Venturi tube another equation, possible?

[thedarkshade]

Take a look at this video and the way that the equation is derived. It seems to be OK, although it's a bit different from the standard one:

 

http://youtube.com/watch?v=tjv5NYItg1w

 

Is it?

[insane_alien]

nope doesn't work. S2^2 is always less than S1^2 which in the case of that equation results in the square root of a negative number so would give imaginary velocities.

 

can't spot a mistake in the maths though, it goes to fast and does several steps at once an i can't be bothered to sit through it slowly to figure it out. also it's maths.

 

i tested it on real data from an experiment i did last year too and it just gave me gibberish using the real equation i get withing 1 percent of the true value(measured using a bucket and a stopwatch )

[thedarkshade]

can't spot a mistake in the maths though, it goes to fast and does several steps at once an i can't be bothered to sit through it slowly to figure it out. also it's maths.

You can do that on your own notebook, it's nothing hard! I watched it and did not really saw a mistake.

And it doesn't mean that S2^2 should always be less, it can be less in S1 than in S2, it doesn't really matter, they just have to differ.

[insane_alien]

no, S2 is the diameter/radius of the constriction in the venturi meter S1 is the diameter/radius of the pipe normally.

 

and if it was bigger then the pressure difference would be negative and you'd still have the same problem.

 

the only way this woks is when the pressure differnce is zero to give a zero velocity.

 

and trust me on this ALL venturi meters, bar some that are made purely for the sake of experimentation, are constrictive (S1>S2)

 

i have never actually seen an expansive venturi meter they're that rare.

 

and then you have to take the fact that the equation did not work for actual data.

[thedarkshade]

no, S2 is the diameter/radius of the constriction in the venturi meter S1 is the diameter/radius of the pipe normally.

 

and if it was bigger then the pressure difference would be negative and you'd still have the same problem.

 

the only way this woks is when the pressure differnce is zero to give a zero velocity.

 

and trust me on this ALL venturi meters, bar some that are made purely for the sake of experimentation, are constrictive (S1>S2)

 

i have never actually seen an expansive venturi meter they're that rare.

 

and then you have to take the fact that the equation did not work for actual data.

OK ok, thanks a lot for all this!

[insane_alien]

s'cool. i like fluid mechanics. one of the few classes i paid attention in in second year. i might look at the maths tomorrow if i feel like it(probably not) and have time(again probably not) so maybe one of our mathmagicians can take a look at it.