Steady Unidirectional Flow Research Articles

Marangoni Bifurcation Flow in a Microchannel T-Junction and Its Micropumping Effect: A Computational Study

We discuss a bifurcation phenomenon of Marangoni flow in a microchannel T-junction and its novel unidirectional pumping effect. The T-junction is formed by a main channel connected with a liquid reservoir and a side channel outlet to the atmosphere. A volatizing meniscus is formed in the side channel and Marangoni convections are generated due to the non-uniform evaporation on the meniscus. It is found for weak evaporations (Ma < 270) the Marangoni convections are symmetrical. However, for intense evaporations (Ma > 270), the initial inward symmetrical Marangoni convection becomes unstable and converted into one single vortex flow. Moreover, the single vortex induces a steady unidirectional flow in the main channel, acting like a pump.

Calcium response in osteocytic networks under steady and oscillatory fluid flow

The fluid flow in the lacunar–canalicular system of bone is an essential mechanical stimulation on the osteocyte networks. Due to the complexity of human physical activities, the fluid shear stress on osteocyte bodies and processes consists of both steady and oscillatory components. In this study, we investigated and compared the intracellular calcium ([Ca2+]i) responses of osteocytic networks under steady and oscillatory fluid flows. An in vitro osteocytic network was built with MLO-Y4 osteocyte-like cells using micro-patterning techniques to simulate the in vivo orderly organization of osteocyte networks. Sinusoidal oscillating fluid flow or unidirectional steady flow was applied on the cell surface with 2Pa peak shear stress. It was found that the osteocytic networks were significantly more responsive to steady flow than to oscillatory flow. The osteocytes can release more calcium peaks with higher magnitudes at a faster speed under steady flow stimulation. The [Ca2+]i signaling transients under the steady and oscillatory flows have significantly different spatiotemporal characters, but a similar responsive percentage of cells. Further signaling pathway studies using inhibitors showed that endoplasmic reticulum (ER) calcium store, extracellular calcium source, ATP, PGE2 and NO related pathways play similar roles in the [Ca2+]i signaling of osteocytes under either steady or oscillating flow. The spatiotemporal characteristics of [Ca2+]i transients under oscillating fluid flow are affected more profoundly by pharmacological treatments than under the steady flow. Our findings support the hypothesis that the [Ca2+]i responses of osteocytic networks are significantly dependent on the profiles of fluid flow.

STANMOD: Model Use, Calibration, and Validation

This article provides an overview of STANMOD, a Windows-based computer software package for evaluating solute transport in soils and groundwater using analytical solutions of the advection-dispersion equation. The software integrates seven separate codes that have been popularly used over the years for a broad range of one-dimensional and multi-dimensional solute transport applications: the CFITM, CFITIM, CXTFIT, CHAIN, and SCREEN models for one-dimensional transport, and the 3DADE and N3DADE models for multi-dimensional transport. All of the models can be run for direct (forward) problems, and several (CFITM, CFITM, CXTFIT, and 3DADE) can also be run for inverse problems. CXTFIT further includes a stochastic stream tube model assuming local-scale equilibrium or nonequilibrium transport conditions. The 3DADE and N3DADE models apply to two- and three-dimensional transport during steady unidirectional water flow assuming equilibrium and nonequilibrium transport, respectively. Nonequilibrium transport can be simulated using the assumption of either physical nonequilibrium (two-region or mobile-immobile type transport) or chemical nonequilibrium (two-site partial equilibrium, partial kinetic sorption). The STANMOD software comes with a large number of example applications illustrating the utility of the different codes for a variety of laboratory and field-scale solute transport problems.

Varying Effects of Hemodynamic Forces on Tissue Factor RNA Expression in Human Endothelial Cells

