Difference between small tubing and big tubing- french laminar flow project

Hello everyone,

I'm completely new to this forum and don't yet know how it works so sorry if my post is not posted correctly!

I'm a french engineering student working on a big project that would involve using around ten laminar jets (that we are planning to build) and music to do a controled choregraphy for an event. The ultimate goal would be to use all those jets and control them indepedently with valvesin order to create a nice musical and visual show!

Me and my team are currently working on a prototype to find ou the perfect parameters that would allow us to have an arch of approximatively 2.5m high and 3m widht. We have a few questions that you guys might be able to answer (maybe they are already on the forum but we couldn't really find an answer):

1) We have created two prototypes with different width of PVC tubings/pipes. The first one we made had a width of 125mm, the second one had a width of 200mm. When we tested them, we didn't see lots of differences but we seemed to agree that the one with larger tubing was the best! The problem is that since we are going to create a whole lot of jets, it would be nice to reduce the costs by using thinner pipes (they cost less and the number of straws necessary to fill them is smaller so cheaper!) What would you recommand and could someone explain why everyone who create big arches of water seems to want to use big diameter tubing?

) Does anyone know a pomp that would work well or give out its caracteristics in terms of flow rate and working preassure?

3) What diameter would be the perfet output nozzle? 12mm, 15mm?

I might have more question to come but will post them when necessary.

Thank you in advance for the help you will give us and sorry again if I'm not using the forum correctly or if the answers to my question can easily be found in this forum!

Can someone please reply! This forum seems to be dead=(

My apologies for not seeing this sooner. This forum is unfortunately not lively at the moment but hopefully we can keep it going a bit.

There is some information in my blog post here: http://scuttlebots.com/2014/06/17/laminar-fountains-what-are-they/

What is really boils down to is for the Reynolds equation to give you a laminar flow you need both a slow flow and a small diameter pipe. Those two things are normally inverses of each other but by making the outer pipe large it slows the flow down and by using the straws they provide the small diameter pipe. The larger the outer pipe the slower the flow.

The question of how big of an outer pipe you need can be calculated using the math from my blog post. You need an outer diameter big enough to give you a slow enough flow to result in a laminar reynolds number. That said, reality also plays a big part. Unless you use military grade tolerances there will be some imperfections in your build. Those imperfections can be aided by achieving a flow that is well within the laminar range instead of barely under the limit. That part you just have to play with. My first fountain that had an arch the size you are talking about was 8 inches across which was great because I only had hand tools available to me when I built it and tolerances were anything by tight. I would not recommend smaller than a 6 inch fountain for that size. It is possible with a 4 inch but your tolerances and build have to be exquisite to maintain a laminar stream.

Hello,

Thank you to answer, your help us.

Since the last post, we advance a lot in our projects. We have decided to keep the 200 mm width pipe. We have also made machined two outer pipe, one of 15 mm and an other of 8 mm. I read your explication about the link between the two diameters, so if me calculs are correct, we should use an outer pipe of 8 mm to get a Reynolds number around 2900.

Do you think it is correct?

Best Regards

That's sounds about right to me off the top of my head. Its a bit worse than what I did for my fountain (200mm external pipe with 6mm inner pipes filling the external pipe). The internal pipes I used were standard drinking straws which are around 6mm in diameter so I'm not sure how the 8mm will work. I'd be interested to find out!

Part of the problem is that once you achieve good laminar flow you can't get "more laminar" so it is hard to determine if my stream was very laminar (in which case the internal pipe size could be bigger) or just barely laminar (in which case any larger inner pipe would be bad).