Water moves through a constricted pipe in steady, ideal flow. At the
lower point shown in figure below, the pressure is P1 = 1.75X104 Pa and the
pipe diameter is 6.00 cm. At another point y = 0.250 m higher, the pressure
is P2 = 1.20X104 Pa and the pipe diameter is 3.00 cm. Find the speed of
flow (A) in the lower section and (B) in the upper section. (C) Find the
volume flow rate through the pipe.

Respuesta :

A) Speed in the lower section: 0.638 m/s

B) Speed in the higher section: 2.55 m/s

C) Volume flow rate: [tex]1.8\cdot 10^{-3} m^3/s[/tex]

Explanation:

A)

To solve the problem, we can use Bernoulli's equation, which states that

[tex]p_1 + \rho g h_1 + \frac{1}{2}\rho v_1^2 = p_2 + \rho g h_2 + \frac{1}{2}\rho v_2^2[/tex]

where

[tex]p_1=1.75\cdot 10^4 Pa[/tex] is the pressure in the lower section of the tube

[tex]h_1 = 0[/tex] is the heigth of the lower section

[tex]\rho=1000 kg/m^3[/tex] is the density of water

[tex]g=9.8 m/s^2[/tex] is the acceleration of gravity

[tex]v_1[/tex] is the speed of the water in the lower pipe

[tex]p_2[/tex] is the pressure in the higher section

[tex]h_2 = 0.250 m[/tex] is the height in the higher pipe

[tex]v_2[/tex] is hte speed in the higher section

We can re-write the equation as

[tex]v_1^2-v_2^2=\frac{2(p_2-p_1)+\rho g h_2}{\rho}[/tex] (1)

Also we can use the continuity equation, which state that the volume flow rate is constant:

[tex]A_1 v_1 = A_2 v_2[/tex]

where

[tex]A_1 = \pi r_1^2[/tex] is the cross-section of the lower pipe, with

[tex]r_1 = 3.00 cm =0.03 m[/tex] is the radius of the lower pipe (half the diameter)

[tex]A_2 = \pi r_2^2[/tex] is the cross-section of the higher pipe, with

[tex]r_2 = 1.50 cm = 0.015 m[/tex] (radius of the higher pipe)

So we get

[tex]r_1^2 v_1 = r_2^2 v_2[/tex]

And so

[tex]v_2 = \frac{r_1^2}{r_2^2}v_1[/tex] (2)

Substituting into (1), we find the speed in the lower section:

[tex]v_1^2-(\frac{r_1^2}{r_2^2})^2v_1^2=\frac{2(p_2-p_1)+\rho g h_2}{\rho}\\v_1=\sqrt{\frac{2(p_2-p_1+\rho g h_2)}{\rho(1-\frac{r_1^4}{r_2^4})}}=0.638 m/s[/tex]

B)

Now we can use equation (2) to find the speed in the lower section:

[tex]v_2 = \frac{r_1^2}{r_2^2}v_1[/tex]

Substituting

v1 = 0.775 m/s

And the values of the radii, we find:

[tex]v_2=\frac{0.03^2}{0.015^2}(0.638)=2.55 m/s[/tex]

C)

The volume flow rate of the water passing through the pipe is given by

[tex]V=Av[/tex]

where

A is the cross-sectional area

v is the speed of the water

We can take any point along the pipe since the volume  flow rate is constant, so

[tex]r_1=0.03 cm[/tex]

[tex]v_1=0.638 m/s[/tex]

Therefore, the volume flow rate is

[tex]V=\pi r_1^2 v_1 = \pi (0.03)^2 (0.638)=1.8\cdot 10^{-3} m^3/s[/tex]

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Answer:

(a)Speed of  flow in the lower section is [tex]v_1=0.638\rm m/s[/tex]

(b)Speed of  flow in the upper section is [tex]v_2=\frac{0.03^2}{0.15^2}\times0.638=2.55\rm m/s[/tex]

(c)Volume flow rate through the pipe is [tex]Q_1=A_1v_1=1.8\times\ 10^{-3} \rm m^3/s[/tex]

Explanation:

Given information:

[tex]\rho =1000\rm kg/m^3[/tex]

[tex]g=9.8\rm m/s^2[/tex]

At lower point,

pressure is

[tex]p_1=1.75\times 10^4 \rm kPa[/tex]

pipe diameter is

[tex]d_1=6\rm cm =0.06m[/tex]

At higher point,

pressure is

[tex]p_2=1.20\times 10^4 \rm kPa[/tex]

pipe diameter is

[tex]d_2=3\rm cm=0.03m[/tex]

speed of the pipe at lower point=

[tex]v_1[/tex]

area of the pipe at lower side =

[tex]A_1=\frac{\pi \times d_1^2}{4}[/tex]

speed of the pipe at higher point=

[tex]v_2[/tex]

area of the pipe at higher side =

[tex]A_2=\frac{\pi \times d_2^2}{4}[/tex]

Flow is irrotational so by use of Bernoulli's equation,

[tex]p_1+\rho g h_1+\frac{1}{2}\rho v_1^2=p_2+\rho g h_2+\frac{1}{2}\rho v_2^2[/tex]

We can write it as,

[tex]v_1^2-v_2^2=\frac{2(p_2-p_1)+\rho g h_2}{\rho}[/tex]

By continuity equation,

[tex]A_1v_1=A_2v_2\\\\\pi \frac{d_1^2}{4}\times v_1=\pi \frac{d_2^2}{4}\times v_2\\\\\frac{v_2}{v_1}=\frac{d_2^2}{d_1^2}=\frac{0.03^2}{0.06^2}=0.25[/tex]

On substitution,

[tex]{v_2}=0.25{v_1}[/tex],

[tex]v_1^2-0.25^2=\frac{2(1.75\times10^4-1.20\times10^4)+1000\times 9.8\times 0.250}{1000}[/tex]

[tex]v_1=0.638\rm m/s[/tex]

(b) By this [tex]A_1v_1=A_2v_2[/tex]

[tex]v_2=\frac{0.03^2}{0.15^2}\times0.638=2.55\rm m/s[/tex]

(c)Volume flow rate is,

[tex]Q_1=A_1v_1=\pi r_1^2\times v_1=\pi 0.03^2\times 0.638=1.8\times\ 10^{-3} \rm m^3/s[/tex]

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