Here are a couple "Schlieren" images I made (really just shadow graphs) using a DSLR camera without a lens, the bright video flash (just a bright white LED) on my cell phone, and compressed air going through the oxidizer orifice that will be used for my rocket igniter (I just drilled it totally free hand, no drill press... 1/16" diameter orifice, I think). You can see nice shock diamonds, especially with the last image (which was taken with less pressure drop, actually... I was running my air compressor in "blow down" mode because it's loud). I need to put my pressure gauge closer to the orifice (should be on the orifice side of my valve) to get a better reading of the pressure drop, but I think the first image is at about 100psi, with the next one being significantly less.
I simply projected the shadow of the end of the orifice directly on the DSLR image sensor. I held the orifice pretty close to the camera. To get more contrast, I could have held it further away (allowing the diffracted vs undiffracted light to diverge more, giving more contrast).
And here's what the orifice actually looks like, next to my breadboard that controls the spark generator (indirectly through a relay, ~31Hz PWM) and the valves, with three LEDs hooked up so I can test the code without the valves and stuff connected:
The purple thing is just the FTDI cable used for programming the ATMEGA chip and powering it with my laptop (in the field, it has its own 5V power supply). FTDI is optional, I'll probably remove it once everything is integrated.
I'll post more later.
