Showing papers on "Pressure gradient published in 2008"
TL;DR: In this article, the overlap parameters for the logarithmic law are obtained for available turbulent pipe and channel flow data using composite profiles fitted to the mean velocity, and their resulting behavior with Reynolds number is examined for these flows and compared to results from boundary layers.
Abstract: The overlap parameters for the logarithmic law are obtained for available turbulent pipe and channel flow data using composite profiles fitted to the mean velocity. The composite profile incorporates κ, B, and Π as the varying parameters and their resulting behavior with Reynolds number is examined for these flows and compared to results from boundary layers. The von Karman coefficient in channel flow is smaller than the well-established value for zero pressure gradient turbulent boundary layers of 0.384, while in pipe flows it is consistently higher. In contrast, the estimates of the wake parameter Π are the smallest for channel flows and largest for boundary layers. Further, the Superpipe data are reanalyzed to reveal that κ=0.41 is a better value for the von Karman constant in pipe flow. The collective behavior of κ in boundary layers, pipes, and channels reveals that the von Karman coefficient is not universal and exhibits dependence not only on the pressure gradient but also on the flow geometry.
348 citations
TL;DR: In this article, the authors present a non-linear 1D model of coronary and systemic arterial circulations, as well as ventricular pressure and an aortic valve that opens and closes independently based on local haemodynamics.
Abstract: There is an important interaction between the pumping performance of the ventricle, arterial haemodynamics and coronary blood flow. While previous non-linear 1D models have focused only on one of these components, the model presented in this study includes coronary and systemic arterial circulations, as well as ventricular pressure and an aortic valve that opens and closes ‘independently’ and based on local haemodynamics. The systemic circulation is modelled as a branching network of elastic tapering vessels. The terminal element applied at the extremities of the network is a single tapering vessel which is shown to adequately represent the input characteristics of the downstream vasculature. The coronary model consists of left and right coronary arteries which both branch into two ‘equivalent’ vessels that account for the lumped characteristics of subendocardial and subepicardial flows. As contracting heart muscle causes significant compression of the subendocardial vessels, a time-varying external pressure proportional to ventricular pressure is applied to the distal part of the equivalent subendocardial vessel. The aortic valve is modelled using a variable reflection coefficient with respect to backward-running aortic waves, and a variable transmission coefficient with respect to forward-running ventricular waves. A realistic ventricular pressure is the input to the system; however, an afterload-corrected ventricular pressure is calculated and results in pressure gradients between the ventricle and aorta that are similar to those observed in vivo. The 1D equations of fluid flow are solved using the locally conservative Galerkin method, which provides explicit element-wise conservation, and can naturally incorporate vessel branching. Each component of the model is verified using a number of tests to ensure accuracy and reveal the underlying processes that give rise to complex pressure and flow waveforms. The complete model is then implemented, and simulations are performed with input parameters representing ‘at rest’ and exercise states for a normal adult. The resulting waveforms contain all of the important features seen in vivo, and standard measures of haemodynamic state are found to be normal. In addition, one or several characteristics of some common diseases are imposed on the model and are found to produce haemodynamic changes that agree with experimental observations in the published literature. Using a patient-specific carotid bifurcation geometry, 1D velocity waveforms are also compared with waveforms obtained from a three-dimensional model. The 1D and 3D results show good agreement. Copyright © 2008 John Wiley & Sons, Ltd.
213 citations
TL;DR: In this article, Fanning friction factor (fm) versus Reynolds number (Rem) was used to compute the two-phase frictional pressure gradient in circular pipes, minichannels and microchannels.
193 citations
TL;DR: The available evidence suggests that orbital tissue pressure provides this moderating influence on intraocular pressure and CSF pressure affects axonal transport, which is known to be important in glaucoma etiology and retinal venous outflow and pressures.
