Hal Robinson's Flow tank

Rcsailing.net General Discussion
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(Duplicate post from the US1meter yahoo forum)

Greetings all,
Its been about 5 years now since I have last posted to the forum. Life got busy with 2 children and buying a house. With that out of the way and a proper basement shop setup, hopefully I can get back into building.

As my first post back, I wanted to share my latest shop acquisition and potentially make its resource available for others to use.
I was fortunate enough to come into possession of (the late, great and infamous US 1 meter character) Hal Robinson’s flow tank designed to study flow around scale size foils (keels and rudders). Beyond the sentimental value it really is neat to be able to examine the stall conditions of different potential keels and rudders.

I have uploaded to the “photos section” 2 pictures of this setup as well as a couple example photos of a test specimen. They are in a folder called “Hal’s Flow Tank”

Photo11 is of the front of the setup with the two larger controller boxes on the middle shelf. The first controller is for the speed of the tank; the second is for the intensity of the linear light source.

Photo12 is an overhead view of the tank to show the layout.
-When filled, the outer section contains water and the center section remains dry. In the upper right hand corner you can see the top of the trolling motor that provides the flow.
-Flow goes counterclockwise.
-The trolling motor is mounted in a special cowl designed to deliver non-spinning flow by the first corner.
-Each corner has flow diverters to soften the corner effects on the water. In the bottom left hand corner there is a wire screen to which another diffuse/flow straightener is normally mounted (currently removed).
-The test section sits in the front section slightly right of center.
-The test section is intentionally made narrower in the front to increase the flow rate through the test section.
-Above the test section is a narrowly focused linear light source that illuminates a very thin plane of light along the sample’s chord for study.
-Flow is visualized through very fine silver powder mixed into the water.

This particular test sample is a cut off of a windsurfing skeg that Hal was evaluating as a potential keel. Results on this sample were quite disappointing. As can be seen in the photo titled test10.jpg, due to a flat spot on the trailing edge, this foil develops a circulating back flow at very low angles of attack.

Currently the tank is only set up to measure angles of stall and flow reattachment after stall at low speeds. Hal was studying foils on the edge of stall condition as he was exploring the fact that when we are sailing fast and pointing, we are always on the hairy edge of stalling.

As Hal regularly opened up his house for others to come over and work on boats and use all of his tools, I thought I would open up my virtual door to others as well. If anyone has test samples they would like to study for stall effects, please feel free to contact me. If anyone is in the metro Boston area that would like to see and use this first hand, please feel free to set up an appointment.

If there is interest, I can try to shoot a video tour of this setup running the next time I have water in it.

Best wishes
Todd Brown
Minuteman Model Yacht Club.

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Hi Todd
Very cool tool. Is it possible to see laminar/turbulent flow differences with this? I have often wondered how our small size and low speed(low Re) affects this. I have it in the back of my mind that given a decent surface the flow over our fins and rudders would always be laminar. Whether this would affect the fin shape is beyond me. I will be very interested in anything you post.
Welcome back
Don

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Oooh, I want one! Can you give us some more details? I’d like to try to build one too.

What is the light source and how is it set up? What is the silver powder? The flow diverters after each corner, what do they look like? Are they after each screen or before?
It looks to be around 2’ x 3’ and 10-12" tall from about 3/8" acrylic with aluminum edge framing?

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You can indeed see the difference between laminar and turbulent flow. Actually, based on how bad this foil performed, you can see even the picture at the lowest angle of attack has turbulence on the trailing edge. One of the ealiest photos shows very strong turbulence with an actual vortex working back up the chord. YIKES.

I can take a “good” keel section and shoot some pictures a 0,1,5 and 10 degrees AOA and then do the same thing with a flat plate of aluminum of “fair” thickness. Maybe over the holiday I can run this comparison for you.

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Ill try to shoot a video tour over the weekend and post to youtube. It should answer all of these questions. Most of your guess are pretty good though.

overall Dims are 32"x48"x12"

IT is made of whatever plastic Hal had around in the shop. I think it is a mix of acrylic and polycarbonate of varying thicknesses. The corners are 2 layers thick so he could make watertight joints that he was sure would hold (wouldn’t want to flood the shop now would we!). The aluminum frame around the top serves 2 purposes. First and foremost it keeps the acrylic from bowing out. Secondly, it gives you something to mount test setups to.

