Wall Shear Force Research Articles

On scaling the mean momentum balance and its solutions in turbulent Couette–Poiseuille flow

The statistical properties of fully developed planar turbulent Couette–Poiseuille flow result from the simultaneous imposition of a mean wall shear force together with a mean pressure force. Despite the fact that pure Poiseuille flow and pure Couette flow are the two extremes of Couette–Poiseuille flow, the statistical properties of the latter have proved resistant to scaling approaches that coherently extend traditional wall flow theory. For this reason, Couette–Poiseuille flow constitutes an interesting test case by which to explore the efficacy of alternative theoretical approaches, along with their physical/mathematical ramifications. Within this context, the present effort extends the recently developed scaling framework of Weiet al. (2005a) and associated multiscaling ideas of Fifeet al. (2005a,b) to fully developed planar turbulent Couette–Poiseuille flow. Like Poiseuille flow, and contrary to the structure hypothesized by the traditional inner/outer/overlap-based framework, with increasing distance from the wall, the present flow is shown in some cases to undergo abalance breakingandbalance exchangeprocess as the mean dynamics transition from a layer characterized by a balance between the Reynolds stress gradient and viscous stress gradient, to a layer characterized by a balance between the Reynolds stress gradient (more precisely, the sum of Reynolds and viscous stress gradients) and mean pressure gradient. Multiscale analyses of the mean momentum equation are used to predict (in order of magnitude) the wall-normal positions of the maxima of the Reynolds shear stress, as well as to provide an explicit mesoscaling for the profiles near those positions. The analysis reveals a close relationship between the mean flow structure of Couette–Poiseuille flow and two internal scale hierarchies admitted by the mean flow equations. The averaged profiles of interest have, at essentially each point in the channel, a characteristic length that increases as a well-defined ‘outer region’ is approached from either the bottom or the top of the channel. The continuous deformation of this scaling structure as the relevant parameter varies from the Poiseuille case to the Couette case is studied and clarified.

Development of Novel Submicron Textured Polyether(Urethane Urea) for Decreasing Platelet Adhesion

Ventricular assist devices have proven to be a useful clinical option for providing circulatory support as a bridge to transplantation and a mode of destination therapy. Thromboembolism is prevented by designing devices that use blood interfaces that either encourage biological material deposition and strong adhesion, or discourage deposition via surface chemistry, surface finish, and fluid flow fields. Minimum continuous or periodic wall shear forces and maximum time at reduced shear are important, and sometimes difficult-to-satisfy, design constraints. We present an approach to reducing platelet adhesion via surface topography, reducing surface area for platelet-material interaction. Large areas of polyether(urethane urea) were textured with two different sizes of ordered pillar arrays via two-stage replication molding without affecting surface chemistry. Pillars had subplatelet dimensions designed to reduce the surface area a platelet may contact. Platelet adhesion was assessed in a physiologically relevant shear stress range from 0-10 dyn/cm2 using a rotating disk and compared to smooth control. Adhesion was highest from 0-5 dyn/cm2. Surface texturing reduced platelet adhesion without increasing platelet activation in bulk suspension. This study demonstrates that material surface texture is an additional variable that may be used to reduce platelet adhesion under low shear stresses potentially reducing thromboembolism.

G603 擬似圧縮性法による3次元CFDコード"MATIS-I"の開発と縮流部流れへの適用(GS-6 数値シミュレーション(1),一般セッション)

To clarify and modify the flow instabilities of the steam control valve in the power plant, we have researched by CFD approach. But there are many troubles in the water system like pressure pulsation caused by pump and valve. For precise researches for them, we constructed CFD "MATIS-I" code for the incompressible fluid. This code is based on pseido-compressible flow method. This method can apply the computational scheme for compressible fluid. After some benchmark tests, we applied the code to the high Reynolds number flow around orifice to investigate the erosion effect for wall wearing. As a result, we found that the wall shear force by the fluid is less than 100Pa, and is very small as compared to the shear stress of the carbon steel.