Abstract: It is generally accepted that glaucoma occurs when intraocular pressure (IOP) is raised above atmospheric pressure beyond tolerable limits for the optic disc. However, the other, unseen side of the optic disc is not air but a set of pressure compartments dominated by the cerebrospinal fluid (CSF) within the subarachnoid space. This invisibility has made investigation difficult; however, in recent decades there has been increased interest in this corollary to IOP. We briefly review the anatomy of the optic nerve subarachnoid space and its pressure relationships to intracranial, retrolaminar, and orbital tissue pressures. The CSF pressure is equivalent to IOP in its influence on translaminar pressure gradient and optic disk surface movement. At low CSF pressure, its influence on retrolaminar tissue pressure is reduced tending to minimize an increase in translaminar pressure gradient. The available evidence suggests that orbital tissue pressure provides this moderating influence. CSF pressure affects axonal transport, which is known to be important in glaucoma etiology and retinal venous outflow and pressures. Recent attempts to develop noninvasive measurement of CSF pressure have increased our knowledge of retinal venous changes in glaucoma. Further work in this area is likely to greatly increase our understanding of glaucoma.
144 citations
TL;DR: Van Gent et al. as mentioned in this paper conducted large-scale physical model tests with different wave periods to examine the physical processes driving dune erosion, which revealed that both short and long waves are important to inner surf hydrodynamics.
129 citations
TL;DR: In this paper, the peristaltic transport of an incompressible viscous fluid in an inclined asymmetric channel through a porous medium is studied under long-wavelength and low-Reynolds number assumptions.
127 citations
TL;DR: In this paper, a direct numerical simulation (DNS) of a channel flow with one curved surface was performed at moderate Reynolds number (Re τ = 395 at the inlet), where the adverse pressure gradient was obtained by a wall curvature through a mathematical mapping from physical coordinates to Cartesian ones.
Abstract: A direct numerical simulation (DNS) of a channel flow with one curved surface was performed at moderate Reynolds number (Re τ = 395 at the inlet). The adverse pressure gradient was obtained by a wall curvature through a mathematical mapping from physical coordinates to Cartesian ones. The code, using spectral spanwise and normal discretization, combines the advantage of a good accuracy with a fast integration procedure compared to standard numerical procedures for complex geometries. The turbulent flow slightly separates on the profile at the lower curved wall and is at the onset of separation at the opposite flat wall. The thin separation bubble is characterized with a reversal flow fraction. Intense vortices are generated not only near the separation line on the lower wall but also at the upper wall. Turbulent normal stresses and kinetic energy budget are investigated along the channel.
124 citations
TL;DR: In this paper, the influence of an endoscope on peristaltic flow of a Jeffrey fluid through tubes is analyzed by plotting graphs and discussed in detail, and exact analytic solutions for velocity components and pressure gradient are established under long wavelength assumption.
117 citations
TL;DR: In this paper, the authors used two topologically-disordered networks representing a sand pack and Berea sandstone to study the flow in porous media of Ellis and Herschel-Bulkley fluids, which model a large group of time independent non-Newtonian fluids.
115 citations
TL;DR: In this article, the transverse jet's enhanced entrainment is explained in terms of the pressure field around the jet, which sets up a pressure gradient that drives downstream crossflow fluid toward the jet.
Abstract: Direct numerical simulation is used to study passive scalar transport and mixing in a round turbulent jet, in a laminar crossflow. The ratio of the jet velocity to that of the crossflow is 5.7, the Schmidt number of the scalar is 1.49, and the jet-exit Reynolds number is 5000. The scalar field is used to compute entrainment of the crossflow fluid by the jet. It is shown that the bulk of this entrainment occurs on the downstream side of the jet. Also, the transverse jet entrains more fluid than a regular jet even when the jet has not yet bent into the crossflow. The transverse jet's enhanced entrainment is explained in terms of the pressure field around the jet. The acceleration imposed by the crossflow deforms the jet cross-section on the downstream side, which sets up a pressure gradient that drives downstream crossflow fluid toward the jet. The simulation results are used to comment on the applicability of the gradient–diffusion hypothesis to compute passive scalar mixing in this flow field. Computed values of the eddy diffusivity show significant scatter, and a pronounced anisotropy. The near field also exhibits counter gradient diffusion.
