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Schlieren photography help please!
Hi! I'm a freshman in college doing some wind tunnel research involving Schlieren photography. However, this isn't really working for some reason. I have a picture of my setup below...imagine, if you will, a wind tunnel between the two elements in the middle of the setup. They each hold a plano-convex lens to focus light from the far left side (that box is a PASCO light source containing a single vertical filament bulb) through the wind tunnel and then down to the razor blade cutoff (the holder for this is the second furthest thing to the right) such that a camera (a Nikon D50 DSLR) on the far right holder will be able to capture a Schlieren image, or so I hope.
schlieren setup.JPG
I'm using a shutter speed of 1/4000 s primarily, though I have tried slower. My F-stop is set at 5.6, though I have experimented with higher and not received good results. I'm using the Nikon 70-300 AF G lens to shoot these with, and I CAN get results from taking an image of a small blowtorch...but anything less than that (I've tried a soldering iron at maximum temperature and a wing) gets me nothing. There are windows on each side of the tunnel, and I've photographed with them on and off, so it's not there that a problem lays, nor with dirt on any optical components (I've cleaned them all and have a very good quality image...but still no Schlieren effects).
Would you folks have any thoughts on what might be needed to get this to work? I'll have a couple of the pictures that I've taken from it below.
schlieren setup.JPG
I'm using a shutter speed of 1/4000 s primarily, though I have tried slower. My F-stop is set at 5.6, though I have experimented with higher and not received good results. I'm using the Nikon 70-300 AF G lens to shoot these with, and I CAN get results from taking an image of a small blowtorch...but anything less than that (I've tried a soldering iron at maximum temperature and a wing) gets me nothing. There are windows on each side of the tunnel, and I've photographed with them on and off, so it's not there that a problem lays, nor with dirt on any optical components (I've cleaned them all and have a very good quality image...but still no Schlieren effects).
Would you folks have any thoughts on what might be needed to get this to work? I'll have a couple of the pictures that I've taken from it below.
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I would try a shorter focal length, a smaller aperture, or a smaller format to try to get as large a depth of field as possible.
I assume you can see the stria in the airflow by eye to focus on? Is focusing done manually, by AF, or by fixed measurement of distance?
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What do you mean about a smaller format to get a larger depth of field? I'm not a camera expert by any means -- I just got a crash course in photography from the friend who lent me his D50, lenses, and IR remote for the project.
Do you think that there is a problem with the masks that I'm using? Currently, I'm covering up the outer 1/4" of lens (radially) on the output side of each to get rid of rays suffering from spherical abberation. I don't think this should cause a problem, however.
Another note about what this system should be able to do: it is possible to take a Schlieren image of a cup of warm water and see the heat waves radiating from it...my professor did something akin to this with a setup similar to mine, so I should be able to get something here...
1/4000 s, F5.6
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I might just be imagining things, but it looks kinda like something happening around the soldering iron here.
0.001s f16 80mm.jpg.JPG
Have you tried a pinhole in front of the light source.
Are you sure the light is collimated?
If you substitute your eye for the camera, what do you see?
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DSCF0314.jpg.JPG
Am I sure the light is collimated...I'm fairly sure it is, but, no, I'm not 100% certain. I've positioned the source and the lens such that they are one focal length (or darn close to it) from one another and then placed my hand on the output side of the lens and only caught a circle of light that did not spread noticably, so I think it is collimated. The setup pictured previously holds everything onto the system's optical axis, so that should not be an issue either.
I can't say that I've ever tried substituting my eye for the camera, but I'll give it a shot. I just did some more work today that had the image projected onto a piece of paper (essentially the same thing sans the focusing the eye does, so we just had to do that by hand (paper and soldering iron placement)) and did not get anything that worked from there either.
It does sound like you are fairly collimated and I agree with your test for that (a distinct shadow without growth across a distance.)
It could still be alignment issue and in an inline system like you have created that can be difficult. That's one area where a mirror based system seems easier.
Here is another pictorial of a system that I presume to be similar to yours:
http://history.nasa.gov/SP-4305/p255b.htm
You might also try a "Focusing Schlieren System" like the following:
http://people.rit.edu/andpph/text-schlieren-focus.html
I realize that time is not your friend just now.
