Just thinking out of the box

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Just thinking out of the box

Post  AndromedaStrain on Thu Jan 07, 2010 6:18 am

Hi All,

fired up with enthusiasm, I've ordered some of the bits needed for my first project. During that process was just pondering - I'm new at this, so bear with me - why does the laminar nozzle have to be circular? I can see that a number of things have to come good in order to get laminar flow at all but with perseverance it is clearly attainable. Is there any physical reason why, the orifice can't be, for example, a thinnish rectangle?, that would lead to the intriguing possibility of a rotating nozzle giving a corkscrew flow, similarly, an arrangement of small circular nozzles - etc etc.

Probably pie in the sky but unless I hear it's against the laws of physics, it's something I'd like to play with someday Mad

Cheers

Ed

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Re: Just thinking out of the box

Post  John on Thu Jan 07, 2010 10:21 am

That would be pretty awesome! I'm not sure if that would be possible or not. As I am just a newbie as well I'm not sure that the flow would work. As I understand, if you have a pipe then after the water travels some distance, it develops a fully developed flow. This velocity profile is a parabola.

How we use this to our benefit is to break up the large velocity profile into lots and lots of little ones, thus the straws. Once it exits the straws the profiles merge and "average" out to a constant profile (same speed). Once it hits the exit it will be a nice clean laminar flow fountain shooting through the air.

Now I'm not sure what would happen with a rectangle output, but I suspect that you would get some water moving faster in the corners. I don't really know just my thoughts.
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Re: Just thinking out of the box

Post  AndromedaStrain on Thu Jan 07, 2010 10:36 am

Hi John,

Likewise, I'm not at all sure if it would be possible - it may be that it would be possible as a vertical drop perhaps, with thin/thick flow rotated at 90 degrees, giving the possibility of some kind of sculpture.

I'd assumed that the orifice would be like a bisected circle with horizontal extensions 'cause I think there is something going on with surface tension in the water to maintain the laminar flow. Like you, I'm just a newbie to this field but we can always dream.

Thanks for your reply

Ed

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Re: Just thinking out of the box

Post  davo on Thu Jan 07, 2010 6:36 pm

Hi Ed,

It would be interesting to see what would happen with a non-circular outlet. I think it would be really cool if you could maintain the shape for any visible length. Unfortunately, I don't think it will maintain for long. What we are creating with the nozzle is a free fluid jet - and there has been quite a bit of research on them. Take a look for instance, at the NASA document http://historical.ncstrl.org/tr/pdf/icase/TR-9-33.pdf. Take, for example, the statement "Natural laminar jets evolve in an uneventful manner from rectangular to circular cross-section." The document basically suggests that within a small number of diameters from the jet outlet (probably hydraulic diameter since we are talking about a rectangular jet) the flow will convert from a rectangular shape to a circle. You will also notice that it was never a sharp rectangle cross-section, but kind of rounded off the corners even at the beginning. It probably won't stay laminar very long after that either. You still might be able to get something interesting looking - but you might need to do it very quickly. There is an interesting phenomenon with rectangular jets called axis-switching. You might be able to get something cool from that - but it wouldn't be a laminar jet.

-David

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Re: Just thinking out of the box

Post  John on Fri Jan 08, 2010 8:27 am

The link is broken. I think you are missing a number after the nine.
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Re: Just thinking out of the box

Post  AndromedaStrain on Fri Jan 08, 2010 8:41 am

Hi David,

Couldn't reach the NASA doc you mentioned (Error 404) but after a bit of digging found http://historical.ncstrl.org/tr/pdf/icase/TR-99-33.pdf which got me there. Interesting that they would tend to circularise very quickly, I'll keep digging for a bit longer and learn some more.

Thanks

Ed

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Re: Just thinking out of the box

Post  davo on Fri Jan 08, 2010 9:30 am

Sorry about that. Yes - the TR-99-33.pdf document is the one that I was referring to.

-David

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Re: Just thinking out of the box

Post  AndromedaStrain on Sun Jan 10, 2010 8:41 am

Hi All,

And there's more...

One of the things I was thinking about in the original post was - suppose we have a central, circular nozzle and then another circular nozzle somewhere along the radius, does this mean than it's possible if the whole face was rotated (ignoring the issues of water sealing for the moment), we could get a spiral flow rotating around of a linear flow? I realise that this might be very difficult for any significant length but at the moment I don't know enough about the subject to say whether it is possible or not - just throwing it open for discussion.

Apparently, a shed-load of straws have been dispatched to me, so real soon I'll be able to start trying to make something work.

I'm interested also in the micro as well as the macro, ie, what's the smallest water leap anyone has attained? Perhaps the Epcot Centre in a 6" bowl? scratch

Cheers

Ed

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