Due a complete lack of wind for what seems forever + lockdowns, combined with numerous ideas wanting to try out, I figured I needed a hydrodynamic test tank to test out ideas when the wind ain't blow'n. I searched for someone doing it before but couldn't find anything helpful. I went to UWA (our local uni) open day recently and checked out their setup....very fancy and expensive looking but not much use for recreational water sports applications..plus I don't want to have to fight students for tank time.

Instead of boring you why or how, or why I shouldn't, the basic concept is I have a pool (which I suspect many have) which, despite spending considerable effort heating and cleaning is highly under utilised except by the local algae. For various reasons I have access to 4 pool pumps but I notice that people are giving away pool pumps for free on gumtree (upgrading to newer models or removing their pool as they are a pain), or selling them very cheaply...I also suspect many might care to donate a pump in exchange for tank time or basic amusement...at present I do not need any pumps...I will let you know if I am on the scrounge.

The key step (in my mind at least) is to break this down into a building block and test that at whatever required flow rate. If successful, add these blocks together to make whatever sized tank is required. Whatever the result it will be a water feature on steroids. To give you a rough idea of the magnitude, if my basic maths is correct, I need around 33 x 1.5 HP pool pumps delivering around 220liters/sec to generate basic planing speed for an average sized windsurfer (without a fin). That will drain my standard sized pool in exactly 3 minutes.

Now, I may not actually get around to making a full sized, medium velocity tank but there is lots of cool stuff that can be done with either a small tank or a larger tank running at slow velocities.

Without further adue I present the first instalment. I spent more time finding drinking straws and editing this video than making what you see, so this is work in progress, not a finished product. I have no real clue what I'm doing, just a desire for the end result. If you feel you contribute, or know of someone doing it before, please let me know.



(since I' new here you'll have to paste that link in your browser I think)

A flow tank always looked like a fun idea to me too, but never got as far as you have.
To me it looks like too much wasted energy by just pumping the water straight into the pool. Have you thought about a circular tank. Then you just need smaller volume high velocity pumps to build the water speed.
Think similar to when you have a circular pool and start running around to create a whirl pool
**** video but you get the idea

Good thinking Darryl. I predict this thread is going to get very interesting!!!!! I put a test tank in the too hard basket many times over the last decade or so. As have a few others here.
Great stuff flex!

Maybe a different method is needed. Most tanks drag the item along the water which negates the flow issues of pumps and drag issues from the walls. But you need a lot of room. Could a circular pond and centrifugal arm with a fin mounted on it give a better flow?
Not sure how long the water would stay still or if there's an AoA issue on the fin.

Or just like the gaastra rig, stick on a plank in front of your dinghy.

Getting a nice laminar flow in the jacuzzi is going to need a lot of straws, but it will be an absolutely epic whirlpool! Keep at it Flex - I am looking forward to the video of the scaled-up version where the lights go dim with the power draw and your back fence gets water blasted into your neighbour's place. I haven't ever looked at the physics involved, but is it possible that if you scale down the fin size, you can scale down the velocity required? (off the top of my head the formula for Reynolds number involves both length of the surface and the velocity)

Flow tanks are sort of like windtunnels. To reduce turbulence run the pump into a larger diameter pipe, through the straws and then contract the flow just before the working channel. Turbulence will build up on the walls again but central flow should have a low enough turbulence to test a thruster fin at least. 33 motors will blow your fuses!

I've been recycling thrown away air con fans in a backyard windtunnel . Working on a 4 to 1 contraction which should reduce relative turbulence at the inlet by 16. Where did you find those tough looking straws?

I tell the neighbours it will be useful for air drying tomatoes.

Don't want to sound like LeeD, but yeah, been there, done that. Photo below is testing half scale windsurfing boards in the Delft University towing tank in 1983.

It's old tech now, many of the old tanks have closed down due to lack of commercial demand, much better off using a modern CFD code such as www.numeca.com/marine-naval

Thanks for the comments so far. responding to the various suggestions below.

I thought about circulating system building a rotating arm spinning around in the pool. I hadn't thought of the whirlpool idea but I think both systems would suffer the same issue: after one rotation whatever subject you are testing will be in the turbulence of the previous run and would continuously build in some sort of positive feedback loop..might be entertaining but make it difficult to work out what is going on.

Knowing that turbulence in air hangs around a long time (which is why aircraft taking off have to be staggered a few minutes apart)..and the experience on Chook pond a few weeks ago where eddies were still present at least a minute afterwards on the return run means a rotating system would only be useful at very low speeds or very large scale.

