How does schlieren photography work and what does it show?

[Clausius2]

I've been googling for some time but I haven't found anything interesting about.

Does anybody know here how is produced a Schileren photograph and which are its physical principles? It is used a lot visualizing flows at wind tunnels. But I don't know what does it represent physically and which is the mechanism for producing the image.

Thanks.

 

[Bystander]

A pressure gradient is necessary to force flow; that gradient implies a density gradient in any real fluid, and that implies a refractive index gradient. Illumination with a collimated source, or imaging with a high f-stop, reveals all density gradients that are not parallel to the direction of illumination as bright (converged light) or dark (diverged rays) on a film or shadow screen.

add this ---- SCHLIEREN

 

[Clausius2]

OOPPS! Thanks for the correction.

And thanks for your explanation. Thank you very much.