Variation Of Pressure Gradient Research Articles

Selection of Intermodal Conductivity Averaging Scheme for Unsaturated Flow in Homogeneous Media

The nonlinear solvers in numerical solution of water flow in variably saturated soils are prone to convergence difficulties. Many aspects can give rise to such difficulties, like very dry initial conditions, which causes a steep pressure gradient and great variation of hydraulic conductivity occurs across the wetting front during the infiltration of water. So, the averaging method applied to compute hydraulic conductivity between two adjacent nodes of the computational grid is one of the most important issues influencing the accuracy of the numerical solution of one-dimension al unsaturated flow equation i.e., Richards’ equation. A number of averaging schemes such as arithmetic, geometric, harmonic and arithmetic mean saturation have been proposed in the literature for homogeneous soil. The resulting numerical schemes are evaluated in terms of accuracy and computational time. It can be seen that the averaging scheme in the framework of arithmetic approaches favorably to other methods for a range of test cases.

Lattice Boltzmann Modeling of Water Cumulation at the Gas Channel-Gas Diffusion Layer Interface in Polymer Electrolyte Membrane Fuel Cells

Several experiments have proved that water in liquid phase can be present at the anode of a PEM fuel cell due to vapor condensation resulting in mass transport losses. Nevertheless, it is not yet well understood where exactly water tends to cumulate and how the design of the gas channel (GC) and gas diffusion layer (GDL) could be improved to limit water cumulation. In the present work, a three-dimensional lattice Boltzmann based model is implemented in order to simulate the water cumulation at the GC–GDL interface at the anode of a PEM fuel cell. The numerical model incorporates the H2–H2O mixture equation of state and spontaneously simulates phase separation phenomena. Different simulations are carried out varying pressure gradient, pore size, and relative height of the GDL. Results reveal that, once saturation conditions are reached, water tends to cumulate in two main regions: the upper and side walls of the GC and the GC–GDL interface, resulting in a limitation of the reactant diffusion from the GC to the GDL. Interestingly, the cumulation of liquid water at the interface is found to diminish as the relative height of the GDL increases.

ANALYSIS OF BLOOD FLOW PASSING THROUGH AORTIC AND MITRAL VALVES USING A COMPUTATIONAL MODEL OF CONCENTRATED PARAMETERS

Blood flow has been extensively studied because of its close relationship with cardiovascular disease. Heart valves blood flow analysis is particularly complex due to the high mobility of its leaflets, a fact that has stimulated the development of computational models aimed to its better understanding. For studying heart valves blood flow, we developed a mathematical model derived from clinical observations based on echocardiographic images, which describe valve leaflets motion and its influence on blood flow. This work presents a concentrated-parameters-based model of heart valves blood flow that takes into consideration five main factors affecting such a flow in the mitral and aortic valves. This model considers factors that are related to blood fluid and valve leaflets characteristics. Considering the main factors involved, it was found that blood flow exhibit an abnormal behavior in response to small variations (less than 10%) in blood pressure gradient or in leaflets stiffness. Likewise, after changing the roughness of the leaflets, the impact is smaller, only slightly affecting blood flow behavior with changes beyond 30%. Moreover, it was observed that the influence of fluid vortices originated behind the valves can be disregarded and the kinetic energy induced by them is almost negligible.

Magnetic field effect on flow parameters of blood along with magnetic particles in a cylindrical tube

Abstract In this paper, the effect of external uniform magnetic field on flow parameters of both blood and magnetic particles is reported through a mathematical model using magnetohydrodynamics (MHD) approach. The fluid is acted upon by a varying pressure gradient and an external uniform magnetic field is applied perpendicular to the cylindrical tube. The governing nonlinear partial differential equations were solved numerically and found that flow parameters are affected by the influence of magnetic field. Further, artificial blood (75% water+25% Glycerol) along with iron oxide magnetic particles were prepared and transported into a glass tube with help of a peristaltic pump. The velocity of artificial blood along with magnetic particles was experimentally measured at different magnetic fields ranging from 100 to 600 mT. The model results show that the velocity of blood and magnetic particles is appreciably reduced under the influence of magnetic field, which is supported by our experimental results.

