# Efficiency of a Siphon

• lapsaJ
I think there are a few reasons. Firstly, the pressure inside the tube is higher at the narrow end than the wide end. This means the fluid is forced to flow in a narrower channel, and the pressure difference causes the flow rate to be higher. Secondly, the fluid is flowing at a much higher speed in a long thin tube than a short thick tube. This means the flow is turbulent, and the pressure waves travel around the tube faster than they do in a thicker tube.

#### lapsaJ

(Hope this is in the right forum)

For my A Level coursework I have to do a practical investigation. I'm thinking of doing one looking at the efficiency of a siphon.

One relationship I am going to be looking at is the rate of flow vs height (difference between the two water levels).

Now I'm having a bit of difficulty thinking what else I can look at to give it some decent scope. My teacher hasn't seen this done before so we're a bit uncertain whether it's worth taking forward.

Any suggestions would be appreciated! Thanks.

You've chosen a rather difficult subject. The flow of liquids in pipes ( fluid dynamics) is a whole branch of physics with more equations than I could shake a stick at.

Thanks for your reply.

I was thinking of applying Bernouilli's equation to the separate parts to derive the flow rate, and possibly the maximum height.

Would it be worth varying the diameter of the siphon tube?

lapsaJ said:
Would it be worth varying the diameter of the siphon tube?

Yes it would. (You might expect the flow would be proportional to the cross section area of the tube, but it won't be, for long thin tubes).

Read up a bit about flow in pipes and the effects of viscosity, laminar and turbulent flow, and Reynolds number before you get into this. Otherwise, you will have a very hard time understanding your measurements.

It won't make a very good project when (not if!) you show that Bernoulli's equation doesn't fit any of your results, but you don't have any understanding of WHY that is the case.

AlephZero said:
Yes it would. (You might expect the flow would be proportional to the cross section area of the tube, but it won't be, for long thin tubes).

Read up a bit about flow in pipes and the effects of viscosity, laminar and turbulent flow, and Reynolds number before you get into this. Otherwise, you will have a very hard time understanding your measurements.

It won't make a very good project when (not if!) you show that Bernoulli's equation doesn't fit any of your results, but you don't have any understanding of WHY that is the case.

Why isn't the diameter of the siphon tube proportional to the rate of flow for long thin tubes? I am just reading up on theories behind the siphon from journals etc..Just wondering why that's the case (altering the diameter).

Thanks