Shear stress, measurements

Konrad was the first to address the issue of pulsed piston transport using the properties of the solids as they slide through the pipe in a plug-like motion. The friction generated in such systems often can be likened to bin and hopper flow and design, requiring shear stress measurements such as carried out by the Jenike shear stress unit. The final expression using the Konrad approach can be written for horizontal flow as... 

The fluidity of the cement paste can be measured in rheological terms by the torque transmitted to a stationary bob inside a revolving outer cylinder placed in a water-cement system as shown in Fig. 1.10. The shear stress measured at the stationary bob is plotted against the rate of applied shear when, for pastes of varying water-cement ratios, the results shown in Fig. 1.11 are obtained for readings taken of the shear stress as the shearing rate is increased (the up curve). 

This equation will be particularly useful for the comparison of extinction angle and shear stress measurements. For the purpose, the second eq. (1.4) is used in the form ... 

With the aid of the two-color Laser-Doppler-Anemometry (LDA), Bewersdorff was able to measure the axial and the radial turbulence intensities simultaneously and also the Reynolds shear stresses. The injection of polymer results in a damping of both intensities in the region of their maxima. In his Reynolds shear stress measurements he showed that the polymer injection results in a drastic damping, and the stress maximum is shifted towards the center of the pipe. In a homogeneous polymer solution the maximum of the Reynolds shear stress remains in the same position-as for water. Only in the region of the buffer zone are the shear stresses reduced. 

As yet there appear to be no reports of wall shear stress measurements in liquid films with cocurrent gas streams. 

In the first of these techniques an approximation to uniform rate of shear throughout the sample is achieved by shearing a thin film of the liquid between concentric cylinders. The outer cylinder can be rotated (or oscillated) at a constant rate and the shear stress measured in terms of the deflection of the inner cylinder, which is suspended by a torsion wire (Figure 9.2) or the inner cylinder can be rotated (or oscillated) with the outer cylinder stationary and the resistance offered to the motor measured. 

Figure 35. Mechanical properties of carbon-carbon epoxy-resin hybrid composites, compared with the properties of the composite skeletons before resin impregnation (61,62). The composite skeletons were prepared from Sigrafil HM 3 PAN based fiber, rigidized with a phenolic resin, and densified by four cycles with coal-tar pitch plus sulfur the carbonization temperature was 1000 C. (b) Flexural strength. (c) Interlaminar shear stress, measured with two sample thicknesses.

In the stratified-flow regime, the shear stress remains constant, equal to the value (Tmax> in both zones. Thus, the shear stress measured in such a flow should be independent of... 

The principle of the rotational method consists of the shear stress measurement between two concentric cylinders, from which one is driven by a constant angle velocity and the viscous liquid situated in between them drags the second one. The shear stress, t, depends on the geometry of the system according to the following equations... 

Tensile stress is used to characterize cohesiveness between particles, or in a certain powder cake, coating resistance in an encapsulated powder. Shear stress refers to the stress component tangential to the plane on which forces act and is mainly used to determine frictional properties (e.g., angle of internal friction) between particles under a pressure load. Furthermore, because individual particles predominantly slide across each other in a shearing action during flow, shear stress measurement allows determination of flow properties. 

Viscosity can be defined as the ratio of shear stress to shear rate (. The shear stress is equal to a force measured in dynes divided over an area upon which it acts in square centimeters. Shear stress has the dimensions of dynes per square centimeter. The shear rate is equal to the velocity of a layer due to shear stress measured in centimeters per second divided by the thickness in centimeters the unit of shear rate is given in reciprocal seconds. Viscosity often is expressed in poises. The poise has the dimension of dyne seconds per square centimeter. Dyne seconds per square centimeter is the dimension for shear stress divided by shear rate and therefore is equal to viscosity. 

The viscosity of the suspension at difierent biomass concentration was measured by Modular Compact Rheometer Physica MCR 300 (Paar-Physica). Controlled shear-stress measurements were done using the concentric cylinder system with a FL 100/6W impeller at temperatures of 30, 40, and 50 °C. Samples were mixed before measurements were taken. Then, an appropriate volume was placed into the viscometer, allowing several minutes for the temperature to stabilize. The rheological measurements were performed three times for each value of biomass concentration using a fresh sample each time. 

The shear stress measurement range of the YR-1 (in Pascals) is determined by the size and shape of the vane spindle and the full scale torque range of the calibrated spring. 

The shear stress measurement range for the three standard vane spindles shown in Fig. 2 at each spring torque is shown in Table 2. 

The viscosity is then measured at ambient temperature at increasing shear rates of 85, 170, 255, 340, and 425 s The shear rate scans are produced in the rotational viscometer by increasing the rotation rate, and the scans are produced in the reciprocating capillary by changing the flow rate in discrete steps. After the initial shear scan, the sample is heated at about 2.7 C/min while being sheared at a constant rate of 170 s The sample temperature is about 80 °C in 25 min time zero is at the start of the ambient temperature scan. Shear scans are taken at 25, 40, 55, and 70 min with the shear rate maintained at 170 s between scans. Power-law parameters n and K are calculated from the shear stress measurements obtained from the scans. The viscosity at 170 s is calculated from these power-law parameters and reported. 

