By the method of Schlieren imaging the field of the density gradient of transparent media can be photographed, revealing flow features such as shock waves, expansions, boundary layers and turbulent structures. As a light source a standard type of LED light has been used, which was pulsed with a high current. By illuminating the flow only 300 nanoseconds per Schlieren image, the flow phenomena that move with a speed faster than the speed of sound are effectively frozen in time.
We captured the start-up of a wind tunnel that establishes a supersonic air flow of Mach 1.7 in the test section. Subsequently, we inject a sonic jet of air transversally in the supersonic flow. The resulting flow is a model flow for studying fuel injection in a scramjet. The flow field is mainly supersonic and highly unsteady. The short pulse time and high light intensity allow detailed imaging of phenomena occurring in the flow with high resolution and minimum motion blur. The results provide new information regarding the nature of the flow, and serve for validation of theoretical studies using computational methods.