The dingy type system suffers that its not a controlled environment. The dingy attitude will be all over the place and more difficult to film/see what's going on. (plus I don't have easy access to a dingy)

Once I get around to explaining the main thing I want to test, means the only options I can come up with are: 1) real world, 2) Dingy or 3) full scale laminar flow tank. Since real world isn't delivering and I have no dingy that leaves #3

I'm with Ian K that to get laminar flow need to feed the flow into a large diameter, through the straws the reduce the flow area dramatically. All DIY examples on the internet follow this approach.

Sourcing the straws locally in WA btw are a pain. Despite not being banned in WA untill 2023 they are nearly impossible to find. All are either made of paper or sugar. The ones I found are the entire WA Officeworks stock...there is one remaining pack sitting in Albany apparently. They come individually wrapped in paper so it becomes very tedious... Only other places in WA that I found that sell plastic straws, with that annoying flexible bit you need to cut off is Reject shop and Spudshed..and they are all sold out. Seems toilet paper is not the only thing Aussies are hoarding. Spotlight claims to sell them but after interrogating their employees it seems they haven't sold for a long while. Online is fine but you have to wait 2+ weeks by which time I've might have lost interest.

Since straws (weirdly) are the main constraint on progress at present, no more glue/silicon in place for these babies.

For sure I've ditched using gutter down pipe as this is designed for zero pressure and leaks like a sieve unless vertical and under any pressure. Going from pressure pipe to DWC is also a pain and unlikely to handle flow rates/pressures need. I think the best approach is pure PVC pressure pipe from pump, through a combiner manifold then through the laminar flow section and major cross sectional change. This way can be sure no leaks to at least 100 psi..I've taken this stuff to 150psi air with some rocket projects so confident a hydraulic system will be rock solid. I started building a 100mm pipe laminar flow system today but then already thinking to reject that and go with 150mm diameter. Then I saw the huge price jump between 100mm and 150mm fittings and might have to work with 100mm. 3d printing the fittings will be hugely cheaper at the expense of time for the bigger build.

The problem with 100mm dia is the transition to a rectangular flow (required if you want to stack modules) means you get roughly max 40mm x 95mm rectangular section (will work it out exactly soon)...150mm dia means a much larger rectangular section possible..i.e. is more 100mm dia modules required at less cost versus less 150mm modules at more cost better?

If you can imagine a 100mm (or 150mm) dia pipe downstream of the laminar flow thingo it needs to transition to some sort of rectangular shape. The rectangles widest point will be less than the diameter so in order to stack the modules will have to be arranged in some sort of fan structure. Even just a single module will need an interface...current plan is 3d print this interface section. Will make more sense once drawn up.

A 1/2 scale or other scale model is the backup plan but really will try and avoid this except to testing extremes. Whilst might be able to work out the maths to compensate a scale model to real world it appears way more attractive to put the real sized thing in a tank to see how the actual device works. Scale models seem to make sense where it is truly not practical at full scale....I.e. one idea is a micro foil....a foil that is so tiny that it only starts functioning above say 30 or 40 knots. Just dumb idea of many...

Hi Pacey, great photo. This was the great era of experimentation and exploration..the approach nowadays seems very boring (but no doubt cheaper for large companies). For sure there is no commercial demand from a very few of us wanting to push the envelope on recreational craft. Your software solution might be great but what is the cost? Quick look at link provided gives no costs, which usually implies very high cost only applicable to major builds not a DIY solution. If cost is sub say $500 for a full no more cost for life software approach I am all in but I suspect price is way way above this..plus it is far less entertaining. I would 'almost guarantee' this software won't include some concepts I want to test..no doubt could be done for extra cost. I have a mate doing this modelling/real world boat building for a living for the last 30 years here in WA and as far as I can tell its a no go area for DIY...really hope to be proven wrong as software will eliminate a bunch of crap ideas quickly. I could dig up my old notes and textbooks on fluid dynamics and write my own code but the bloody rats have eaten then all! I figure by the time I download and relearn all this stuff and apply it I would have built this system and got some results.

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Pacey said..

It's old tech now, many of the old tanks have closed down due to lack of commercial demand, much better off using a modern CFD code such as www.numeca.com/marine-naval

If numeca.com's CFD was the be all and end all of nautical design why did the AC75 protocol have to specifically ban tank testing? Why did they limit teams to building two boats? And 3 foils? And why did all the boats all end up looking and performing differently?

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Flex2 said..
The ones I found are the entire WA Officeworks stock...there is one remaining pack sitting in Albany apparently.

Thanks I won't start looking. Bunnings might have the answer.

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Ian K said..

Pacey said..