Impact of Shear Stress on Early Vein Graft Remodeling: A Biomechanical Analysis

In this study, an analytical technique was developed to estimate the dynamic, three-dimensional wall shear and tensile forces within the arterial circulation and applied to our established model of flow-mediated graft remodeling within the rabbit carotid system. Through selective unilateral distal ligation, vein grafts were exposed to distinct flow environments, characterized by a sixfold difference in mean flow rate. Implanted vein grafts were harvested at 1, 3, 7, 14, and 28 days to evaluate graft morphology. Hemodynamic and real-time imaging data, obtained at graft implantation and harvest, served as input for estimation of dynamic shear stress and wall tension. Marked differences in the remodeling process were observed in high vs. low flow grafts, with low flow grafts demonstrating accelerated intimal hyperplasia and reduced outward remodeling. The impact was a peak in the maximum and minimum shear stress at Day 7, with a delayed increase in lumen diameter leading to partial normalization of the wall shear by Day 28. Intramural wall tension reached a maximum at Day 3, with an increase in wall thickness leading to a significant reduction in these forces by Day 14. Despite the significant morphologic changes, no differences in the incremental moduli of elasticity were observed.

Drag Force of Non-newtonian Fluid on a Continuous MovingSurface with Strong Suction/Blowing

A theoretical analysis for the laminar boundary layer flow of anon-Newtonian fluid on a continuous moving flat plate with surfacestrong suction/blowing is made. The types of potential flows necessaryfor similar solutions to the boundary layer are determined and bothanalytical and numerical solutions are presented. It is shown that thesolution of the boundary layer problem depends not only on the ratio ofthe velocity of the plate to the velocity of the free stream, but alsoon the suction/blowing parameter. The skin friction decreases with increasing the parameters of power law and blowing. In the case ofexisting suction, the shear force decreases with the increases oftangential velocity, the largest shear force occurs at wall and thesmallest shear force occurs at the edge of the boundary layer. However,in the case of existing surface blowing, the shear force initiallyincreases with tangential velocity and the biggest shear force occursat the interior of the boundary layer, the skin friction approaches tozero as the blowing rate approaches the critical value.

STRESS CONCENTRATION DUE TO ADVANCING HEART PULSE THROUGH A BLOOD VESSEL JOINT

The effect of a vessel joint of different properties on the heart pulse propagation and anastomosis junction is investigated. The system under consideration is modelled by an elastic piecewise-constant vessel consisting of two perfectly jointed vessels of different thicknesses and radii. A mathematical model is developed, to perform numerical calculations and to obtain some qualitative and quantitative estimations. The effect of different thicknesses and radii of jointed blood vessels on the concentrations of pressure, radial vessel wall displacement, bending moments and shear forces are analyzed in detail.

Mixing And Emulsification Processes InMicromixers

Mixing and emulsification processes in micromixers developed by the Institute of Microtechnology Maim (IMM) are investigated by means of computational fluid dynamics (CFD). For both miscible and immiscible fluids the CFD results are compared to experimental data obtained with transparent micromixer made from photosensitive glass. It is found that in the case of miscible liquids the accuracy of numerical predictions is limited by artifacts from numerical diffusion. However, streamline tracking techniques offer an alternative method for computing flow patterns in micrornixers. Corresponding flow patterns comply with photographic images of the mixing zone. Subsequently, the case of immiscible liquids is investigated using volume tracking techniques. The simulations, based on a 3D model for a planar interdigital micromixer, describe the time evolution of droplet formation. Data on droplet size and the decay wavelength of the liquid lamellae are in agreement with experimental results. Both the simulations and the experiments indicate that surface tension forces play the dominant role for droplet formation. Thus, the results suggest that even in the presence of wall boundaries and shear forces the Rayleigh-Plateau instability governs the dynamics.

