schlieren optics

By December 6, 2017NEWS
schlieren optics

Aerodynamicists use wind tunnels to test models of proposed aircraft and engine components. During a test, the model is placed in the test section of the tunnel and air is made to flow past the model. In some wind tunnel tests, the aerodynamic forces on the model are measured. In some wind tunnel tests, the model is instrumented to provide diagnostic informationabout the flow of air around the model. In some wind tunnel tests, flow visualization techniques are used to provide diagnostic information about the flow around the model.

This page describes an older flow visualization technique called schlieren photography. Schlieren photography is similar to the shadowgraph technique and relies on the the fact that light rays are bent whenever they encounter changes in density of a fluid. Schlieren systems are used to visualize the flow away from the surface of an object. The schlieren system shown in this figure uses two concave mirrors on either side of the test section of the wind tunnel. A mercury vapor lamp or a spark gap system is used as a bright source of light. The light is passed through a slit which is placed such that the reflected light from the mirror forms parallel rays that pass through the test section. On the other side of the tunnel, the parallel rays are collected by another mirror and focused to a point at the knife edge. The rays continue on to a recording device like a video camera.

Now if the parallel rays of light encounter a density gradient in the test section, the light is bent, or refracted. In our schematic, a shock wave has been generated by a model placed in the supersonic flow through the tunnel test section. Shock waves are thin regions of high gradients in pressure, temperature and density. A ray of light passing through the shock wave is bent as shown by the dashed line in the figure. This ray of light does not pass through the focal point, but is stopped by the knife edge. The resulting image recorded by the camera has darkened lines that occur where the density gradients are present. The model completely blocks the passing of the light rays, so we see a black image of the model. But more important, the shock waves generated by the model are now seen as darkened lines on the image. We have a way to visualize shock waves.

The earliest schlieren photographs of shock waves were black and white images. The image shown here is a color schlieren image produced by putting a prism near the slit and breaking the white light into different colors. Notice that the resulting image is two dimensional while, in reality, shock waves are three dimensional. So the schlieren photographs provides some valuable information about the location and strength of the shock waves, but it requires some experience to properly interpret the results of the process.

 

Schlieren Imaging: How to See Air Flow

 

This project documents the construction, science, and application of a Schlieren aparatus. In short, a Schlieren system leverages the principal of refraction, and some clever optics to allow for the visualization of differences in density of transparent media.

I also made a video for this project that goes over all the major points and if you skip to the 2:54 mark, you can see some of the results:

Step 1: Materials

Here’s what you’ll need to create a Schlieren system of your very own:

  1. Parabolic/spherical telescope mirror with long focal length
  2. Camera
  3. Telephoto lens
  4. Razor blade
  5. Point light source

Parabolic/Spherical Mirror – For this project, a bigger mirror, and a longer focal length, are both desirable. You also want a high quality mirror for this project, so I’d recommend buying a telescope mirror — its okay if it has a few scratches. Telescope mirrors are made from high quality glass blanks, precision ground to their final shape, and finally coated with a reflective surface. I bought mine on ebay for $60. It has a diameter of 160mm and a focal length of 1300mm. Price of telescope mirrors scale roughly with the square or cube of their radius (which makes sense). Here’s a similar listing: http://www.ebay.com/itm/Telescope-Spherical-PRIMAR…

Camera – Almost any camera would do — what’s more important is being able to zoom in and focus on the location of your mirror. So having a camera with detachable lenses, or at least a good optical zoom feature, is important.

Telephoto lens – Because I’m using a DSLR, I’m using a telephoto zoom lens. It’s a cheap lens, and its not fancy by any means. It has a maximum focal length of 300mm, and I need almost all of it in order to fill my picture with the mirror.

Razor blade – Any small, very sharp edge will do. If you want to have more fun, you can even use a color-gradient filter and place this at the focal point instead.

Point light source – I think this is the most difficult component on the entire list. The smaller and brighter your source of light, the better your results. I first tried lasers, but soon discovered that the speckling of laser light is far too distracting for it to be worthwhile. Interestingly, there’s a newer type of Schlieren imaging called Background Oriented Schlieren that leverages this speckling. Ultimately I settled on using a 3W LED coupled to some scrap fiber optic I had laying around. Other people have ground down LEDs and used tinfoil apertures. Whatever you end up doing, remember that you want your aperture as near to where the light is generated as possible, because intensity scales with 1/r^2.

