# Force in bent pipe

1. Apr 7, 2016

### foo9008

1. The problem statement, all variables and given/known data

refer to part 8.2 in this problem , why P1 = P2 ?

http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Examples/eg6_ans.htm

3. The attempt at a solution
there's some elevation between inlet and outlet , how can P1 = P2 , is the author wrong ?

2. Apr 7, 2016

### Staff: Mentor

The author is right. There is atmospheric pressure at inlet and outlet.

In their analysis, they are also neglecting the elevation difference between inlet and outlet and also the weight of the fluid.

3. Apr 7, 2016

### foo9008

why the elevation difference between inlet and outlet can be neglected ?

4. Apr 7, 2016

### foo9008

P1 and P2 are not atmospheric pressure as shown in the calculation , why P1 = P2?

5. Apr 7, 2016

### Staff: Mentor

Because the static pressure difference between the inlet and outlet elevations is small compared to the pressure exerted on the surface of the blade, and the weight of the fluid in the control volume is small compared to force exerted by the blade to change the direction of the fluid jet.

6. Apr 7, 2016

### Staff: Mentor

Look at the figure. P1 and P2 are both atmospheric. This isn't an enclosed tube. It's a jet open to the atmosphere.

7. Apr 7, 2016

### foo9008

this is the(hand-written) question that i wish to ask , but then , i found the somewhat the same typed question on the internet , so i copy the link and ask here . ok , it's much clearer now .

how do we know that the static pressure difference between the inlet and outlet elevations is small compared to the pressure exerted on the surface of the blade???

8. Apr 7, 2016

### Staff: Mentor

We can calculate each of them and compare their magnitudes. The problem statement also inherently implies that you can neglect the elevation change, since it doesn't even give you the inlet and outlet elevations, and it assumes that the velocity and thickness of the jet doesn't change: "75mm wide and 25mm thick, strike the vane with a velocity of 25m/s."

Last edited: Apr 7, 2016