Question

The figure shows schematic of a set-up for visualization of non-uniform density field in the test section of a supersonic wind tunnel. This technique of visualization of high speed flows is known as: 

• A. schlieren
• B. interferometry
• C. shadowgraph
• D. holography

Hint:

Shadowgraph = brightness variation due to density gradients; Schlieren = requires knife edge; Interferometry = uses fringes.

Answer 1

The Correct Option is C

The schematic shows a setup where parallel light rays pass through a region of varying density inside a supersonic wind tunnel. After passing through the test section, these rays are received on a screen, producing regions of different brightness.
1. Working principle:
Density gradients in high-speed flows cause changes in refractive index. Light rays bend due to these gradients. The resulting curvature in the ray paths creates bright and dark regions on the screen. This visualization method does not require knife edges or focusing filters—only intensity variation on the screen.
2. Why this is shadowgraph (Option C):
Shadowgraph relies on the second derivative of density with respect to spatial coordinates. It shows shock waves, expansion fans, and other compressible flow features as shadows due to refracted rays forming intensity variations. The setup in the figure matches this exactly.
3. Why the other options are incorrect:
- Schlieren (A): Requires a knife edge to cut refracted light and produces sharper images. The shown setup has no knife edge.
- Interferometry (B): Uses coherent light (laser) and interference fringes—not simply rays and a screen.
- Holography (D): Requires reference beams and recording medium—not shown here.
Therefore, the technique shown is a shadowgraph.