gradually enhances, while that for the fractures weakens. The reduction extent in permeability of the rough-walled fractures is more remarkable than that of the matrix under an applied confining pressure.
70 citations
TL;DR: In this paper, the authors investigated the evolution of coal permeability under different flow regimes and showed that the relative contribution of the slip flow to total flow increases with the reduction in pore pressure, increasing from 0.02 to 0.18.
67 citations
TL;DR: In this article, a self-consistent physical mechanism is presented to explain the emergence of the liquid jet as a consequence of the collapse of the gas cavity driven by the low capillary pressures that appear suddenly around its base when the cap, the thin film separating the bubble from the ambient gas, pinches.
Abstract: Here we provide a theoretical framework describing the generation of the fast jet ejected vertically out of a liquid when a bubble, resting on a liquid–gas interface, bursts. The self-consistent physical mechanism presented here explains the emergence of the liquid jet as a consequence of the collapse of the gas cavity driven by the low capillary pressures that appear suddenly around its base when the cap, the thin film separating the bubble from the ambient gas, pinches. The resulting pressure gradient deforms the bubble which, at the moment of jet ejection, adopts the shape of a truncated cone. The dynamics near the lower base of the cone, and thus the jet ejection process, is determined by the wavelength , the jet is ejected after a bubble is pinched off; in this regime, viscosity delays the formation of the jet, which is thereafter emitted at a velocity which is inversely proportional to the liquid viscosity.
64 citations
TL;DR: In this paper, a multi-linear transient pressure model of multistage fractured horizontal well with considering low-velocity non-Darcy flow and stress sensitivity is established, which is verified by comparing the result of proposed model with that of Ekaterina Stalagorova's model.
60 citations
TL;DR: In this article, a constitutive relation between fluid power flux and pressure gradient was discussed, and a conservative equation of fluid mechanical energy was introduced to describe convectional phenomena that are essential to convective heat and mass transfer.
57 citations
TL;DR: In this paper, a mathematical model is developed to investigate the electroosmotic flow of pseudoplastic aqueous nanoliquids in microchannel, where a tangent hyperbolic fluid model is employed to describe the rheological behavior of the pseudplastic fluid and analytical solutions for potential distribution, temperature and nanoparticle fraction are derived and perturbation solution for stream function, pressure gradient and volumetric flow rate are obtained.
Abstract: The study of electroosmotic flow of biorheological fluids has been employed in the advancement of diversified biomicrofluidics systems. To explore more in this field, a mathematical model is developed to investigate the electroosmotic flow of pseudoplastic aqueous nanoliquids in microchannel. A tangent hyperbolic fluid model is employed to describe the rheological behavior of the pseudoplastic fluid. Here, analytical solutions for potential distribution, temperature and nanoparticle fraction are derived and perturbation solution for stream function, pressure gradient and volumetric flow rate are obtained. The convective boundary condition is applied on the channel walls. The authentic assumptions of Debye–Huckel linearization, long wavelength and small Reynold’s number are employed in the dimensional conservative equations. The influences of various emerging parameters are graphically computed for axial velocity, pressure gradient, thermal temperature, nanoparticle volume fraction, skin friction coefficient and Nusselt profiles. To observe the thermal radiation effects, a thermal radiative flux model is also deployed. It is noticed that the heat transfer Biot number increases with increasing thermal temperature; however, a reversed behavior is reported for the nanoparticle volume fraction. Therefore, the present model does not only provide a deep theoretical insight to interpret the electroosmotic flow systems, but it will also be applicable in designing the emerging tool for biomicrofluidic devices/systems under peristalsis mechanisms.
54 citations
TL;DR: A mathematical model is developed to describe the blood flow inside a curved microvessels driven by electroosmosis and results indicate that pressure gradient and wall shear stress reduce with increasing the curvature effects however the trapping is more with high curvature of the microvessel.
52 citations
TL;DR: In this article, the peristaltic pumping characteristics and the trapping of fluid bolus have also been thoroughly examined in the light of electroosmosis, and the authors showed that even mild electroo-mosmosis can cause higher pressure gradient in the axial direction.
TL;DR: In this article, the authors investigated motility-induced phase separation of active Brownian particles, which are modeled as purely repulsive spheres that move due to a constant swim force with freely diffusing orientation.
