Unsteady shock waves at hypersonic speeds

– Sidrat Tasawoor Kanth

When rockets and aircraft move faster than the speed of sound, strong disturbances called shock waves are created, which propagate in the air along with the object in flight. Shock waves are essentially sharp physical boundaries in space across which a rapid increase in air pressure, temperature, and density occurs.

A selection of schlieren images, shown in sequence at 16 different instances over one time period of shock-wave pulsation, illustrate the unsteady nature of the flow. Here t denotes time and T is the pulsation period.

Under certain conditions, shock waves can naturally exhibit periodic oscillatory motion relative to the moving object. Such behaviour was previously studied for objects with simple geometric shapes that can be described by a single 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 resembles a cone set on a cylinder – the cone angle and difference in the cone base and cylinder diameters constitute the two parameters. The experiments were performed in a high-speed wind tunnel where the object was subject to air flowing at six times the speed of sound (Mach number = 6). To visualise the shock waves, an optical imaging technique called Schlieren was employed using lenses, a laser, and a high-speed camera.

Photo of group members (gathered around the Roddam Narasimha Hypersonic Wind Tunnel, where the experiments for this study were performed)

The researchers found that the different air flow and shock wave patterns created by the two shapes interacted to affect the unsteady shock-wave dynamics. For certain combinations of the parameters, the air flow and shock waves were highly disturbed, resulting in ‘pulsations’, which have much higher amplitude of unsteadiness than the relatively low-amplitude ‘oscillations’ which occurs at other combinations. While this study was aimed at investigating fundamental aspects of shock waves, the findings are relevant to some aspects of design of rockets and high-speed aircraft.

Short video of unsteady shock wave motion from this study: https://www.youtube.com/watch?v=Cr8zacOvbIk

Reference:

Sasidharan, V., & Duvvuri, S. (2021). Large- and small-amplitude shock-wave oscillations over axisymmetric bodies in high-speed flow. Journal of Fluid Mechanics, 913, R7.

https://doi.org/10.1017/jfm.2021.115

Lab website:

http://aero.iisc.ac.in/ds/