It's old tech now, many of the old tanks have closed down due to lack of commercial demand, much better off using a modern CFD code such as www.numeca.com/marine-naval

If numeca.com's CFD was the be all and end all of nautical design why did the AC75 protocol have to specifically ban tank testing? Why did they limit teams to building two boats? And 3 foils? And why did all the boats all end up looking and performing differently?

The reasons for banning tank testing, limiting the number of boats and foils is simply to keep the costs down. I don't know what Team NZ's budget was for this AC, but the two AC campaigns I worked on (Alinghi in 2007 and Oracle in 2013) were both $100 million dollar budgets just for the winning team.

The boats looked and performed differently because it was the first year of a new class. Given a few generations of the same class rule the boats would converge on an near optimal design, differing primarily in the wind ranges they were designed for.

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Ian K said..

Flex2 said..
The ones I found are the entire WA Officeworks stock...there is one remaining pack sitting in Albany apparently.

Thanks I won't start looking. Bunnings might have the answer.

Ian K, that might turn out to be a good idea for a larger scale...been playing around in Fusion 360 and the 3D printer currently pumping out one of the below modules for 100mm pipe. The trouble with the stacking module approach is the interface between the modules (red arrows) will create a step change in flow and no doubt lots of turbulence. Might be able to chamfer them to a knife edge to minimise the effect as per second image but for a 100mm ramp as pictured this is 10deg flow difference between modules. I can print max 220mm so can reduce that down further. No clue what the resulting flow will look like.

Might be better long term to stack these Polycarb sheets to whatever volume + a bit is required and mount in a sealed container. Since this stuff is strong as, might be also be able to build the whole structure out of it. A bit like the 3rd image...with the tubes being the feeds from the pumps and the rectangular section holding the stacks of polycarbonate sheeting with the flow coming out a cutout of desired dimensions. Making the rectangular container water tight under pressure will be the challenge.

I've not heard of a stackable system in windtunnels. 2D contractors have been used. They'd stack side by side. 4:1 is considered the smallest contraction ratio to get turbulence down to a reasonable level. The join indicated in red will generate turbulence. Without a wall it should eventually dissipate in the free stream. but maybe not before it gets lost in fully developed channel flow. Maybe a slight contraction downstream of the outlets equal to the sum of the joiner widths might help? Maybe not.

I'd think you can get away with stacking but are best with a single custom built contractor as the last step.

Contractorless windtunnels have been built - making much use of those polycarb sheets from Bunnings.

publications.csiro.au/rpr/download?pid=csiro:EP124393&dsid=DS4

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Ian K said..
I've not heard of a stackable system in windtunnels. 2D contractors have been used. They'd stack side by side. 4:1 is considered the smallest contraction ratio to get turbulence down to a reasonable level. The join indicated in red will generate turbulence. Without a wall it should eventually dissipate in the free stream. but maybe not before it gets lost in fully developed channel flow. Maybe a slight contraction downstream of the outlets equal to the sum of the joiner widths might help? Maybe not.

I'd think you can get away with stacking but are best with a single custom built contractor as the last step.

Contractorless windtunnels have been built - making much use of those polycarb sheets from Bunnings.

publications.csiro.au/rpr/download?pid=csiro:EP124393&dsid=DS4

Interesting Ian, are you one of the authors of this paper? Have only skimmed it super quick but bed time reading I think. Did I read it correctly though you are using 90mm pipe for the flow straightener and the Bunnings polycarbonate sheets as the tunnel container (i.e. at 90 deg to air flow) where as here we are talking the water flowing through the channels (parallel to flow) to replace the hard to get straws?

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Flex2 said..

Ian K said..
I've not heard of a stackable system in windtunnels. 2D contractors have been used. They'd stack side by side. 4:1 is considered the smallest contraction ratio to get turbulence down to a reasonable level. The join indicated in red will generate turbulence. Without a wall it should eventually dissipate in the free stream. but maybe not before it gets lost in fully developed channel flow. Maybe a slight contraction downstream of the outlets equal to the sum of the joiner widths might help? Maybe not.

I'd think you can get away with stacking but are best with a single custom built contractor as the last step.

Contractorless windtunnels have been built - making much use of those polycarb sheets from Bunnings.

publications.csiro.au/rpr/download?pid=csiro:EP124393&dsid=DS4

Interesting Ian, are you one of the authors of this paper? Have only skimmed it super quick but bed time reading I think. Did I read it correctly though you are using 90mm pipe for the flow straightener and the Bunnings polycarbonate sheets as the tunnel container (i.e. at 90 deg to air flow) where as here we are talking the water flowing through the channels (parallel to flow) to replace the hard to get straws?

