Schlieren video clip

[James D]

Some of you maybe interested in this, Its a really simple technique which involves just a camera and a grid pattern.
It seems quite sensitive, What do you think?

[larry cottrill]

That is very cool indeed. Ben and I did some crude observations of the Dynajet tail end using an 8-inch diameter telescope mirror in a more traditional Schlieren setup. It worked great, except that (of course) the rapid cycling of the engine made it look like pure chaos! The answer (I thought) is to strobe the light source to match the engine cycles - I actually bought an adjustable-rate Radio Shack strobe light to modify for this purpose, but never got around to trying it. Strobing is interesting because if you can set up a condition where the strobe rate is SLIGHTLY off from the cyclical rate of what you're observing, you see the illusion of slow-motion operation, which makes a very interesting study.

Since then, Bruno and others have explained that adjacent pulsejet cycles are far from perfect duplicates of one another, so the whole idea of strobed observation may be far less workable than I originally imagined, although I'm sure something useful could be learned.

L Cottrill

[Dave_G]

Hi James,

Did you use the method here:

http://www.rit.edu/~andpph/text-schlieren-focus.html

or a different setup?

Dave

[James D]

Hi Dave
It's a bit more simple/crude than a focusing schlieren setup, What I basically did was, point a digital camera at a grid of vertical lines on my computer screen, then moved the camera around until it formed a sort of 'Moiré pattern' ,Density gradients between the screen and camera showed up as a disturbance of the moiré pattern.
It takes a bit of fine tuning but can give half decent results.

Larry
I like the strobe idea, I wonder if you could use a system based around a microphone to keep the strobe in time with the pulsejet, and some kind of programmable variable delay?

[Jim Berquist]

James D.

Try this! Build a frame of some sort. Card board would work. Cover with Automotive Polarized Film. Back lighted by a 2 tube fluorescent lite set up or some other soft light system. You can purchase clear or near clear. They use it to cut glare. This should provide the same effect as your computer grid and more refined detail. I have a screen on my door made from black fiber glass window screen . It's just a peace of screen that hangs by weights out side the house. When the wind fold it over it's self I see the same patterns. Polarization!!!!!!!!!

Larry! A simple SCR can trigger your strobe. !!!



Jim

[pezman]

Technically, that's a shadowgram.

The difference is that Schlieren images show the gradient of the density field's projection onto a 2d surface, while shadowgrams show the 2nd derivatives of the density field's projetion (i.e. I think that the brightness is technically the Laplacian of the projected density field).

-- a minor distinction of your goal is to visualize the flow qualitatvely, but important of you are trying to get quantitative information.

Another simple shadowgram device is a collimated laser pointer (project through concave lens to expand beam and pass the beam through flow-field).

edited for spelling, grammar etc.

[Jim Berquist]

Ben !

What dose that do? Mess with the light patterns? Wouldn't that work with some visible visible laser on the subject being photographed???? Red , Green, Blue type light?? ....Come to think about it , Laser is just polarized light.

[Jim Berquist]

A hologram is created when two in phase laser sources are put on the same target from two different angles . The resulting photograph of the target is seen as the combination of the reflected light of the two sources. This is seen as the 3d effect. Expert I are not. But this is due to the interference created by each other by the two sources , right?

What we saw in the video was the interference of the light by the gas and or heat from the flame right?

Yes laser is both polarized and coherent in nature.

What effect does the Schlieren effect exhibit?

Refracted light

Reflected light

Phase changed light

Impeded light

Jim

[pezman]

Schlieren images show differences in refractive index of a fluid. The measurement is possible because light tends to bend towards regions of higher refractive index. For air (and most other fluids), the refractive index is proportional to the fluid's density -- so Schlieren images allow us to measure how much density changes over a given region.

There are dozens of ways to generate a schlieren image, but most of these fall into a few categories:

Foucault method: Plenty of descriptions of the test out there. Here's one that's an easy read: http://home.moravian.edu/users/phys/mej ... tester.htm
If you perform one of these tests through a varying medium (for example the heat plume of a candle), the image of the knife edge gets shifted a little to the right or left, depending on the local density gradient of the heated air. This small shift is revealed as a large jump in the brightness of the foucault image, so it paints a picture of the local density gradient.

Grid cut-off method: In this method, you take two grids -- one is a background (vertical stripes of alternating back and white, for example) and the other is a cut-off grid. The cut-off grid is set up so that it obscures half of each bar on the background, so a small shift of the background image in one direction will cause a decrease in brightness (more black background is revealed and more white background is hidden). A small shift in the other direction has the opposite effect (more white is revealed and more black is hidden). Note that "schlieren" is the german word for "strip" so it apears that the term Schlieren image is a reference to this grid cut-off technique.

Synthetic schlieren: A digital image of a patterened background is taken when there is no disturbance present. Later, another image is taken when a disturbance is present. The shift of each feature in the background is carefuly calculated by a computer, and these shifts can be used to calculate the changes in density of the fluid betwen the camera and the background. Synthetis schlieren is probably the best shoice for amateur because it uses cheap equipment, it's easy to set up, it can cover an incredible range of sizes (essentially no upper or lower limit) and is easy to use for quantitative analysis.

The technique that JamesD posted looks as though it uses the spaces between elements on the camera's CCD sensor as a ready-made cut-off grid for the stripes on the computer screen. I actually thought of something like that a while back when I was playing actively with Schlieren images, but discarded it as being impossible to set up and never bothered exploring it, which I guess shows that you can over-think stuff. I'm amazed at how well it works, although I'd be surprised if you can get much more than qualitative flow information from it.

A cousin of the Schlieren image is the shadowgram. If you pass a point light source through a flow-field, rays will bend towards dense regions and away from sparse regions. Therefore, regions that represent a local peak in density will be bright (rays on each side are bent towards the peak) and regions that represent a local minimum in density will appear dark (rays on each side are bent away from the region).

It is possible to use laser interferometry to measure extremely small differences in fluid density, but it is hard to set up and tends to be a little too sensitive. On the other hand, a hologram might be able to capture fluid density variations in three dimnstions, but I suspect that it would be a little slow.

Collimated laser beams also make excellent light sources for shadow-grams, since the coherent light passes through the medium as parallel rays and the lasr provides pretty intense light in a small inexpensive package.

[Jim Berquist]

As I have a Micro Wave back ground I can deal with that! We ran space diversity. Top antenna and bottom. The top would be active in the day time and the bottom at night! Refraction!!!

So what your saying is refraction is impedance and change of phase!

There fore the cause or impedance! Is the image you see is the change in the lights path....????

Light can be bent! What you see may not be what is there! The light from the sun is 8 minutes old!

how far can light be bent? Are we as far from the stars as we think?


Jim