Distance from end of straws to nozzle?

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Distance from end of straws to nozzle?

Post  AndromedaStrain on Mon Jan 04, 2010 12:31 pm

Hi All,

first post, expecting to have toe shot off but I've read all the posts so far and I'm trying to get a feel for the distance between end of straws and the exit nozzle and what then is the effect of more straws/tube diameter/straw diameter? It seems to me that there must be some size (diameter) of tube where there is an optimal exit laminar flow to give any desired effect. I'm sure someone has done some experiments in this direction, having seen very small units with a few reducing hole size disks as well as the *normal* straw tubes.

Unfortunately, back in the mists of time when I did physics at school, I was probably asleep when subjects like Reynolds number and Bernoulli thingies were discussed, so I guess I'll have to get my feet and other bits wet and jump in at the deep end.

By the by - It was a Utube video, a bundle of years ago, of the Epcot Centre pulsing fountains that first ignited an interest but at that time there seemed to no way of following it up on the Net, now this forum has got me going again.

Best Wishes

Ed

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Re: Distance from end of straws to nozzle?

Post  John on Mon Jan 04, 2010 2:42 pm

I hope you don't shoot your toe off! Especilly since this project doesn't include any high explosives or any fast moving ballistics. I suppose there is some danger in the tools that you will be using, but I suspect that you are competent using them. Plus, I don't think those would classify as "shooting your toe off" more like chop, or saw, or well you get the idea. Everyone here is very friendly so please don't hesitate to ask questions. That's the purpose of a forum right? Very Happy Welcome.

Down to business.

I'm currently building a 8" diameter nozzle with a .5" output. I have the normal size straws, not too large, and not too small. Currently my straws are about 2" from the exit. I've tried the smaller coffee stir straws that were about 6.5" in length in a 4" diameter nozzle. I had some good results with that, but I needed more. That is why I went with the 8" version.

The ideal nozzle would be to have the smallest diameter straws in the largest diameter nozzle possible. I would like to try sometime using coffee stir straws in my 8" nozzle, but now is not the time. Too many projects, I need to finish one someday. Very Happy I don't know if there is an optimal straw and diameter size because the larger pipe you use the more straws you can put in the pipe, and the less turbulent you will make it. In turn the further you would be able to "shoot" it. (I don't think that this water will "shoot your toe off") The smaller diameter the straws are the better, but at the cost of having more material in the nozzle and increasing the head loss in your nozzle.

Not everyone uses straws. As you have already seen there are posts about people using a wire mesh instead of straws. I don't know how it is working out as I have not made one of those. The smallest ones here on the forum are the ones made of the mesh.

Sometimes I forget to mention certain details, so if that doesn't make sense or if you have other questions, just ask!

John
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Re: Distance from end of straws to nozzle?

Post  AndromedaStrain on Tue Jan 05, 2010 9:03 am

Hi John,

Thanks for the comprehensive reply and your welcome - I've been in other fora over the years where bitchiness seems to be the norm - not my cup of tea. Like I said in my post I have read *all* of the posts in this forum and the responses have been unfailingly polite and helpful.

Your reply makes perfect sense and I will have a go at making an 8" beastie just as soon as I can, various jobs permitting, thanks.

I've noticed in the (professional) head and tail cut jumping water videos that the water cuts are almost always ragged and can see possible causes for the phenomena, I'm going to dip my little toe in the water again, this time just to see if anyone notices but has anyone tried using something that used to be common in cameras? ie a shutter - I seem to remember that they opened/closed in thousandths of a second?

Good New Year to All.

Ed

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Re: Distance from end of straws to nozzle?

Post  John on Tue Jan 05, 2010 9:09 am

Before you start out to build your 8" beastie, you might want to consider your goals, what are you trying to accomplish with your nozzle?

The most successful streams have been either 6" or 8", in my opinion. If you don't really need a large throw you might want to consider building a 6". If you want something like a 6"x6" arc then you probably want to build a 8". FYI

I know there have been a couple of people talking about building a photo shutter cutter, but I haven't seen any of those results as of yet.
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Re: Distance from end of straws to nozzle?

Post  AndromedaStrain on Tue Jan 05, 2010 9:23 am

Thanks John,

We've a relatively smallish garden where I'd like to add the water which has a central feature of the the Chinese God of of longevity, Fou Shou Lao and I'd like some nozzles around the periphery which would shoot into an area just in front. The maximum distance would be about 10 feet though that could be reduced to make the thing circular - haven't decided yet as it's way too early, first I have to crawl and if I master that, perhaps do some toddling. Our dogs may be in for some surprises when random water starts dropping.

Cheers

Ed

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Re: Distance from end of straws to nozzle?

Post  davo on Tue Jan 05, 2010 9:56 am

Hi Ed,

I can't give you any specific numbers on the distance between the end of the straws and the outlet nozzle. I would, however, suggest that you want to make that a fairly minimal distance. The reason why is because the Reynolds number of the flow (when not in the straws) is way into the turbulent range. Given enough pipe to flow through, you will lose the laminar nature that you gained in the straw section. We go to a large pipe diameter after inlet to drop linear velocity (one of the factors in the Reynolds calculation) - but that alone isn't enough to get into the laminar range. Once we put that reduced velocity flow through a sufficiently small diameter (another factor in the Reynolds calculation), then it becomes laminar. When the water exits the straws, the laminosity will immediately start to deteriorate. It has a long way to deteriorate before it becomes transitional, so you have some room to spare - but I would still say less is better.

There is also a question of if the water should pass through anything after the straws. Anything you pass through will cause a quicker shift to transitional. It might be worthwhile though because the velocity out of each straw is not even. (There is a peak velocity at the center of each straw.) I would expect that passing the water through some mesh would be good - but that it should still be kept minimal. I have not yet done any experiments to confirm that.


-David

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Re: Distance from end of straws to nozzle?

Post  AndromedaStrain on Tue Jan 05, 2010 10:46 am

Hi David,

Thanks for the reply, I obviously have a lot to learn yet. I suppose the intent of my original post was really asking why the straw bundle diameter should be any different from that of the nozzle. There must be some secondary effects that I'm not aware of, for example, fundamentally, is laminar flow something other than the addition, in X and Y of individual streams that have been 'linearised' so that the outlet from each straw moves at the same velocity?, if it is this addition, then I'd expect that the tube bundle would be about the same diameter as the nozzle, though I'd guess that the unused flow contributes in some way to binding the individual streams.

Doh, I'm having a Homer Simpson moment, for now I don't know what I don't know.

Forgive the Geeky reply, it's just my upbringing, I like to try and understand the things I don't understand study

Cheers

Ed

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