A fast pressure-correction method for incompressible two-fluid flows

We have developed a new pressure-correction method for simulating incompressible two-fluid flows with large density and viscosity ratios. The method's main advantage is that the variable coefficient Poisson equation that arises in solving the incompressible Navier–Stokes equations for two-fluid flows is reduced to a constant coefficient equation, which can be solved with an FFT-based, fast Poisson solver. This reduction is achieved by splitting the variable density pressure gradient term in the governing equations. The validity of this splitting is demonstrated from our numerical tests, and it is explained from a physical viewpoint.In this paper, the new pressure-correction method is coupled with a mass-conserving volume-of-fluid method to capture the motion of the interface between the two fluids but, in general, it could be coupled with other interface advection methods such as level-set, phase-field, or front-tracking. First, we verified the new pressure-correction method using the capillary wave test-case up to density and viscosity ratios of 10,000. Then, we validated the method by simulating the motion of a falling water droplet in air and comparing the droplet terminal velocity with an experimental value. Next, the method is shown to be second-order accurate in space and time independent of the VoF method, and it conserves mass, momentum, and kinetic energy in the inviscid limit. Also, we show that for solving the two-fluid Navier–Stokes equations, the method is 10–40 times faster than the standard pressure-correction method, which uses multigrid to solve the variable coefficient Poisson equation. Finally, we show that the method is capable of performing fully-resolved direct numerical simulation (DNS) of droplet-laden isotropic turbulence with thousands of droplets using a computational mesh of 10243 points.

Scale analysis of turbulent channel flow with varying pressure gradient

In this paper orthogonal wavelet transformations are applied to decompose experimental velocity signals in fully developed channel flows with varying pressure gradient into scales. We analyze the time series from turbulent data, to obtain the statistical characteristics, correlations between the adjacent scales and the principal scale of coherent structures in different scales by wavelet transformations. The results show that, in the counter gradient transport (CGT) region, skewness factors and flatness factors deviate strongly from the corresponding values of Gaussian distribution on certain scales. PDFs on each scale confirm this observation. Scale-scale correlations show further that the fluctuations on some certain special scales are more intermittent than nearby. Principal scale of coherent structure is coincident with the scales on which the statistical properties depart from Gaussian distribution. These features are the same for different families of wavelets, and it also shows some different features in the region between favorable pressure gradient and adverse pressure gradient.

Hall Effects on Unsteady Rotating MHD Flow Through Porous Channel with Variable Pressure Gradient

Hall effects on an unsteady MHD flow of a viscous incompressible electrically conducting fluid in a horizontal porous channel with variable pressure gradient in a rotating system have been studied. We have considered three different cases (i) impulsive change of pressure gradient (ii) cosine oscillations of pressure gradient and (iii) sine oscillations of pressure gradient. The governing equations are solved analytically using the Laplace transform technique. It is found that interplay of Coriolis force and hydromagnetic force in the presence of pressure gradient and Hall currents plays an important role in characterizing the flow behavior. Effects of the parameters of these forces on the velocity distributions and shear stresses have been depicted graphically and discussed. It is found that the primary velocity increases with an increase in Hall parameter for the impulsive change, cosine and sine oscillations of the pressure gradient. The secondary velocity increases for the impulsive change and cosine oscillations of the pressure gradient while it decreases for sine oscillations of the pressure gradient with an increase in Hall parameter. Further, the shear stress due to the primary flow at the lower wall reduces for both the impulsive change and cosine oscillations of the pressure gradient whereas it increases for sine oscillations of the pressure gradient with an increase in Hall parameter.

Transient Generalized Couette Flow of Viscoelastic Fluid Through a Porous Medium with Variable Viscosity and Pressure Gradient

The transient generalized Couette flow through a porous medium of a non-Newtonian viscoelastic fluid between two parallel porous plates is studied with heat transfer. A uniform suction from above and injection from below are applied perpendicular to the plates which are maintained at two fixed, but different, temperatures while the viscosity of the fluid is assumed to vary exponentially with temperature. The fluid is driven by a uniform horizontal exponential decaying pressure gradient. The coupled set of equations of motion and the energy equation is solved numerically using finite differences. The influence of the different physical parameters of the model on the velocity and temperature fields is investigated and presented.

Observations of barrier island length explained using an exploratory morphodynamic model

Barrier coasts display a chain of islands, separated by tidal inlets that connect a back‐barrier basin to a sea or ocean. Observations show that barrier island length generally decreases for increasing tidal range and increasing basin area. However, this has neither been reproduced in model studies nor explained from the underlying physics. This is the aim of our study. Here we simulate barrier coast dynamics by combining a widely used empirical relationship for inlet dynamics with a process‐based model of the tidal hydrodynamics. Our model results show stable inlet systems with more than one inlet open that support the observed qualitative relationships and fit in existing barrier coast classifications. To explain this, we identify a competition between a destabilizing mechanism (bottom friction in inlets, tending to reduce the number of open inlets) and a stabilizing one (spatially varying pressure gradients over the inlets, tending to keep the inlets open).

