Seeing the invisible

[flynfrog]

http://www.rps.psu.edu/may97/see.html

Cool link I found I cant quite figure out how it works

maybe some on here can explain

Ciro?

[Ciro Pabón]

Gladly, Flyn (hi! long time no talk). It's a Schlieren system, as the article states somewhere, invented by Mach himself.

You create a collimated light source (this means the rays are parallel), shine it into a transparent object, focus it, and put a knife edge at the focus, blocking about half the light.

If you don't use the knife, anyway you have shadows (duhhh...), in what is called a shadowgram.

You can get approximate shadowgrams with a source of light as common as the sun: the sun is so far that its rays are approximately collimated (parallel). Sun rays are not exactly collimated because the Sun it's not a point, but you get the idea.... when the light is "very parallel", any change in air density "twist" part of the light ray, so the ray interferes with itself, creating a shadow.

The shadows show the "second spatial derivative (Laplacian) of the refractive index field in the transparent medium under study", which translated, means, erm... a shadow.

In air, refraction is proportional to air density, which means you are getting a (sort of) map of air densities.

You can watch the shadows of air at your next sunny day barbecue. It's almost the same thing, except that when you use a knife edge to block half of the light ray (and thus move from shadowgrams to Schlieren) you get the first derivative (that is, the rate of variation) of the air refraction. Don't ask me why.



All the aero guys at the forum surely will jump to correct this... (thanks in advance, you guys).

I have the strange ability (which I suppose some other forum members have) of being able to see the shadows of heated air some days, early in the morning, right after I've woke up.

I noticed it by chance: I have to take my eyes out of focus, "gaze down" and "watch" the top border of my "eye image". I'm sure I'm seeing something real, as the twisting and twirling of the air that I see it's more intense near heat sources.

I've never heard that happening to anybody, but I swear it happens to me sometimes. That, or I'm nuts... Actually, I would be more confident about my sanity if someone says it happens to him too... So, if you don't have a Schlieren apparatus you can hire me (only from 4 to 6 am... ).

[segedunum]

Yep Ciro, that happens to me too. It has to be in my peripheral vision. If I try and look directly at them, I can't see them.

[JimmyK]

Unfortunately I was skimming (whilst being tired) and misread it as 'propane leaking from a horse'. Thus ended me reading of the article.

[flynfrog]

so is it possible to make my own

i have alot of camera optics im guessing the knife edge part would be rather difficult to get adjust correctly

now that i think about it when i worked in the wind tunnel there was a high mach tunnel with alot of mirrors around it im guessing they did something simaler

[Ciro Pabón]

Segedunum: how glad I am that you see the same. This means either I'm sane or we both are crazy... BTW: Are you from Tyne or do you live there? I was once there visiting Hadrian's wall.

Flynfrog: I spoke with a friend and he explained me that, in practice, you don't create the Schlieren image as Mach said (or Wikipedia). It's much more simple. You need a lens and a flood lamp, a couple of plastic translucent sheets and a little patience. This is how you can construct yours:

1. Make black vertical stripes (using some tape?) on the translucent sheet.
2. Take a photo of your sheet and produce a negative of the image (the black stripes will be white and the white spaces between the lines will be black, of course).
3. Put a lens between both plastic sheets . Put a white screen (the wall or a paper) beyond the negative, to receive the light that has passed through the first sheet, the lens and the second "negative" sheet.
4. Put the negative after the lens. Move it forward and backward until the black stripes of the negative block the light from the white stripes of the original plastic sheet. When you "get it", the white screen should be uniformly gray: the light that passes between the stripes should be blocked by the black stripes on the negative. I'm told that's the hard part... The sheets should be framed (in wood or something) and you need a base with screws or something that allow you to move the negative sheet slowly. A workbench clamp could work, or so I think.
5. Put your object between the first sheet and the lens (a candle or something hot). Presto! A Schlieren image.

The distance between the first sheet and the white screen (where the image will be projected) should be more or less 4 times the focal length of your lens. The object (candle or whatever) should be in the middle of both (at 2 times the focal length of the lens from the first sheet).

The "negative" second sheet should be at 1.3 times the focus length from the lens (or at 0.7 times the focus "before" the white screen).

I saw the "array" at the University of Los Andes this Saturday and that distances is what a student told me worked for them.

BTW, thanks for asking, I would never have found that if you haven't asked. Next time you participate in SAE racing you can tinker a little to try to get a cool way to check your models aero...

[flynfrog]

i used to have a Polaroid landscape camera that would have been perfect for this

[flynfrog]

And thank you Ciro i know you love this kind of stuff as much as I do.

As for Fsae im done now went out with a 7th and an 18th place finish but ive moved on to new and better things.

Im gong to do some more research into this looks like it could be entertaing to say the least

[salilp]

2. Take a photo of your sheet and produce a negative of the image (the black stripes will be white and the white spaces between the lines will be black, of course).

Thank you ciro for the simplification.

cant we print the same pattern in inverse for our second sheet? since ony white and black are involved, why do we need to take a photo and produce negative?

[flynfrog]

2. Take a photo of your sheet and produce a negative of the image (the black stripes will be white and the white spaces between the lines will be black, of course).

Thank you ciro for the simplification.

cant we print the same pattern in inverse for our second sheet? since ony white and black are involved, why do we need to take a photo and produce negative?

im guessing it has to do with the distortion from the lens :dunno:

I really want to try this but wont have time for at least a few weeks

[deluge]

F1 Technology?

[flynfrog]

F1 Technology?

I was thinking a candle but it could be the new hotness in F1

[Belatti]

What happened with the white vortexes that where formed in F1 rear wings, some years ago??? Where did tey go? Huhh???

On light grey cloudy days, I sit and look to the sky with my eyes focused to nothing, looking to that huge mass of vapor.
Maybe I´m nuts too, or not, but suddenly I began seeing what I use to call "ethereal sparks", thousands of them falling down slowly and always in perispherical vision, impossible to focus one of them. Strange, at least...
Did someone experienced something like that in this esoteric fourm???

[SoftBatch]

There is a perfectly good explanation for those but I can't remember what it is, it has to do with something in your eye, your not seeing stuff in the vapor. Ask the doc at your next eye checkup.

[bar555]

Belatti wrote

What happened with the white vortexes that where formed in F1 rear wings, some years ago??? Where did tey go? Huhh???

I think the white vortexes where actually air moist ( water vapors mixed with air molecules) which was turned from vapor phase to liquid or being condensed because the pressure was raised above a critical limit (phase transit phenomenon) . Now the rear wings have more sophisticated shapes ( not just horizontal profiles ) and vortexes can only be seen in rainy races .