Abstract: We investigate motility-induced phase separation of active Brownian particles, which are modeled as purely repulsive spheres that move due to a constant swim force with freely diffusing orientation. We develop on the basis of power functional concepts an analytical theory for nonequilibrium phase coexistence and interfacial structure. Theoretical predictions are validated against Brownian dynamics computer simulations. We show that the internal one-body force field has four nonequilibrium contributions: (i) isotropic drag and (ii) interfacial drag forces against the forward motion, (iii) a superadiabatic spherical pressure gradient, and (iv) the quiet life gradient force. The intrinsic spherical pressure is balanced by the swim pressure, which arises from the polarization of the free interface. The quiet life force opposes the adiabatic force, which is due to the inhomogeneous density distribution. The balance of quiet life and adiabatic forces determines bulk coexistence via equality of two bulk state functions, which are independent of interfacial contributions. The internal force fields are kinematic functionals which depend on density and current but are independent of external and swim forces, consistent with power functional theory. The phase transition originates from nonequilibrium repulsion, with the agile gas being more repulsive than the quiet liquid.
TL;DR: In this article, time-resolved planar and volumetric flow field measurements were performed in the near-wall and log-law region of an adverse pressure gradient turbulent boundary layer following a zero pressure gradient chaotic boundary layer at a friction Reynolds number and to relate them with well known coherent flow motions near the wall.
Abstract: With the aim to characterize the near-wall flow structures and their interaction with large-scale motions in the log-law region, time-resolved planar and volumetric flow field measurements were performed in the near-wall and log-law region of an adverse pressure gradient turbulent boundary layer following a zero pressure gradient turbulent boundary layer at a friction Reynolds number and to relate them with well known coherent flow motions near the wall. The space–time results confirm that the turbulent superstructures have a strong impact even on the very near-wall flow motion and also their alternating appearance in time and intensity could be quantified over long time sequences. Using the time record of the velocity field, rare localized separation events appearing in the viscous sublayer were also analysed. By means of volumetric particle tracking velocimetry their three-dimensional topology and dynamics could be resolved. Based on the results, a conceptual model was deduced that explains their rare occurrence, topology and dynamics by means of a complex interaction process between low-momentum turbulent superstructures, near-wall low-speed streaks and tilted longitudinal and spanwise vortices located in the near-wall region.
TL;DR: In this paper, the effect of wall slip conditions, porous media, and heat transfer on peristaltic inflow of MHD Newtonian fluid with suspended particles in a catheterized tube has been studied with long-wavelength and low-Reynolds number approximations.
Abstract: The effect of wall slip conditions, porous media, and heat transfer on peristaltic inflow of MHD Newtonian fluid with suspended particles in a catheterized tube has been studied with long-wavelength and low-Reynolds number approximations. The analytical solution has been derived for velocity and temperature. The amplitude ratio, particle concentration, catheter size, and the dimensionless flow rate were used to discuss the pressure gradient. The solutions for velocity and temperature derived in the analysis have been computed numerically and investigated. The tube surface is maintained at a fixed temperature. The variations of physical variables with the pertinent parameters were discussed graphically. The mathematical model presented corresponds to the flow in the annular space between two concentric tubes. It has been deduced that the thermal energy is reduced with particles’ concentration and with slip condition through the catheterized tube. The flow accelerates with the magnetic field and slip condition at the wall, whereas it decreases at the catheter. The catheter size has a different effect on both pressure drop and friction force.
TL;DR: The authors challenge the lithostatic pressure paradigm by showing that there can be significant outcrop-scale pressure gradients due to compression- and reaction-induced stress in mechanically heterogeneous rock.
Abstract: Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients. The geodynamic evolution of mountain ranges can be reconstructed using the pressure recorded by minerals in metamorphic rocks, under the key assumption that rock pressure is lithostatic. Here, the authors challenge the lithostatic pressure paradigm by showing that there can be significant outcrop-scale pressure gradients due to compression- and reaction-induced stress.
TL;DR: In this paper, the authors modeled the temperature gradient, pressure gradient, and liquid flow under wet frost exposure for highly saturated concrete with different amount of entrained air and found that the water uptake happens at the melting stage and for non-air-entrained CBM, the hydraulic pumping effect is dominant and will suck the water from wet surface.
TL;DR: In this article, a relation between coal permeability and the expansion of the gas-invaded area with the matrix is established, which is defined as local swelling/shrinking.
TL;DR: In this article, a theoretical study for magnetohydrodynamic pumping of electro-conductive couple stress physiological liquids (e.g. blood) through a two-dimensional ciliated channel is conducted.
