Fluid Mechanic Integration

**chronicals** · July 19th 2010, 07:24 AM

I have a fluid mechanics question, i don't know which equation i should integrate and how?

For turbulent flow in a smooth, circular tube with a radius R, the velocity profile varies according to the following expression at a Reynolds number of about 10^5.

Vx= Vxmax * [(R-r)/R)]^(1/7)

where r is the radial distance from the center and Vmax the maximum velocity at the center. Derive equation relating the average velocity ( bulk velocity ) Vav to Vmax for an incompressible fluid.
( Hint: The integration can be simplified by substitution z for R-r )

**chisigma** · July 19th 2010, 07:49 AM

Is...

$\displaystyle v_{m} = \frac{v_{0}}{R} \ \int _{0} ^{R} (1-\frac{r}{R})^{\frac{1}{7}}\ dr$ (1)

... that setting $x=\frac{r}{R}$ becomes...

$\displaystyle v_{m} = v_{0} \ \int_{0}^{1} (1-x)^{\frac{1}{7}}\ dx = \frac{7}{8}\ v_{0} \ |-(1-x)^{\frac{8}{7}}|_{0}^{1} = \frac{7}{8}\ v_{0}$ (2)

Kind regards

$\chi$ $\sigma$

**chronicals** · July 19th 2010, 08:15 AM

Originally Posted by chisigma

Is...

$\displaystyle v_{m} = \frac{v_{0}}{R} \ \int _{0} ^{R} (1-\frac{r}{R})^{\frac{1}{7}}\ dr$ (1)

... that setting $x=\frac{r}{R}$ becomes...

$\displaystyle v_{m} = v_{0} \ \int_{0}^{1} (1-x)^{\frac{1}{7}}\ dx = \frac{7}{8}\ v_{0} \ |-(1-x)^{\frac{8}{7}}|_{0}^{1} = \frac{7}{8}\ v_{0}$ (2)

Kind regards

$\chi$ $\sigma$

but the result is (49/60)* Vmax, we should use hint, i think

**chisigma** · July 19th 2010, 09:00 AM

Effectively I tried to symplify the problem integrating in the only $r$ variable

... integrating in the $r$ and $\theta$ variables it results...

$\displaystyle v_{m} = \frac{v_{0}}{\pi \ R^{2}}\ \int_{0}^{2 \pi}\ \int_{0}^{R} (1-\frac{r}{R})^{\frac{1}{7}}\ r \ dr \ d\theta$ (1)

... that setting again $x= \frac{r}{R}$ becomes...

$\displaystyle v_{m} = \frac{v_{0}}{\pi}\ \int_{0}^{2 \pi}\ \int_{0}^{1} x\ (1-x)^{\frac{1}{7}} \ dx \ d\theta=$

$\displaystyle = 2\ v_{0}\ \int_{0}^{1} x\ (1-x)^{\frac{1}{7}}\ dx =$

$\displaystyle = 2\ v_{0} \ \frac{7}{8}\ |x\ (1-x)^{\frac{8}{7}} - \frac{7}{15} \ (1-x)^{\frac{15}{7}}|_{0}^{1}= \frac{49}{60}$ (3)

Kind regards

$\chi$ $\sigma$

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