Shear Stress Parameters Research Articles

Wall shear stress and early atherosclerotic lesions in the abdominal aorta in young adults

To study the correlation between wall shear stress and early atherosclerotic lesions in the abdominal aorta. Blinded histomorphometric studies. Comparison with in vitro data. Abdominal aortic haemodynamics were simulated in a realistic pulsatile flow model. Abdominal aortas from 10 young adults with no signs of atherosclerotic disease were obtained during autopsy. Quantitative wall shear stresses were measured at rest and exercise in one suprarenal and two infrarenal positions using laser Doppler anemometry. Intimal thickening indices were measured blindly at the corresponding locations using histomorphometric methods, and compared to wall shear stress variables using linear regression analysis. Intimal thickness index increased significantly with age. Intimal thickness index was significantly lower in the suprarenal than the infrarenal aorta, and higher at the distal posterior vessel wall compared to the anterior wall. Intimal thickness index correlated significantly with mean, minimum and oscillating wall shear stresses measured at rest. Intimal thickness in the undiseased abdominal aorta correlated significantly with mean, minimum and oscillating wall shear stresses at rest measured in a pulsatile flow model. No correlations were found with maximum shear stress parameters. Exercise changed the local wall shear stresses away from the characteristics associated with intimal thickness index.

Mixed binary convection in a rotating disk chemical vapor deposition reactor

This study examines the effects of gas mixing on flow behavior in a rotating disk reactor. We restrict the study to an isothermal binary gas system flowing over a high speed rotating disk in a cylindrical reactor. Complex flow fields are produced as a result of the interaction that occur between the solutal buoyant force, the forced flow, and the flow induced by the rotation of the disk. Deviation from the ideal rotating disk flow is quantified with a radial shear stress parameter.

Stress‐induced azimuthal anisotropy in borehole flexural waves

A nonlinear elastic model for acoustic waves in a stressed medium is used to calculate tectonic stress‐induced changes in borehole flexural dispersions. Our theoretical analysis shows that a horizontal uniaxial stress in the formation causes a crossover in flexural dispersions for the radial polarization aligned parallel and normal to the stress direction. This crossover in flexural dispersions is caused by stress‐induced radial heterogeneities in acoustic wave velocities that are different in the two principal stress directions. Other sources of borehole flexural anisotropy caused by finely layered dipping beds, aligned fractures, or microstructures found in shales, exhibit neither such radial heterogeneities nor flexural dispersion crossovers. Consequently, a crossover in flexural dispersion can be used as an indicator of stress‐induced anistropy. In this situation, the fast shear polarization direction coincides with the far‐field uniaxial stress direction. The analysis also yields an expression for the largest shear stress parameter in terms of the fast and slow seismic shear‐wave velocities with shear polarization parallel and perpendicular to the far‐field stress direction.

Field Experiments on Erosion by Overland Flow and Their Implication for a Digital Terrain Model of Channel Initiation

Dietrich et al. (1992, 1993) proposed a digital terrain model for predicting the location of channel heads on the basis of the assumption that they occur where saturation overland flow exerts a boundary shear stress in excess of a critical value. Flume experiments were conducted in the modeled field site to evaluate the threshold hypothesis and to constrain critical shear stress and flow resistance parameters. Under complete grass cover, microtopography and grass stems were found to prevent significant sediment transport at all but the highest flows. When the grass stems were cut close to the ground, flow resistance and critical shear stress for significant sediment transport were reduced by up to an order of magnitude, but the remaining dense root mat prevented deep scour. These field experiments support the threshold assumption and the model estimations of the critical shear stress if local topographic convergence of flow is taken into account. The experiments also support the interpretation that significant degradation of vegetation cover is required for channel incision.

The effect of osmotic and mechanical stresses and enzymatic digestion on the electro-rotation of insect cells ( Spodoptera frugiperda)

The electro-rotation technique was used to obtain information on the variation in electrical properties between individuals of a cell line derived from ovary cells of the insect Spodoptera frugiperda. Cell rotation was induced in media of low electrical conductivity by means of the interaction of an induced cellular dipole with a rotating electrical field. Measurements of the field frequency giving fastest rotation and of this maximum speed itself allow membrane parameters such as membrane capacity to be determined. The influence on the membrane parameters of prolonged centrifugation, shear stress, osmotic stress, and partial digestion of the membrane proteins by trypsin were studied. It was found that centrifugation at up to 3000 × g and hypertonic stress up to 520 mosmol kg −1 do not cause any permanent change in the membrane structure, whereas hypotonic stress (down to 80 mosmol kg −1) and shear stress (5.9 N m −2) lower the membrane conductivity and accelerate the rotation speed of the cells. The membrane properties were changed after trypsin treatment, presumably due to digestion of membrane proteins.