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Alignment may be an issue...I have things mounted such that when the lens holders are pressed flush with the wind tunnel, everything should be perpendicular and in line, but it's on my list of things to double check now. I'd be using a Z-type mirror system, but 1) off-axis parabolic mirrors are REALLY EXPENSIVE (and I had a $100 budget for all this...and blew it all on a pair of $40 lenses with $25 of shipping fees), 2) I figured that I could eliminate coma from this system a lot easier, and 3) it was looking to be a LOT easier to set up, given the wind tunnel.
I'm also going to do some experimenting with my light source and see if that does anything...a query, though: how critical is it that the razor blade be perpendicular to the oncoming light? I wouldn't think the system would be that sensitive to this, but it's a thought that has come up.
http://history.nasa.gov/SP-4305/p255b.htm -- yep, that's it. Almost exactly that.
On the plus side, since no one else is as close to us to finishing their projects (this is for an engineering design class), we're all granted the option of an extension...so we might be able to figure this out during the summer. On the minus side, I'm not going to be in town anymore...but I'll still be in touch with folk.
I would be surprised if you get anything to show up with a Schlieren photo of an airfoil at 80 mph. You'll need to be just under Mach 1 before you start seeing compressibility effects.
You mention in a different post that you were testing with a blow torch and a soldering iron. I would suggest trying a candle instead. It will probably have a strong Schliiren result, should be simpler to manage, and is (arguably) safer than a blow torch.
Sorry I don't have any specific experience with Schlieren photography equipment, but you seem to be in good hands with Ziggy. You might also try speaking with some grad students to see if they can look at your setup.
Good luck!
Mike
A sodium light source is a good start partly because the spectrum is pretty narrow and partly because you can find sodium sources to be fairly inexpensive. The human eye is also pretty sensitive to the sodium spectra and should make the effects more visible. A lot of physics departments have sodium light sources.
A yellow-green LED is another possible source and may be small enough so that you won't need a slit or pinhole.
http://commons.wikimedia.org/wiki/File:Yellow-Green_LED_Spectrum.gif
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As for the airspeed...I'd think, though, if I could see optical inhomogeneities from a schlieren image of a wine glass with warm water in it, that I could see pressure changes going up from wing section developing upwards of two pounds of lift (or from a soldering iron tip). Given, it'll be harder to get on the wings we want to use (smaller wings at 20 mph), but those wings are going to be different in that they will have a turbulent airflow and thus a far easier flow to visualize. We're not looking for a shockwave; we're looking for where the air's going in order to verify results from a computer fluid dyanmics solver.
Hehe...my college doesn't have a grad school, nor are there grad schools around town. However, we are getting in touch with Gary Settles, the author of a rather authoritative book on schlieren photography and shadowgraph techniques and a professor at PSU.
It has been about 40 years since I built a Schlieren system for the supersonic wind tunnel at my undergraduate university. Needless to say, I don't remember all of the details but there are a few things you might check.
1. I presume that the room is dark when you are doing your experiments. Extraneous light can mess things up.
2. The knife edge should be at the focus point of the second lens. You can check this by putting a piece of paper at the knife edge location and moving it forward and backward until you get the smallest possible image of the light source.
3. The vertical location of the knife edge determines how much Schlieren effect you get. I had a micrometer attachment to move my knife edge up and down by very small amounts until I got the maximum effect. This was very sensitive.
Finally, I'm not sure about projecting directly into the camera. I don't remember but I think I had a translucent screen (like a piece of wax paper) behind the knife edge. This let me see the effect when I was adjusting the knife edge. I think that I then just photographed the screen.
I have some old textbooks and notes somewhere in my attic. I'll try to find them and get back to you.
Steve Davis
Air flow around the soldering iron is due to heating of the air, causing it to become less dense. It is considered buoyancy driven flow. It is very easy to visualize with Schilieren or shadowgraph because of the density differences caused by the heating.
Air flow across an airfoil is pressure driven flow. The pressure differences on a 20 mph flow across an airfoil will cause very, very small density changes. I don't think there will be enough density change to show up in a Schilieren photograph. Have you found any photos?
That's a great idea. You might ask him about visualization of low speed airflows to make sure you should expect to see it with the Schilieren technique.