Atherosclerotic lesions predominantly localize in areas exposed to distinct hemodynamic conditions. In such lesions, tissue factor (TF) is over-expressed. Therefore, we hypothesized that varying types of mechanical forces may induce different effects on TF expression in endothelial cell, and may also influence the effects of chemical stimuli. TF RNA expression in human umbilical vein endothelial cells (HUVEC) exposed to mechanical stress in the presence or absence of chemical stimulation with thrombin (Th) was determined. The forces examined were: steady unidirectional laminar flow (LF), pulsatile unidirectional laminar flow (PF), constant oscillatory flow (OF), pulsatile to-fro flow (TFF), and cyclic strain (CS). Mechanical stimulation of HUVEC with LF for 2 h induced an 8.7 ± 0.7-fold increase in TF RNA expression, while PF induced 4.7 ± 0.9 and TFF induced 8.6 ± 1.7-fold, respectively. These responses were significantly higher than static controls. Exposure to OF or CS did not result in any significant increase, whereas chemical stimulation with Th led to significant TF expression (4.9 ± 0.3-fold). The combination of mechanical-chemical stimuli induced significantly higher TF expression than mechanical stresses alone, and this effect was synergistic. Combination of LF+Th for 2 h induced significantly increased TF expression (16.6 ± 1.7-fold), as did PF+Th (14.8 ± 2.4) and TFF+Th (17.4 ± 1.0). Furthermore, after 6 h exposure, only TFF demonstrated significantly higher TF expression both with and without Th. While uniform laminar flow resulted in transient TF expression, disturbed flow induced sustained amplification of TF expression. Further investigation is needed to elucidate the mechanism of localized atherosclerosis in areas exposed to disturbed flow.

WAVE-FLUME EXPERIMENTS OF DISSIPATING WAVES ON SOFT MUD

A series of laboratory wave-flume experiments was conducted to investigate the dissipation of monochromatic regular waves on a horizontal muddy bed of commercial kaolinite. The rheological parameters of kaolinite samples with different water content ratios were obtained from controlled shear rate tests using the rheometer. The flow curves of shear stress versus shear rate were found to be essentially Bingham viscoplastic medium for steady unidirectional mud flows of the rheological tests. Assuming Bingham behavior for kaolinite, a wave-mud interaction model was employed to obtain the values of the wave attenuation coefficients. Comparisons between the measured and simulated results show a good agreement.

Steady and Oscillatory Fluid Flows Produce a Similar Osteogenic Phenotype

Mechanical loading induces positive changes in the skeleton due to direct effects on bone cells, which may include regulation of transcription factors that support osteoblast differentiation and function. Flow effects on osteoblast transcription factors have generally been evaluated after short exposures. In this work, we assayed flow effects on osteogenic genes at early and late time points in a preosteoblast (CIMC-4) cell line and evaluated both steady and oscillatory flows. Four hours of steady unidirectional flow decreased the level of RANKL mRNA 53 ± 7% below that of nonflowed cells, but increases in Runx2 and osterix mRNA (44 ± 22% and 129 ± 12%, respectively) were significant only after 12-19 h of continuous flow. Late flow effects on RANKL and osterix were also induced by an intermittent flow-rest protocol (four cycles of 1 h on/1 h off + overnight rest). Four hours of oscillatory flow decreased RANKL mRNA at this early time point (63 ± 2%) but did not alter either osterix or Runx2. When oscillatory flow was delivered using the intermittent flow-rest protocol, Runx2 and osterix mRNA increased significantly (85 ± 19% and 161 ± 22%, respectively). Both β-catenin and ERK1/2, known to be involved in RANKL regulation, were rapidly activated by steady flow. Inhibition of flow-activated ERK1/2 prevented the increase in osterix mRNA but not Runx2; Runx2 phosphorylation was increased by flow, an effect which likely contributes to osterix induction. This work shows that both steady and oscillatory fluid flows can support enhancement of an osteogenic phenotype.

Differential Response of Endothelial Cells to Simvastatin When Conditioned with Steady, Non-Reversing Pulsatile or Oscillating Shear Stress

Few studies have investigated whether fluid mechanics can impair or enhance endothelial cell response to pharmacological agents such as statin drugs. We evaluated and compared Kruppel-like factor 2 (KLF2), endothelial nitric oxide synthase (eNOS), and thrombomodulin (TM) expression in human abdominal aortic endothelial cells (HAAEC) treated with increasing simvastatin concentrations (0.1, 1 or 10μM) under static culture and shear stress (steady, non-reversing pulsatile, and oscillating). Simvastatin, steady flow, and non-reversing pulsatile flow each separately upregulated KLF2, eNOS, and TM mRNA. At lower simvastatin concentrations (0.1 and 1μM), the combination of statin and unidirectional steady or pulsatile flow produced an overall additive increase in mRNA levels. At higher simvastatin concentration (10μM), a synergistic increase in eNOS and TM mRNA expression was observed. In contrast, oscillating flow impaired KLF2 and TM, but not eNOS expression by simvastatin at 1μM. A higher simvastatin concentration of 10μM overcame the inhibitory effect of oscillating flow. Our findings suggest that oscillating shear stress renders the endothelial cells less responsive to simvastatin than cells exposed to unidirectional steady or pulsatile flow. Consequently, the pleiotropic effects of statins in vivo may be less effective in endothelial cells exposed to atheroprone hemodynamics.