Simplified Solution for Stresses and Deformations

A shortcut to the classical Hertzian solution for local stress and deformation of two elastic bodies in contact is presented. The shortcut is accomplished by using simplified forms for the ellipticity and for the complete elliptic integrals of the first and second kinds as a function of the geometry. Thus the interdependence of these variables can be uncoupled, and the resulting transcendental equation, which must be solved through use of the computer or design charts, avoided. Simplified formulas that make the elastic deformation at the center of contact easy to calculate have been previously reported by the authors. However, the range of applicability was limited to ellipticities greater than or equal to 1. This paper extends the range of validity to include ellipticities less than 1, that is, where the semimajor axis in the elliptical contact lies in a direction parallel to the rolling direction rather than being perpendicular as in previous studies. Furthermore an auxiliary shear stress parameter is expressed in simplified form as a function of the geometry. This enables a shortcut calculation to be made for the location and magnitude of the maximum subsurface shear stress.

A Film Thickness Analysis for Line Contacts Under Pure Rolling Conditions With a Non-Newtonian Rheological Model

The non-Newtonian constitutive equation proposed by Winer and Bair [1] is applied in a conventional isothermal film thickness analysis of line contact lubrication of rolling elements. The present analysis provides four different dimensionless film thickness equations for four different regimes of lubrication. Due to the formulation technique used in deriving the governing pressure-gradient equation, the present study is recommended for high viscosity, high rolling speed, and low limiting shear stress cases where Newtonian models fail to match the experimental data. Comparison of the present film thickness equations with the Newtonian correspondences in each lubrication regime shows a considerable difference, but the analysis suffers from the fact that the limiting shear stress parameters of these high viscosity lubricants need to be determined experimentally. The present analysis assumes a reasonable range of limiting shear stress which is smaller than the corresponding values for low viscosity lubricants which are predominantly Newtonian in behavior (unless severe rolling and/or sliding with high loads is applied).

Open Channel Flow with Varying Bed Roughness

The distribution of velocity and boundary shear stress in a rectangular flume has been examined experimentally, and the influence of varying the bed roughness and aspect ratio assessed. The resistance of the channel bed was varied by means of artificial strip roughness elements, and measurements made of the wall and bed shear stresses. Dimensionless plots of both shear stress and shear force parameters are presented for different bed roughnesses and aspect ratios, and these serve to illustrate the complex way in which such parameters vary. The definition of a wide channel is also examined, and a graph giving the limiting aspect ratio for different roughness conditions is presented. The boundary shear stress distributions and isovel patterns are used to examine one of the standard side-wall correction procedures. One of the basic assumptions underlying the procedure is found to be untenable due to the cross channel transfer of linear momentum.

Analytical procedure for calculation of attached and separated subsonic diffuser flows

An analytical procedure for computing the static and total pressure distributions in subsonic diffusers with axisymmetric or two-dimension al cross sections is described. It includes velocity profile and shear stress parameters which apply to attached and separated flows and allows for a strong interaction between the inviscid and viscous portions of the duct flow. With this approach, solutions can be obtained for separated regions where conventional boundary-layer methods fail. Analytical and experimental static and total pressure distributions are compared for plane-wall and conical diffusers. An approximate technique is used to apply the analysis to actual aircraft diffusers. A Ae a0 al9...,a7 blfb2,b3 CD D d

Heat and Mass Transfer at a General Three-Dimensional Stagnation Point

Theme 1 presented detailed results for compressible laminar three-dimensional stagnation-point flow of a gas with constant properties (i.e., density-viscosity product p\i is constant and Prandtl number a is unity). This work was extended to include mass transfer (injection only) and nonunity Prandtl number by Libby. While Foots solved the governing equations numerically, using the forward integration scheme, Libby used the quasilinearization technique. In recent years, Libby and his coworkers' have extended the quasilinearization technique to study the laminar boundary-layer problem with variable gas properties at an axisymmetric stagnation point with hydrogen injection. This three-dimensional problem with variable gas properties, employing model gas equations (^ oc /f, p oc h~, a = 0.7, where h is the enthalpy and co is the viscosity exponent) without mass transfer, was considered by Wortman' and Wortman et al. With mass transfer (i.e., only injection with or without a foreign gas), it was considered by Wortman and Mills and Wortman. The governing equations in Refs. 5-9 were solved by a method based on functional analysis. The main objective here is to obtain values of critical wall parameters which were not obtained by the previous studies of this problem. With this in mind, the solution of the preceding problem with variable gas properties for both suction and injection has been obtained by the use of the method of parametric differentiation. The values of the shear stress and heat transfer parameters for various values of physical parameters of the flow are presented in tabular form. These results show that, irrespective of the nature of the stagnation point, the critical wall values are affected appreciably with the variations of co, the exponent in power law, only at low wall temperatures, their variations with co being considerably small at high values of the wall temperature. Another important feature that is observed is that the variation of the density-viscosity product (i.e., co ^ 1) across the boundary layer gives rise to a point of inflexion in the velocity and enthalpy profiles.