111 citations
TL;DR: In this paper, a method to determine the position of the separation point on the rotating blade, based on the chordwise pressure gradient in the separated area, is proposed to evaluate rotation and turbulence effects on a wind turbine blade aerodynamics, focusing particularly on stall mechanisms.
TL;DR: In this article, the effects of an adverse pressure gradient (APG) on a turbulent boundary layer were investigated using a fully implicit fractional step method to simulate the flows and the instantaneous flow fields and vorticity fluctuations were analyzed to characterize the response of the outer turbulence to an APG.
TL;DR: In this paper, a fractal model for the starting pressure gradient for Bingham fluids in porous media based on the fractal characteristics of pores in the media and on the capillary pressure effect was presented.
TL;DR: Deng et al. as mentioned in this paper used the non-hydrostatic Global Ionosphere Thermosphere Model (GITM) to solve the complete vertical momentum equation, and showed that after a sudden intense enhancement of high-latitude Joule heating, the vertical pressure gradient force can locally be 25% larger than the gravity force, resulting in a significant disturbance away from hydrostatic equilibrium.
Abstract: [1] Under hydrostatic equilibrium, a typical assumption used in global thermosphere ionosphere models, the pressure gradient in the vertical direction is exactly balanced by the gravity force. Using the non-hydrostatic Global Ionosphere Thermosphere Model (GITM), which solves the complete vertical momentum equation, the primary characteristics of non-hydrostatic effects on the upper atmosphere are investigated. Our results show that after a sudden intense enhancement of high-latitude Joule heating, the vertical pressure gradient force can locally be 25% larger than the gravity force, resulting in a significant disturbance away from hydrostatic equilibrium. This disturbance is transported from the lower altitude source region to high altitudes through an acoustic wave, which has beensimulated inaglobal circulation model forthefirst time. Due to the conservation of perturbation energy, the magnitude of the vertical wind perturbation increases with altitude andreaches 150(250) m/sat300(430) kmduringthe disturbance. The upward neutral wind lifts the atmosphere and raises the neutral density at high altitudes by more than 100%. These large vertical winds are not typically reproduced by hydrostatic models of the thermosphere and ionosphere. Our results give an explanation of the cause of such strong vertical winds reported in many observations. Citation: Deng, Y., A. D. Richmond, A. J. Ridley, and H.-L. Liu (2008), Assessment of the non-hydrostatic effect on the upper atmosphere using a general circulation model (GCM), Geophys. Res. Lett., 35, L01104, doi:10.1029/2007GL032182.
TL;DR: In this paper, the effect of rarefaction on hydrodynamicaly developing flow field, pressure gradient and entrance length is analyzed, and a correlation for the fully developed friction factor is presented as a function of Knudsen number (Kn) and aspect ratio (α).
TL;DR: In this article, a flow analysis for an inclined slider bearing with the aim of showing the governing mechanism at conditions where an optimum in load carrying capacity is achieved is carried out, and the effects of surface texture on pressure buildup and load-carrying capacity are explained for a textured slider bearing geometry.
Abstract: A flow analysis is carried out for an inclined slider bearing with the aim of showing the governing mechanism at conditions where an optimum in load carrying capacity is achieved. The effects of surface texture on pressure buildup and load carrying capacity are explained for a textured slider bearing geometry. Numerical simulations are performed for laminar, steady, and isothermal flows. The energy transferred to the fluid from the moving wall is converted into pressure in the initial part of the converging contact and into losses in the second part. The convergence ratio can be increased, in order to get the greatest pressure gradient, until the limiting value where flow recirculation begins to occur. The texture appears to achieve its maximum efficiency when its depth is such that the velocity profile is stretched at its maximum extent without incurring incoming recirculating flow. The wall profile shape controlling the velocity profile can be optimized for many hydrodynamic contacts.
TL;DR: In this paper, the peristaltic flow of a third order fluid in an asymmetric channel is considered in the presence of a slip condition, and the series solution of the stream function and longitudinal pressure gradient is given under long wave length and low Reynolds number approximations.