Geoelectrical, strain and tilt investigations of hydrological processes at the broadband Geodynamical Observatory Moxa, Germany

The Geodynamic Observatory Moxa, located in Thuringia/Germany, is dedicated to studies of temporal deformations of the earth's crust and of variations of the gravity field. One of the essential issues with respect to these investigations is the reduction of the hydrological impact on the data of the gravimeters , strainmeters and tiltmeters . In order to optimise the reductions, we investigated the changes in the hydrological conditions in the woody mountain slope above the observatory with time-lapse electrical resistivity tomography (ERT), and analysed the strain and tilt measurements for prominent signatures of pore pressure induced subsurface deformations. Here we present the results for two profiles – parallel and perpendicular to the slope – measured with ERT during 33 campaigns between June 2007 and April 2010. Resistivity changes and variations of apparent soil moisture, inferred from ERT sections, were found to primarily occur in the first two metres of the subsurface. These variations can be related to subsurface flow in the upper two metres induced by precipitation events and snowmelts. Trees close to the profiles only show a minimum impact on the resistivity and soil moisture changes. Furthermore, systematic hydrologically induced deformations can be observed in hodographs of strain and tilt measurements for large precipitation events (> 80 mm) and snowmelts. In the strain data a short-term (< 3 days) dilatational signal is found with an amplitude of 20 nstrain to 60 nstrain and a long-term (> 7 days) compressional signal between 40 nstrain and 180 nstrain. The preferential N–S direction of long-term deformational signals (> 1 week) is also observed in the tilt data. The direction of tilt changes (25 nrad–120 nrad) is nearly parallel to the drainage direction of the nearby Silberleite creek indicating variations of pore pressure gradients during hydrological events. The results of these hydrological studies at the Geodynamic Observatory Moxa can be used for removing the time dependent hydrological signal in strain and tilt data and, thus, better correction algorithms for hydrological impacts can be developed to enhance the value of the data for geodynamic studies. • We present investigations of hydrological processes at the Geodynamic Observatory Moxa, Germany. • We present results of time-lapse electrical resistivity tomography. • Strain and tilt data are analysed for prominent hydrologically induced deformations. • Systematic hydrologically induced deformations are found.

Numerical Study on Internal Flow of Small Axial Flow Fan with Splitter Blades

The splitter blades are widely used in axial compressors and play an active role in the improvement of the overall performance of compressors. However, little research on the application of splitter blades to small axial flow fans is conducted. This paper designs a splitter blade small axial flow fan (model B) with a small axial flow fan as the prototype fan (model A) by adding short blades at the second half part of the passageway among long blades of model A. The steady simulation for the two models was conducted with the help of RNG k-e turbulence model provided by software Fluent, and static characteristics and internal flow characteristics of the two models were compared and analyzed. Results show that splitter blades can improve the unsteady flow in the small flow rate region and also have a positive role to increase static pressure rise and efficiency in the higher flow rate region. The variation of static pressure gradient on the meridian plane in model B is well-distributed. The static pressure on the blade surface of model B distributes more uniformly. Splitter blades can suppress the secondary flow from pressure side to suction side in the leading edge because the pressure difference between suction side and pressure side in model B is generally lower than that of model A. And it also can restrain the vortex shedding and flow separation, and further it may be able to get the aerodynamic noise lower because static pressure gradient on the blade surface is well-distributed and the vortex shedding is not developed. Therefore, the performance of the fan with splitter blades is better than that of the prototype fan. The findings of this paper can be a basis for the design of high performance small axial flow fans.

Molecular dynamics simulations of metal-organic frameworks as membranes for gas mixtures separation

We use molecular dynamics simulations to analyze the dynamic behavior of the CO2/N2 and CO2/CH4 gas mixtures through IRMOF-1 and Cu-BTC metal-organic frameworks-based membranes. Two approaches are considered to simulate the membrane system: permeation at constant pressure gradient and permeation at variable pressure gradient. The results show that both materials would work similarly to separate CO2/N2 and that Cu-BTC is the most acceptable to separate CO2/CH4 gas mixtures. The roles of diffusion and adsorption are clearly elucidated from the simulations. These simulation results also demonstrate how metal-organic framework properties influence the separation ability of the membrane and help to understand the dynamic mechanism of the process at a molecular level thus providing orientations that may guide experimental work.

