Schlieren imaging is a classic tool visualizing supersonic flows. One fascinating case is the interaction between a shock wave and an expansion fan in supersonic flow. Shocks create very sharp density gradients, while expansions are more subtle, making it tricky to capture both in a single visualization.

In this case, researchers used colored masks to help bridge this gap. By extending the “visible range” of density gradients, they reveal details that a simple knife-edge cut-off would miss. Rotating the masks changes the system’s sensitivity, letting researchers highlight gradients in different directions. This makes it possible to see not just the shock and expansion themselves, but also their deflections, curvature, and how they interact.

Among the approaches studied, the band lattice mask proved especially effective—showing both the strong shock structures and the more delicate expansion fringes in one image. This demonstrates how clever use of color can turn schlieren from a basic visualization method into a deeper diagnostic tool for understanding supersonic flows.

Original article by Lara Nel, Beric Skews & Kavendra Naidoo:
Schlieren techniques for the visualization of an expansion fan/shock wave interaction