Schlieren Optics Homework Help: Visualizing a Supersonic Wind Tunnel

[Harry Fry]

Homework Statement

My final year project at university is to visualise the test section of a supersonic wind tunnel by schlieren method, though I'm having a bit of trouble setting it up.

The light source I am using is a halogen bulb, in an old lamp housing, with a condenser on the end with a focal length of 300mm

According to sources I have been using, a pin hole should be used to make it a point source the distance of the focal length away

The light rays will then be reflected off a spherical mirror, the focal length of the mirror away (930mm) through the test section and onto a spherical mirror on the other side.

Homework Equations

The Attempt at a Solution

I've struggled to get the whole thing going, I put the light source, pin hole and mirror the appropriate distances away, but the light image I'm getting on the other side of the pin hole is rubbish.

Any thoughts?

Thanks in advance.

 

[Quantum Defect]

Homework Statement

My final year project at university is to visualise the test section of a supersonic wind tunnel by schlieren method, though I'm having a bit of trouble setting it up.

The light source I am using is a halogen bulb, in an old lamp housing, with a condenser on the end with a focal length of 300mm

View attachment 79868

According to sources I have been using, a pin hole should be used to make it a point source the distance of the focal length away


View attachment 79874

The light rays will then be reflected off a spherical mirror, the focal length of the mirror away (930mm) through the test section and onto a spherical mirror on the other side.

View attachment 79870

Homework Equations

The Attempt at a Solution

I've struggled to get the whole thing going, I put the light source, pin hole and mirror the appropriate distances away, but the light image I'm getting on the other side of the pin hole is rubbish.

Any thoughts?

Thanks in advance.

Where's the very sharp edge? (razor blade)

http://sciencedemonstrations.fas.ha...ram_name=indepth.html#a_icb_pagecontent221620

 

[Harry Fry]

The sharp edge is due to go on the other side of the test section.

This is the set up I'm going for

 

[Quantum Defect]

The sharp edge is due to go on the other side of the test section.

This is the set up I'm going for
View attachment 79880

I have never done Schlieren optics, but it is something that I am interested in. I have done mostly experiments with lasers over my research career, which has involved a lot of finicky optical setups.

It seems like the simpler set up that is shown in the materials at Harvard might be easier to implement. The images in the YouTube video above are pretty good, I thought. Unless you are measuring much smaller effects in the wind tunnel which require the longer path length in your setup, you might consider simplifying.

There are a lot of mirrors in your scheme, and the optical path length is very long, so alignment will be difficult. You are also using some things in your path that are going to be problematical -- not so stable (the lab jack) which may make your life miserable. With the setup shown in the Harvard demo, there are relatively fewer optical components, with less to go wrong. The Schlieren effect is obtained from tiny changes in refractive index (wiggling of the light beam) in the observation window. What happens if your optics are not stable? Your beam will have additional sources of wiggle. Your "pin hole" looks pretty ragged and is also quite large.

Another thing to think about is the age of the design above. So much has been changed by the advent of really amazing consumer technology. When I was in graduate school (mid 80s) , we were beginning to see some of the very first CCD detectors being used in experiments. You now can buy these for tens of dollars, and they are a feature on everyone's smart phone. I grew up in the age of film, I took polaroid pictures of the oscilloscope traces for some work as an undergrad. I remember being frustrated with not being able to take holiday pictures indoors without a good flash, etc. etc. All that I am saying is that if the setup above might be designed to work well with yesteryear's photographic technology, and you might be able to do much better without any significant increase in expense.

See also: http://scitation.aip.org/content/asa/journal/jasa/129/4/10.1121/1.3588692
http://www.mne.psu.edu/psgdl/Pubs/2003-Pandya-JASA.pdf
http://alexandria.tue.nl/repository/freearticles/617406.pdf

 

[artax]

Hello Harry, How are you getting on?...I think you would be better off with a brighter light source and a smaller pinhole . Have you tried White LED's? You will also need to get the room really dark. I don't know the power of your halogen bulb or your pinhole size.

This looks very similar to the way I do a Foucault test on my telescope mirrors to test for sphericity... In fact I've played in the garage imaging the heat rising from a teacup using my latest 12 inch f= 70 inch mirror in the same way shown in the video with the hairdryer. However there's only one parabolic mirror with the light source at centre of curvature, not focus. A good quality single LED flashlight is best... the brighter the better with a piece of aluminum foil for a pinhole that's had a needle pushed through it. A good quality bike shop will have bright LED lights for around 20 dollars.

Keith Penny,
Sheffield University.

 

[Harry Fry]

Thanks Keith,

I managed to get it to work in the end. The main issue was I wasnt getting the light rays to converge at the pinhole (even though the pinhole was probably larger than preferable). Then swapped the spherical mirrors for convex lenses and plain mirrors. The output wasn't brilliant but I managed to get the schlieren image so I could see the oblique shock waves in the end

.