Numerical Simulation of Peristaltic Urine Flow in a Stented Ureter

The capacity of ureteral stents to enhance the conveyance of urine from kidney to bladder is the critical function for patients that require them. The flow path in and around the stent is not a trivial one, particularly if some elements of peristalsis are present in the ureter. This paper details a numerical flow simulation for an axially symmetric stented ureter segment. The flow of urine through a stented, elastically-modeled ureter was considered under varying pressure gradients, bore (lumen) obstructions, and peristaltic deflections (waves). Peristaltic waves are combined with the pressure gradient developed between the kidney and bladder to provide a more accurate representation of the complex flow mechanics found within the ureter. Although it is recognized that peristalsis ceases or diminishes greatly after prolonged presence of a stent, in the time frame that it is active, detrimental consequences like reflux may occur. Several relationships from varying control parameters are determined to predict the onset of reflux as flow conditions within the ureter change. It was determined that occurrence of reflux is more likely as the peristaltic deflection or the obstruction of the stent bore increases. The threat of reflux is low if the pressure gradient between the kidney and bladder remains large. These simulations provide insight into the fluid behaviour within a stented ureter that could lead to optimized stent designs and reduce the possibility of reflux, infection, and discomfort.

Unsteady Pulsatile Flow through a Vertical Constricted Annulus with Heat Transfer

This paper investigates the effects of heat transfer and pulsatile flow of a Newtonian fluid (blood) through a vertical annulus with mild constriction on the outer wall while the inner wall represents the catheter tube. An analytical expression in terms of Bessel functions of the first and second kind is obtained for the heat and the velocity components. The variation of pressure gradient with steady flow rate is given as well as the wall shear stress distribution for different values of time. The pressure gradient and the wall shear stress are calculated, and their variations are discussed with respect to the Grashof number, the height of the constriction, and the size of the catheter. It is observed that an increase in the catheter size increases the pressure gradient as well as the wall shear stress

LES modeling of converging-diverging turbulent channel flow

The paper presents the results of the application of large-eddy simulation (LES) to turbulent channel flow with a varying pressure gradient obtained by an appropriately specified shape of one of the walls. The main objective of the paper is to assess various subgrid scale (SGS) models implemented in two different codes as well as to assess the sensitivity of the predictive accuracy to grid resolution. Additionally, the role of SGS viscosity, controlled by a constant parameter of the SGS model, was investigated. The simulations were performed with inlet conditions corresponding to two Reynolds numbers: and . The consistency and the accuracy of simulations are evaluated using direct numerical simulation (DNS) results. It is demonstrated that all SGS models require a comparable minimum grid refinement in order to capture accurately the recirculation region. Such a test case with a reversal flow, where the turbulence transport is dictated by the dynamics of the large-scale eddies, is well suited to demonstrate the predictive features of the LES technique.

Endoscopic third ventriculostomy in hydrocephalus associated with achondroplasia

Hydrocephalus in patients with achondroplasia is thought to be due to increased dural sinus venous pressure resulting from narrowing of the jugular foramen. In this setting, where hydrocephalus is presumed to be "vascular" in origin and therefore communicating, endoscopic third ventriculostomy (ETV) would seem contraindicated. The authors describe 3 patients in whom ETV was successfully performed, resulting in MR imaging-documented decreases in ventricle size. The patients were 11 months, 33 months, and 13 years at the time of surgery. All patients had serial preoperative MR images demonstrating progressive hydrocephalus in a "triventricular" pattern with a small fourth ventricle but an open aqueduct. All patients had undergone suboccipital decompression for foramen magnum stenosis prior to the treatment of hydrocephalus. Preoperative retrograde venography revealed variable pressure gradients across the jugular foramen. It is postulated that the increase in intracranial venous pressure resulting from jugular foramen stenosis may lead to disproportionate venous engorgement of the cerebellum and some degree of obstructive hydrocephalus amenable to ETV. The authors discuss the role of suboccipital decompression in the progression of hydrocephalus in patients with achondroplasia.

