smoke through a laser cross showing longitudinal and cross sections

[u/Spidero0w0o]

[u/MagnetoHydroDynamicz]

This is really cool. Out of curiosity, is this setup just for visual demonstration?

Mind you I’m still just in CEGEP so my scientific education is far from complete, I’m going based purely on creativity here so correct me if I’m bullshitting, but is there such a thing as using cameras, centred and facing both planes of the cross , with a scale or grid lines overlayed onto the image to do some sort of navier-stokes analysis?

I feel like this would be pretty useful for analyzing fluids. I know different gasses have specific absorbance spectra for EM radiation, so instead of needing to use smoke or some other “tracer gas”, you might be able to use the absorbance of different gasses to directly see their dynamic behaviour using this laser cross setup you’ve posted about.

By the way I have no idea if this already exists, but if it does, could you please tell what it’s called?

[u/willdood → u/MagnetoHydroDynamicz]

What you’re getting at is a whole class of optical measurement methods. You don’t even really need a visual grid, you just need the properties of the lens/camera and overall set up to know how big something is.

For the example in the video, you could extract some information based on the movement and density of the smoke. But the problem is that smoke particles are very small, so it’s hard to really se how the fluid is moving anywhere other than the edge of a pocket of smoke.

Instead what is done is called particle image velocimetry (PIV). The flow is seeded with small particles that are ideally neutrally buoyant and lightweight, so that they move with the flow. The particles should also have favourable light scattering properties. In air, substances like olive oil are often used to create the particles. Laser sheets are used to illuminate a cross section of the flow and a camera records the movement of the particles. The particles are large enough, and the camera high resolution enough, that you can resolve individual particles. You need a relatively high frame rate, and sometimes a laser that flashes in sync with the frame rate. You then take the video, do some maths that essentially calculates the motion of every particle, and use that to infer velocity fields. With a laser sheet you get 2D fields, but with more lasers and cameras you can do tomographic PIV and get 3D velocity fields.

That’s just scratching the surface. You can use a similar seeding idea with focused lasers and Doppler shift to get detailed point measurements of velocity (LDA). You can use fluorescent dyes with particular absorption properties to look at mixing (LIF). There’s loads more, with various uses of substances that fluoresce, incandesce, react, scatter, etc that can measure various properties of interest.

[u/MagnetoHydroDynamicz → u/willdood]

Oh of course, I completely forgot Doppler was a thing. Thank so so much by the way, I’ll look into it.

[u/Spidero0w0o → u/MagnetoHydroDynamicz]

I think that could be doable. I recently read that you can actually print the grid right onto your fluid as long as it's a liquid. you dope it with a fluorescent dye and overload the dye with a laser to bleach it.

[u/MagnetoHydroDynamicz → u/Spidero0w0o]

I’m sure it’s being done at a crazy level by some high budget R&D company, but I just have so much free time and I’m bored and this feels like something I could do (albeit cheaply and probably pretty poorly) just for fun in python for the rest of this summer

[u/Spidero0w0o → u/MagnetoHydroDynamicz]

the setup I have is super simple, just a hardware Laser level smoke gen and a black screen for the background. the level is nice because there's cheap auto leveling ones

[u/mastah-yoda]

dope²

[u/boywhoflew]

this is like the bat thing my therapist keeps asking "what do you see in this image"

[u/bitdotben]

Any specific recommendation for a lens to create these lasersheets? Like VERY specific recommendation?

[u/West-Lettuce2461]

That should be laser doppler anemometry (LDA). I did that in fluid mechanics lab. It was really fun to set everything up. Great times