Surface shear stress

Film is locally removed by dissolution, surface shear stress or particle/bubble impact but it can repassivate. Erosion corrosion rate is a function of the frequency of film removal, bare metal dissolution rate and subsequent repassivation rate. 

The surface shear stress t is a consequence of the velocity difference between the metal surface and the fluid velocity. For tubular geometries it can be obtained from pressure drop measurements or calculated ... 

Equation (15) indicates that the wall shear stress changes with position on the cell surface and at each position the stress changes with time. Figures 9 and 10 show the distributions of the fluid shear rate in the eddy and the cell surface shear stress as a function of the dimensionless radius, R, and time, yt, respectively. 

When heat and mass are transferred simultaneously, the two processes interact through the Gr and Gq terms in Eq. (10-12) and the energy and diffusion equations. Although solutions to the governing equations are not available for spheres, results should be qualitatively similar to those for flat plates (T4), where for aiding flows (Gr /Gq > 0) the transfer rate and surface shear stress are increased, and for opposing flows (Gr Gq < 0) the surface shear stress is predicted to drop to zero yielding an unstable flow. 

Fig. 4.3 A finite-volume control volume for the radial Couette-Poiseuille problem. Surface shear stresses, normal pressure, and heat fluxes are illustrated with arrows indicating in their positive directions.

At a fixed radius of r = 1 cm, evaluate the surface shear stress as a function of the strain rate a. Explain the functional form of the result in the context of the nondimensional solution. 

When only approximate values of the overall features of the flow, such as the surface heat transfer rate and surface shearing stress, are required, it is possible to apply the boundary layer assumptions in a different way to obtain, relatively easily, approximate solutions for these quantities. The derivation of the equations required in this approximate solution procedure will be discussed in the present section (21,[7),[13]. 

Effect of Surface Shear Stress on Film Condensation on... 

The determination of the surface shear stress t, from Eq, 6-9 is not practical since it requites a knowledge of the flow velocity profile. A more practical approach in external flow is to relate Tj to the upstream velocity V as... 

The coefficient of friction Cy for a fluid flowing across a surface in terms of the surface shear stress, is given by (n) 2pV /-r, (6) 2r,/pi c)l7jpViAT... 

Silverman [21] derived velocity correlations between a rotating cylinder (mO, pipe flow (m2), annulus flow (1/3), and an impinging jet (wall jet region only, 1/4), as listed in Table 2. These equations assume that the appropriate transformations are to be made on the basis of equal mass-transfer rates for the different geometries. Silverman [21] also explored the case where the equality of surface shear stress is the appropriate criterion, on the basis that the equality of the shear stress will ensure the same corrosion processes for the various geometries. We stress that the equations listed in Table 2 must be used with great caution because they are based on the... 

Within the stagnation region, the surface shear stress Try is given by... 

The so-called 0°-canopy consisted of Up -orientated trees rotated to the wind direction a = 0° (that is, parallel to it). Hence, the resistance to the flow was mainly caused by the pressure drag force acting within the stem space and, within the crown space, by the surface shear stress on the triangular surfaces. No significant features of this canopy flow were found at the location X=10 rows to distinguish it from the flow associated with the canopy consisting only of stems . 

Raupach et al 1992 [528] showed that in this region two other mechanisms become important in determining Ac x, c). The first is the changes induced by the hill in the eddy flux field Af(x, z) and hence in its divergence. The second is the change to the flux of c from the surface that occurs because the surface shear stress Ar(x, z) varies as the hill is traversed. The mechanism for this in the rough hill model of [528] is the... 

Torsion tests were carried out at the temperatures of 200, 225, 250, 275, 300 °C and strain rate range 10 3-1 s 1. The samples were heated using an induction furnace the temperature stabilization time was 6 min. The gauge section of samples was a solid cylinder with a length (L) of 7 mm, and a radius (R) of 4 mm the fillet radius between the gauge section and the shoulders was 0.5 mm. The torque (T) and the twist angle (0) were converted to surface shear stress (x) and strain (y) by the relationships ... 

Equivalent stresses (a) and strains (e) were derived from surface shear stresses and strains by means of the Von Mises yield criterion ... 

Here p is the dimensionless pressure in the gas, i.e., p /(/j,uc/s2ic) and r is the dimensionless surface shear stress, r /(p,uc/e c)- In (6-21), we have also introduced the dimensionless surface-tension function b such that a = o b. We denote the value of a at the ambient temperature (and/or in the absence of surfactants) as a0. Hence, if there are no changes in a from its ambient value, the function Vsb = 0. Once again, it will be noted that we have retained terms that include the small parameter e. These are again terms that could be responsible for the motion of the film, and in this case, the appropriate choice for uc would be either eao//j or 3 er0//b depending on whether the dominant effect is Marangoni or capillary-driven motion, respectively. 

The effects of the viscosity variation across the boundary layer on the surface shear stress and heat flux are shown in Fig. 6.7. The shear stress is normalized by the value obtained from the Bla-... 

For a given value of x and the same flow properties, the boundary layer on a cone (k = 1) is thinner by a factor of 1/V3, and the surface shear stress and heat transfer are larger by a factor of V3. The local skin friction and heat transfer coefficients are related similarly ... 

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