Fluid Mechanics Homework

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Fluid Mechanics Homework

Post by SEAL » Sat Jun 19, 2021 7:51 pm

So right now I'm taking a fluid mechanics class, and this was one of the homework questions in my textbook:
IMG_1009.jpeg
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They actually got the internals pretty spot on. I wonder if whoever wrote that problem is familiar with iSoaker/the community? Also wondering how accurate the pressure calculation is. Obviously it disregards stuff like friction and wind effects.
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Re: Fluid Mechanics Homework

Post by CDMT » Sat Jun 19, 2021 8:28 pm

It sure is cool that your teacher included that graphic..
I think that sadly the teacher probably isn't a Super Soaker enthusiast since you can find the same problem on multiple sites..

remember seeing this graphic before SEAL. I think it was a simplification of this graphic:

https://www.animatedsoftware.com/pumpglos/supersoa.htm

Then a few years later the graphic your teacher has showed up. I remember it most because at the time it annoyed me that the copied graphic had the nozzle off center and the coloration was changed from blue to a lime green in the graphic. Which was different than the original Super Soaker 100.

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Re: Fluid Mechanics Homework

Post by SSCBen » Sun Jun 20, 2021 9:23 am

Nice find. I vaguely recall having a similar problem at the University of Texas at Austin. I wouldn't expect this pressure calculation to be accurate. My guess is that whoever made the problem wants you to assume that there are no flow losses and no drag, both of which are significant factors in the pressure calculation. But it would give you a rough idea about some of the factors involved. One major factor missing from the calculation is the nozzle diameter. If there's no drag then the range is a function of only the firing height, velocity at the nozzle, gravity, and the firing angle, as I recall. But in reality the drag is going to depend a lot on the jet and droplet sizes. Plus, the nozzle diameter is a factor in the flow losses.

If you took flow losses and drag into account and calibrated the resulting model well (that is, used the right loss coefficient, drag coefficient, etc.), then I'd expect the calculation to be reasonably accurate.

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Re: Fluid Mechanics Homework

Post by Drenchenator » Sun Jun 20, 2021 9:31 am

Interesting! The graphic is fairly accurate, though I would not think it would be difficult nowadays to find schematics for designs like this.

The problem doesn't give enough information. You have to assume at least something, and that means that the answers could vary by a lot. It gives a 30 foot horizontal range but does not specify at what height it is fired. If I assume a height of 5 ft, use a dragless trajectory, and use Bernoulli's equation (bad idea!) I get about 20 psi. That number seems reasonable but it is certainly only an approximation given all the assumptions involved. Ben is right about the nozzle diameter too. Without that you can't know the actual velocity at the nozzle. I would advise against using Bernoulli's equation here too (as I just did), since it does not apply in pipe flows. Control-volume analysis would work a lot better but would require things like the nozzle diameter.
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Re: Fluid Mechanics Homework

Post by SEAL » Sun Jun 20, 2021 12:39 pm

CDMT wrote:
Sat Jun 19, 2021 8:28 pm
It sure is cool that your teacher included that graphic..
I think that sadly the teacher probably isn't a Super Soaker enthusiast since you can find the same problem on multiple sites..

remember seeing this graphic before SEAL. I think it was a simplification of this graphic:

https://www.animatedsoftware.com/pumpglos/supersoa.htm

Then a few years later the graphic your teacher has showed up. I remember it most because at the time it annoyed me that the copied graphic had the nozzle off center and the coloration was changed from blue to a lime green in the graphic. Which was different than the original Super Soaker 100.
This actually wasn't one of the problems my teacher assigned; I just stumbled upon it while flipping through the text book. That website is cool though. Super old school. $60 for a CD? Haha.

Yeah I didn't think you'd get a very accurate result. I think the SS series typically operated around 50 psi, right? (That's what it says on the Power Drencher, anyway.) Which makes sense 'cause the actual number would be higher in order to overcome drag, etc. Control volume analysis is actually what I'm doing in the class right now. I definitely think a lot of this knowledge can be useful when designing water guns, although we already know a lot about what makes a good soaker just through observation.
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