Good luck with your project. It sounds like a lot of fun!
Mike
Steve,
Is this only true for a slit or knife edge at the source? I don't believe that I had problems with orientation using a pinhole at the source.
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A micrometer, though??!? Wow...I have a 3/8-16 nut for my cutoff adjustment. I know that I'm intercepting half (or dang close to it) of the light at the focal point, though, and capturing it with a camera shouldn't be an issue. It's the same idea as the paper, just that there's a photosensor on the other end...kinda like if you put your eye there and fixed it at a certain focal length.
I could see how heating the wing should help with the Schlieren photograph while having negligible effect on the airflow.
I found my reference materials.
1. The textbook "Elements of Gas Dynamics" by Liepmann & Roshko, Wiley, 1965. This was the standard text for Aeronautical Engineers for a long time. Pages 157 to 162 is on Schlieren systems. There might be a copy in your school library.
2. A pamphlet "Schlieren Photography" by the Eastman Kodak company from 1960. It was Kodak Pamphlet No. P-11. I don't know where I got this. You probably can't find this (although a phone call to Kodak might turn one up). It is a shame because it contains a lot of practical information.
Here are some things that I found that might be helpful. Alignment and adjustment are critical in these systems. In particular:
The light source should be focused onto a slit or knife edge. This should be located at the focal point of the first lens to provide a collimated beam. The second lens focuses the image of the slit onto the plane of the knife edge. The knife edge should be parallel to the slit or first light source. This location and orientation is critical.
The camera is focused on the model in the wind tunnel (your soldering iron). I finally remember. In my day, I had an old (even then) Speed Graphic 4X5 press camera. I used the ground glass to set things up and a Polaroid back to take photos. You may need to use manual focus and live view to set things up.
Finally, the Kodak pamphlet says, " Proper positioning of the second knife edge can be checked by observing the image on the ground glass while moving the knife edge gradually into the light beam slightly. When properly located, this movement will produce uniform darkening. If the image darkens nonuniformly in the direction of the knife edge motion, the position of the knife edge should be shifted along the beam until uniform darkening is seen. If darkening is non-uniform in the direction perpendicular to the knife edge motion, the knife edge should be rotated until this condition is corrected."
I hope some of this is useful.
By the way, the Kodak pamphlet contains a Schlieren photo of a hot soldering iron and it has lots of turbulent eddies.
Steve
Ziggy,
For a pin hole, I would guess that the orientation of the knife edge would be less critical than for a slit or knife edge.
However, the second knife edge blocks deflected light rays. Therefore it darkens and lightens areas with density gradients perpendicular to the knife edge. Also, how far the knife edge protrudes into the image of the source will determine how much deflection will be blocked. The trade off seems to be between darkening the whole image and getting more sensitivity.
It is obvious that the poster needs to make some adjustment/alignment to get more sensitivity. I'd like to help (Schlieren photos are great fun!) but I can't tell what is off.
Steve
I'll put getting in touch with Kodak on the list of things to do. Unfortunately, I'm leaving school and the other three people on the team are going to be responsible for making schlieren work, but I can pass along what I can still.
As for photography, we are using manual focus and TTL viewing (is this the same as live viewing?) to get the camera on target. When looking at the front of the lens, the light can be seen to be going onto the center of the camera lens.
We've tried a vertically (parallel to the filament) and a horizontally oriented razor blade to no effect, though perhaps the holder isn't good enough to hold everything as precisely as it should. I'm trying to get my hands on another 3/8-16 nut to fix that, however. Testing on a smaller system, we fixed a razor blade onto a magnetic strip and moved it steadily up and down to try and get a schlieren system to work on a smaller scale (projecting onto a piece of paper) with a soldering iron again and got nothing from that either. The entire system was fixed onto a solid magnetic track that held all components in place, square to a common edge, and a measured distance apart. Again, we'll have a shot and keep you all updated...thanks for the help!!
Langley: http://www.nasa.gov/centers/langley/about/contact.html
Or search people.nasa.gov to bring up Dr. Weinstein's contact information.
Thanks to all, and God bless!
Fantastic and thanks for getting back to us.
If you should ever get any images feel free to post them.
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