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Fluids problem  gauge pressure 
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#1
Aug907, 07:48 AM

P: 16

1. The problem statement, all variables and given/known data
Water flows steadily from an open tank. The elevation of point 1 is 10.0 m, and the elevation of points 2 and 3 is 2.00 m. The crosssectional area at point 2 is 0.0480 m[tex]^2[/tex]; at point 3 it is 0.0160 m[tex]^2[/tex]. The area of the tank is very large compared with the crosssectional area of the pipe. Part 1 Assuming that Bernoulli's equation applies, compute the discharge rate in cubic meters per second. Part 2 What is the gauge pressure at point 2 2. Relevant equations [tex]A_1v_1=A_2v_2[/tex] [tex]p_1+\rho gh_1+\frac{1}{2}\rho v_1^2=p_2+\rho gh_2+\frac{1}{2}\rho v_2^2[/tex] 3. The attempt at a solution I have already calculated the discharge rate (at point 3) to be 0.200 m[tex]^3[/tex]/s which I know to be correct but I am stuck with the second part. I used Bernoulli's equation with points 2 and 3 and came up with an answer of 17.8 Pa but this appears to be wrong. I took [tex]h_2=h_3=0[/tex] to simplify Bernoulli's equation to get [tex]p_3p_2=\frac{1}{2}\rho v_2^2\frac{1}{2}\rho v_3^2[/tex] and used the volume flow rate equation to get the velocity at point 2 to be 0.067m/s and then used that in Bernoulli's equation to get the answer I have, am I doing something wrong here or should I even be using Bernoulli's equation to solve this part of the question. [tex]p_3p_2[/tex] should be equal to the gauge pressure shouldn't it? Any guidance whatsoever would be greatly appreciated. 


#3
Aug907, 06:51 PM

P: 16

Oh, thank you, I accidentally put in the volume flow rate in as v instead of the velocity of the fluid, I have the correct answer now, Thanks.



#4
Oct2907, 03:21 PM

P: 150

Fluids problem  gauge pressure



#6
Oct2907, 10:22 PM

P: 150

Then p_3 is atmospheric pressure .. is it 101.3 kPa? so then p_2 = (.5(1000(4.7^212.5^2)))101300) = 168380 Pa?



#7
Oct2907, 10:49 PM

P: 16

The question asked for gauge pressure, which is the pressure at 2 minus atmospheric pressure given by p_2p_3 in the top equation, which is 6.97*10^4. So yeah, the absolute pressure at 2 would be the gauge pressure plus atmospheric which is pretty much what you got.



#8
Oct2907, 10:53 PM

P: 150

OHHHH i finally get it!



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