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Skin Friction and Drag

The u velocity profile in figure 6.18 has nearly the same shape as the velocity difference U-u for the impulsively moved plane surface (see figure 6.14). From this velocity profile, we can calculate the value of the wall shear stress to be:


where the numerical coefficient is obtained from the numerical solution to the Blasius equation 6.87. It is customary to define a dimensionless wall shear stress, called the skin friction coefficient , by dividing by the dynamic pressure of the oncoming flow:


The flat plate skin friction coefficient can be found by substituting equation 6.88 into 6.89:


Engineers may be interested in the total drag force on a flat plate. For a plate of length L in the streamwise direction and width W perpendicular to the flow, the drag would be:


where the factor 2WL is the total plate area of the upper and lower surfaces exposed to the flow. The dimensionless form of the drag force,

called the drag coefficient , is the ratio of the drag force to the product of the exposed area A times the dynamic pressure :


For the flat plate, the drag coefficient is calculated by substituting equation 6.91 into 6.92 to obtain:


where is the plate length Reynolds number.


Nishikant Sonwalkar
Thu Feb 16 17:47:33 EST 1995