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482 Chapter 9 ■ Flow over Immersed Bodies<br />

1.0<br />

Linear<br />

Cubic<br />

Sine wave<br />

y __<br />

δ<br />

0.5<br />

Blasius<br />

Parabolic<br />

0<br />

0<br />

0.5<br />

u__<br />

U<br />

1.0<br />

F I G U R E 9.12 Typical<br />

approximate boundary layer profiles<br />

used in the momentum integral equation.<br />

to express the wall shear stress. From Eq. 9.30 we obtain the approximate value<br />

while the Blasius solution result is given by<br />

c f 12C 1 C 2 A<br />

m<br />

rUx 12C 1C 2<br />

1Re x<br />

c f 0.664<br />

1Re x<br />

(9.32)<br />

These results are also indicated in Table 9.2.<br />

For a flat plate of length / and width b, the net friction drag, d f , can be expressed in terms<br />

of the friction drag coefficient, C Df , as<br />

The friction drag<br />

coefficient is an integral<br />

of the local<br />

friction coefficient.<br />

or<br />

/<br />

C Df <br />

d b<br />

f t w dx<br />

1<br />

2 rU 2 b/ 0<br />

1<br />

2 rU 2 b/<br />

C Df 1 / /<br />

c f dx<br />

0<br />

(9.33)<br />

TABLE 9.2<br />

Flat Plate Momentum Integral Results for Various Assumed<br />

Laminar Flow Velocity Profiles<br />

Profile Character<br />

DRe x<br />

12<br />

x<br />

c f Re x<br />

12<br />

C Df Re <br />

12<br />

a. Blasius solution 5.00 0.664 1.328<br />

b. Linear<br />

uU yd<br />

3.46 0.578 1.156<br />

c. Parabolic<br />

uU 2yd 1yd2 2<br />

5.48 0.730 1.460<br />

d. Cubic<br />

uU 31 yd22 1yd2 3 2 4.64 0.646 1.292<br />

e. Sine wave<br />

uU sin3p1 yd224<br />

4.79 0.655 1.310

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