Boundary Shear in Circular Pipes Running Partially Full

The distribution of boundary shear stress in circular conduits flowing partially full, with and without a smooth flat bed simulating deposited sediments, has been examined experimentally ranging from 0.375 < F < 1.96 (F = Froude number) and 65,000 < R < 342,000 (R = hydraulic radius), using the Preston tube technique. The invert level of the flat bed and the water depth have been varied to simulate a wide range of possible flow conditions that may occur in culverts, sewers, and hydropower tunnels. The distribution of boundary shear stress around the wetted perimeter is shown to be highly sensitive to changes in cross-sectional shape. The results have been analyzed in terms of the variation of local/global shear stress versus perimetric distance, and the percentage of the total shear force acting on the wall or bed of the conduit. The %SFw results (%SFw = the percentage of the average wall shear force of the overall shear force) have been shown to agree well with Knight's empirical formula for prismatic channels. The influence of secondary flows on the distribution of boundary shear stress and the implications of this for sediment transport have also been examined.

Wall Remodeling after Wall Shear Rate Normalization in Rat Mesenteric Arterial Collaterals

Previous studies have demonstrated endothelial and smooth muscle hyperplasia occur during arterial luminal expansion associated with elevation of arterial wall shear rates. The current study investigated whether remodeling induced by elevated wall shear would ultimately result in a vessel with intimal and medial cell densities and other wall characteristics similar to control arteries. A rat mesenteric model was used in which collateral wall shear is restored to normal 4 weeks after arterial occlusion. Twelve weeks after shear elevation, paired in vivo measurements indicated that the maximum collateral inner diameter was increased 27–75%. Morphometric evaluation of collateral cross sections indicated that, relative to control arteries, luminal and medial areas were increased 79 ± 22 and 56 ± 15%. Collateral medial cell density was decreased (1.12 ± 0.044 vs. 1.35 ± 0.005 nuclei/1,000 µm²) but intimal cell density was similar (2.86 ± 0.166 vs. 2.49 ± 0.102 nuclei/100 µm luminal perimeter). Medial thickness to radius ratio was also similar between control and collateral arteries. Thus, for the wall characteristics evaluated, there are many similarities between enlarged collaterals and control arteries. Comparison of nuclear numbers in arterial cross sections of the current and previous studies suggest that intimal and medial cellular regression is correlated with a decrease in wall shear force toward normal levels.

Mechanical effects of heart pulse propagation on a vessel-graft suture line stress

The effects of vessel joint where the both sides have different wall properties on the heart pulse propagation are investigated. Such a local disturbance can influence post-transplantion pathology and evolution of the organ inconsistence. Using a mathematical model, developed in a previous article, we perform analytical analysis and present some qualitative and quantitative estimations. The effects of jointed vessels with different thicknesses and radii on the local concentration of the pressure, radial wall displacement, bending moment and shear force are analyzed in detail. In particular, it is obtained that the bending and shear stresses at a joint sharply and strongly increase in comparison with the uniform vessel ones. (Int J Artif Organs 1998; 21: 114–8)

Human melanoma integrins contribute to arrest and stabilization potential while flowing over extracellular matrix.

To form distant metastases, tumour cells must stabilize adhesive interactions that prevent detachment at secondary sites. Primary receptor-ligand interactions alone may not maintain prolonged adhesive contacts without secondary events that lead to adhesion stabilization. Computerized imaging methods enable us to examine various substrates for: (i) the wall shear adhesion threshold (WSAT), a measure of the dynamic adhesive potential of tumour cells; (ii) the number of tumour cells that adhered; and (iii) the adhesion stabilization lag time (ASLT) or length of time required for tumour cells to stabilize adhesive contacts capable of withstanding high wall shear force (up to 100 dynes/cm2). The relative WSAT ratios found were: wheat germ agglutinin (WGA) > laminin > fibronectin > vitronectin > collagen I > collagen IV > von Willebrand factor (vWF) (the greater the shear rate the higher the adhesive potential). The relative stabilization ratios found were as follows: laminin < fibronectin < vitronectin < collagen IV < collagen I < vWF < WGA (shorter times correlate with greater stabilization potential). Stabilization data using fibronectin as a substrate correlated the best with metastatic potential. Using three melanoma lines of different metastatic potential semiquantitative reverse transcriptase-polymerase chain reaction (PCR) showed a two- to four-fold increase in alpha1, alpha3, alpha4, alpha5, alpha6, and ICAM-1 in the highly metastatic 70W cells compared to the MeWo and non-metastatic 3S5 melanoma cells. There were no differences in alphav, beta1 and beta3 levels among the three melanoma lines, and PCR products for alphaIIb, alpha2, CD36, or ICAM-2 were not detected. The 70W cells also had higher levels of alphax and beta2 (CD11/CD18 and p150 leukocyte antigen) than either the MeWo or 3S5 cells. The data indicate that melanoma cells exhibit differences in the adhesion properties under fluid shear and differences in the expression of adhesion components that correlate with their metastatic potential.

