# Dimensional Analysis problem - choosing variables

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1. Nov 4, 2014

### GBA13

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

I have a dimensional analysis problem where water is draining from a open topped tank and in a certain amount of time,t it drops from h1 to h2. and I need to find a bunch (6) of variables which apply to this problem. I have some but Im not sure about the other ones.

2. Relevant equations
I have to present my answer as deltaH (the change in height from h1 to h2) / d (the size of the hole from which water is flowing) = f(?,?,?)

3. The attempt at a solution

I have chosen, the time it takes for the water to drop from a level, h1 to a level h2.

the diameter of the tank

the diameter of the hole which the water is draining from, which is on the floor of the tank in the middle.

the density of the water

and then the last two I'm not sure about. (It says its not viscous so I don think I can use that)

I chose g, gravity as I think that might effect flow
and P the pressure, which I am taking is the air pressure. [I think its possible exit velocity of the water should replace this but I'm really not sure.

this gives me:

delta H (the change in height from H1 to H2) = f(t,D,d,P,g, rho)

Could you guys give some advice about which equations I should use?

Thanks very much!

2. Nov 9, 2014

### Greg Bernhardt

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Nov 10, 2014

### Staff: Mentor

Why don't you just solve this problem using Bernoulli's equation first? This will give you all the parameters involved, and then you can back out the dimensionless groups from that solution.

Chet

4. Nov 10, 2014

### JohnJJ

Ive never heard of Bernoulli's equation, ill have to check it out!

5. Nov 10, 2014

### GBA13

Hi everyone, thanks for posting, I've managed to do it but thanks for posting. I've never been told about Bernoulli's equation so I'll have to check it out, it sounds very useful if it can give you the variables! Thanks! :)