Coating of Electronic Components by the RTV Dispersion—Part I: Physical Model of the Deposition Process Determined by Drop Tests

1993 ◽  
Vol 115 (3) ◽  
pp. 233-239
Author(s):  
J. A. Owczarek
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Yield Stress ◽  
Physical Model ◽  
Volumetric Flow Rate ◽  
Deposition Process ◽  
Test Method ◽  
Electronic Components ◽  
Drop Tests ◽  
Dispersion Part

This paper describes a study of the process of deposition of RTV dispersion on electronic components placed on substrates. The objective was to develop a technique for the consistent manufacture of encapsulant coating of a desired thickness and extent. In addition, it was desired to obtain an understanding of the phenomenon of run-over, or wicking, of the RTV dispersion onto external leads of circuits being encapsulated, and of means to control it. In this paper physical properties of the RTV dispersion which influence the deposition process were determined using a novel drop test method. These properties allow building of a physical model of the deposition process, and its analysis. The results of drop tests show that the RTV dispersion behaves like a plastic “false body” material which possesses yield stress after a long rest, and which retains residual yield stress after shearing. Part I of this paper is concerned with building of the physical model of the encapsulant deposition process. It also deals with the derivation of an equation relating the wall shear stress to the encapsulant volumetric flow rate.

Coating of Electronic Components by the RTV Dispersion—Part II: Determination of the Residual Yield Stress After Deposition, and Method to Produce a Drop of Required Cured Skin Thickness and Diameter

1993 ◽  
Vol 115 (3) ◽  
pp. 240-248
Author(s):  
J. A. Owczarek
Keyword(s):  
Yield Stress ◽  
Deposition Time ◽  
Correlation Equation ◽  
Test Method ◽  
Skin Thickness ◽  
Electronic Circuits ◽  
Electronic Components ◽  
Drop Spreading ◽  
Dispersion Part

In this part of the paper the drop test method is used to show that RTV dispersions are in fact plastic “false body” materials, and to determine the magnitudes of the residual yield stresses after shearing for different RTV dispersion lots. With the aid of the equation for the wall shear stress derived in Part I of this paper [1], a correlation equation between the residual yield stress and the deposition variables is obtained, and analysis is made of the drop spreading phenomenon. It is shown that the drop spread, which is responsible for the run-over, or wicking, of external leads of electronic circuits, can be decreased by decreasing the deposition rate of the encapsulant. Finally, a method is developed to determine the required deposition flow rate and deposition time to produce a drop having required final diameter and cured skin thickness.

scholarly journals Pipe rheology of microfibrillated cellulose suspensions

Cellulose ◽  
2019 ◽  
Vol 27 (1) ◽  
pp. 141-156 ◽  
Author(s):  
Tuomas Turpeinen ◽  
Ari Jäsberg ◽  
Sanna Haavisto ◽  
Johanna Liukkonen ◽  
Juha Salmela ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Yield Stress ◽  
Power Law ◽  
Slip Flow ◽  
Microfibrillated Cellulose ◽  
Flow Index ◽  
Shear Rheology ◽  
Power Law Fluids ◽  
Wall Shear

Abstract The shear rheology of two mechanically manufactured microfibrillated cellulose (MFC) suspensions was studied in a consistency range of 0.2–2.0% with a pipe rheometer combined with ultrasound velocity profiling. The MFC suspensions behaved at all consistencies as shear thinning power law fluids. Despite their significantly different particle size, the viscous behavior of the suspensions was quantitatively similar. For both suspensions, the dependence of yield stress and the consistency index on consistency was a power law with an exponent of 2.4, similar to some pulp suspensions. The dependence of flow index on consistency was also a power law, with an exponent of − 0.36. The slip flow was very strong for both MFCs and contributed up to 95% to the flow rate. When wall shear stress exceeded two times the yield stress, slip flow caused drag reduction with consistencies higher than 0.8%. When inspecting the slip velocities of both suspensions as a function of wall shear stress scaled with the yield stress, a good data collapse was obtained. The observed similarities in the shear rheology of both the MFC suspensions and the similar behavior of some pulp fiber suspensions suggests that the shear rheology of MFC suspensions might be more universal than has previously been realized.

scholarly journals Comparative Analysis of Mathematical Models for Blood Flow in Tapered Constricted Arteries

2012 ◽  
Vol 2012 ◽  
pp. 1-34 ◽  
Author(s):  
D. S. Sankar ◽  
Yazariah Yatim
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Yield Stress ◽  
Flow Rate ◽  
Core Region ◽  
Core Radius ◽  
The Core ◽  
Casson Model ◽  
Longitudinal Impedance ◽  
Two Fluid