Synergistic action of severe wall injury and shear forces on thrombus formation in arterial stenosis: Definition of a thrombotic shear rate threshold

Objectives. This study attempted to determine the influence of progressive degrees of stenosis on platelet deposition onto a severely damaged vessel wall.Background. The severity of wall injury and increased shear forces have been proposed as the determinants of thrombus formation and growth in arterial stenosis.Methods. Carotid angioplasty was performed in 15 mongrel dogs to produce severe wall damage. Group I (n = 9) had arteries with damage only. In group II (n = 14), progressive degrees of stenosis were produced at the center of the dilated area. Acute thrombus formation was evaluated by angiography at the time of angioplasty and platelet deposition/cm2quantified by indium-111 labeling 1 h after the procedure.Results. Severe wall damage (group I) produced a significant increase in platelet deposition compared with control arterial segments (8.19 ± 3.82 vs. 3.62 ± 2.52 platelets × 106/cm2[mean ±SD], p < 0.05), and the presence of a stenosis (group II) further increased platelet deposition (36.98 ± 3.82 platelets × 106/cm2, p < 0.05). Angiographic filling defects or total occlusion was found in seven of the arteries in group II but in none in group I (p < 0.05). A shear rate of ~5,000 s−1, corresponding to a critical stenosis of 70% and a 1.5- to 1.6-mm diameter, was found to identify the arteries in which thrombosis was likely to occur (p < 0.05). Four of 5 arteries <1.5 to 1.6 mm in diameter had angiographic filling defects or occlusion compared with 1 of 13 with less severe stenosis (p < 0.01).Conclusions. In low shear rate conditions, deep arterial injury will lead to mural thrombosis without further thrombus growth. When deep arterial injury occurs under critical local shear conditions, platelet deposition will be enhanced, and thrombosis may progress to total occlusion.

An Experimental Investigation of Pure-Substance, Adiabatic Two-Phase Flow in a Vertical Pipe

Measurements of pressure drop, temperture, and average void fraction are presented for adiabatic, vertical-upwards, two-phase flow of Refrigerant 114 in a pipe. An experimental method has been developed according to which the evolution of flow states occurring in long pipes can be realized in a test section of limited length. The experiments cover the range of the flow from flashing to near choking. The measurements indicate existence of macroscopic thermodynamic equilibrium, except in the immediate neighborhood of flashing. Compressibility due to phase change is shown to play a very important role in the development of the flow. Three regions are recognized based on the measured energetics of the flow. Each region is dominated by potential energy changes, dissipation, and kinetic energy changes, respectively. The evolution of the flow is governed by hydrostatic effects in the initial region after flashing and by high, phase-change-induced kinetic energy increases far downstream as the flow approaches choking. In the intermediate region, viscous, inertial and gravitational effects play a role of comparable importance. The interfacial and wall shear forces have also been calculated from the measurements. The former dominate the initial regions of the flow, while the latter are strongest at high vapor contents.

In vivo shear flow and erythrocyte membrane fluidity in hypertensive patients.