I am unsure what the silver flakes are but if I remember correctly it is a paint additive. I know Hal tries alot of different silver flake and bubble type solutions. This was by far the winner. I have a peanut butter jar full of the stuff that I am treating like gold! Luckily, if I let the silver settle to the bottom of the tank before draining, I dont loose much each time.

The light source was harvested from an old photocopier. It passes a fine line of high intensity light over the paper as it transfers the image ESPECIALLY THE OLD ONES that used the heat to transfer pigment. Most of these run off of 120v but get very hot. Hal armed this one with a muffin fan for cooling and then hooked it up to a variac (large grey box with big black dial on the shelf) that can adjust the voltage from 60 volts to 140 volts. This allows the user to adjust the brightness of the light.

As for the corners, they are constructed of 1/4 round 1/4" think acrylic that Hal had. It started as a large diameter tube that he cut into 4 sections and used in each corner.

In the lower left hand corner, I am reconstructing the final straightner prior to the test section. Hal’s was good, but even he agreed it could be better. In one of my last evenings with Hal, we discussed what the new design should be. He never got around to it. . . I figure it should be first on my list. When I work on it, I will post pictures.

~tb

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Is the test section long enough to test a hull? I had consided building something like this but I was going to use a hot tub pump. I couldn’t figure an easy way to control the speed. The trolling motor is a clever solution. Does it run off a battery or? I think that your local automotive paint supplier will have a wide assortment of “silver” particles. I believe most are mylar now. Whats the SG of mylar- hmmmm
Don

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Oh, I know that silver metallic! It’s used in making pearl metallic paint. (I used to mix automotive paint some 20 odd years ago). One thing, just be prepared for “sticker shock” when you ask how much for a bottle. The stuff isn’t cheap.

I have a peanut butter jar full of the stuff that I am treating like gold!

Good for you! Because it used to be more expensive than gold (seriously). Although with today’s market of gold prices it might be equal. :rolleyes:

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One of the many reasons I pushed so hard to get this out of Hal’s house and into my basement was a want to test real drag numbers on test hulls. I THINK I could get a footy in there. I have sketchs for the drag measurement apparatus as well as the servo control loop to keep the boat centered in the tank. I also have a test hull from Hal’s shop that was a US1m scaled down to one foot. To test anything beyond a footy, you would have to develop a 7-12" scale model and only be able to be A-B comparisons and not absolute value drag numbers.

The thing that I do not like about the trolling motor is that it is only a 2 blade prop that leads to a LOT of buffeting of the current. a hot tub pump impeller would probably deliver a more uniform flow front.

The trolling motor is controlled by the old looking AC-DC varibale power supply on the second shelf on the right hand side. It allows me to foot the trolling motor up to a 120V outlet and convert to 0-12vDC for the trolling motor. This allows me to vary the speed from barely moving to “too fast” for the corners of the tank (will splash water out of the tank in the corners as well as start to cavitate the prop and suck air down into the flow at the prop.

Unfortuneately this doesn’t get me nearly as much speed as I had hoped. I am going to calculate the speed based on my pictures. I believe the photos below were shot at about a quarter of a knot.

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Thinking out loud… I wonder if the backside and return loop could be made from 6" dia or 8" dia PVC pipe with only a 4" section of clear plexi for the test section. That way you can get nice sweeping corners, no splashing, and easy to seal. If you used the stuff with a rubber seal, it could even be disassembled. You could probably get a faster water speed through the system if there is less turbulance too.

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That is an interesting thought, and if starting from scratch might be worth a try. I think what you might find though is that unless the test section is smaller then the ID of the tubing you are using, that the “diffuser” will create a fair amount of turbulence unless carefully dealt with.

Could make the construction a LOT simpler though.

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For your metallic flakes, I suppose one could purchase some of the mirrored Mylar used for growing plants (ahem) in a basement under “grow lamps” and run a bunch through a cross-cut paper shredder. Remove from the bin and run again and again until the stuff is chopped pretty fine. Still would need to deal with static electricity though. Only a thought as I empty my wastebasket shredder here at work.

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Very Very interesting, but no where is reported the water speed.
Since the speed is the major contributor to the foils performance ! since our sailing boats are traveling at realative low speed , let says 0.3m/s up to 1.5 m/s, how this can be compared with the water flow tank ?
Thanks a lot
Cheers
ClaudioD