Computational fluid dynamic simulation of small leaks in water pipelines for direct leak pressure transduction

Computational fluid dynamic simulation of small leaks in a pipe having 0.1m diameter has been performed under realistic velocities and pressures using a 3D turbulent flow model of well tested CFD code. The steady state simulations have shown clear signature in the pressure and pressure gradient variations along the pipe. For very small leaks (below 1l/min), this signature in not very strong in the pressure variation, but very clear in the pressure gradient. The averaged power spectral density (PSD) as well as FFT of the pressure fluctuations based on transient Detached Eddy Simulations (DESs) are presented for different locations around the leak position. These results show that the presence of a leak causes measurable differences in the magnitude and frequency of the pressure signal spectrum and also in the averaged PSD for the range of 220–500Hz frequency.

Implications of channel flow analogue models for extrusion of the Higher Himalayan Shear Zone with special reference to the out-of-sequence thrusting

The Higher Himalayan Shear Zone (HHSZ) contains a ductile top-to-N/NE shear zone—the South Tibetan detachment system-lower (STDSL) and an out-of-sequence thrust (OOST) as well as a top-to-N/NE normal shear at its northern boundary and ubiquitously distributed compressional top-to-S/SW shear throughout the shear zone. The OOST that was active between 22 Ma and the Holocene, varies in thickness from 50 m to 6 km and in throw from 1.4 to 20 km. Channel flow analogue models of this structural geology were performed in this work. A Newtonian viscous polymer (PDMS) was pushed through a horizontal channel leading to an inclined channel with parallel and upward-diverging boundaries analogous to the HHSZ and allowed to extrude to the free surface. A top-to-N/NE shear zone equivalent to the STDSU developed spontaneously. This also indirectly connotes an independent flow confined to the southern part of the HHSZ gave rise to the STDSL. The PDMS originally inside the horizontal channel extruded at a faster rate through the upper part of the inclined channel. The lower boundary of this faster PDMS defined the OOST. The model OOST originated at the corner and reached the vent at positions similar to the natural prototype some time after the channel flow began. The genesis of the OOST seems to be unrelated to any rheologic contrast or climatic effects. Profound variations in the flow parameters along the HHSZ and the extrusive force probably resulted in variations in the timing, location, thickness and slip parameters of the OOST. Variation in pressure gradient within the model horizontal channel, however, could not be matched with the natural prototype. Channel flow alone presumably did not result in southward propagation of deformation in the Himalaya.

Spatiotemporal Characteristics of Near-Bed Pressure Gradients on a Barred Beach

Observations of the cross-shore variations of pressure gradients measured on a fixed barred beach in a large-scale laboratory wave flume are presented in this paper. The wave-induced components (0.1–1.0 Hz) of the free surface elevation, near-bottom vertical and cross-shore velocities, and pressure gradient are used in the analysis. The cross-shore variation of the pressure gradient showed the maximum value in the area of wave breaking over the bar, and the pressure gradients were considered to be influenced by the fluctuation of the water surface elevation, which are shown to be a rough proxy for the water surface slope. The cross correlation related to the vertical velocity showed no correlation in the wave breaking area. Surprisingly, although several measuring points were located in the wave breaking area or in the inner surf zone, the spatial variation of the pressure gradient and water surface elevation had a correlation coefficient greater than 0.8 with time lags approximately 0.1 of the wave perio...

A biphasic model for sinusoidal liver perfusion remodeling after outflow obstruction

Liver resection can lead to focal outflow obstruction due to transection of hepatic veins. Outflow obstruction may cause additional damage to the small remnant liver. Drainage of the obstructed territories is reestablished via dilatation of sinusoids. Subsequently, sinusoidal canals are formed draining the blood from the obstructed territory to the neighboring unobstructed territories. We raised the phenomenological hypothesis that the blood pressure gradient is the main driving force for the formation of sinusoidal vascular canals. We generated a biphasic mechanical model to describe this vascular remodeling process in relation to the variable pressure gradient. Therefore, we introduced a transverse isotropic permeability relation as well as an evolutional optimization rule to describe the relationship between pressure gradient and the direction of the sinusoidal blood flow in the fluid phase. As a next step, we developed a framework for the calculation concept including the representation of the governing weak formulations. Then, we examined a representative numerical example with simulation of the blood flow under both conditions, the physiological situation as well as after outflow obstruction. Doing so, we were able to reproduce numerically the experimentally observed process of reestablishing hepatic venous drainage via redirection of blood flow and formation of new vascular structures in respect to the fluid flow. The calculated results support the hypothesis that the reorientation of blood flow mainly depends on the pressure gradient. Further investigations are needed to determine the micromechanical influences on the reorientation of the sinusoids.