SHOCK WAVES.

Film Loop: Formation of Shock Waves

Length: 3:45 Min., Black and White, No Sound

A pulsed air jet producing a periodic circular wave first moves over the water surface at about 1/3 (and then 2/3) of the wave velocity; the wave fronts ahead of the source get closer together. When the source velocity exceeds the wave velocity (by about 5%) a shock wave builds up and moves along with the source. When the ratio of source to wave velocity is about 1.6 the cone of the shock wave is quite sharp. At one point the motion is frozen and animation is superposed to show the relationship of the shock wave angle to the wave and source velocities.

APPARATUS. Same as for Film-Loop 80-237.

NOTES. It took only 2.5 sec for the source to move across the tank at 1.6 times the wave velocity. In order to prevent stoboscopic effects and to be able to observe the effect for a reasonable time the sequence was photographed with a high speed camera. The film is designed to be screened at 16 frames per second (silent speed); the projected phenomena are slowed down by about a factor of 6. The ratio of source to wave velocity is usually called the Mach number. For Mach numbers greater than 1 the reciprocal of that number is equal to the sine of the half angle for the shock cone.

DATA AND DISCUSSION. In the sequence where we first see the shock wave (about Mach 1.05), the measured half angle of the shock cone is 73degrees. In the second sequence (about Mach 1.6) the measured half angle is about 40 degrees; see Fig. 1. At Mach 1.6 we can see a circular concave wave to the rear of the source and moving in the same direction; this is the first circular wave formed as the source originally starts to move across one edge of the tank. What would you observe from the following vantage points: (a) outside the Mach cone, (b) inside the cone, (c) anywhere in the cone-shaped shock itself?