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kingjewel1 March 21, 2012 15:48

Laser light sheet for flow vis
 
How can we make a laser light sheet for flow vis http://www.youtube.com/watch?v=_AJgEa2dbJU&feature=related? Are they expensive to buy off the shelf or can they be made?

Also what sort of seeding material do you use?

Regards,

Rami April 2, 2012 08:37

Hi Kingjewel1,

I am not an experimentalist, but laser sheets were used in my group. Actually, it seems very simple and inexpensive. Use a glass cylinder as a lens. Aim the laser normal to its cylindrical surface, and - viola!

As for your other question about seeding - sorry, I can't help in this issue.
Rami

mettler April 2, 2012 09:13

actually, it is neither simple nor inexpensive. LaVision makes a system that does this, and it is awesome to use. But, it is extremely complicated. You also have to have cameras with filters so you can see the fluorescent particles as they are excited by the laser. You are setting the shutter speed to be in time with the laser. As a result, the excited fluorescent particles can be mapped by the software. The laser is a pretty powerful laser too. It is called PIV - particle induced fluorescence. There is also PLIF - Planer Laser Induced Fluorescence - that is heat transfer stuff and is just as awesome - except for the rhodamine that you have to use is quite messy. I believe the system I used back in 2006 was over $100k.

Rami April 2, 2012 10:53

Quote:

Originally Posted by mettler (Post 352676)
actually, it is neither simple nor inexpensive. LaVision makes a system that does this, and it is awesome to use. But, it is extremely complicated. You also have to have cameras with filters so you can see the fluorescent particles as they are excited by the laser. You are setting the shutter speed to be in time with the laser. As a result, the excited fluorescent particles can be mapped by the software. The laser is a pretty powerful laser too. It is called PIV - particle induced fluorescence. There is also PLIF - Planer Laser Induced Fluorescence - that is heat transfer stuff and is just as awesome - except for the rhodamine that you have to use is quite messy. I believe the system I used back in 2006 was over $100k.

Hi Mettler,

What you describe is too expensive for our pocket... Our group used smoke in a swirling air flow and a simple camera, resulting in a reasonable visualization to show a novel concept called a tornado-type solar receiver (see a paper in J Solar Energy Engineering 2002 by A. Kogan). What you suggest (PIV and PLIF) is probably more informative but more expensive too, of course.

mettler April 2, 2012 10:59

Yes, I agree..If you want simple visualization, then the smoke method you mentioned is great. I did a flow visualization through porous media using fluorescent ink from a highlighter pen and a black light. That worked just fine. The nice thing about PIV/PLIF is that you get an actual measurement. We were measuring boundary layer height with the PIV. We could actually measure the height with the software and then compare it to known equations.

Fugacity April 2, 2012 12:42

If you just want to observe the flow, a small laser with a cylindrical lens and a smoke machine will work fine for air. Liquid would be more difficult, as very small glass or plastic beads are typically used as seed particles and they don't play well with filters or pumps.

If you want actual data, then it gets expensive. Yes, the LaVision PIV systems run about $100k. But, you can create your own PIV system with a few simple ingredients:

-DSLR Camera with a remote shutter trigger & appropriate lens and filter to only let the laser light in.
-Smoke Generator
-Pulsed Laser-typically a nd:YaG(green), but can be any kind; This & associated optics are the big ticket items as anything with enough oomph to illuminate a 2'x3' flow field is $50k+ last time I checked.
-DAQ System & Matlab.

The PIV process is a just cross-correlation between 2 photos with a known delay between the exposures. Importing the pictures into Matlab will make them real-numbered matrices, which makes the math rather straightforward, though a bit cumbersome. There's even an piv Matlab library floating around now; mpiv. We had to write our own code in grad school... That is, after we walked 3 miles uphill in the snow to get to class...:rolleyes:

http://en.wikipedia.org/wiki/Particle_image_velocimetry

mettler April 2, 2012 12:50

you have to be able to time the trigger of the camera with the pulse of the laser otherwise you will end up with a lot of empty photographs. Also, depending on the flow-field size, you might need some good zoom lenses - as with my experiment, I had to be in the order of the boundary layer thickness. Our fluorescent spheres were 50microns. We had no problems with them passing thru our pump. And, the laser can't be 'any' kind. The laser is what is energizing the fluorescent spheres, so it has to be a certain wavelength. It'd definitely be neat to build one on your own. Again, tho, the lavision thing was awesome, and their tech support is pretty outstanding. Rich, the engineer from LaVision, came to the our lab and did a three day high speed/cram PIV/PLIF class with us and showed us how to run it.

kingjewel1 April 2, 2012 16:13

Quote:

Originally Posted by mettler (Post 352676)
actually, it is neither simple nor inexpensive. LaVision makes a system that does this, and it is awesome to use. But, it is extremely complicated. You also have to have cameras with filters so you can see the fluorescent particles as they are excited by the laser. You are setting the shutter speed to be in time with the laser. As a result, the excited fluorescent particles can be mapped by the software. The laser is a pretty powerful laser too. It is called PIV - particle induced fluorescence. There is also PLIF - Planer Laser Induced Fluorescence - that is heat transfer stuff and is just as awesome - except for the rhodamine that you have to use is quite messy. I believe the system I used back in 2006 was over $100k.

My tongue is hanging out after looking at the LaVision website. Early Christmas present?

We're looking to characterise some indoor air flow (about 4m x 3m sheets), so anything from pure visualisation to full PIV is interesting.

If I pointed a laser at a rotating cylindrical lens would it do the trick do you think... How would you choose the laser power, lens curvature etc?

mettler April 2, 2012 16:19

with LaVisions set-up you can choose the laser power and the focal length of the laser. Rich helped us out with that, but there is a lot of trial and error too. We were trying to shine it thru nanofluids, so we had it cranked. But, it started to melt the lexan channel. Depending where you are, you might have to take a laser certification/safety class. We were at a University, so I had to take the class.

mettler April 3, 2012 08:05

if you look at my profile there are some pictures from the PIV system that we took during the project.


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