An Experimental Study on the Effect of Oscillatory Flow on Turbulence Propertiesinduced by Porous Media

The effect of oscillatory flow properties, the Reynolds number and the KC number, on turbulence generated by a porous media was investigated by hydraulic experiment. Violent turbulence was generated when interstitial velocity exceeded a critical level. Turbulent energy generated in pores increased as the Reynolds number increased. In a porous media, difference between minimum and maximum values of turbulent energy was increased as the KC number increased. And outside porous media, turbulent energy spread widely as the KC number increased. Under unsteady oscillatory flow, maximum turbulent energy was larger than that of unidirectional steady flow.

A thin rivulet of perfectly wetting fluid subject to a longitudinal surface shear stress

The lubrication approximation is used to obtain a complete description of the steady unidirectional flow of a thin rivulet of perfectly wetting fluid on an inclined substrate subject to a prescribed uniform longitudinal surface shear stress. The quasi-steady stability of such a rivulet is analysed, and the conditions under which it is energetically favourable for such a rivulet to split into one or more subrivulets are determined.

Endothelial Mechanisms of Flow-Mediated Athero-Protection and Susceptibility

See related article, pages 97–105 The arterial endothelium survives remarkably well as the interface between blood and vessel wall in an environment of constantly changing biomechanical stresses as well as acute and chronic exposure to inflammatory stimulants (eg, cytokines and hypercholesterolemia respectively).1 Cell turnover, which tends to occur in regional clusters,2 is otherwise very low in this monolayer. The endothelium also plays an important regulatory role in the pathogenesis of vascular disease. The cells readily respond to diverse stimuli through a repertoire of mechanisms to enhance their own survival even as they facilitate inflammatory, proatherogenic responses in the subendothelial tissue. The necessity to be a responsive cellular interface probably accounts for much of the endothelial phenotype heterogeneity that exists between vascular beds as well as within discrete regions of the arterial circulation.3,4 Hemodynamic characteristics that vary with blood vessel geometry predict the location of arterial sites that are susceptible to atherosclerosis.5 Curved and branching vessel geometries create sites of flow separation that contain transient flow reversals, lower average shear stresses, and occasional turbulence, (collectively, disturbed flow) and that are predictive of lesion formation. In contrast, pulsatile unidirectional laminar flow (and higher average shear stresses) is associated with regions where atherosclerosis rarely occurs, despite there being equivalent exposure to plasma risk factors such as hypercholesterolemia throughout the circulation. Although the signatures of endothelial phenotype in such regions in vivo are varied and complex, data are emerging from genomic5–7 and protein8 analyses of endothelium at such sites that identify molecular differences. Some of these are accessible for study in vitro to investigate detailed mechanisms under more controlled conditions. …

Modelling offshore sand wave evolution

We present a two-dimensional vertical (2DV) flow and morphological numerical model describing the behaviour of offshore sand waves. The model contains the 2DV shallow water equations, with a free water surface and a general bed load formula. The water movement is coupled to the sediment transport equation by a seabed evolution equation. Using this model, we investigate the evolution of sand waves in a marine environment. As a result, we find sand wave saturation for heights of 10–30% of the average water depth on a timescale of decades. The stabilization mechanism, causing sand waves to saturate, is found to be based on the balance between the shear stress at the seabed and the principle that sediment is transported more easily downhill than uphill. The migration rate of the sand waves decreases slightly during their evolution. For a unidirectional steady flow the sand waves become asymmetrical in the horizontal direction and for a unidirectional block current asymmetrical in the vertical. A sensitivity analysis showed the slope effect of the sediment transport plays an important role herein. Furthermore, the magnitude of the resistance at the seabed and the eddy viscosity influence both the timescale and height of sand waves. The order of magnitudes found of the time and spatial scales coincide with observations made in the southern bight of the North Sea, Japan and Spain.