You’ll be surprised — even though your point light source looks exceedingly dim, it will still create an image in your camera.

Step 2: Optics Setup

Step-by-step setup

  1. Place your point light source on a stable surface on one side of a room.
  2. Place your mirror on another very stable surface on the opposite side of the room, facing your point light source
  3. Vertically position a white piece of paper or posterboard near your point light source
  4. Dim your lights and adjust your mirror until your focusing the light back to a point very near to the point light source itself — this is why we want a white surface as an aid.
  5. Position the razor edge vertically near the focal point, but don’t obscure the light just yet.
  6. Place your camera in the path of the light, behind the focal point.

Consider building a mirror mount

I built a mount for my mirror that allows me to make fine-adjustments with some wing nuts. I’d recommend doing this, but it’s not completely necessary — I may make a separate instructable on how I build mine. All you need is a way to orient your mirror somewhat precisely.

The importance of stability

In the end, we’re relying on the bisection of a focal point less than 1mm in diameter. Thus, having a very stable system is important for good results. I used a table and a counter top on opposite sides of the room for my setup, but realized as I would walk around my apartment, that the floor, unbeknownst to me, would bend ever so slightly, but enough to change the path of light resulting in a washed-out image or a completely black image.

I dealt with it, but had to take my movements into consideration when producing the videos.

Step 3: Physics

I think its important to understand a little how Schlieren systems work before actually setting things up.

Refraction is our friend

Light refracts, meaning it bends, when it passes through matter with different density. That’s why a straw in a glass of water looks bent or displaced when viewed from the side.

When light passes through air of differing density, it also refracts, diverging from the path it was once traveling. In a Schlieren configuration, this means that this diverted light will no longer pass through the same focal point. However, the amount of refraction is so small, that we normally don’t notice it as the light still passes very near to our original focal point.

Thus we need a means by which to amplify the effects of this small refraction. That’s where the razor blade comes into play. If we bisect our focal point with a sharp edge, because our focal point will actually be finite-sized, then small changes in the path the light takes could have big consequences.

If the light refracts and bends one direction, it will hit the razor blade and never make it to our camera. On the other hand, if light refracts and bends the other direction, light that was previously being rejected, is now finding its way to our camera. But because it’s analog and not binary, we end up with shades of gray corresponding to differences in density.

It’s really that simple!

Astigmatism is not our friend

To reduce astigmatism, make sure your point source is close to your focus point. The closer the better. The greater this distance is, the greater the difference in angle between horizontal rays hitting your mirror, and the more pronounced the astigmatism will become.

Astigmatism results in a soft focus, which is no good for a Schlieren setup.

Step 4: Results!

Now that you understand the physics, you should know why we’re using the razor blade in the way that we are.

Image acquisition

Getting your camera lined up can be tricky. I find it easiest just to move my camera around until I see my mirror light up. Trust me, you’ll know. The mirror will go from being dark, to suddenly illuminating. Once you find the sweet spot, find a way to support your camera in that position. A tripod wasn’t an option in my setup, so I just used some notebooks and shims from scraps laying around.

Schlieren acquisition

Now move your razor blade to intercept your focal point. If you have a way to simultaneously view the output from your camera, this will make this process easier. You want to position the razor roughly half way into your focal point, and you’ll know you’ve gotten it right when you can see imperfections in the shape of your mirror. In my case, I saw a concentric pattern from imperfect polishing.

Go have fun!

This is the best part. See what you can visualize with your setup! Candles are a good benchmark, as they should produce very high contrast plumes. Other things I’ve tried include:

  1. Propane (gas)
  2. Dry ice
  3. Camp stoves (lit)
  4. Exhaling/Inhaling
  5. Fans
  6. Vacuum
  7. Lighter/candle/etc
  8. Isopropyl vapor from bottle (this one is fun)
  9. Farting — wasn’t me!
  10. Boiling water in a kettle
  11. CO2 from vinegar/baking soda reaction
  12. Freshly baked cookies
  13. Body heat

I’ve tried many other things as well. Not everything is possible with my unsophisticated setup. I was really hoping to be able to view sound waves from banging two blocks of wood together, but either the system isn’t sensitive enough, or the frame rate on my camera is far too slow. If you have a high-speed camera you’d be willing to let me borrow though, let me know ;).

Share your results!