Sorry, got the tunnels and the references mixed up. Was a few years back. This one used polycarbonate sheets and a 7:1 contraction. link.springer.com/article/10.1023/A:1011605719943 The one without a contractor used 90mm pipe as a flow straighteners. A bit dodgy, but good enough for purpose.

Unfortunately I didn't get the chance to fiddle about with either of them, the first was commandeered by a phd student once completed and I got hit with a redundancy as I finished the second one.

Was checking out prices in Bunnings today. I've got the square entrance to a 660mm contractor to fill and no govt. visa card. 20mm electrical conduit would be about half the price cut into 200 mm lengths. Although the smaller the diameter the better, small eddies are quicker to succumb to viscosity. Especially if you have a contractor to stretch them out.

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Ian K said..

Flex2 said..

Ian K said..
I've not heard of a stackable system in windtunnels. 2D contractors have been used. They'd stack side by side. 4:1 is considered the smallest contraction ratio to get turbulence down to a reasonable level. The join indicated in red will generate turbulence. Without a wall it should eventually dissipate in the free stream. but maybe not before it gets lost in fully developed channel flow. Maybe a slight contraction downstream of the outlets equal to the sum of the joiner widths might help? Maybe not.

I'd think you can get away with stacking but are best with a single custom built contractor as the last step.

Contractorless windtunnels have been built - making much use of those polycarb sheets from Bunnings.

publications.csiro.au/rpr/download?pid=csiro:EP124393&dsid=DS4

Interesting Ian, are you one of the authors of this paper? Have only skimmed it super quick but bed time reading I think. Did I read it correctly though you are using 90mm pipe for the flow straightener and the Bunnings polycarbonate sheets as the tunnel container (i.e. at 90 deg to air flow) where as here we are talking the water flowing through the channels (parallel to flow) to replace the hard to get straws?

Sorry, got the tunnels and the references mixed up. Was a few years back. This one used polycarbonate sheets and a 7:1 contraction. link.springer.com/article/10.1023/A:1011605719943 The one without a contractor used 90mm pipe as a flow straighteners. A bit dodgy, but good enough for purpose.

Unfortunately I didn't get the chance to fiddle about with either of them, the first was commandeered by a phd student once completed and I got hit with a redundancy as I finished the second one.

Was checking out prices in Bunnings today. I've got the square entrance to a 660mm contractor to fill and no govt. visa card. 20mm electrical conduit would be about half the price cut into 200 mm lengths. Although the smaller the diameter the better, small eddies are quicker to succumb to viscosity. Especially if you have a contractor to stretch them out.

Hi Ian, the last link seems to be a paid link..tried a bit to search for a free download but failed. Not sure I really understand your reference to a 660mm square contractor...it appears you are building a system for air at present?? Or are you building a epic big hydro tank?

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Didn't get what I aimed to do done today but after editing progress seems enough. For those that can't sleep

Ian K, the constrictor in video is just under 4:1.is it enough you think or is more contraction better..your references seem to indicate 7:1 but these are air systems.does water follow the same basic rules.logic would suggest not but this fluid stuff seems to work non intuitively...

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Flex2 said..

these are air systems.does water follow the same basic rules.logic would suggest not but this fluid stuff seems to work non intuitively...

No only intending it for air. But the basic principles of getting a low turbulence flow will be the same. I was sort of inspired by the plywood tunnel Ralph Bagnold constructed when researching his classic "The Physics of Windblown Sand and Desert Dunes". His had a square cross section of 1 foot, I've gone metric. A cross section of 333 mm and a working section of 4.8 metres length. Out of the fans it diverges to 666mm square hence the 4:1 contraction.

Ralph Bagnold did a pretty good job on sand, he has a dune field on Mars named after him, I'm not optimistic of coming up with anything new but should be fun trying. He pretty well covered every aspect. Though I've included a divergence in the second half of the working section to gently slow things down to maybe take a better look at settling.

I did a preliminary run without a contractor with a bag of brickies sand from Bunnings hoping to see a bit of ripple formation . I'll need to construct a waste gate. 3 speed fans are only giving 20 to 40 knots. Too much. Sand blasted everywhere!

Here's Curiosity Rover's pic of the Bagnold dunes.

in the interest of speed, printing a nested insert to fit into existing constrictor. This one will have about 6:1 ratio versus 3.86:1. Could also put another inside that but that ratio will be nearly 12:1 which will be overkill. Once a decent ratio determined can optimise shape of constrictor to maximise width (in order to stack)..for 6:1 out of DWV pipe it looks like 96mm x 15mm constrictor is the widest. For 150mm DWV its about 133x22mm for 6:1. I think for a practical tank using DWV modules to build flow volume will quickly get out of hand and expensive. The biggest pipe you can buy off the shelf easily is 300mm DWV, roughly that would equate to an outlet 50mm x 280mm at 5:1, or 42 x 280 at 6:1. Sufficient for some fin testing and not impossible to stack 2 or 3 of those for a board test jig.