Unidirectional flow of a thin rivulet on a vertical substrate subject to a prescribed uniform shear stress at its free surface

We use the lubrication approximation to analyze the steady unidirectional flow of a thin rivulet on a vertical substrate subject to a prescribed uniform longitudinal shear stress at its free surface. In particular, we determine the quasisteady stability of a rivulet of prescribed flux, and calculate when it is energetically favorable for a rivulet to split into two narrower rivulets.

On applicability of reciprocating flow turbines developed for wave power to tidal power conversion

Tidal power generation with reciprocating turbines in a simple system is investigated on a performance simulation in order to enlarge the capability of practical use of tidal power with extra-low head and time-varying energy density characteristics. Four reciprocating turbines, which are two types of impulse and a Wells developed for wave power conversion systems, and a cross-flow type of Darrieus for extra-low head hydropower are focused for utilizing extra-low head tidal power. Their turbine characteristics in a unidirectional steady flow obtained by physical test models are compared in non dimensional forms and power plant performance with the turbines are numerically simulated on equivalently scaled turbines based on the low of similitude on turbine performance with the non dimensional characteristics under one of the simplest controls in combination with suitable reservoir ponds area. The output of the power plant depends on tidal difference and a pond inundation area. The results are summarized and discussed on the averaged electric output of the power plant and the optimum scale of pond inundation area.

Surface-tension-driven flow outside a slender wedge with an application to the inviscid coalescence of drops

We consider the two-dimensional inviscid flow that occurs when a fluid, initially at rest around a slender wedge-shaped void, is allowed to recoil under the action of surface tension. As noted by Keller &amp; Miksis (1983), a similarity scaling is available, with lengths scaling like in the presence of viscosity is simply connected (Billingham 2005), the free surface must first pinch off at some finite time and then continue to do so at a sequence of later times. We investigate this using boundary integral solutions of the full inviscid initial value problem, with smooth initial conditions close to those of the original problem. In addition, we show that the inner asymptotic scalings that we developed for the steady problem can also be used in this time-dependent problem. The unsteady inner equations reduce to those for steady unidirectional flow outside a region of constant pressure, and can be solved numerically.We also show how the slender wedge solution can be related to the small-time behaviour of two coalescing drops, and describe the relationship between our solutions and those found by Duchemin, Eggers &amp; Josserand (2003), for which a similar unsteady inner region exists. In each case, the free surface pinches off repeatedly, and no similarity solution exists.

Numerical modeling of flow and scour below a pipeline in currents: Part II. Scour simulation

A vertical two-dimensional (2D) numerical model for time dependent local scour below offshore pipelines subject to unidirectional steady flow is developed. The governing equations for the flow and sediment transport are solved by using finite difference method in a general curvilinear coordinate system. The performance of two turbulence models, the standard k– ɛ model and Smagorinsky subgrid scale (SGS) model, on modeling time dependent scour processes is examined. Both suspended load and bed load are considered in the scour model. The suspended-load model is verified against two channel sediment transport cases. The change of bed level is calculated from the continuity equation of total sediment transport. A new time marching scheme and a sand slide scheme are proposed for the scour calculation. It is found that the proposed time marching scheme and sand slide model work well for both clear-water and live-bed scour situations and the standard k– ɛ turbulence closure is more preferable than the SGS model in the 2D scour model developed in this study.

2D CFD analysis and experimental analysis on the effect of hub to tip ratios on the performance of 0.6 m impulse turbine

The objective of this paper is to present the performance comparison of 2D Computational Fluid Dynamics (CFD) analysis with experimental analysis of 0.6 m impulse turbine with fixed guide vanes for both 0.6 and 0.7 hub to tip ratio (H/T). Also the comparison of 2D CFD analysis of the said turbine with different values of H/T ranging from 0.5 to 0.7. A 2D-cascade model was used for CFD analysis while uni-directional steady flow was used for experimental analysis. The blade and guide vane geometries are based on 0.6 m rotor diameter, with optimum profile, and different H/T of 0.5, 0.6 and 0.7. It was concluded from 2D CFD analysis that 0.5 H/T ratio performances was higher than that of 0.6 and 0.7 H/T at peak efficiency and the operational flow range for 0.5 H/T was found to be wider than that of 0.6 and 0.7 H/T ratio.

