600 reasons to go with the flow

This is the 600th post here at 30 Squares, and I know that isn't many posts compared to lots of other blogs, but to me it seems like a huge number. 

Before I started 30 Squares, I dipped my toe in the blogging waters with a blog called Separated Flow. It was supposed to be about scale modelling, aerodynamics, blimps, model railroading, science and the intersection of those subjects. The genesis idea was a project a friend and I built back in the early '90s: a small, flow-visualization windtunnel. The project embodied all the ideas the blog might touch on and seemed a good place to start. 

I posted about that project along with some others, but 10 or so posts in, the tone seemed too ironic and cynical. I didn't like where my head was at, so I deleted Separated Flow. After a year or so, I started 30 Squares of Ontario - today's 30 Squares - for model railroad posts, and retroDynamics for non-model railroad modelling, science, aerodynamics and other things that Separated Flow might have discussed.

Early in retroDynamics' life I reposted the windtunnel project from the remains of Separated Flow as it again seemed integral to its heart and soul. A few years later, 30 Squares was becoming my main blog, so I closed down retroDynamics and figured I'd broaden the scope of 30 Squares a little. 

So, now that I'm 400 posts away from a thousand, it seemed like as good a time as any to again re-post the genesis project that got me started in this blogging thing :-)

A Tale of Two 'Vettes: How to Build a Small Flow Visualization Windtunnel
from 18 Dec 2009 in retroDynamics and sometime in 2008 in Separated Flow

I recall watching an episode of Mythbusters where Adam and Jamie were building and flying concrete gliders. In one scene they were attempting to measure the lift of their airplanes with a makeshift windtunnel and scale. Well, it wasn't so much a windtunnel as a large fan with a flow-straightener made of drinking straws placed in front of the breeze. The rig was ingenious, but it got me reminiscing about a small-scale windtunnel that a friend of mine, David A., and I built in 1988. At that time we had some hopes that we might be able to build a business around flow-visualization photography with small windtunnels and digital analysis of the pictures. Nothing came of that wacky business idea, but we did build an interesting little windtunnel.

One thing I should note: all the pictures in this post are scans of old Kodachrome and Ektachrome 35mm slides made with a Canon CanoScan 4400F scanner.

[The windtunnel without the blower and the flow viz gear.]

These are politically correct times, so here is my disclaimer before you read on: fire and smoke was involved in this project, so don't try this at home. And especially don't try it in suburban Toronto garages containing firewood and gasoline as we did. No accidents occurred during this project, but we probably just benefited from dumb luck.

A good place to start this story is at the end. In this case, with some pictures taken in the windtunnel of the air flow around a scale model Corvette and Chevette.

[Flow over a mid-1980s Corvette]

There are a couple more flow viz photos at the end of this story, but we’ll get to them soon enough.

[Flow over a late 1970’s to early 1980’s Chevette]

The windtunnel was sized to be able to take pictures of the flow around 1/24 scale model cars. At that time I was interested in the air flow around automobiles and had a stash of around two dozen plastic model kits my cousin had bestowed on me. It turns out there was a Corvette and Chevette in the pile. Dave wanted to own a Corvette, but an ancient Chevette was his actual ride, so the choice of subjects to start with was pretty clear.

[The difference in hatch slope is quite pronounced.]

There was also some aerodynamic reason to the choice. It turns out that just because a car has a sloped rear window or hatchback doesn't mean that the air flow will stay attached all the way along the surface. There's a range of angles and length where separated flow is possible, and it looked like the Chevette's hatchback had such a configuration - which turned out to be more-or-less true if the photos are to be believed. In contrast, a gradual slope that terminates in a sharp 90-degree down turn at the end of the car - like that on the Corvette - can actually result in the flow being attached all the way along the slope. We thought we'd see if we could demonstrate this with the windtunnel and hoped that it would make for some interesting pictures.

[Unpainted windtunnel at a secret backwoods testing facility :-) ]

I like to use the cheapest and most readily available materials I can get away with for any project. It usually takes a few iterations to get things right, so not blowing all my money on the first prototype is very important. In this case, bristol board that you can buy at just about any craft store or art supply was used to build the tunnel.

The dimensions were scoped out using Rae & Pope's standard text: Low-Speed Wind Tunnel Testing. I don't seem to have my design notes from the time, so I can't properly comment on the thinking that went into the tunnel, but numerous compromises were made to keep it small. One requirement was to be transportable in the trunk of my car.

[It fit pretty well with the backseat turned down.]

But, I think all this emphasis on smallness resulted iin making the cross-section of the test-section where the model car sits too small. I suspect the walls of the test section are close enough to the model to have an effect on the overall flow. However, the ratio of the height to width of the test section is built in accordance with guidelines given in Rae & Pope so that standard correction factors could be applied to measurements. All things considered though, if I built this again I'd make it bigger - but, not so big that I'd need a Hummer to transport it.

[Looking into the test-section at the Chevette]

In the above photo you’ll notice that the long edge of the test section is horizontal and the short edge is vertical. When it came time to actually use the tunnel, as you’ll see in the photos that follow, we flipped it 90-degrees.

[Initial build (no, they weren’t our sponsors - we were just fans with high hopes)]

The problem with using the cheapest materials possible is that they often have poor performance, which was the case with the initial build. Relatively thin bristol board allowed for easy layout and shaping of the main components with simple tools, but there were stiffness problems.