TL;DR: In this article, the authors proposed a dynamic framework for an atmospheric general circulation spectral model in which a reference stratified atmospheric temperature and a reference surface pressure are introduced into the governing equations so as to improve the calculation of the pressure gradient force and gradients of surface pressure and temperature.
Abstract: This paper describes a dynamic framework for an atmospheric general circulation spectral model in which a reference stratified atmospheric temperature and a reference surface pressure are introduced into the governing equations so as to improve the calculation of the pressure gradient force and gradients of surface pressure and temperature. The vertical profile of the reference atmospheric temperature approximately corresponds to that of the U.S. midlatitude standard atmosphere within the troposphere and stratosphere, and the reference surface pressure is a function of surface terrain geopotential and is close to the observed mean surface pressure. Prognostic variables for the temperature and surface pressure are replaced by their perturbations from the prescribed references. The numerical algorithms of the explicit time difference scheme for vorticity and the semi-implicit time difference scheme for divergence, perturbation temperature, and perturbation surface pressure equation are given in detail. The modified numerical framework is implemented in the Community Atmosphere Model version 3 (CAM3) developed at the National Center for Atmospheric Research (NCAR) to test its validation and impact on simulated climate. Both the original and the modified models are run with the same spectral resolution (T42), the same physical parameterizations, and the same boundary conditions corresponding to the observed monthly mean sea surface temperature and sea ice concentration from 1971 to 2000. This permits one to evaluate the performance of the new dynamic framework compared to the commonly used one. Results show that there is a general improvement for the simulated climate at regional and global scales, especially for temperature and wind.
TL;DR: A theory is proposed that reconciles the view that hydrocephalus is caused by obstruction to the flow of CSF with the observed absence of large pressure gradients across the cerebral mantle and predicts that very small stresses are sufficient to produce large deformations of the brain if these are allowed to occur slowly.
TL;DR: In this paper, a core displacement experiment, simulated oil, formation water, injected water, and distillated water were used as fluids to measure and analyze the threshold pressure gradient (TPG) for both a single and two-phase fluid flow.
Abstract: In a core displacement experiment, simulated oil, formation water, injected water, and distillated water were used as fluids to measure and analyze the threshold pressure gradient (TPG) for both a single- and two-phase fluid flow. A certain Chinese oil field core sample was used, which represents a typical ultra-low permeability reservoir: “block 119.” The study indicates that different types of fluids give a different TPG versus permeability power function, with index equal to approximately −1. The study also indicates that, due to the capillary pressure and the Jamin effect, the TPG for the two-phase oil and water is greater than that for the single-phase flow. By combining laboratory and field data, the effect of the TPG in the development of the ultra-low permeability reservoirs can be explained.
TL;DR: In this paper, the authors investigated the influence of external conditions on a transitionally rough favorable pressure gradient turbulent boundary layer, and found that the velocity profiles show the effects of the upstream conditions imposed on the flow when normalized with free-stream velocity.
Abstract: Laser Doppler anemometry measurements are carried out in order to investigate the influences of the external conditions on a transitionally rough favorable pressure gradient turbulent boundary layer. The acquired data is normalized using the scalings obtained by the means of equilibrium similarity of the outer flow. The point at hand is to not only understand the interaction between the rough surface and the outer flow but also to include the external pressure gradient as the flow evolves in the streamwise direction. It is found that the velocity profiles show the effects of the upstream conditions imposed on the flow when normalized with the free-stream velocity. However, the profiles do collapse when normalized with U ∞ δ*/δ, demonstrating that this scaling absorbs the roughness effects and upstream conditions. An augmentation in the Reynolds stresses occurs with an increase in the roughness parameter and a decrease due to the external favorable pressure gradient. However, close to the wall, there is an...
TL;DR: In this article, the authors studied the Cauchy problem for the n-dimensional Navier-Stokes equations and proved some regularity criteria involving the integrability of the pressure or the pressure gradient for weak solutions in the Morrey, Besov and multiplier spaces.