Was just searching for a bit of scrap polycarbonate to make a pre straw flow straightener when I came across some Corflute which is essentially the same thing except infinitely cheaper than polycarbonate (and a hell of a lot easy to cut) and comes in 2.5,3,5,10mm..looks like Bunnings only stock 2.5,3 and 5mm but since I only have enough poly scrap to make a 200mm long section I might also try a post straw flow straighter made out of 3mm core flute...since straws are approx 5mm dia, the 5mm stuff could replace the hard to find straws.

www.bunnings.com.au/project-panel-white-corflute-900mm-x-600mm-x-3mm_p0390160

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Flex2 said..
in the interest of speed, printing a nested insert to fit into existing constrictor. This one will have about 6:1 ratio versus 3.86:1. Could also put another inside that but that ratio will be nearly 12:1 which will be overkill. Once a decent ratio determined can optimise shape of constrictor to maximise width (in order to stack)..for 6:1 out of DWV pipe it looks like 96mm x 15mm constrictor is the widest. For 150mm DWV its about 133x22mm for 6:1. I think for a practical tank using DWV modules to build flow volume will quickly get out of hand and expensive. The biggest pipe you can buy off the shelf easily is 300mm DWV, roughly that would equate to an outlet 50mm x 280mm at 5:1, or 42 x 280 at 6:1. Sufficient for some fin testing and not impossible to stack 2 or 3 of those for a board test jig.

Can you use that handy printer to make contractors rather than constrictors? You can google "Borger windtunnel contractor" to get the mathematically ideal shape but in practice most smooth eyeballed contractions do the job. Try to asymtote the inlet and outlet to be parallel to the flow. I formed one to the Borger specification back in the day but this time I just clamped layers of mdf between two offset parallel planes so they formed a natural smooth curve. You want to generate a smooth pressure drop between inlet and outlet to accelerate the mean flow but not introduce turbulence. The pressure drop also reduces the boundary layer. That's why a contractor is kept as short as possible to minimise it rebuilding.

Ian, yes, no worries to print a contractor instead of constrictor. Only went in interest of speed as internet has many examples with the constrictor and they all seem to work fine, plus the pressure load is taken by plastics deigned for this. The final result with a contractor would need piles more meat (plastic) as there would be some serious forces contracting 1/4 tonne per second fluid at between 4:1-8:1 ratio over the length of say 50cm in fractions of a second.. Still trying to absorb the information but what is the rough efficiency gain switching from constrictor to contractor?

The main issue with 3D printing (for me) are my printer can only print 210x210x250mm. Could farm it out but will avoid if at all possible as the idea is DIY cheap that others can follow if they choose. This would mean need to print in multiple parts...which leads to weaknesses/leaks. BTW all great info you are sharing, very helpful.

got this contractor for 100mm DWV printing now. Exactly 5:1 ratio. The clamp thing is just so I don't have to glue it to the pipe.

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Flex2 said..
got this contractor for 100mm DWV printing now. Exactly 5:1 ratio. The clamp thing is just so I don't have to glue it to the pipe.

That looks great. Hopefully the difference is obvious as soon as you hook it up. Those printers are sure handy. I need one!

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Ian K said..
Those printers are sure handy. I need one!

Probably cheaper than you think now. I just ordered one of these www.prusa3d.com/original-prusa-i3-mk3/ for $US749, very highly rated.

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Pacey said..

Ian K said..
Those printers are sure handy. I need one!

Probably cheaper than you think now. I just ordered one of these www.prusa3d.com/original-prusa-i3-mk3/ for $US749, very highly rated.

yes the Prusa mentioned above is the one I've got. Been running mine nearly 24/7 for 2 years now..brilliant. Plus the support, knowledge base and community is great. Pays for itself in no time by giving you the ability to both print any plastic thing you can buy much cheaper but also if you can think it you can build it. Printing custom O ring's and the ability to 'minionize' them is very useful. Yes, minionizing is a thing with o rings to stop them rubber banding. Will post some updates soon on this project which has said O rings (delays testing due to actual wind and heavy rain) but here's a taster..starting to look the business.

bit slow on updating but made some progress....could waffle on with words but have made a vid if anyone interested. Still struggling to get nice laminar flow at high flow rates....