[The walls got sucked in as soon as the blower was switched-on]

Originally the tunnel didn't have any external bracing, and thin clear plastic was used for the test section windows. The motor and blower setup had more power than was originally thought, and the resulting flow sucked in the tunnel's sidewalls as soon as the blower was switched on. I can't find my notes on the pressure differential inside and outside the tunnel, but it was obviously appreciable. We did make a simple manometer to measure the inside and outside pressure differential, but that data has been lost.

[New and improved windtunnel with braces and glass test section]

This lead to folded bristol board external frames to be added to the test section and the transition piece. As well, the test section windows were replaced with glass panes liberated from a couple of picture frames. Those enhancements did the trick and kept the walls from buckling.

[Flow straightening grid attached to the tunnel inlet]

The blower could indeed pull a lot of air through the tunnel, but it had some serious angular velocity components. To straighten out the flow in the test section a couple more enhancements were added. First, a grid panel with 1 inch by 1 inch openings was built - again from bristol board stock - and to one side a sheet of metal screen door mesh was attached. This assembly was then attached to the front of the tunnel. As well, inside the tunnel, at the end of the test section just before the transition piece, another grid frame was inserted - this one was a plastic parts divider from a storage box. I don't recall if a metal mesh was attached to this insert, but it could have been.

[Smoke jet in the test section]

The combined effect of both of those grids was to straighten the flow out fairly well as can be seen from the photos of the smoke jet.

A simple wooden stand was built to support the tunnel while in use. This structure is pretty simple: some adjustable shelf brackets were mounted to a freestanding wooden frame in order to allow for height adjustment. I don’t know why I painted it blue. I think I had some old blue paint around that needed to be used up. And the pin striping? Who knows why, but I it does look faster :-)

[The motor and blower assemblies]

The blower and motor assembly was Dave's handiwork and this part functioned very well right from the get-go.I don't have any specs on the set up, but I believe the motor was a 1/4 hp unit. No special power supplies were required, it was configured so it could be plugged into a standard wall outlet. As you can see in the photo, the blower exhaust went straight down so we had to put in an angled board to defect the stream. When we operated the tunnel in the garages we made sure that both the main door and the side-doors were opened so that you'd get flow through the whole space and didn't recirculate the air. Typically we'd set things up so that the exhaust from the blower was near the side-door.This made sure that smoky air exiting from the tunnel got vented directly to the outside. No doubt that today this would earn us a visit from the local police or fire department.

[An early test of the smoke generator with a bunsen burner as a heat source]

The smoke generator was admittedly a Rube Goldberg affair. And it took a bit of trial-and-error to get a rig that generated enough smoke. That picture above is one of the earliest attempts. We filled a simple Badger airbrush with mineral oil and shot it into a heated brass tube. In the earlier prototypes we just used a high school bunsen burner to heat the input end of the tube and you can see that it generated only a little puff of smoke.

[Heating the entire length of the tube generates the most smoke]

From those early attempts we moved on to some more heavy duty smoke production as you can see in the above photo. To be honest I can’t quite remember what we used as a heat source on this prototype - no doubt I inhaled when I shouldn’t have! - but I think it was some sort of electrical set up, but I also seem to recall that it got too hot. As you can see it generated lots of smoke which is what we needed. The key idea was that the entire tube needed to be heated so that all the mineral oil was burned and converted to smoke. In the end we settled on a rig where the tube was heated by a couple of propane torches with flame spreaders attached. You can sort of see the end result in the photo below. As I mentioned previously, at one end of the rig, mineral oil was blown into the heated tube with an airbrush and emerged from the other end as smoke. The cooler output end of the heated tube was connected to a plastic tube, which in turn was connected to a long, small diameter aluminum tube - which was supported by a tripod - whose output end was snaked through the flow straightening grid into the test section. This was quite a rig when it was all up and running. Once again, these day's I'd think of some other, safer way to do this.

[Full setup in action.]

The photo above shows all the components set up and ready to do a test run. To the far left - and unfortunately in heavy darkness - is the workmate with the smoke generator clamped to its top. To the right is a set of four photo lamps aimed more-or-less straight down the tunnel inlet, below and to the right of the lamps is the smoke wand inserted into the flow straightening grid and into the test section and finally to the right of that is the windtunnel itself. A photo lamp is also clamped to the windtunnel support stand and aimed down into the upper viewing port on the test section. A black blanket was taped on the garage wall opposite the windtunnel window in order to minimize extraneous reflections from appearing in flow visualization photos.

To finish-up, here are a couple more photos of the Corvette and Chevette. In these pictures the smoke wand is placed a little higher relative to the roof on each car. The result is a somewhat better defined smoke stream along the entire length of the body.

In the end the project was more fun than scientific. There are still numerous unresolved technical issues surrounding the use of crudely detailed models, the small size of the tunnel, and so on and so on. But, maybe the world would be a better place if every suburban house had its own windtunnel room - with appropriate smoke filtering of course :-)

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It won't be an anniversary post without ending with a list of the top 5 most popular of the 600 posts. From 1st to 5th, they are,

E. L. Moore's Legacy in the 21st Century: The Elizabeth Valley Railroad

A review of Model Railroader's 75-year collection, part 1

E. L. Moore's Legacy in the 21st Century: Balsa, cost, a handcar shed and organic veggies

Amtronic Ranch

HO-scale logging airships?