Heated laminarizing gas flow in a square duct

Experiments in a vertical, rounded-corner square duct are reported for heating rates which cause significant property variation in helium and nitrogen. New data are presented for adiabatic wall friction in laminar, transitional and turbulent flow and for heated flows in the low Reynolds number, turbulent range. The validity of an existing correlation for variable properties turbulent flow is extended down to Re i = 4000. In laminar flow, effects on local heat transfer parameters are slight but local friction factors vary strongly as the wall-to-bulk temperature ratio varies. Data concentrated in the range 3000 < Re i < 10 4 are examined to determine heating rates which cause premature laminarization as the Reynolds number decreases axially along the tube. Laminarization criteria based on inlet conditions correspond to values of the “critical” acceleration parameter observed for two-dimensional, accelerated, external flows. Local heat transfer measurements show a wall shear stress parameter provides a better indication of incipient laminarization along the duct than the acceleration parameter.

Turbulent Flow Characteristics of Aim Impinging JET

Measurements of shear stress, turbulence energy and other parameters were made in the impingement region of a plane two-dimensional jet confined between parallel walls. Hot wire anemometer arrays were used to examine the turbulance levels in the three directions. The object of the experiments was to make comparisons with a prediction method previously proposed. Measurements were also made in the free jet and these agreed well with the many earlier results available. In the impingement zone where the turbulence levels are very high the order of agreement is encouraging despite the uncertainties present both in the theory and the experiment. In particular, the assumption of a constant turbulence length scale in such a complex flow is not confined by the measurements.

Turbulent Flow Characteristics of an Impinging Jet

Measurements of shear stress, turbulence energy and other parameters were made in the impingement region of a plane two-dimensional jet confined between parallel walls. Hot wire anemometer arrays were used to examine the turbulance levels in the three directions. The object of the experiments was to make comparisons with a prediction method previously proposed. Measurements were also made in the free jet and these agreed well with the many earlier results available. In the impingement zone where the turbulence levels are very high the order of agreement is encouraging despite the uncertainties present both in the theory and the experiment. In particular, the assumption of a constant turbulence length scale in such a complex flow is not confined by the measurements.

Statistical Properties of Dunes Generated by Undirectional Water Currents: ABSTRACT

Field and laboratory studies of ripples and dunes formed in sand by unidirectional water currents show for coarse and very fine sand that the ripple index is related to median particle size and to flow intensity as measured by a dimensionless shear stress parameter. However, for medium-grained sand (0.25-0.50 mm.) End_Page 476------------------------------ the ripple index is approximately constant and is independent of flow intensity. For medium-grained sand statistical properties of the ripples and dunes other than the ripple index are required to correlate the characteristics of the bed configurations with flow parameters. Two methods of analysis for medium sand have been developed. In the first, spectral analyses of profiles of the channel bed lead to a linear second-order Markov model which describes the essential statistical properties of the dunes. All the parameters of the model are simple linear functions of the water discharge per unit width of channel. One of the parameters, the standard deviation of the bed elevation about the mean, shows excellent correlation (correlation coefficient of 0.99) with unit water discharge for flows ranging from 0.7 to 10 cfs. per foot of width and for average dune lengths from 2-25 feet. The second method of analysis consists of defining distributions of dune lengths, amplitudes, ripple indices, and angles of downstream faces, and relating the moments of these distributions to flow characteristics. All the distributions are skewed, and the skewness correlates roughly with flow intensity. This analysis gives useful indications of the variations in the geometric properties of ripples and dunes, but it is time-consuming and requires extensive data. Moreover, the parameters describing the ripples and dunes show only a crude relation to flow characteristics. Of the two methods, the spectral analysis appears to be more useful. End_of_Article - Last_Page 477------------

Two dimensional flow of Reiner-Philippoff fluids in the inlet length of a straight channel

In this paper, two dimensional boundary layer equations for Reiner-Philippoff fluids have been obtained for the two limiting cases when the shear stress parameter is small or large. These equations have been integrated using Karman-Pohlhausen's method in the inlet of a straight channel. Analytical expressions for the inlet length and the boundary layer thickness in both cases have been obtained. The pressure drop has also been obtained from kinetic energy end-correction.