TL;DR: In this paper, the authors studied the effect of a prescribed flow pulsation (defined by two control parameters: velocity pulsation frequency and pressure gradient amplitude at the inlet section) on the heat transfer rate behind a backward facing step in the unsteady laminar 2-D regime.
TL;DR: In this paper, the authors consider axisymmetric relativistic jets with a toroidal magnetic field and an ultrarelativistic equation of state, and study the lateral structure of jets whose pressure is matched to the pressure of the surrounding medium.
Abstract: We consider axisymmetric relativistic jets with a toroidal magnetic field and an ultrarelativistic equation of state, to study the lateral structure of jets whose pressure is matched to the pressure of the surrounding medium. We find all self-similar steady state solutions of the relativistic MHD equations for this setup. One of the solutions is a parabolic jet being accelerated by the pressure gradient as it propagates through a medium with pressure declining as p(z) ∝ z−2. As the jet material expands due to internal pressure gradients, it runs into the ambient medium resulting in a pileup of material along the jet boundary, while the magnetic field acts to produce a magnetic pinch along the axis of the jet. Such jets can be in a lateral pressure equilibrium only if their opening angle θj at distance z is smaller than about 1/γ , where γ is the characteristic bulk Lorentz factor at this distance; otherwise, different parts of the jet cannot maintain causal contact. We construct maps of optically thin synchrotron emission from our self-similar models. We suggest that the boundary pileup may be the reason for the limb-brightening of the subparsec jet of M87. We find that if the synchrotron emissivity falls with the distance from the jet axis, the polarization fraction rises toward the edge, as seen in 3C273 and Mrk501. Projection effects and the emissivity pattern of the jet have a strong effect on the observed polarization signal, so the interpretation of the polarization data in terms of the geometry of magnetic fields is rather uncertain. For example, jets with toroidal magnetic fields display the "spine-sheath" polarization angle pattern seen in some BL Lac objects.
TL;DR: In this article, a fundamental shift in the view of edge transport physics was made; transport near the last-closed flux surface may be more appropriately described in terms of a critical gradient phenomenon rather than a diffusive and/or convective paradigm.
Abstract: Recent experiments have led to a fundamental shift in our view of edge transport physics; transport near the last-closed flux surface may be more appropriately described in terms of a critical gradient phenomenon rather than a diffusive and/or convective paradigm. Edge pressure gradients, normalized by the square of the poloidal magnetic field strength, appear invariant in plasmas with the same normalized collisionality, despite vastly different currents and magnetic fields—a behavior that connects with first-principles electromagnetic plasma turbulence simulations. Near-sonic scrape-off layer (SOL) flows impose a cocurrent rotation boundary condition on the confined plasma when B×∇B points toward the active x-point, suggesting a link to the concomitant reduction in input power needed to attain high-confinement modes. Indeed, low-confinement mode plasmas are found to attain higher edge pressure gradients in this configuration, independent of the direction of B, evidence that SOL flows may affect transport and “critical gradient” values in the edge plasma.
TL;DR: This paper investigates viscoelastic electroosmotic flows through a rectangular straight microchannel with and without pressure gradient and shows that the volumetric flow rates of viscoELastic fluids are significantly different from those of Newtonian fluids under the same external electric field and pressure gradient.
Abstract: Many lab-on-a-chip based microsystems process biofluids such as blood and DNA solutions. These fluids are viscoelastic and show extraordinary flow behaviors, not existing in Newtonian fluids. Adopting appropriate constitutive equations these exotic flow behaviors can be modeled and predicted reasonably using various numerical methods. In the present paper, we investigate viscoelastic electroosmotic flows through a rectangular straight microchannel with and without pressure gradient. It is shown that the volumetric flow rates of viscoelastic fluids are significantly different from those of Newtonian fluids under the same external electric field and pressure gradient. Moreover, when pressure gradient is imposed on the microchannel there appear appreciable secondary flows in the viscoelastic fluids, which is never possible for Newtonian laminar flows through straight microchannels. The retarded or enhanced volumetric flow rates and secondary flows affect dispersion of solutes in the microchannel nontrivially.