Pulsatile flow of blood in narrow tapered arteries with mild overlapping stenosis in the presence of periodic body acceleration is analyzed mathematically, treating it as two-fluid model with the suspension of all the erythrocytes in the core region as non-Newtonian fluid with yield stress and the plasma in the peripheral layer region as Newtonian. The non-Newtonian fluid with yield stress in the core region is assumed as (i) Herschel-Bulkley fluid and (ii) Casson fluid. The expressions for the shear stress, velocity, flow rate, wall shear stress, plug core radius, and longitudinal impedance to flow obtained by Sankar (2010) for two-fluid Herschel-Bulkley model and Sankar and Lee (2011) for two-fluid Casson model are used to compute the data for comparing these fluid models. It is observed that the plug core radius, wall shear stress, and longitudinal impedance to flow are lower for the two-fluid H-B model compared to the corresponding flow quantities of the two-fluid Casson model. It is noted that the plug core radius and longitudinal impedance to flow increases with the increase of the maximum depth of the stenosis. The mean velocity and mean flow rate of two-fluid H-B model are higher than those of the two-fluid Casson model.

scholarly journals Effects of Surface Tension and Yield Stress on Mucus Plug Rupture: A Numerical Study

2020 ◽  
Vol 142 (6) ◽  
Author(s):  
Yingying Hu ◽  
Francesco Romanò ◽  
James B. Grotberg
Keyword(s):  
Surface Tension ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Yield Stress ◽  
Pressure Difference ◽  
Rupture Process ◽  
Viscoplastic Fluid ◽  
Air Interface ◽  
Mucus Plug ◽  
Wall Shear

Abstract We study the effects of surface tension and yield stress on mucus plug rupture. A three-dimensional simplified configuration is employed to simulate mucus plug rupture in a collapsed lung airway of the tenth generation. The Herschel–Bulkley model is used to take into account the non-Newtonian viscoplastic fluid properties of mucus. Results show that the maximum wall shear stress greatly changes right prior to the rupture of the mucus plug. The surface tension influences mainly the late stage of the rupture process when the plug deforms greatly and the curvature of the mucus–air interface becomes significant. High surface tension increases the wall shear stress and the time needed to rupture since it produces a resistance to the rupture, as well as strong stress and velocity gradients across the mucus–air interface. The yield stress effects are pronounced mainly at the beginning. High yield stress makes the plug take a long time to yield and slows down the whole rupture process. When the effects induced by the surface tension and yield forces are comparable, dynamical quantities strongly depend on the ratio of the two forces. The pressure difference (the only driving in the study) contributes to wall shear stress much more than yield stress and surface tension per unit length. Wall shear stress is less sensitive to the variation in yield stress than that in surface tension. In general, wall shear stress can be effectively reduced by the smaller pressure difference and surface tension.

Hemodynamic impacts of hematocrit level by two-way coupled FSI in the left coronary bifurcation

2020 ◽  
Vol 76 (1) ◽  
pp. 9-26
Author(s):  
Saeed Bahrami ◽  
Mahmood Norouzi
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Yield Stress ◽  
Blood Circulation ◽  
Arbitrary Lagrangian Eulerian ◽  
Coronary Bifurcation ◽  
Shear Rates ◽  
3D Geometry ◽  
Wall Shear ◽  
Rigid Walls

Cardiovascular disease is now under the influence of several factors that encourage researchers to investigate the flow of these vessels. Oscillation influences the blood circulation in the volume of red blood cells (RBC) strongly. Therefore, in this study, its effects have been considered on hemodynamic parameters in the elastic wall and coronary bifurcation. In this study, a 3D geometry of non-Newtonian and pulsatile blood circulation is considered in the left coronary artery bifurcation. The Casson model with various hematocrits is analyzed in elastic and rigid walls. The wall shear stress (WSS) cannot show the stenosis artery alone, therefore, the oscillatory shear index (OSI) is represented as a hemodynamic parameter of WSS individually of time. The results are determined using two-way fluid-structure interaction (FSI) coupling method using an arbitrary Lagrangian-Eulerian method. The most prominent difference in velocity happened in the bifurcation and at hematocrit 30 with yield stress 6.59E-04 Pa. The backflow and vortex flow in the LCx branch grown with increasing shear rates. The likelihood of plaque generation at the ending of the LM branch is observed in hematocrits 10 and 20, while the WSS magnitude is normal in the hematocrit 60 with the greatest yield stress in the bifurcation. The shear stress among the rigid and elastic models is the highest at the ending of the LM branch. The wall shear stress magnitude among the models decreased at most of 24.49% by dividing the flow. Time-independent results for models showed that there is the highest value of OSI at the bifurcation, which then quickly dropped.

