Microscopic Scale

A quite simple experimental photographic technique which enables the visualization of the flow of air in the vicinity of a moving body produces a picture called a "shadowgraph".

This technique requires a short duration flash of light, which must originate from a point. As shown in the figure , not even a photographic lens is required. The shadowgraph of the bullet, passing at very close distance in front of a film or photographic plate, visualizes the pressure differences of the flowfield particularly well.

The pictures shown in the three following figures were taken by applying this simple but effective technique.

The first photograph shows a .308 Winchester (7.62 x 51 Nato) FMJ bullet traveling at approximately 2800 ft/s (approx. 850 m/s) (see picture ; Be patient! Depending on your connection, loading of this high-resolution grey scale picture may take some time!).

One may distinguish at least three different shock waves. The first and most intensive one emerges from the bullet's nose and is called the Mach cone. A second shock wave originates from the location of the cannelure, and the third shock wave forms behind the bullet's base. Additionally one can see highly turbulent flow behind the base, which is called the wake.

The flow type at the bullet's surface changes from a laminar boundary layer at the forward region of the bullet, which is characterized by parallel stream lines, into a turbulent flow showing vortexes, beginning at the cannelure.

For a 9 mm Luger FMJ pistol bullet, moving slightly faster than the speed of sound (see picture  Be patient! Depending on your connection, loading of this high-resolution grey scale picture may take some time!) one finds the following significant differences: the Mach cone is still present but no longer attached to the bullet' s tip, and the opening angle of this cone has increased. The wake is still visible, but the boundary layer appears to be laminar from the tip to the base, all along the bullet' s surface.

Finally, for a cal. .32 ACP pistol bullet, moving at a speed considerably below the speed of sound (see picture  Be patient! Depending on your connection loading of this high-resolution grey scale picture may take some time!), all shock waves are absent, and what remains is the turbulences behind the bullet's base.