TL;DR: In this article, the authors study fluid-structure interactions in a long and shallow microchannel, conveying a non-Newtonian fluid, at steady state, with a linearly elastic and compliant top wall.
Abstract: We study fluid–structure interactions (FSIs) in a long and shallow microchannel, conveying a non-Newtonian fluid, at steady state The microchannel has a linearly elastic and compliant top wall, while its three other walls are rigid The fluid flowing inside the microchannel has a shear-dependent viscosity described by the power-law rheological model We employ lubrication theory to solve for the flow problem inside the long and shallow microchannel For the structural problem, we employ two plate theories, namely Kirchhoff–Love theory of thin plates and Reissner–Mindlin first-order shear deformation theory The hydrodynamic pressure couples the flow and deformation problem by acting as a distributed load onto the soft top wall Within our perturbative (lubrication theory) approach, we determine the relationship between the flow rate and the pressure gradient, which is a nonlinear first-order ordinary differential equation for the pressure From the solution of this differential equation, all other quantities of interest in non-Newtonian microchannel FSIs follow Through illustrative examples, we show the effect of FSI coupling strength and the plate thickness on the pressure drop across the microchannel Through direct numerical simulation of non-Newtonian microchannel FSIs using commercial computational engineering tools, we benchmark the prediction from our mathematical theory for the flow rate–pressure drop relation and the structural deformation profile of the top wall In doing so, we also establish the limits of applicability of our perturbative theory
TL;DR: Numerical calculations confirm that the response to a nearly instantaneous pressure drop during an explosive eruption can be non-monotonic as the rock around the chamber relaxes at different rates, behaviour that has been observed in some magmatic systems.
Abstract: Geodetic observations on volcanoes can reveal important aspects of crustal magma chambers. The rate of decay of deformation with distance reflects the centroid depth of the chamber. The amplitude of the deformation is proportional to the product of the pressure change and volume of the reservoir. The ratio of horizontal to vertical displacement is sensitive to chamber shape: sills are efficient at generating vertical displacement, while stocks produce more horizontal deformation. Geodesy alone cannot constrain important parameters such as chamber volume or pressure; furthermore, kinematic models have no predictive power. Elastic response combined with influx proportional to pressure gradient predicts an exponentially decaying flux, leading to saw-tooth inflation cycles observed at some volcanoes. Yet many magmatic systems exhibit more complex temporal behaviour. Wall rock adjacent to magma reservoirs cannot behave fully elastically. Modern conceptual models of magma chambers also include cumulate and/or mush zones, with potentially multi-level melt lenses. A viscoelastic shell surrounding a spherical magma chamber significantly modifies the predicted time-dependent response; post-eruptive inflation can occur without recharge if the magma is sufficiently incompressible relative to the surrounding crust (Segall P. 2016 J. Geophys. Res. Solid Earth, 121, 8501-8522). Numerical calculations confirm this behaviour for both oblate and prolate ellipsoidal chambers surrounded by viscoelastic aureoles. Interestingly, the response to a nearly instantaneous pressure drop during an explosive eruption can be non-monotonic as the rock around the chamber relaxes at different rates. Pressure-dependent recharge of a non-Newtonian magma in an elastic crust leads to an initially high rate of inflation which slows over time; behaviour that has been observed in some magmatic systems. I close by discussing future challenges in volcano geodesy. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.
TL;DR: In this paper, the authors assume that there exists a self-similarity scaling law which captures the first order properties of sea ice and derive an equation for the kinematic viscosity, which is proportional to the wave frequency and the ice thickness squared.
TL;DR: In this paper, the mixed convective MHD fluid through the vertical micro-channel with electrically conducting and non-conducting walls is formulated by momentum and energy equations and key rheological parameters effects such as magnetic Prandtl number, pressure gradient, Reynolds number and radiation parameter are examined on heat and mass transfer.
TL;DR: In this article, a turbulence model based on OpenFOAM has been developed for the jet impingement heat transfer, which is based on the standard SST k-ω model and considers the effects of cross-diffusion term and the Kato-Launder model.
TL;DR: In this article, an effective numerical simulation method is presented based on the subsurface Darcy's flow module in COMSOL multiphysics, where the non-Darcy kinematic equation at the full pressure gradient range is expressed at the interface of the gravitational acceleration vector in Darcy law.
TL;DR: In this article, the authors investigate the behavior and evaluate the dynamics of individual oil drops in an oil-in-water dispersion within an electrical submersible pump (ESP) impeller.
