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Problem 1.51 An approximation for the boundary-layer shape in Figs. 1.5b and P1.51 is the formula u(y) Usin( 0y8 where U is the stream velocity far from the wall and is the boundary layer thickness, as in Fig. P1.51. If the fluid is helium at 20C and 1 atm, and if U= 10.8 m/s and 8= 3 mm, use the formula to (a) estimate the wall shear stress , in Pa; and (b) find the position in the boundary layer where is one-half of Tw Fig. 1.5 Shear stress causes continuous shear deformation in a fluid: (a) a fluid element straining at a rate 80&r; (b) newtonian shear distribution in a shear layer near a wall. Fig. Pl.51 18 (y) Velocity profile -No slip at wall (b)