Question

^{In Fig. 6.2-2, why does the f versus Re curve for turbulent flow lie above the curve for laminar
flow rather than below?}

^{1.0 0.5 0.2 0.1 Laminar 0.05 16 f= Re 0.02 Turbulent k/D 0.004 0.01 0.001 f 0.0791 Rel/4 0.0004 -0.0001 Hydraulically smooth" 0.005 0.002 10 106 105 0.001 102 103 104 Reynolds number Re D<ū> p/u Fig. 6.2-2. Friction factor for tube flow (see definition of f in Eqs. 6.1-2 and 6.1-3. ICurves of L. F. Moody, Trans. ASME, 66, 671-684 (1944) as presented in W. L. McCabe and J. C. Smith, Unit Operations of Chemi- cal Engineering, McGraw-Hill, New York (1954). Friction factor j Usually unstable}

Answer 1

Friction factor is a measure of pressure drop.

When the fluid moves from laminar region to turbulent region, i.e if the fluid is in transition region, then the pressure drop suddenly increases. That's why at the transition velocity, friction factor increases and therefore the turbulent curve lies about laminar curve at transition Reynolds number.