TL;DR: In this paper, the authors investigated the origin of sea ripple formation by means of direct numerical simulation in which the sediment bed is represented by a large number of fully resolved spherical grains (i.e. the flow around each individual particle is accounted for).
Abstract: Sea ripples are small-scale bedforms which originate from the interaction of an oscillatory flow with an erodible sand bed. The phenomenon of sea ripple formation is investigated by means of direct numerical simulation in which the sediment bed is represented by a large number of fully resolved spherical grains (i.e. the flow around each individual particle is accounted for). Two sets of parameter values (differing in the amplitude and frequency of fluid oscillations, among other quantities) are adopted which are motivated by laboratory experiments on the formation of laminar rolling-grain ripples. The knowledge of the origin of ripples is presently enriched by insights and by providing fluid- and sediment-related quantities that are difficult to obtain in the laboratory (e.g. particle forces, statistics of particle motion, bed shear stress). In particular, detailed analysis of flow and sediment bed evolution has confirmed that ripple wavelength is determined by the action of steady recirculating cells which tend to accumulate sediment grains into ripple crests. The ripple amplitude is observed to grow exponentially, consistent with established linear stability analysis theories. Particles at the bed surface exhibit two kinds of motion depending on their position with respect to the recirculating cells: particles at ripple crests are significantly faster and show larger excursions than those lying in ripple troughs. In analogy with the segregation phenomenon of polydisperse sediments, the non-uniform distribution of the velocity field promotes the formation of ripples. The wider the gap between the excursion of fast and slow particles, the larger the resulting growth rate of the ripples. Finally, it is revealed that, in the absence of turbulence, the sediment flow rate is driven by both the bed shear stress and the wave-induced pressure gradient, the dominance of each depending on the phase of the oscillation period. In phases of maximum bed shear stress, the sediment flow rate correlates more with the Shields number while the pressure gradient tends to drive sediment bed motion during phases of minimum bed shear stress.
TL;DR: In this paper, the authors proposed a new type of high-speed railway tunnel to mitigate the aerodynamic effects by enlarging the cross-section at both ends and reducing the crosssection in the middle portion of the tunnel.
TL;DR: In this paper, it was shown that odd or Hall viscosity gives rise to new terms in the pressure gradient of the flow thus modifying the evolution equation of the liquid-gas interface accordingly.
Abstract: Thin viscous liquid films sitting on a solid substrate support nonlinear capillary waves, driven by surface shear stresses at a liquid–gas interface. When surface tension is spatially dependent other mechanisms, such as the thermocapillary effect, influence the dynamics of thin films. In this article we show that in liquids with broken time-reversal symmetry the character of the aforementioned waves and of the thermocapillary effect are significantly modified due to the presence of odd or Hall viscosity in the liquid. This is because odd viscosity gives rise to new terms in the pressure gradient of the flow thus modifying the evolution equation of the liquid–gas interface accordingly. These terms in turn break the reflection symmetry of the evolution equation leading the system to evolve from a pitchfork to a Hopf bifurcation. The odd-viscosity incipient waves can stabilize unstable thin liquid films. For instance, we show that they can suppress the thermocapillary instability. We establish the parameter ranges that odd viscosity has to satisfy in order to initiate those waves that will lead to stability.
TL;DR: In this paper, a large-eddy simulation of turbulent flow over a series of increasingly high, two-dimensional bumps is studied by well-resolved largeeddy simulation and the mean flow and Reynolds stresses for the lowest bump are in good agreement with experimental data.
Abstract: Turbulent flow over a series of increasingly high, two-dimensional bumps is studied by well-resolved large-eddy simulation. The mean flow and Reynolds stresses for the lowest bump are in good agreement with experimental data. The flow encounters a favourable pressure gradient over the windward side of the bump, but does not relaminarize, as is evident from near-wall fluctuations. A patch of high turbulent kinetic energy forms in the lee of the bump and extends into the wake. It originates near the surface, before flow separation, and has a significant influence on flow development. The highest bumps create a small separation bubble, whereas flow over the lowest bump does not separate. The log law is absent over the entire bump, evidencing strong disequilibrium. This dataset was created for data-driven modelling. An optimization method is used to extract fields of variables that are used in turbulence closure models. From this, it is shown how these models fail to correctly predict the behaviour of these variables near to the surface. The discrepancies extend further away from the wall in the adverse pressure gradient and recovery regions than in the favourable pressure gradient region.