Effects of turbine damping on performance of an impulse turbine for wave energy conversion under different sea conditions using numerical simulation techniques

This paper presents the work carried out to predict the behavior of a 0.6 m impulse turbine with fixed guide vanes with 0.6 hub to tip (H/T) ratio under real sea conditions.This enhances the earlier work done by authors on the subject by including the effects of damping applied by the turbine. Real wave data for different wave sites were used as the input data. A typical oscillating water column (OWC) geometry has been used for this simulation. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. Considering the quasi-steady assumption, uni-directional steady flow experimental data were used to simulate the turbine characteristics under irregular unsteady flow conditions. The test rotor used for this simulation consisted of 30 blades with elliptical profile with a set of symmetric, fixed guide vanes on both up-stream and down-stream sections of the rotor, with 26 vanes each. The results show that the performance of this type of turbine depends on the level of damping applied by the turbine and the prevailing wave site conditions. The objective of this paper is to predict the effects of applied damping on the behavior of impulse turbine under irregular, unsteady conditions for wave power conversion using numerical simulation.

Effects of turbine damping on performance of an impulse turbine for wave energy conversion under different sea conditions using numerical simulation techniques

This paper presents the work carried out to predict the behavior of a 0.6 m impulse turbine with fixed guide vanes with 0.6 hub to tip (H/T) ratio under real sea conditions.This enhances the earlier work done by authors on the subject by including the effects of damping applied by the turbine. Real wave data for different wave sites were used as the input data. A typical oscillating water column (OWC) geometry has been used for this simulation. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. Considering the quasi-steady assumption, uni-directional steady flow experimental data were used to simulate the turbine characteristics under irregular unsteady flow conditions. The test rotor used for this simulation consisted of 30 blades with elliptical profile with a set of symmetric, fixed guide vanes on both up-stream and down-stream sections of the rotor, with 26 vanes each. The results show that the performance of this type of turbine depends on the level of damping applied by the turbine and the prevailing wave site conditions. The objective of this paper is to predict the effects of applied damping on the behavior of impulse turbine under irregular, unsteady conditions for wave power conversion using numerical simulation.

Homotopy analysis of MHD flows of an Oldroyd 8-constant fluid

This paper deals with some steady unidirectional flows of an Oldroyd 8-constant magnetohydrodynamic (MHD) fluid in bounded domains. The fluid is electrically conducting in the presence of a uniform magnetic field. Three nonlinear flows are produced by the motion of a boundary or by sudden application of a constant pressure gradient or by the motion of a boundary and pressure gradient. The governing nonlinear differential equations are solved analytically using homotopy analysis method (HAM). Expressions for the velocity distribution are given. It is noted that for steady flow the solutions are strongly dependent on the non–Newtonian and magnetic parameters. The MHD solutions for a Newtonian fluid, as well as those corresponding to the Oldroyd 3 and 6-constant fluids, a Maxwell fluid and a second grade one, appear as limiting cases of our solutions. Finally, a physical interpretation of the results is given with the help of several graphs.

Steady unidirectional laminar flow inhibits monolayer formation by human and rat microvascular endothelial cells.

Endothelialization of artificial vascular grafts is rapid and complete in numerous animal models, including dogs and rats, but not in human patients. One possible explanation for this well-known, yet puzzling observation might be that monolayer formation of human endothelial cells (ECs), and of canine or rodent ECs, is affected differently by flow-induced shear stress. To begin testing this hypothesis, the authors wounded confluent monolayers of cultured rat and human ECs and exposed these cultures for 20 h to unidirectional steady laminar shear stress of 10 dyn/cm(2) induced by fluid flow perpendicular to the wound boundaries. In comparison to experimental control cultures simultaneously maintained under static (no-flow) conditions, flow-induced shear stress attenuated the monolayer formation (sheet migration) in both human and rat ECs. In brief, compared to control, the average human EC monolayer formation under shear was reduced by 33% whereas the average rat EC monolayer formation was reduced by 34%. Furthermore, the cell responses showed a dependence on fluid flow direction that differed per species. When exposed to shear stress, human EC monolayer formation was reduced by 16% in the upstream direction (opposing the direction of flow) and reduced by 50% in the downstream direction (with the direction of flow), whereas rat EC monolayer formation was reduced by 64% upstream and showed no change downstream. These findings suggest that although overall monolayer formation is inhibited by fluid-induced shear stress to the same extent in both species, there are cell type- and/or species-dependent migration responses to fluid-induced shear stress, and that different flow conditions possibly contribute to species-specific patterns of endothelialization.