Parametric Wall Shear Stress Characterization of the Rotating Cage Test Method

CORROSION ◽  
10.5006/2864 ◽  
2019 ◽  
Vol 75 (6) ◽  
pp. 580-586
Author(s):  
Allan Runstedtler ◽  
Jinxing Huang ◽  
Patrick Boisvert ◽  
Nicholas Senior
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Test Method ◽  
Wall Shear ◽  
Cage Test

The Impact of Hemorheology on Wall Shear Stress in a Separated and Reattached Flow Region

2013 ◽  
Author(s):  
Khaled J. Hammad
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Yield Stress ◽  
Flow Region ◽  
Shear Thinning ◽  
Progression Rate ◽  
Vortex Center ◽  
Wall Shear ◽  
Separated And Reattached Flow ◽  
The Impact

Wall-bounded separating and reattaching flows are encountered in biological applications dealing with blood flows through arteries and prosthetic devices. Separated and reattached flow regions have been associated in the past with the most common arterial disease, atherosclerosis. Previous studies suggest that local wall shear stress (WSS) patterns affect the location and progression rate of atherosclerotic lesions. A parametric study is performed to investigate the influence of hemorheology on the wall shear stress distribution in a separated and reattached flow region. Recent hemorheological studies quantified and emphasized the yield stress and shear-thinning non-Newtonian characteristics of unadulterated human blood. Numerical solutions to the governing equations that account for yield stress and shear-thinning rheological effects are obtained. A low WSS region is observed around the flow reattachment point while a peak WSS always exists close to the vortex center. The yield shear-thinning hemorheological model always results in the highest observed peak WSS. The yield stress impact on WSS distribution is most pronounced in the case of severe restrictions to the flow.

scholarly journals Analysis of Flow Characteristics of the Blood Flowing through an Inclined Tapered Porous Artery with Mild Stenosis under the Influence of an Inclined Magnetic Field

2014 ◽  
Vol 2014 ◽  
pp. 1-9 ◽  
Author(s):  
Neetu Srivastava
Keyword(s):  
Magnetic Field ◽  
Blood Flow ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Flow Rate ◽  
Flow Characteristics ◽  
Volumetric Flow Rate ◽  
Mhd Equations ◽  
Inclined Magnetic Field ◽  
Wall Shear

Analytical investigation of MHD blood flow in a porous inclined stenotic artery under the influence of the inclined magnetic field has been done. Blood is considered as an electrically conducting Newtonian fluid. The physics of the problem is described by the usual MHD equations along with appropriate boundary conditions. The flow governing equations are finally transformed to nonhomogeneous second-order ordinary differential equations. This model is consistent with the principles of magnetohydrodynamics. Analytical expressions for the velocity profile, volumetric flow rate, wall shear stress, and pressure gradient have been derived. Blood flow characteristics are computed for a specific set of values of the different parameters involved in the model analysis and are presented graphically. Some of the obtained results show that the flow patterns in converging region (ξ<0), diverging region (ξ>0), and nontapered region (ξ=0) are effectively influenced by the presence of magnetic field and change in inclination of artery as well as magnetic field. There is also a significant effect of permeability on the wall shear stress as well as volumetric flow rate.

Mathematical model for a Herschel-Bulkley fluid flow in an elastic tube

Open Physics ◽  
2011 ◽  
Vol 9 (5) ◽  
Author(s):  
Kuppalapalle Vajravelu ◽  
Sreedharamalle Sreenadh ◽  
Palluru Devaki ◽  
Kerehalli Prasad
Keyword(s):  
Shear Stress ◽  
Fluid Flow ◽  
Wall Shear Stress ◽  
Yield Stress ◽  
Newtonian Fluid ◽  
Power Law ◽  
Fluid Model ◽  
Elastic Tube ◽  
Yield Stress Fluid ◽  
Power Law Index

AbstractThe constitution of blood demands a yield stress fluid model, and among the available yield stress fluid models for blood flow, the Herschel-Bulkley model is preferred (because Bingham, Power-law and Newtonian models are its special cases). The Herschel-Bulkley fluid model has two parameters, namely the yield stress and the power law index. The expressions for velocity, plug flow velocity, wall shear stress, and the flux flow rate are derived. The flux is determined as a function of inlet, outlet and external pressures, yield stress, and the elastic property of the tube. Further when the power-law index n = 1 and the yield stress τ 0 → 0, our results agree well with those of Rubinow and Keller [J. Theor. Biol. 35, 299 (1972)]. Furthermore, it is observed that, the yield stress and the elastic parameters (t 1 and t 2) have strong effects on the flux of the non-Newtonian fluid flow in the elastic tube. The results obtained for the flow characteristics reveal many interesting behaviors that warrant further study on the non-Newtonian fluid flow phenomena, especially the shear-thinning phenomena. Shear thinning reduces the wall shear stress.

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