1. To evaluate the response of red blood cells subjected to the shear flow in hypertension, the relationships between wall shear phenomena determined in vivo in the brachial artery of hypertensive patients and the modifications of the membrane dynamics measured in vitro in erythrocyte ghosts of 32 patients were investigated. 2. Two fluorescent probes, diphenylhexatriene (DPH) and its trimethylamino-derivative (TMA-DPH), localized respectively in the lipid membrane core and at the lipid-water interface, were used. 3. Shear rate, shear stress and blood velocity were positively correlated with TMA-DPH anisotropy (P = 0.015, 0.005 and 0.026, respectively), but not with that of DPH. This indicates that wall shear forces were associated with the microviscosity of the outer part of the cell membrane. 4. The changes in wall shear forces and erythrocyte membrane microviscosity probed by TMA-DPH or DPH were observed to vary in parallel under nitrendipine therapy. 5. These results suggest that in vivo shear forces participate in the control of erythrocyte membrane fluidity or that erythrocytes adapt their membrane properties to blood flow conditions.

Boundary Layer Flow Over Long Cylinders With Suction

The full, axisymmetric boundary layer equations over a long circular cylinder have been repeatedly solved in the past by a variety of approximate Pohlhausen-type and series expansion methods. These methods are not suitable however, for axisymmetric solutions with wall suction boundary condition. In the present paper, the basic equations are solved by a simple approximate method, based on successive derivatives of the wall compatibility condition. A family of velocity profiles, which coincides with the asymptotic suction profile at infinity, is assumed, and the wall shear force is found by integrating an ordinary differential equation. The computed boundary layer properties for the impermeable (zero-suction) cylinder case are shown to be in good agreement with solutions of other, more sophisticated techniques. The case of an infinite cylinder with constant suction asymptotically approaches the well-known constant thickness boundary layer solution, in analogy with the case of constant suction on a flat plate. The present method is further explored to exhibit the drag reduction properties of wall suction. An optimal suction profile is found, as a function of free stream conditions and the cylinder fineness ratio. For example, in the case of a cylinder fineness ratio 5, and Re = 25 × 106, the flow can be laminarized with only 80 percent of the mass sucked off, relative to uniform suction.

A study of the inelastic seismic response of reinforced concrete coupled frame-shear wall structures

Three 12 storey frame-shear wall buildings with varying wall size were designed according to capacity design principles. Their response to the El Centro NS 1940 and Pacoima Dam S15°W 1970 accelerograms was investigated using a 2-dimensional nonlinear dynamic analysis program. Particularly significant observations were the high levels of wall shear forces and generally low levels of column bending moment encountered. Wide spread beam flexural yielding involving moderate levels of inelastic deformations and interstorey drifts controlled by the walls indicate the potentially desirable inelastic performance of these hybrid structures. Design schemes are proposed whereby improved estimation of maximum inelastic actions can be made using the traditional elastic analysis for equivalent lateral static loading on the structure.

Aggregation of platelets in damaged vessels.

The only certain physiological function of platelets is their aggregation in injured vessel walls as haemostatic plugs. The association of thrombocytopenia with petechial haemorrhages suggests that platelets are somehow required for the functional integrity of small vessels, but no mechanism has yet been established. The pathological aggregation of platelets as thrombi in atherosclerotic arteries is commonly, if not always, initiated by haemorrhage. In artificial vessels, platelets tend to aggregate on the walls wherever blood flow is non-laminar. The mural aggregation of platelets is not prevented by unphysiologically high wall-shear forces. The facts suggest, on the contrary, that the process depends in some way on abnormal haemodynamic conditions. This contribution is mainly concerned with questions about how haemodynamic conditions in and around vascular leaks affect arriving platelets that aggregate there, and about the chemical agents responsible for making the platelets reactive. The effects of these agents are known mainly from in vitro experiments in which aggregation can be quantitatively correlated with biochemical effects by simple and reproducible methods; the relevance to their reactions in haemostasis and thrombosis is uncertain. It is difficult to devise quantitative methods for analysing these processes in vivo because of the very low concentrations at which endogenous agents can activate platelets and haemostasis factors in the plasma; the rapidity with which platelets aggregate in a damaged blood vessel; and the complexity and inconstancy of the haemodynamic situation. All these factors must be accounted for in hypotheses of haemostasis. New experimental approaches towards analysing the haemostatic mechanism in vivo are described.