Information obtained from the in vitro flow visualization, and velocity and shear stress measurement studies will be discussed in the text. 

Yoganathan et al., have also made velocity and shear stress measurements downstream from a 27 mm (model 1260) valve in an aortic chamber under steady flow conditions (45,46). Experiments were conducted at steady flow rates of 10 and 25 1/min. They... 

Figliola has made steady flow velocity and shear stress measurements downstream from a 25 mm spherical disc aortic valve (47,89). At a flow rate of 25 1/min he measured a maximum wall shear stress of 722 dynes/cm and an occluder wall shear stress (resolved on the upper side of occluder) of 440 dynes/cm. He also monitored a maximum turbulent shear stress of 545 dynes/ cm2, a 25 mm downstream from the valve. His velocity measurements also showed a large region of stagnation across the outflow face of the disc. Tillman has measured the "wall" (i.e. surface) shear stresses along the orifice ring in the major and minor outflow regions of an aortic valve under pulsatile flow... 

Flow visualization studies in aortic and mitral chambers under both steady and pulsatile flow indicate smooth central type flow downstream from the valve (48,85,88,130). Initial velocity and shear stress measurements have been made by Yoganathan et al., with size 27 and 25 aortic valves (38,131). 

Yoganathan, A.P., Woo, Y.R., and Sung, H.W. 1986. Turbulent shear stress measurements in the vicinity of aortic heart valve prostheses. J. Biomech. 19 433—442. 

Cardiovascular disease, namely, coronary artery disease, remains the leading cause of death in the developed nations. Over the last few years, MEMS sensors have advanced the understanding of blood flow, namely, fluid shear stress, in arterial circulation. Fluid shear stress is defined as the frictional force acting tangentially on the surface of a blood vessel wall. Furthermore, the measurement of wall shear stress is important to study the durability of prosthetic valves and to monitor thrombosis or blood clots in cardiopulmonary bypass machines, artificial hearts, and left ventricular assist devices. Luminal shear stress measurement predicts the development of atherosclerotic plaque in patients at risk for acute heart attacks. In this context, the application of microscale hot-wire anemometry bridges fluid mechanics of blood flow with vascular biology. 

The waU shear stress is an essential quantity of interest in a wall-bounded flow. The time averaged value of wall shear stress can be used to determine the skin friction drag acting on the body by the fluid flow. The time-resolved behavior of surface shear stress indicates the unsteady flow structures responsible for individual momentum transfer events and turbulence activities. The instantaneous shear stress values at distributed locations on the surface can be used to feedback control the turbulence events inside a boundary layer. Shear stress measurement also helps in assessment and control of power consumption rate and therefore is an important quantity of interest in various industrial applications. 

The importance of shear stress measurement is even more crucial for small-scale devices, i.e., MEMS applications, due to their higher surface to volume ratio. There have been many efforts in literature for successful shear stress measurements. The success of these efforts primarily relies on the complexity of the flow, the nature of solid boundaries, and limitations of the measurement techniques. The other drawback of shear stress measurements is its smaller magnitude, i.e., the estimated value of shear stress of a typical car moving at 100 km/h is about 1 Pa. Hence, highly sensitive shear stress measuring devices are required for successful measurements of surface shear stress, and it is essential to have proper understanding of the various noise sources that can effect shear stress measurements. 

Various shear stress measurement techniques have been proposed in the literature. Some of the principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry, and shear stress-sensitive liquid crystal [5]. 

These shear stress measurement techniques are not ideal MEMS-based techniques. 

Velocity measurements close to the wall or the velocity profile measurement in the near-wall region can be used to determine the wall shear stress. Clauser proposed an approach for shear stress measurement of turbulent flow [1]. Here, the mean velocity measurements away from the wall are used with the assumption that the mean velocity (u) varies with the logarithmic distance from the wall (y), i.e.. 

The PIV techniques provide instantaneous velocity field information. The spatial and temporal resolution of PIV can be controlled with proper selection of lighting source, i.e., laser imaging system, i.e., camera and fi ame grabber, and optical arrangements/compo-nents. The continuous development of this hardware in the near future is expected to result in very high spatial and temporal resolution velocity measurements. Therefore, the p-PIV and p-holographic PIV techniques will develop as high-resolution indirect instantaneous shear stress measurement techniques in the near future. 

The thermal sensors for shear stress measurements can have two principal modes of variation elevated hot wire probe and surface-mounted hot wire probe. The principle of operation of these two types of thermal shear stress sensors is discussed in the following sections. 

In experimental aerodynamics, the surface hot wire probe has proved to be the most successful standard measurement technique to determine the laminar-to-turbulent flow transition, local separation, and shear stress fluctuations. The flush-mounted thermal shear stress sensor is one of the most successful techniques for shear stress measurement and is available in various forms, i.e., sensor skin, etc. [4], due to the rapid development of MEMS manufacturing processes. 

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