TL;DR: The toroidal plasma rotation generated by the external momentum input and by the plasma itself (intrinsic rotation) has been separated through a novel momentum transport analysis in the JT-60U tokamak device.
Abstract: The toroidal plasma rotation generated by the external momentum input and by the plasma itself (intrinsic rotation) has been separated through a novel momentum transport analysis in the JT-60U tokamak device. The toroidal rotation, which is not determined by the momentum transport coefficients and the external momentum input, has been observed. It is found that this intrinsic rotation is locally determined by the local pressure gradient and increases with increasing pressure gradient. This trend is almost the same for various plasmas: low and high confinement mode, co and counterrotating plasmas.
TL;DR: An analytically-based computational model based on the Womersley equations for pulsatile blood flow within elastic and viscoelastic arteries concludes that longitudinal wall displacements comparable to the radial displacements can be present and should be considered when studying the effects of tissue movement on Doppler ultrasound clutter.
Abstract: The existing computational model studies of pulsatile blood flow in arteries have assumed either rigid wall characteristics or elastic arterial wall behavior with wall movement limited to the radial direction. Recent in vivo studies have identified significant viscoelastic wall properties and longitudinal wall displacements over the cardiac cycle. Determining the nature of these movements is important for predicting the effects of ultrasound clutter in Doppler ultrasound measurements. It is also important for developing an improved understanding of the physiology of vessel wall motion. We present an analytically-based computational model based on the Womersley equations for pulsatile blood flow within elastic and viscoelastic arteries. By comparison with published in vivo data of the human common carotid artery as well as uncertainty and sensitivity analyses, it is found that the predicted waveforms are in reasonable quantitative agreement. Either a pressure, pressure gradient or volumetric flow rate waveform over a single cardiac cycle is used as an input. Outputs include the pressure, pressure gradient, radial and longitudinal fluid velocities and arterial wall displacements, volumetric flow rate and average longitudinal velocity. It is concluded that longitudinal wall displacements comparable to the radial displacements can be present and should be considered when studying the effects of tissue movement on Doppler ultrasound clutter.
TL;DR: In this article, the pore-scale effects of seismic stimulation on two-phase flow are modeled numerically in random 2D grain0pack geometries using the lattice Boltzmann model.
Abstract: The pore-scale effects of seismic stimulation on two-phase flow are modeled numerically in random 2D grain0pack geometries. Seismic stimulation aims to enhance oil production by sending seismic waves across a reservoir to liberate immobile patches of oil. For seismic amplitudes above a well-defined (analytically expressed) dimensionless criterion, the force perturbation associated with the waves indeed can liberate oil trapped on capillary barriers and get it flowing again under the background pressure gradient. Subsequent coalescence of the freed oil droplets acts to enhance oil movement further because longer bubbles overcome capillary barriers more efficiently than shorter bubbles do. Poroelasticity theory defines the effective force that a seismic wave adds to the background fluid-pressure gradient. The lattice-Boltzmann model in two dimensions is used to perform pore-scale numerical simulations. Dimensionless numbers (groups of material and force parameters) involved in seismic stimulation are defined carefully so that numerical simulations can be applied to field-scale conditions. Using the analytical criteria defined in the paper, there is a significant range of reservoir conditions over which seismic stimulation can be expected to enhance oil production.
TL;DR: In this article, the peristaltic flow of a Maxwell fluid in an asymmetric channel is studied, and an analytic solution is obtained through a series of the wave number, where the first and second order terms are the viscoelastic contribution to the flow.
Abstract: The peristaltic flow of a Maxwell fluid in an asymmetric channel is studied. Asymmetry in the flow is induced by taking peristaltic wave train of different amplitudes and phase. The viscoelasticity of the fluid is induced in the momentum equation. An analytic solution is obtained through a series of the wave number. The leading velocity term denotes the Newtonian result. The first and second order terms are the viscoelastic contribution to the flow. Expressions for stream function and longitudinal pressure gradient are obtained analytically. Numerical computations have been performed for the pressure rise per wavelength and discussed.