Fluid Dynamics 101?

[Aqua Pro Builder]

Is it true that in some cases, a smaller drain tube will be draining faster than a wider drain tube provided that they are at same water height?

For example, in the scenario that the wider one is not fully filled with water while the narrower one is.

 

[Simon Garratt]

Within reason yes,

A wide drain tube that is sucking in air causing turbulance and counter friction (the bubble wants to go up whilst the water wants to go down) as water passes down it, can have a slower rate of flow than a smaller tube that isnt suffering that effect, and has a constant flow driven and aided by suction as the unimpeded falling water draws following water behind it at a rate allowable by gravitaional force, offset against internal tube wall friction. , but it would be very difficult to come up with a set of rules for a given sized pipe in relation to another and even worse if there are any elbows or joints involved..

With the big pipe, you would need to know the intial in-flow rate, minus the volume of air sucked in over the same time, minus the friction factor of the pipe, its length and fittings present..

all in all not an easy task imo.

even then it wont stay as a constant when allowing for biological buildup inside the pipe over time.

Regards

Simon. (sometimes i can be a real geek)

 

[kimoyo]

Your question seems a little ambiguous. Can you explain what you mean by faster? Neglecting air resistance or pipe drag, things fall at the same rate. In a full pipe water will fall at a higher speed because there's no air. So, if both pipes were completely fill then the flow rates would be the same but the total volume flowing would be greater in the bigger pipe because of the increased area. But if one pipe isn't completely filled, you'd have to figured out how much extra area is needed to compensate for the air to get the same total volume.

 

[goosey]

Toricelli's Rule

http://demonstrations.wolfram.com/TorricellisTheorem/

Click on "watch web preview" and note that velocity only changes with water height not pipe size.

Therefore, a wider pipe will flow more liquid at the same velocity.

 

[Aqua Pro Builder]

Your question seems a little ambiguous. Can you explain what you mean by faster? Neglecting air resistance or pipe drag, things fall at the same rate. In a full pipe water will fall at a higher speed because there's no air. So, if both pipes were completely fill then the flow rates would be the same but the total volume flowing would be greater in the bigger pipe because of the increased area. But if one pipe isn't completely filled, you'd have to figured out how much extra area is needed to compensate for the air to get the same total volume.

OK faster as in terms of total water volume flow thru.

The question comes about when I observed the what comes in a std bulk head kit. The elbow that comes with most of them is always a reducer, meaning the designer what it to be a reducer not that it's just more choice of a reducer and a same ID size elbow. I did an experiment by mounting a bulkhead at the surface of the water and connect it with a wide drain pipe same ID as the bulkhead. The pipe is never filled and the total water drained is very slow. At this point I was wondering whether the siphoning effect would have helped the situation if I were to switch is to the reducer. HHHHHAAAHAHAA, I forgot to make enough length of the PVC insert so that I can connect the reducer elbow that comes with it. :shhh::irked::splitspin