Viscous Effects on Slip Factor of Centrifugal Blowers

For the design of centrifugal blowers it is essential to estimate the slip factor correctly. There are several empirical equations as well as theoretical ones for estimating slip factors. These equations predict the slip factor correctly for conventional blowers and pumps, and they are widely used for design. According to our experience for high specific speed blowers, however, the experimental slip factor was considerably smaller than the prediction based on these equations, and as a result the blowers which were designed using the predicted slip factor did not accomplish the design goal. In this paper the flow in shrouded centrifugal impellers with backward leaning blades is analyzed assuming that the flow is quasi-two-dimensional, steady, subsonic and inviscid, and then the slip factor is corrected to include viscous effects such as blockage of flow passage and variation of blade shape due to boundary layer, as well as the change of moment of momentum due to the wall shear force inside the impeller. By incorporating these corrections the slip factor based on inviscid theory agrees well with experimental slip factor for various impellers in a wide range of specific speed.

Endothelium and Atherosclerosis

To review the effect of damaged endothelium on the development of atherosclerotic disease.Atherosclerotic cardiovascular disease is a complex problem involving lipid deposition, blood pressure, rheologic forces, carbohydrate tolerance and thrombogenic factors. The loss of functional, if not structural, integrity of the vascular endothelium is closely related to the initiation of atherosclerosis. The endothelium contributes to local vascular regulation. Normal endothelial cells are thrombo-resistant; they prevent leukocyte adhesion and control vascular tone by converting angiotensin I into angiotensin II, inactivating bradykinin, norepinephrine, serotonin and ADP, and by secreting vasodilator substances, such as prostacyclin and endothelium-derived relaxing factor (EDRF), and contracting factors such as endothelin. Endothelin is a potent vasoconstrictor peptide that increases intracellular calcium, causing a rapid and transient increase in c-fos and c-myc messenger (m)RNA levels and DNA synthesis in rat vascular smooth muscle cells. In isolated vessel segments, altered endothelial vasoreactivity is usually demonstrated following mechanical trauma to the endothelium.We investigated the function of normal and damaged endothelium in pigs, following superficial balloon injury, which produces a significant alteration in endothelium-dependent coronary vasoreactivity. The anesthetized pigs were given an intracoronary (left anterior descending artery) infusion of acetylcholine. The balloon injury caused a local transient spasm while the distal uninjured vessel did not change. When acetylcholine was given before the balloon injury, the diameter of the left anterior descending artery did not change, even after preconstriction in vivo with prostaglandin (PG) F2 alpha, but acetylcholine given after the balloon angioplasty caused dose-dependent vasoconstriction. It is known that vasoconstriction in arteries can reduce blood flow and increase arterial wall-shear forces, which increase platelet deposition in injured arteries and may precipitate rupture of atherosclerotic plaques.The intact endothelium is one of the greatest sources of protection from arterial thrombosis, atherosclerosis and vasoconstriction.

Adiabatic, evaporating, two‐phase flow of steam and water in horizontal pipe

AbstractThe adiabatic, evaporating, two‐phase flow of steam and water in horizontal pipe including the critical flow was successfully described by a system of nonlinear differential equations which include different phase velocities, fluid acceleration, wall shear forces, interface shear forces, and mass and energy transfer between the phases. The solution of the differential equations by the method of Runge‐Kutta was facilitated with a high‐speed digital computer. Experimental data were obtained which included void fraction measurement by X‐ray attenuation and showed that the theoretical equations described the complex flow phenomena to ± 10% when radial temperature gradients were small. Design charts based on the numerical solution of the theoretical equations are presented for rapid evaluation of the flow variables for the system steam‐water including critical flow for inlet pressures from 30 to 150 lb./sq.in.abs., mass flow rates per unit area from 300 to 1,000 lb./sq. ft. sec., and L/D ratios from 25 to 1,000. The critical flow is defined as the maximum flow rate for a given pressure at the point of initial vaporization and L/D ratio.