When rockets and aircraft move faster than the speed of sound, strong disturbances called shock waves are created around the flight body. Under certain conditions, shock waves can exhibit periodic oscillatory motion relative to the body. Such behaviour is of fundamental interest, and was previously studied for simple bodies that can be described by a single geometric parameter.

Now, researchers in the Department of Aerospace Engineering, led by Duvvuri Subrahmanyam, have uncovered new and interesting dynamics that govern unsteady shock-wave motion when two geometric parameters are at play. Their moving object is a cone set on a cylinder – the cone angle and the ratio of cone base and cylinder diameters constitute the two parameters. It was subject to air flow at six times the speed of sound in a hypersonic wind tunnel. An optical imaging technique called Schlieren was used to study the shock-wave behaviour.

The researchers found that the different air flow patterns created by variation in the governing parameters resulted in two disparate states of shock-wave unsteadiness. In a certain parameter regime, the shock waves were highly disturbed, resulting in ‘pulsations’, which have much higher amplitude of unsteadiness than the relatively low-amplitude ‘oscillations’ that occurred in a different regime.