So I have been going down a slow-motion rabbit hole. I recently posted about the Slow Mo Guys’ recent video where they filmed eggs being shot by 9mm bullets and filmed them at various frame rates up to 1 million frames per second. Well, Destin of Smarter Every Day has a video he made back in 2018 looking at shockwave shadows coming off supersonic and subsonic bullets.
Slow Motion @TFB:
- Slow Mo Guys 1 Million FPS – 9mm vs Egg and Newton’s Cradle
- Wheelgun Wednesday: Slow Motion S&W 500 – Mind The Gap
- Rifle Dynamics Slow-Motion AK Rifles
In the Slow Mo Guys’ 9mm vs Egg video, there were some shadows cast on the egg. They hypothesize these may be the shadows of shockwaves coming off the 9mm bullet. There is a way to see shockwave shadows but it requires a special setup that capitalizes on something called Schlieren imagery. According to Wikipedia:
Schlieren are optical inhomogeneities in transparent media that are not necessarily visible to the human eye.
The easiest way to capture shockwave shadows is using a concave mirror, a single point source of light, a razor’s edge and a camera.
Here is a video that explains how a Schlieren imaging setup works.
Now that you know how to set this up, let’s see what Destin was able to capture. He uses a Phantom v2511 high-speed camera.
Here is his concave mirror.
Here is his single-point light source and razor blade.
His buddy Coop shoots a Barrett 82A1 through his shop so the bullet flies past the concave mirror.
Below is one of the images captured by the Phantom v2511. If you notice this was captured at 171,504 frames per second.
Destin provides a great graphic explanation of how the shockwave shadows create a mach angle. Based on the angle you can measure the speed of the object.
Below is the equation. 28.85º is the angle of the shockwave shadow and using the equation below gives you a speed of 2.072 MACH.
What About Subsonic Shockwave Shadows?
This is what led me down the path to finding Smarter Every Day’s video. I was curious if the shockwave shadows are exclusive to objects traveling faster than the speed of sound. They are. However, Destin captured something interesting.
In order to compare supersonic vs subsonic, they used .300BLK bullets.
This image was only captured at 28,546.3 frames per second, it is the supersonic .300BLK bullet.
Now compare that to the image below of the subsonic .300BLK bullet. Notice there is no mach angle? Yet there are some lines emanating off the bullet.
Destin then consults with Dr. Kanistras who is an aerodynamics professor at the University of Alabama in Huntsville.
According to Dr. Kanistras, the small peaks emanating from the subsonic shockwave shadows are actually supersonic shockwaves due to the air moving over the bullet.
Revolver Shockwave Shadows
At the end of the video, Destin fires a revolver and films the shockwaves coming off of the cylinder gap.
Here is the Smarter Everyday video on shockwave shadows.
This is pretty cool and now I want to try this, albeit at a slow frame rate and lower resolution, with my Chronos 1.4 slow-motion camera.